In the practice of diagnosing shock absorbers and suspension, the method of measuring the grip of wheels with the road and the method of measuring the amplitude are used.
The diagram of the method of diagnosing the grip of wheels with the road is shown in the figure:
Rice. Scheme of the method for diagnosing shock absorbers by wheel adhesion: 1 - car wheel; 2 - spring; 3 - body; 4 - shock absorber; 5 - the axis of the car; 6 - measuring platform
With this method, the oscillation base is rigid in the lower part and spring-loaded only in the upper part. The technology for checking shock absorbers and suspension when using the wheel adhesion method is as follows. First, the vehicle wheel to be tested is installed exactly in the middle of the measuring platform of the shock absorber. At rest, the static weight of the wheel is measured. Then the drive for moving one of the platforms in the vertical direction (first left, then right) is turned on. With the help of an electric motor, periodic excitation of oscillations with a frequency of 25 Hz is carried out; in this case, the measuring platform moves as a rigid link. The resulting dynamic wheel weight (weight on the plate at 25 Hz) is compared to the static weight by dividing the former by the latter.
Example. Let the static weight of the wheel at 0 Hz be 500 kg and the dynamic weight at 25 Hz be 250 kg. Then the wheel weight drop coefficient (in percent), measured by the wheel adhesion method, will be (250/500) * 100 = 50%.
The obtained values of the weight drop coefficient of the left and right wheels and their difference (in percent) are displayed on the monitor screen.
The state of shock absorbers is characterized by the following ratios:
- good - not less than 70% (for sports suspension - not less than 90%)
- weak - from 40 to 70 (from 70 to 90)
- defective - less than 40% (from 40 to 70%)
The results of assessing the condition of shock absorbers should not differ by more than 25% on the sides of the vehicle. The processing of the results is based on empirical values that have been obtained through serial studies of vehicles from various manufacturers. It is assumed that in an average car, the stiffness of the shock absorbers, as a rule, increases with increasing axle load.
The considered method has the following disadvantages: the measurement results depend on the air pressure in the tire of the vehicle being diagnosed; when diagnosing, it is imperative that the wheel be located exactly in the middle of the shock absorber platform; the application of constant external forces, lateral forces, affects the lateral movement of the car, which affects the test results.
Diagnosis by the method of measuring the amplitude, used on the equipment of the firms "Bogue" and MAHA, is more progressive. The platform of the stand is suspended on a flexible torsion bar, the oscillation base is spring-loaded both in the upper and lower parts, which makes it possible to measure not only the weight, but also the amplitude of oscillations at operating frequencies.
The technology for checking shock absorbers and suspension using the amplitude measurement method is as follows. The car wheel, installed on the platform of the stand, oscillates with a frequency of 16 Hz and an amplitude of 7.5 ... 9.0 mm. After turning on the electric motor of the stand, the wheel of the car oscillates relative to the resting masses of the car, the oscillation frequency increases until it reaches the resonant frequency (usually 6 ... 8 Hz).
Rice. Scheme of the method for diagnosing shock absorbers by amplitude fluctuations (the designations are the same as in the previous figure)
After passing the resonance point, the forced excitation of oscillations is stopped by turning off the electric motors of the stand. The oscillation frequency increases and crosses the resonance point at which the maximum suspension travel is reached. In this case, the frequency amplitude of the shock absorber is measured.
The performance characteristics of the shock absorber are determined in the "throttle" and "valve" modes. In throttle mode, when the maximum piston speed is not more than 0.3 m/s, the rebound and compression valves in the shock absorber do not open. In valve mode, when the maximum piston speed in the shock absorber is more than 0.3 m / s, the rebound and compression valves open, and the more, the greater the piston speed.
Diagrams when testing the shock absorber on the stand are recorded in throttle mode at a frequency of 30 cycles per minute, a piston stroke of 30 mm, a maximum speed of 0.2 m/s. In the case where the shock absorber is tested in a shock absorber strut, the piston stroke is 100 mm. The charts are recorded in valve mode at 100 cycles per minute, the same piston stroke as in throttle mode, and at a maximum piston speed of 0.5 m/s.
When testing shock absorbers, a defect is the appearance of liquid on the rod and at the upper edge of the strut cuff or shock absorber gland, provided that the liquid appears again after wiping the leak. A defect is the presence of knocks, squeaks and other noises, with the exception of sounds that are associated with the flow of fluid through the valve system, as well as the presence of an excess amount of fluid ("backup"), fluid emulsification, insufficient fluid ("failure").
The deviation of the shape of the curved diagrams from the reference is also considered a defect. The figure shows the reference chart shape and the shock absorber chart shape with defects.
Rice. Diagrams of the operation of serviceable and defective shock absorbers: I, II, III - sections indicating the presence of liquid emulsion, "failure" and "backup" respectively; Ro, Ps - resistance forces during the rebound and compression stroke
The oscillation amplitude is determined by the movement of the test platform following the wheel and is recorded. In this case, the maximum deviation (maximum oscillation amplitude) is also measured. It is recalculated and displayed on the monitor screen separately for the left and right shock absorbers. According to the oscillation graph on the monitor screen, you can evaluate the effectiveness of shock absorbers, even without knowing the parameters set by the manufacturer: the smaller the resonance amplitude on the graph, the better the shock absorber works.
Rice. Shock absorber amplitude
An example of documenting the results of checking the shock absorbers of the front and rear axles of a vehicle on the stand is shown in the figure.
Rice. Shock absorber control data
The oscillation amplitude values measured for each wheel at the resonant frequency are displayed in millimeters. In addition, wheel travel differences are displayed for both shock absorbers on the same axle. Thanks to this, it is possible to judge the mutual influence of both shock absorbers of one axle.
The state of the shock absorbers in terms of the amplitude indicator is determined as follows:
- good - 11 ... 85 mm (for the rear axle weighing up to 400 kg - 11.75 mm)
- bad - less than 11
- worn out - more than 85 mm (for a rear axle weighing up to 400 kg - more than 75 mm).
The wheel travel difference should not exceed 15 mm.
On shock absorber test benches, for example from MAHA, you can search for suspension noise. In this mode, the operator can set the rotor speed himself (from 0 to 50 Hz). Without the noise search mode, the source of the noise must be searched for in a split second, while the vibrations of the suspension are damped.
Maintenance of stands for testing shock absorbers and suspension includes checking the fastening of the stand to the base, as well as all threaded connections every 200 hours of operation and at least once a year. Every 200 hours of operation, the levers of the stand are lubricated with thick grease.
Long-term operation of the vehicle on dirt roads or with poor asphalt pavement will inevitably lead to increased wear on the vehicle's suspension. Against the background of significant loads, long-term work, the entire chassis of the car, including shock absorbers, “crumbles”. Consider in this article the obvious signs of a malfunction of shock absorbers and the existing diagnostic methods.
You can diagnose the signs of a malfunction of shock absorbers in different ways:
- perform a visual inspection;
- "test" suspension response in wiggle mode;
- when driving, the controllability of transport is assessed;
- carry out instrumental control (bench diagnostics).
Let's take a look at the presented approaches.
The most reliable option is a visual inspection at the repair pit. By the way, this method is the cheapest. When examining the shock absorbers, it is necessary to reveal darkening from oil on the surface of the parts. Remember that oil streaks should not be observed here. This factor marks the loss of tightness of the shock absorber components. So, such a shock absorber will not last long. If in doubt about the result, such a shock absorber should be wiped dry, and after a couple of days, a visual inspection should be repeated. Considering the design, evaluate the condition of the anthers, the rebound buffer - traces of oil are also possible here. You can evaluate the shock absorber by examining the condition of the tires. If uneven wear spots are visible on the edge of the tire, this is a “defect” caused by the influence of a faulty shock absorber.
Let us find out the essence of the wiggle test.
This simple method allows you to identify a clearly "killed" element. It is necessary to swing the car around the corner. Then they let the car go down. Too long coasting or abrupt stop in any one position are clear signs of shock absorber failure on the side of the car where you made efforts to rock it. An uncharacteristic rattle, clatter, clicks or even a knock in the process of rocking the car is also a reason for a more detailed diagnosis of the state of the shock absorbers. Extraneous sounds in the suspension when the car is moving over bumps is also a clear sign of problems in the shock absorbers.
Assessing handling while driving.
The design is considered faulty if, at a speed of more than eighty km / h, the car begins to “scour” from side to side, as if in a rut. This behavior is also observed at lower speeds but on the road with a large number of bumps along the course of movement - stability decreases sharply, vertical swing occurs, and extraneous sounds appear. On high-speed turns, the reaction of the car to the steering wheel is reduced. Often, the development of symptoms occurs gradually and the driver gets used to such behavior of the car, not paying attention to the development of destructive processes in the design of shock absorbers.
Instrumental control (diagnostics on the stand).
By far the most accurate and complete diagnostic method. With the help of test benches, the damping properties of each shock absorber are evaluated individually. The vibration stand at the exit will provide a diagram of the results of measurements of axial vibrations. It will be possible to determine the condition of the components by comparing the diagram and the allowable amount of axial oscillation of a working shock absorber.
P.S. I suggest you watch a funny video that tells how to identify signs of shock absorber failure using a rather unusual method - the stick method!
WHA classic. Diagnostics of malfunctions of shock-absorbers by means of a stick!
Article info:
Long-term operation of the vehicle on dirt roads or with poor asphalt pavement will inevitably lead to increased wear on the vehicle's suspension. Against the background of significant loads, long-term work, the entire chassis of the car, including shock absorbers, “crumbles”.
Signs of bad shock absorbers
Publication date: 01/08/2016
The test bench is the most accurate and complete diagnostic method. With the help of test benches, the damping properties of each shock absorber are evaluated individually. The vibration stand at the exit will provide a diagram of the results of measurements of axial vibrations.
After cleaning, the parts are subjected to control and sorting (troubleshooting).
Troubleshooting - determination of the technical condition of parts; sorting them into suitable, requiring repair and unusable; determining the route for parts requiring repair.
To fit includes parts whose deviations in size and shape are within the allowable wear specified in the technical specifications for the repair of the machine.
Parts subject to repair, the wear of which is higher than permissible, or there are other recoverable defects.
Unfit parts are those whose restoration is impossible or economically unreasonable due to high wear and other serious defects (deformations, fractures, cracks).
The reasons for the rejection of parts are mainly various types of wear, which are determined by the following factors:
constructive- the limiting change in the dimensions of the parts is limited by their strength and structural change in the interface;
technological- the limiting change in the dimensions of parts is limited by the unsatisfactory performance of its service functions in the operation of a unit or unit (for example, the wear of the pump gears does not provide pressure or injection performance, etc.);
quality- a change in the geometric shape of parts during wear impairs the operation of a mechanism or machine (wear of hammers, jaws of crushers, etc.);
economic- the permissible reduction in the size of parts is limited by a decrease in the productivity of the machine, an increase in the loss of transmitted power due to friction in the mechanisms, an increase in lubricant consumption and other reasons, which affects the cost of the work performed.
Troubleshooting of equipment parts is carried out in accordance with the technical specifications, which include: general characteristics of the part (material, heat treatment, hardness and main dimensions); possible defects, permissible size without repair; the maximum allowable size of the part for repair; signs of final marriage. In addition, the technical specifications provide instructions on the permissible deviations from the geometric shape (ovality, taper).
Specifications for troubleshooting are drawn up in the form of special cards, in which, in addition to the above data, methods for restoring and repairing parts are indicated.
The data given in the specifications relating to the allowable and limiting values of wear and dimensions should be based on materials according to
study of wear taking into account the operating conditions of parts.
Parts defect and control visually and with a measuring instrument, and in some cases with the use of devices and measuring instruments. Visually check the general technical condition of the parts and identify visible external defects. For better detection of surface defects, it is recommended to pre-clean the surface thoroughly and then pickle it with a 10-20% sulfuric acid solution. In addition, with the visual method, defects are detected by tapping and feeling parts.
The control of latent defects is carried out by hydraulic, pneumatic, magnetic, luminescent and ultrasonic methods, as well as by X-rays.
Hydraulic and pneumatic troubleshooting methods are used to control parts and assemblies for tightness (water and gas tightness) and to detect cracks in body parts and vessels. To do this, use special stands equipped with tanks and pumping systems.
The magnetic method for troubleshooting parts is based on the appearance of a magnetic stray field when a magnetic flux passes through a defective part. As a result, the direction of the magnetic field lines on their surface under these defects changes (Fig. 22) due to unequal magnetic permeability.
/ control method- to detect defects (cracks, etc.), the surface of the part is coated with ferromagnetic powder (calcined iron oxide-crocus) or a suspension consisting of two parts of kerosene, one part of transformer oil and 35-45 g / l of finely crushed ferromagnetic powder (dross). For a clearer detection of magnetic field disturbances on light parts, it is recommended to use black magnetic powders, and red ones on dark surfaces. This type of control is more sensitive in detecting internal defects of the part and is used when the magnetic characteristics of the material of the part are unknown.
2 way control - detection of surface cracks and small and medium parts made only from high carbon and alloy steels. It is more productive and more convenient than the I method. To better detect defects, various types of magnetization of parts are used. Transverse cracks are better detected when
longitudinal magnetization, and longitudinal and angled - with circular magnetization.
Longitudinal magnetization is carried out in the field of an electromagnet or
Rice. 23. Schemes of methods for magnetizing parts:
a, b - longitudinal; V. G - circular; d, e - combined; 1 - magnetized part; 2 - solenoid solenoid (Fig. 23, a, b) circular - by passing alternating or direct current of high power (2000-3000 A) through a part or a copper rod installed in a hole in hollow parts - bushings, springs, etc. (Fig. 23, c, d). To detect a defect of any direction in one step, combined magnetization is used (Fig. 23, d, f).
After magnetic flaw detection, the parts must be washed in clean transformer oil and demagnetized. The scheme of the magnetic flaw detection device is shown in fig. 24. The device consists of a device for magnetizing 2, magnetic starter 3 and transformer 4.
The device for circular magnetization is a stand to which a table with a lower contact copper plate and a movable head with a contact disk moving along the stand are fixedly fixed. Part 1 is tightly clamped between the contact and the plate and the transformer (or battery) is turned on. The current from the secondary winding of the transformer with a voltage of 4-6 V is supplied to the copper plate and the contact disk and in contact with the workpiece 1 magnetization occurs, which lasts 1-2 s. Then the part is immersed in a suspension bath for 1-2 minutes, removed and inspected to determine the location of the defect.
At repair enterprises, the universal magnetic
flaw detector type M-217, which allows for circular, longitudinal and local magnetization, magnetic testing and demagnetization.
The flaw detector consists of a power unit, with the help of which a magnetic field is created, a magnetizing device (contacts and a solenoid) and a bath for magnetic suspension.
The industry also produces other magnetic flaw detectors: stationary - MED-2 and 77PMD-ZI, as well as portable 77MD-1Sh and semiconductor PPD.
Portable flaw detectors make it possible to inspect parts directly on machines, especially large parts that are difficult or impossible to remove and examine using stationary installations.
Only steel and cast-iron parts can be inspected by the method of magnetic flaw detection, establishing external and internal defects up to 1-10 microns in size.
The luminescent method of parts control is based on the ability of certain substances to fluoresce (absorb) radiant energy and give it off in the form of light radiation for some time when the substance is excited by invisible ultraviolet rays.
This method reveals surface defects such as hairline cracks on parts made of non-magnetic materials. A layer of fluorescent liquid is applied to the surface of the part under study, which penetrates into all surface defects in JO-15 min. After that, excess liquid is removed from the surface of the part. Then on
a thin layer of developing powder is applied to the wiped surface, which draws out the fluorescent liquid that has penetrated there from cracks and other defects. After irradiating the surface of the part with ultraviolet light, the places from which the fluorescent liquid was drawn begin to glow, indicating the localization of surface defects.
A mixture of 85% kerosene, 15% low-viscosity mineral oil with the addition of 3 g per liter of OP-7 emulsifier is used as a fluorescent liquid, and developing powders consist of magnesium oxide or silica gel. The sources of ultraviolet radiation are mercury-quartz lamps of the PRK-1, PRK-4, 77PLU-2 and SVDSh types with a special light filter UFS-3. Also apply
portable unit LUM-1 and stationary flaw detector LDA-3.
Using the luminescent method, it is possible to determine surface defects with sizes of 1–30 µm.
The ultrasonic testing method is based on the reflection of ultrasonic vibrations from the existing internal defects of the part when they pass through the metal due to a sharp change in the density of the medium.
Rice. 25. Schemes of operation of ultrasonic flaw detectors:
a - shadow method (defect not detected); b - shadow method (defect detected);
- reflection method
In the repair industry, there are two methods of ultrasonic flaw detection: sound shadow and reflection of pulses (signals). With the way of sound shadow(Fig. 25, a, b) ultrasonic generator / acts on a piezoelectric plate 2,
which in
in turn acts on the part under study. 3.
If along the path of ultrasonic waves 4
turns out to be a defect 6,
then they will be reflected and will not fall on the receiving piezoelectric plate 5, as a result of which a shadow will appear behind the defect, which is marked by the recording device 7. "
With reflection method(Fig. 25, V) from the generator 12
through a piezoelectric transducer 9
ultrasonic waves are transmitted to the workpiece 3,
passing it and reflected from its opposite end, they return to the receiving probe 8.
If there is a defect 6
ultrasonic pulses will be reflected earlier. Caught on the receiving probe
8
and the pulses converted into electrical signals are fed through an amplifier 10
into a cathode ray tube 11.
Using a sweep generator 13,
switched on simultaneously with the generator 12,
the signals receive a horizontal sweep of the beam on the screen of the tube 11, where the initial pulse appears in the form of a vertical peak. Reflected from the defect, the waves return more quickly, and a second pulse appears on the screen, spaced from the first at a distance /j. The third pulse corresponds to the signal reflected from the opposite side of the part. The distance / 2 corresponds to the thickness of the part, and the distance / t corresponds to the depth of the defect. By measuring the time from the moment the pulse is sent to the moment the echo is received, the distance to an internal defect can be determined.
For repair purposes, an improved ultrasonic flaw detector UZD-7N is used.
The maximum penetration depth for steel is 2.6 m with flat and 1.3 m with prismatic probes, the minimum depth is 7 mm. In addition, our industry produces ultrasonic flaw detectors DUK.-5V, DUK-6V, UZD-YUM, etc. with high sensitivity, which can be used in repair production.
X-ray control is based on the properties of electromagnetic waves to be absorbed differently by air and solids (metals). Beams passing through materials slightly lose their intensity if they encounter voids in the controlled part in the form of cracks, shells and pores on their way.
The output beams projected onto the screen will show darker or more brightly lit areas that differ from the general background.
These spots and streaks of varying brightness indicate defects in the material. In addition to X-rays, rays of radioactive elements - gamma rays (cobalt-60, cesium-137, etc.) are used in flaw detection. This method is complicated and therefore rarely used at repair enterprises (when inspecting seams near the body of rotating furnaces and mills, etc.).
Troubleshooting parts with paint is widely used in repair practice when repairing equipment at the installation site or in stationary conditions when inspecting large parts such as frames, beds, crankcases, etc.
The essence of the method lies in the fact that the investigated surface of the part degreased with gasoline is painted with a special bright red liquid, which has good wettability and penetrates into the smallest defects (within 10-15 minutes). Then it is washed off the part and the latter is painted with white nitro enamel, which absorbs the coloring liquid that has penetrated into the defects of the part. The liquid, speaking on the white background of the part, indicates the shape and size of the defects. Determination of defects with the help of kerosene and chalk coating is based on this principle.
The control and troubleshooting of various parts of equipment are characterized by certain features in which specialized tools and equipment are used.
Shafts. The most common shaft defects are curvature, wear of bearing surfaces, keyways, threads, splines, threads, necks and cracks.
The curvature of the shafts is checked in the centers of a lathe or a special machine for runout, using for this purpose an indicator mounted on a special stand.
The ovality and taper of the crankshaft necks are determined by measuring a micrometer in two sections spaced from the fillets at a distance of 10-15 mm. In each belt, the measurement is made in two perpendicular planes. The limit dimensions of seats, splines, keyways are estimated using limit brackets, templates and other measuring tools.
Shaft cracks are detected by external inspection, magnetic flaw detectors and other methods. Shafts and axles are rejected if cracks are found with a depth of more than 10% of the shaft diameter. Reducing the diameter of the shaft journals during turning (grinding) in the case of shock loading is allowed by no more than 5%, and with a calm load, not
more than 10%.
Gear wheels. The suitability of gears for work is judged mainly by the wear of the tooth in thickness (Fig. 26). The teeth are measured in thickness with caliper gauges, tangential and optical gear gauges, and templates. Tooth thickness of spur gears
measured in two sections. Three teeth are measured for each gear, located one relative to the other at an angle of 120 °. Before starting the measurement, the most worn teeth are marked with chalk. The maximum tooth wear in thickness (counting along the pitch circle) should not exceed: for open gears (III-IV classes) Rolling bearings. To control rolling bearings, devices of various types are used, on which the radial and axial backlashes in the bearings are determined. Radial A)
backlash is checked using the device shown in fig. 27. The bearing to be checked is mounted on the mandrel with the inner ring and clamped with a nut. On top of one end of the rod 4
rests against the surface of the outer ring of the bearing, and the other against the foot of the control minimeter 5.
Bottom one end of the rod 2
rests against the surface of the outer ring of the bearing, and the other end is connected to the lever system. Kernel 4
goes through the tube 3,
a rod 2
- in the head. A tube 3
and rod 2
connected with a ruler by means of levers 1,
on which the goods move R. If the cargo R located on the right side, the tube 3
presses on the outer ring of the bearing from above - the ring will move down, as a result of which the rod 4
will also move down and on the minimeter 5
fix the indication of the arrow. If the cargo R moves to the left side, then the rod presses on the outer ring of the bearing 2
- the ring will move up. Kernel 4
also moves up, while again fixing the minimeter reading. The difference between the indications of the arrow of the minimeter and will be the radial clearance in the bearing under test.
Repair planning
Maintenance and repair of equipment with PPR systems is planned by an annual plan (PPR schedule), which is an integral part of the technical and industrial financial plan of the enterprise. It is developed for a year. Equipment repairs are scheduled monthly. The planning of repair work and maintenance of equipment is reduced to determining the number and types of repair and maintenance, setting the deadlines for the completion of these works, determining their labor intensity, rational distribution of repair workers and on-duty personnel by workshops and sections, calculating the necessary material resources and cash costs. This plan is developed on the basis of the planned number of hours of operation of the machine for the year, data on the number of hours worked by machines at the beginning of the year from the start of operation (or after a major overhaul).
The annual repair plan for the equipment of the enterprise is developed at the end of each year for the subsequent planning period by the department of the chief mechanic (CMO) of the plant with the participation of shop mechanics, coordinated with the planning and production department and approved by the chief engineer of the enterprise. The elements of the plan are first developed for the workshops of individual industries and auxiliary sections of the enterprise, and then a master plan for the PPR for the whole enterprise is drawn up.
Based on the annual plan for the maintenance and repair of equipment, an annual schedule for the overhaul of equipment is compiled, which serves as the main document for financing the overhaul of equipment.
Monthly equipment repair plans for workshops are drawn up at the end of each month for the next month on the basis of annual and quarterly plans by the department of the chief mechanic with the participation of workshop mechanics. The monthly plan for the repair of equipment serves for the operational management and control of the implementation of the PPR system in the workshops of the enterprise (preparation for the replacement of repaired machines, etc.).
The plan for the mechanical repair shop and the electrical shop for the next month is developed on the basis of a general plan for the repair of machines and assemblies, orders from mechanics for the manufacture of spare parts, etc. Modernization of some types of equipment is carried out according to a separate plan linked to the repair plan for the main equipment.
The preparation of the annual plan is based on the actual condition of the equipment, as well as the repair standards given in the current instructions and regulations for the PPR.
The alternation of repairs, inspection and overhaul periods for machines is different, which is explained by the different conditions of their operation, as well as the service life of parts.
To take into account the planning of repair work, it is necessary to know the complexity of their implementation.
For preliminary calculations of the volume of repair work, the equipment is divided into groups (categories) of repair complexity, taking into account the degree of complexity and repair features of the machines. The more complex the equipment, the larger its main dimensions and the higher the required accuracy or quality of the products, the higher the category of complexity of its repair. The repair complexity group shows how many conditional repair units are contained in the total labor intensity of repairing a given machine.
A quantitative characteristic of the complexity of repair r of specific models of equipment is the complexity of their overhaul (QH). The relationship between the category of repair complexity and the complexity of their overhaul is determined by the "dependence
where K k is the norm of labor intensity of a repair unit during a major overhaul.
The norms of labor intensity of a conditional unit of repair complexity in different industries of building materials are different, which is explained by the specifics of the equipment and the conditions of their work. Thus, in the asbestos-cement industry, the sheet-forming machine SM-943 was adopted as a reference unit, the repair complexity of which is 66 units with a unit of labor equal to 35 man-hours. This conventional unit of repair complexity of the mechanical part is assigned to the 4th or 5th category of the seven-digit grid of the pieceworker, when 65% falls on locksmith and other work and 35% on machine work.
In the industry of prefabricated reinforced concrete, one conventional unit of repair complexity for the mechanical part of technological equipment at the cost of overhaul is taken equal to 50 man-hours, assigned to the 4th category of the pieceworker's tariff scale.
Table 3
Distribution of a conditional unit of repair complexity of mechanical (A "n), electrical (R" e) equipment for the precast concrete industry
The group of repair complexity r for equipment of factories of industrial building materials is given in the sectoral provisions of the PPR.
The labor intensity of a conditional unit of repair complexity for precast concrete equipment for various repairs is given in Table. 3.
The total labor intensity of repair (man-hours) of any machine, taking into account the repair of its electrical equipment
Qk \u003d KmChm + KeChe, (40)
where Km and Ke are the labor intensity of a conventional unit of repair complexity of mechanical and electrical equipment, man-hour; Chm and Che - groups of repair complexity of mechanical and electrical equipment.
Table 4
Equipment downtime rates per conventional unit of repair complexity
Note. When the enterprise operates according to the regime of a six-day working week with one day off, the machine downtime rates are accepted with a coefficient of 1.15.
The duration of machine downtime during repairs depends on the complexity of the repair, the composition and qualifications of the repair team, the repair technology and the level of organizational and technical measures. Downtime rate (days) of equipment under repair (with a 5-day working week with two days off)
where N is the downtime rate for precast concrete equipment, determined from Table. 4; r - group of repair complexity of the mechanical or electrical part of the equipment.
The time of operational testing of the machine after repair does not count towards the total downtime if it worked normally.
Downtime (days) of equipment under repair can also be determined by the formula
where ti is the norm of time for performing locksmith work for machines of the first group of repair complexity; r m - machine repair complexity group; M - coefficient taking into account the method of performing repair work (when working without metalwork preparation of parts M = 1; with preliminary preparation of parts M = 0.75-0.8; with the nodal repair method M = 0.4-0.5); nc - the number of locksmiths working in one shift; tcm - shift duration, h; C is the number of work shifts per day; Kp - coefficient taking into account the overfulfillment of the standards for the production of locksmiths (K = 1.25).
The PPR system of equipment is based on the theory of wear of machine parts. The construction of the structure of the repair cycle for the machine is based on the analysis of changes in the performance of the machine during the entire repair cycle.
An important condition that determines the possibility of using a preventive system is the frequency and frequency of maintenance and scheduled repairs in the repair cycle. This condition is generally determined by the dependence
where N is the number of parts to be replaced during the repair cycle; TC - the time of operation of the machine between the two most complex repairs (repair cycle); ti - average service life (resource) of parts of this group before replacement; ni is the number of parts with an average service life.
The construction of a rational schedule for the repair cycle is possible if the values of Тц and tt are multiples of each other and are equal to an integer:
Pi \u003d Tc / ti - (44)
The value of Pi is called the shift factor and shows how many times the service life of the parts of this group is less than the service life until the next most difficult repair. This value determines the nature of maintenance and repair measures, as well as the structure of the repair cycle.
The main indicator of the PPR system is the duration of the overhaul period. It takes into account the reliability of the equipment and methods of its operation.
The overhaul period should be determined by the limit value of the wear curve of a characteristic part and service life (resource), using the rules of mathematical statistics.
For a justified construction of a PPR system, it is necessary to choose the optimal structure of the repair cycle and have the value of the resources of the units for calculating the duration of the overhaul period.
In practice, the structure of the repair cycle and the intervals between overhaul periods are established on the basis of statistical data on the actual average service life of machine parts.
At present, the task is to set the parameters of the repair cycle by economic calculations, and when creating a new machine, to design parts with certain service lives corresponding to the repair schedule.
Has it knocked? so you need to find in time, according to the source of extraneous knocking, a malfunction in the car.
There are many sources of extraneous knocking, from wear of parts, in a car, and if it is possible to identify the malfunction in time and replace the worn part, repairs will be much cheaper. But for many beginners, this is not so easy, and many drive until the moment when the car finally gets up. Only now the fuss with the repair will be much more and it will already cost much more. In order not to bring to this, you need to be able to identify at least the main malfunctions of the chassis, which are described in this article.
I already wrote about extraneous sources of noise in the engine and those who wish can read by clicking here. In the same article, we will talk about the main malfunctions of the chassis of the car and the knocks that the worn parts of the chassis emit. And let's try to figure out the causes of knocks that can occur in the front suspension and steering of cars that have MacPherson suspension. These are the majority of foreign cars and our front-wheel drive domestic cars (VAZ 2108; 210.9; 2110, etc.). Although we will also touch on the rear-wheel drive classics a little (read the ball joints below).
By the way, even for car repairmen, finding the real cause of knocking in the MacPherson front suspension is not so simple. And they often sin on a completely serviceable shock absorber strut, but the true reason for the knock is completely different. They probably think that because of its complex structure, it is unreliable and short-lived. But fragility can still be somehow attributed to domestic cars, but on foreign cars this part works out to the fullest, and the reason for the knock most often comes from other chassis elements.
In general, any knock that appears in the suspension of the car must be immediately found and eliminated, as it serves as an alarm signal for more serious malfunctions. But let's start in order.
Steering.
In addition to the device and steering malfunctions, I advise you to read here in And I started with the steering because steering rack knock, often confused with rack banging MacPherson type. And they confuse because when the car is moving along small bumps in the road, the knock from the steering rack is heard only from one side, that is, just like when the shock absorber strut malfunctions, and this is what misleads many beginners. But after all, shaking is also felt on the “steering wheel” itself (steering wheel).
The main causes of knocking in the steering are increased clearance in the engagement of the steering rack and gear, from the wear of the teeth of these parts, or from the wear of the rack support bushings (often these bushings are not made of bronze, as before, but of some incomprehensible shit). To check exactly what is worn out in this node, a simple trick will help: pull the tie rods up and down, watching the movements of the rack at this moment. If she stands still, then everything is fine, but if she goes up and down, then her bushings are worn out. Well, if the steering rack also turns, then this means there is an increased gap between the teeth of the gear and rack. But this can be corrected by adjustment. Also, during this check, it is possible to identify worn bushings for fastening the steering rods to the rack itself.
The source of an extraneous knock can also come from a worn steering joint, and it is also easy to check. To do this, we seat the assistant behind the wheel, and he must vigorously and without interception (without changing speed) turn the steering wheel left and right. At this time, you should feel the steering rod joints, that is, grab the hinge with your hand so that you simultaneously hold both the hinge body and its pin, or parts of the steering rigidly connected to it. With this check, you will clearly feel even a minimal play in the steering joint (of course, if it is worn out).
Upper shock absorber mount.
The device of the upper support can be seen in Figure 1. It consists of a rubber support - damper 2 and bearing 3. Over time, due to the loss of elasticity of the damper rubber, a muffled knock appears when it hits medium and large road irregularities. To be sure of the cause of the knock, you need to measure the gap between support 2 and limiter 1 (this cannot be done on a VAZ 2110 car, since the engineers wanted to close this assembly). And if measurements show that the gap exceeds 1 centimeter (10 mm), then the rubber support (depfer) must be replaced. It should be noted that often the gap is not uniform in a circle (more on one side and less on the other). So we choose the average value.
And yet, what causes this knock, because there is no contact between metal parts during a breakdown? But it should be borne in mind that the hydraulic system of the shock absorber does not have time to extinguish short and sharp movements of the piston in the shock absorber cylinder. For this, there is a rubber support, which, while not old, has the necessary elasticity. If the energy intensity of the rubber decreases over time, then the impacts are already damped worse and harder transmitted to the car body, and the metal body responds to this with a rumble or knock.
Knock from bearing wear. This knock is almost the same as with the loss of elasticity of the support-depfer, but it is more sonorous and sharp. But to fully assess the real state of the bearing, you can only dismantle the rack. And moreover, the bearing wears out unevenly and uneven wear appears in its raceways, and it is in the area where the bearing works the most, that is, during the rectilinear movement of the machine. Based on this, it is possible to identify a bearing malfunction, that is, if you notice that the knock appears only during rectilinear movement, and disappears when cornering, then the cause of the knock is the support bearing.
Even when checking, you can use this technique. Ask an assistant to rock the car body up and down, and in the meantime, feel the shock absorber rod with your hand. The knock of a worn support bearing will be transmitted to the rod, which means that by comparing the knock at different angles of rotation of the wheels, it is possible to identify the condition of the bearing (here, too, with even wheels, the knock will appear, and with the wheels turned, the knock will disappear).
I also advise you to check the tightness of the nut of the upper support, sometimes it is unscrewed and a similar knock appears.
Ball bearings.
This is a common source of knocking, but it often occurs not on front-wheel drive cars, but on classic (rear-wheel drive) ones. Although it is also found on front-wheel drive cars, it is much less common. When hitting even small bumps, a worn ball joint makes a sharp knock. The simplest diagnostic method is known to many: you need to jack up the car and pull the front wheel that is hanging out (we pull in the transverse direction). And for beginners, in order not to confuse the play in the ball joint with the play in the wheel bearing, I advise you to ask an assistant to fix the wheel with the brake pedal when you pull the wheel when checking. The ball joint with play must be replaced. If you do not find play in the ball joint, then pay attention to its rubber boot. If it is torn, then the hinge with a torn anther will not last long (after all, dust and dirt are an abrasive).
Shock absorber.
Let me remind you once again that she is often accused of other people's sins, but she is not cheap. And this node is rarely the cause of knocks (about 10 - 15 percent of cases). But this is a rather important detail of the machine and therefore deserves detailed consideration.
Not even an empty (not leaking) shock absorber strut, but pretty worn out, causes well-audible knocks, or even bumps. And how does it all work out on a trip? For example, the wheel of your car falls into a pit, and the rebound force at the worn out rack is rather small, and such a rack can no longer prevent (extinguish) the fact that the suspension spring, sharply straightening, shoots the car wheel down. And the wheel either touches the bottom of the pit, if it is not deep, or hangs in the air and stretches the shock absorber to the end. In both cases, the driver hears and feels a strong impact.
There are several ways to diagnose this problem, and the quickest and easiest way is to sharply push down with your hands on the car body. And if the body at the same time smoothly rises to its original state and stops, then the shock absorber strut is in order.
It is very rare, but it still happens that the strut knocks due to a malfunction inside the shock absorber, for example, the nut that holds the piston is loosened. But usually with more serious defects in the rack, it is not knocks that appear, but other malfunctions that can be checked as described above. That is, the force of resistance to the action of the suspension spring drops, and the body sways during the test (described above), or when the car is moving. The troubles are obvious: the stability of the car is deteriorating, the reliable contact of the wheels with the road is disturbed, the ride and handling are deteriorating. In this case, the rack must be replaced or repaired.
Very often, the failure of the shock absorber occurs due to careless operation of the car. I don't mean racing on bad roads, which we have in abundance. To slow down on bumps is understandable, it's about something else. We should not forget that oil is not only in such important units as the engine, gearbox and rear axle. It is also found in shock absorbers, and for the normal operation of the shock absorber, the oil must have a certain viscosity, depending on temperature.
What is the temperature on a frosty morning? And often drivers tear away, forgetting that in cold weather the oil in the shock absorbers has an ambient temperature, and when the temperature drops, its viscosity increases. And in the shock absorber cylinder, the oil stands like a stake, turning into a gel at minus 20 degrees below zero. Now imagine what loads a shock absorber will experience on a bad road, filled not with a liquid, but with a thick substance that cannot be pumped through the holes or the piston valve.
Under extreme loads, which are many times higher than normal, the thinnest and most fragile parts break first - the disk plates of the shock absorber valves. Well, in order to prevent this, the driver only needs to drive carefully for the first few minutes, avoiding pits and sharp bumps and shocks (especially in severe frost). With a gradual warming up of the oil, from the operation of the piston in the shock absorber (you can feel this, because the suspension will work softer), you can safely add gas.
Also keep in mind that if you have to repair the shock absorber, do not try to fill in thicker oil (allegedly thicker oil has less chance of leakage through the seals). The result can be this - a breakdown of the reed valves, as well as when driving through pits with oil thickened from frost (as described above). And with thicker oil, the handling and stability of the car will deteriorate.
After all, a stiffer shock absorber does not guarantee good performance under heavy loads. In addition, the compression force of the suspension increases, and, accordingly, the force on the car body increases, and this is fraught with the appearance of cracks on the body, in the area where the rack is attached. From a more viscous oil, the rebound force also increases, which is also not good.
To the more viscous oil that some “kulibins” pour into their shock absorbers, it’s worth adding a frost of about 20 degrees, no more, and you can imagine how the car will behave and what will happen to the suspension. I do not argue that hard shock absorbers are installed on sports cars, but they are not hard from oil, but initially from their design, which is developed on a special stand that determines the characteristics of shock absorbers and they are intended for sports cars, with reinforced suspension and body elements.
Other sources of chassis knocks.
The source of the knock may be due to a broken anti-roll bar bracket. This part consists of two silent blocks (rubber-metal hinges), which are deployed relative to each other by a certain degree and are interconnected by a rod or tube. When operating on our roads, it even happens that this part breaks at the place where the hinge is welded to the rod. At the same time, knocks are clearly audible when driving over bumps and when turning. You can identify the malfunction visually, and if it is not possible to see, then you should simply pull the end of the stabilizer link with your hand (it is more convenient to do this with the front wheels turned out to the end). If the welding is intact, then I advise you to also check the silent blocks themselves (if the rubber-metal hinges are broken).
Knock from broken engine mounts (pillows), manifests itself with a sharp gas supply, sudden braking, or simply when driving through strong bumps. The engine at such moments knocks on the body, touching it with an oil pan, generator or other part (depending on the design of the car). Often this source of knocking is not known to many beginners. The check is simple: you need to open the hood and, pressing with your whole body, pull the engine with your hands.
I also advise you to read the article - the suspension and its malfunctions, the article is located. Some malfunctions are also described there, from which knocks and extraneous noises coming from the chassis appear. And you can read about suspension repair.
In conclusion of the article, I want to say that there are a lot of sources of noise in a car, and sometimes the reasons are very insignificant and simply banal. For example, the fastening of the expansion tank or washer tank may unscrew in motion. And he dangles and knocks under the hood, hitting the body. There can be many reasons for knocking, and it’s impossible to list everything in one article. But to immediately respond to a knock and find this source of a knock is the responsibility of any driver. And I hope this article will help in this, especially for beginners; Good luck everyone!
The shock absorber is designed to ensure safety and driving comfort: it must ensure optimal grip of the tire with the road surface, prevent body vibrations and wheel separation from the road.
During the operation of the car, the shock absorber inevitably loses its original performance and, ultimately, fails. The main signs of the inoperability of the shock absorber:
- loss of tightness by the shock absorber;
- increased friction in pairs "rod-guide" and "piston-cylinder";
- change in the characteristics of the shock absorber;
- knocking inside the shock absorber;
- spontaneous withdrawal from a given trajectory - the car "scours";
- low position of the car body;
- the performance of the new shock absorber does not correspond to the manufacturer's parameters (typical for CIS conditions).
Diagnostics of operational
defects and methods for their elimination
The loss of tightness is diagnosed by a simple inspection of the shock absorber. Characteristic signs of leakage are: a decrease in gas pressure inside the housing (for gas design options) and leakage of the working fluid, accompanied by streaks on the outer surface of the shock absorber housing. This happens when the stem seal and/or the outer seal of the body is broken. Initially, a slight loss of fluid progresses over time, during the operation of the shock absorber, a “failure” occurs - a zone of reduced resistance in the range of the stroke of the rod. Indirect signs of loss of tightness: when rocking in the corners, the car makes several oscillations (which is acceptable for cars manufactured by US and Canadian companies for the domestic market), when driving on the road, the vehicle spontaneously withdraws from a given trajectory, “yaw”. Note that there are shock absorber designs (for example, Monroe Sensa-trac), in which the rebound force changes in a certain zone of the rod stroke depending on the load and position of the car body, fig. 1 (Reimpel J., 1986).
When using single-tube structures in the suspension of a car, the working fluid first leaks, and the gas exits only when it is completely lost. One of the characteristic signs of the depressurization process that has begun is wedging in the area of the stroke of the rod, which is clearly manifested when using single-tube plug-in cartridges from the Plaza company (St. Petersburg), which structurally repeat the Bilstein scheme, fig. 2 (Reimpel J., 1986), suspended on spring strut guides (MacPherson suspension).
Work with increased friction in most cases is observed in cars with broken body geometry or with deformation of suspension units and parts, as a result, with modified geometry and suspension kinematics. Accurate diagnostics is possible only with special stands and stocks. Characteristic features of these defects:
- there are noticeable deformations of the suspension units (including shock absorber deformations);
- the wheel alignment angles differ from those prescribed by the vehicle manufacturer and they cannot be adjusted in the entire range of working adjustments;
- two identical shock absorbers are installed on one axle of the car, while one of them regularly fails with low mileage (no more than 5-10 thousand km), and the other remains operational;
- when the wheel is suspended, the force of the spring is not enough to extend the stem completely, while at the same time, in the suspension of another similar car, the strut works normally: the kinematics of the suspension is broken.
A change in the performance of a shock absorber is the most common defect and can be caused by the following reasons:
- breakage, wear and deformation of parts inside the shock absorber;
- loss of initial properties of the working fluid;
- gas outlet for gas structures;
- when working in difficult road conditions, the shock absorber heats up (sometimes up to 80-100 degrees Celsius) and the damping properties of the vibration damper decrease or complete "off"; when the temperature drops, the performance is restored;
- spontaneous disassembly of the piston group or bottom valve (in the case of a two-pipe scheme); usually observed in shock absorbers manufactured at CIS factories, in addition, similar cases have been noted in Boge designs;
- valve leaks.
For some reasons for changing the performance of the shock absorber, let's make an explanation.
Breakage, accelerated wear and deformation of parts during the operation of the shock absorber usually occurs when the car is operated in difficult road conditions, which is generally characteristic of the CIS conditions, and the peculiar mentality of domestic drivers (“more speed - less holes”). Other reasons may be a violation of the kinematics of the suspension, deformation of the car body, as well as the use in the design of the vibration damper of materials whose physical properties do not correspond to the working conditions and the resulting loads (a distinctive feature of the products of the factories of the CIS, Poland, Turkey and the Czech Republic). All this, as a rule, leads to a decrease in the effectiveness of the shock absorber and is often accompanied by knocking.
The working fluid is operated in severe, harsh conditions, while it must have sufficient stability of properties when operating in a wide temperature range (approximately from -40 to +100 degrees Celsius). Over time, the liquid decomposes into fractions with precipitation. In addition, when the temperature changes, a significant fluctuation in the properties of an improperly selected working fluid is possible, as well as leakage of valves (“hanging”, deformation), as a result, a change in the characteristics of the vibration damper.
The cause of valve leaks is the wear process, accompanied by the separation of small particles from the shock absorber parts, which, falling on the valve seat, lead to loss of tightness, as well as deformation of the parts. A distinctive feature of shock absorbers manufactured at CIS factories is the ingress of dirt or chips inside during assembly, as well as the use of substandard parts.
Note that the causes that cause a change in the operating characteristic, as a rule, reduce the effectiveness of vibration damping. However, sometimes there is an increase in damping properties, "tightening" of the shock absorber. The reasons for this are the reduction of gaps during the mutual running-in of parts, as well as the filling of gaps that arise with liquid decomposition products. The processes that cause a decrease or increase in damping properties occur simultaneously, and at the moment it is not possible to predict the current state of the shock absorber.
In most cases, the causes of knocking lie in defects in ball bearings, silent blocks and other chassis components and have nothing to do with the shock absorber. Knocking inside the shock absorber can be caused by the following reasons:
- the piston ring is installed in the piston groove with a gap;
- breakage of the bypass valve spring, while the valve closes with a blow;
- discrepancy between the efforts of the valves: bypass piston and compression of the bottom valve;
- increased backlash in pairs "rod-guide" and "piston-cylinder";
- failures along the stroke of the rod due to fluid leakage; for the products of CIS plants - insufficient amount of filled liquid;
- when the stem is fully extended, a sharp metallic knock is heard;
- "morning sickness" shock absorber;
- the performance characteristics, dimensions and stroke of the shock absorber rod do not correspond to those of the vehicle suspension.
Let's take a closer look at some of the shock absorber defects that cause knocking.
The presence of a gap between the piston ring and the side walls of the piston groove allows the ring to move from one wall to another when changing the direction of piston movement. During this movement, the force on the shock absorber rod is reduced due to the reduction in sealing efficiency. At the moment when the ring touches the side wall of the piston groove, the force on the rod increases sharply, which gives a distinctly audible knock. As a rule, this defect manifests itself if the specified gap exceeds one millimeter.
During the movement of the car, the rebound and compression strokes of the suspension alternate with each other. When changing the direction of movement of the rod, there are some dead spots in which the piston speed is zero. For example, consider the compression stroke of a two-tube shock absorber. When the piston approaches the bottom dead center, the flow of fluid in the working cylinder into the over-piston cavity from the cavity located below the piston decreases so much that the bypass valve of the piston group closes under the action of the spring. If the spring is broken or absent at all, the valve "hangs" and does not fall into its seat at the described time. In this case, the valve remains in the open position even after the piston has passed the bottom dead center (i.e. already during the suspension rebound), while the speed of the rod in the opposite direction is negligible. Then it closes, and a bang is heard. The bypass valve of the bottom valve will be the source of knocking in a similar situation during the rebound stroke of a twin-tube shock absorber.
The purpose of the bypass valve of the piston of a two-tube shock absorber is to pass part of the working fluid into the over-piston space during the compression of the shock absorber, while another part of the fluid is forced out into the compensation cavity - the space between the housing and the working cylinder. A reinforced bypass valve is used if it is necessary to use a compression adjustment that requires a greater opening force of this valve if for some reason (usually in order to reduce metal consumption) it is undesirable to increase the stem diameter. In such an embodiment, this valve complements the compression resistance of the bottom valve. If a reinforced piston valve and a bottom valve with a relatively small opening force (force mismatch) are used in the design, during compression, an insufficient amount of liquid enters the over-piston space, since it flows through an element with lower hydraulic resistance, i.e. through the bottom valve. As a result, a volume filled with gas appears above the piston; when the rod moves upwards, the gas is first displaced, and then the liquid. As a result, at first the force developed by the shock absorber is small, and then it increases abruptly, which leads to a knock. This phenomenon is usually observed when the car is moving at low speed over bumps with a significant difference in height.
The source of knocking when changing the direction of the transverse force acting on the rod is usually the backlash in the “piston-cylinder” pair. Its causes: wear on the cylinder wall, wear of the piston and piston ring. In the case of using the Bilstein struts in the MacPherson suspension (see Fig. 2), the source of the knock will be lateral play in the cylinder guides.
Separately, we single out the Monroe Sensa-trac design with a bypass groove on the inner wall of the working cylinder and similar ones, which are used, as a rule, in car suspensions manufactured by US and Canadian companies. For this design, the appearance of backlash in the “piston-cylinder” pair is typical due to the destruction of the piston ring during its repeated movement along the bypass groove. However, a similar Boge solution (see Fig. 1), used, for example, in the A-pillars of the FIAT Croma, leads to the destruction of the piston ring much less frequently. Reason: better choice of ring material or groove shape.
The trend in modern shock absorber designs is the ring vulcanized to the piston. This solution is used by firms in North America, Korea, Japan (usually KYB, Tokico), and more recently in Europe (Sachs). The reasons for the destruction of the ring and the appearance of play in the “piston-cylinder” pair: excessive loads during operation on the roads of the CIS, violation of the geometry of the body or suspension kinematics, insufficient strength of the ring material.
Separately, we note the design features of KYB shock absorbers (Japan) - some parts (for example, bushing 1, Fig. 3) are made of soft metal with special properties. The purpose is to ensure the constancy of the annular gap in the “sleeve-washer” pair of the piston group in a wide temperature range, and, consequently, to increase the stability of the shock absorber performance. During operation, soft parts are deformed, and the initial tightening of the piston assembly fastening nut is loosened. As a result, the piston under the action of the load moves along the axis of the shock absorber, which causes knocking. The fastening nut of the KYB piston assembly is loosened with a significant deformation of the threaded end of the rod, so the complete disassembly of the piston group does not occur.
If a double-tube shock absorber with a large angle of inclination to the vertical (more than 45 degrees) is installed in the suspension, with the rod fully extended, the liquid level in the compensation cavity may drop below the level of the bottom valve. At the same time, a certain amount of air enters the space under the piston of the working cylinder during the operation of the shock absorber, forming an air cushion, and during the compression stroke, a failure is observed, causing a knock. Single-tube shock absorbers with a separating piston, as well as double-tube shock absorbers of a special design with a sealed gas element inside, which can be installed in any position, do not have this defect, fig. 4 (Reimpel J., 1986).
A sharp metal knock when the shock absorber rod is fully extended can be caused by the following reasons: destruction of the elastic rebound buffer on the rod (used to reduce the noise level during rebound), fig. 5, or by mutual contact of the metal parts of the suspension (as a rule, when using vibration dampers, the travel of which exceeds the travel of the suspension). Destruction of the rebound buffer can be caused by insufficient efficiency of the damping properties of the shock absorber, improperly selected buffer material, or when exposed to loads exceeding the allowable ones.
Let us note the design features of the hydraulic rebound buffer used in the front pillars of VAZ vehicles, manufactured by the Skopinsky Auto-Aggregate Plant (SAAZ): this design uses a ceramic-metal plunger installed in a cylinder with a small gap (Fig. 6) and provides additional resistance during rebound. With an increase in the gap or with a significant loss of operational properties of the working fluid, the efficiency of this device decreases, which causes knocking.
"Morning sickness" is typical for twin-tube shock absorbers and is as follows. When the car is parked for a long time, the liquid cools (its volume decreases) and drains through the throttle holes and leaky seals; as a result, a cavity filled with gas appears. At the beginning of the movement, the effectiveness of the shock absorber decreases and is restored only after a while. Some manufacturers (Sachs, Boge) have design options that prevent the occurrence of this phenomenon. For example, the angle ring used in some Boge shock absorbers serves as a reservoir for collecting fluid from the guide, fig. 7 (Reimpel J., 1986). The liquid from this reservoir prevents the formation of an air bubble in the working cylinder when the shock absorber cools down to ambient temperature at the end of the trip and the subsequent decrease in the volume of liquid in the cylinder. Other manufacturers do not use similar designs. This indirectly indicates that the noted phenomenon is not a serious operational problem.
The installation of shock absorbers in the suspension of a car, whose performance characteristics, and sometimes the dimensions and stroke of the rod do not correspond to those prescribed by the car manufacturer, is quite common in the CIS due to the low solvency of the population. As a rule, this is a replacement with domestically produced components of similar ones used on foreign cars; The main criterion for selection is the proximity of dimensions. Example: on a rear-wheel drive BMW 3-series car (body designation E21), the rear suspension often uses the rear strut of the front-wheel drive VAZ 2108, which has a maximum length and stroke that exceeds similar BMW parameters by about 50 and 30 mm. A rear-wheel drive car has a different axle weight distribution, different sprung and unsprung masses, different driving dynamics and a different top speed than a front-wheel drive car. In addition, the kinematics and characteristics of the BMW independent suspension differ from those of the VAZ dependent suspension. The BMW driving wheels are driven by constant velocity joints (CV joints), which have a limit on the maximum angle between the shafts. When using longer racks, this angle exceeds the allowable one, which leads to accelerated wear of the CV joint under the action of torque. Therefore, such a replacement is dangerous for other road users. In the case of using shock absorbers with smaller overall dimensions in the suspension, premature operation of the compression or rebound buffers is possible, which also causes knocking.
In the vast majority of cases, the reason for the low position of the car body is a decrease in rigidity or a breakdown of the elastic suspension element. If the shock absorber plays the role of an additional elastic element in the suspension (for example, variants of the rear suspensions of the Subaru Forester, Honda Legend models), then, as a rule, it has a fairly high internal pressure (about 1.5-2.0 MPa versus the usual 0.4- 0.6 MPa). Therefore, when the pressure decreases, the car "falls". In this case, when using a shock absorber that does not have high pressure, it is necessary to simultaneously use a suspension spring of a different stiffness.
Conclusion
Note that in almost all of these cases, thorough diagnostics and a set of works on the entire undercarriage of the car are necessary. It is possible to give a conclusion about the performance of the shock absorber only after testing on the stand, and to evaluate the joint operation of the vehicle suspension with the selected type of shock absorber - after sea trials, which are desirable to be carried out with the participation of several drivers in order to minimize the role of the subjective factor. In our opinion, the best way to repair a shock absorber is to manufacture it using new parts. The usual practice of repairing a shock absorber, which involves the continued use of used parts, is not justified - such parts have wear and therefore it is impossible to fine-tune the performance of the shock absorber.
