Belting is a transfer flexible connection(Fig. 5.2), consisting of a leading 1 and slave 2 pulleys and a belt put on them 3 . The transmission may also include tensioners and fences. It is possible to use several driven pulleys and several belts. The pulleys are rigidly fixed on the driving and driven shafts.
Main purpose- transfer of mechanical energy with a decrease in the rotational speed.
According to the principle of action, transmissions are distinguished friction(most gears) and engagement(toothed belt). Depending on the shape of the cross section of the belt, belt drives are distinguished: flat, wedge, polyclinic, round, square. V-ribbed, multi-ribbed, toothed and high-speed flat belts are made infinitely closed. Flat belts are mainly produced as final ones - in the form of long ribbons.
Advantages belt drive friction: no lubrication system, simplicity and low cost design, protection against sudden load fluctuations and shocks, the ability to transmit movement over long distances, overload protection due to slipping of the belt along the pulley, smooth and low noise operation.
Flaws: low durability of belts in high-speed gears; significant dimensions; inconstancy of the gear ratio (due to belt slippage on the pulleys); the need to protect the belt from oil; significant forces acting on shafts and supports.
To determine the gear ratio of a belt drive, it is assumed that the belt does not stretch and does not slip on the pulleys. Such an assumption does not introduce a significant error in the calculations, since the linear velocity [m/s] of any point lying on the surface of a rotating body (in our case, the drive pulley) is defined as
Where - angular velocity, rad/s; - pulley diameter, m; - number of revolutions per minute, rpm.
Since any point of the belt that coincides with the considered point of the driving pulley moves with the same linear speed (and therefore those points of the belt that are in contact with the driven pulley and the points of the driven pulley that coincide with them have the same linear speed).
Accordingly, the linear speed of any point of the rim of the driven pulley is also determined: In this case, the ratio of the linear speeds of both the driven and the driving pulleys is , or and, therefore, or .
The gear ratio of the transmission is expressed by the ratio of the diameters of the driven and driving pulleys:
The angles and (see Fig. 5.2) corresponding to the arcs along which the belt and pulley touch are called wrap angles.
Since the belt drive transmits rotation due to the forces of friction between the belt and the pulley, its performance depends significantly on the wrap angles, the determining one of which is the wrap angle on the smaller pulley. Its value primarily depends on the distance between the centers of the pulleys (center distance) and the gear ratio. Practice has shown that a flat belt drive works fine if the wrap angle is at least 120 degrees. This requirement is met if the following conditions are met: the center distance is not less than twice the sum of the pulley diameters.
It is possible to ensure the operability of a flat-belt transmission even with large gear ratios by using a tension roller 4 (see fig. 5.3) which will increase the wrap angle on the smaller pulley.
The limiting circumferential speed of a flat-belt transmission, depending on the material of the belt, lies in the range of 20 ... 40 m / s.
A more perfect type of transmission of motion by a flexible connection is a V-belt, where grooves are made on the rim of the pulleys, into which a belt enters, having a trapezoid shape in cross section. In these broadcasts payload is transmitted due to frictional forces between the side surfaces of the belt and the grooves of the pulleys. The trapezoidal section of the belt, due to wedging, increases its adhesion to the pulley and increases the traction ability of the transmission. This makes it possible to implement higher gear ratios (up to 7 and even up to 10), the possibility of using at small center-to-center distances.
If for a flat belt drive, the center distance
then for V-belt transmission, which allows one gear to rotate several driven shafts without the use of tension rollers.
On kinematic diagrams, belt drives have corresponding conventions(in Fig. 5.4, A flat, and in Fig. 5.4, b - with V-belts).
Recently, toothed belt drives have become widely used. On working surface the belt has protrusions - teeth that engage with similar teeth on the pulleys. Such gears work without slip, which ensures the constancy of the gear ratio.
In some cases, a more complex belt drive is used - multi-stage (Fig. 5.5), consisting of several stages (pairs of pulleys).
The gear ratios of individual steps ( , , ) are expressed in terms of the ratio of the diameters of the driven () and driving () pulleys. For the entire transmission - the diameter of the driving pulley, and - the diameter of the driven pulley, however, their ratio will not be the desired gear ratio the entire transmission, since these pulleys are not connected by a single belt.
We determine the required ratio, taking into account that the drive shaft (not the pulley!) of each subsequent stage is simultaneously the driven shaft of the previous one.
Gear ratio of the first pair of pulleys
Gear ratio of the second pair of pulleys
Since the pulleys with a diameter and are fixed on the same shaft, .
Gear ratio of the third pair of pulleys
and consequently, .
Gear ratio of the entire transmission
Thus, the gear ratio of a multi-stage belt drive is equal to the product of the gear ratios of its individual stages.
Automotive mechanics includes a fairly large number of mechanisms that transmit various rotational or translational movements to other devices. One such device is the V-belt drive. In this article we will try to tell you in as much detail as possible what it is, what it is for and how it works?
What is V-belt transmission and how does it work?
Belt transmission is a way of transferring rotating mechanical energy from its source to another mechanism. IN this case, such energy is torque. Any belt drive consists of one belt and at least two pulleys.
The belt, as a rule, is made of rubber that has undergone a special treatment, which allows it to endure not too strong mechanical influences tensile and some thermal deflections. There are many varieties of belt drives, but we will focus on the most common option - V-belt, which has become quite widespread in the automotive industry.
V-belt transmission is made in the form of a wedge-shaped belt and corresponding pulleys. The V-belt pulley is a metal disk with special branches around the circumference designed for the belt itself. The belt, in turn, has two versions: a toothed belt or a smooth one.
Initially, such a belt was driven a large number of various vehicle mechanisms. The main to this day remain the generator and water pump. On trucks and many others modern cars with the help of such a belt, special and air compressors for amplifiers brake system car.
The main feature of the V-belt pulley should be a special ditch for the belt. Without her, this belt simply jump off the mechanism, as it has a relatively small thickness. This approach allows to reduce the space occupied by the belt drive by reducing its dimensions.
The dimensions of the pulleys depend on the gear ratio. If the gear is downshift, then the drive pulley must be smaller than the driven pulley and vice versa.
The belt must have a certain softness in various weather conditions. Since the vehicle is intended for operation in winter and summer period, which means that the belt should not lose its elastic properties under any circumstances. The use of any other belt in V-belt transmission is unacceptable.
Video - Belt drive device - pulleys and belts
Advantages and disadvantages of belt drive
Like all mechanisms, belt transmission also has its advantages and disadvantages, all of which, unfortunately, cannot be solved, which makes it possible to use this mechanism only in certain activities.
Advantages:
- Increased smoothness of work. Since rubber has sufficient elasticity, this allows it to reduce shock loads and reduce vibrations that occur.
- Possibility of inaccurate installation of pulleys. The elastic belt allows for slight misalignment, which will not affect common work mechanism. That is why this transmission has the ability to change the gear ratio on the go and is so widely used on CVT gearboxes.
- No noise. Always and everywhere belt drive was famous for the lack of noise. This forced the developers of the VAZ 2105 to release it with a timing belt drive.
- Complete absence of overload. The fact is that the belt can slip during its operation, which reduces the load on the mechanism and protects expensive metal parts of the device from wear. For example, if the rotation is too fast crankshaft, does not receive the same torque, but rotates at its initially obtained speed, since by increasing the traction, the belt begins to slip relative to the second pulley. In addition, in walk-behind tractors, a belt drive is used as a clutch drive, as it works much softer and smoother.
- Economic expediency. The fact is that pulleys and belts are quite cheap and do not need to be replaced so often. Perhaps the belt drive is the most economical of all.
- Belt drive does not need to be lubricated. Moreover, lubrication will negatively affect the operation of the belt, as it will begin to slip more often and will not be able to transmit the required torque.
- In case of damage to the belt, it simply flies off the mechanism without consequences, unlike the chain, which breaks everything that it “gets”.
- over a sufficiently large distance. Not only that, some belts have the ability to stretch, which makes them even softer over time.
Flaws:
- Belt drive pulleys have much larger size than the pulleys of any other gears. This makes this design too large, although the load on both types of gears is exactly the same.
- The low strength of the belt and accelerated wear. When tightening, the belt constantly heats up and breaks, which causes the mechanism to stop.
- Violation of the gear ratio due to slippage of the belt relative to other pulleys. This problem almost completely absent in the toothed belt version.
- Need for additional devices: belt tensioning device, devices that dampen vibrations and hold the belt in the grooves.
- Too little bearing capacity.
That's all that is a V-belt transmission. IN modern mechanical engineering she plays an important role, so do not underestimate her.
Belt drive is a mechanism for transferring energy with the help of drive belt, using forces of friction or engagement. The amount of load to be transferred depends on the tension, wrap angle and coefficient of friction. The belts go around pulleys, one of which is the leading one, and the other is the driven one.
Advantages and disadvantages
Belt drive has the following positive properties:
- noiselessness and smoothness in work;
- high manufacturing precision is not required;
- slippage during overloads and vibration smoothing;
- no need for lubrication;
- low cost;
- opportunity manual replacement transfers;
- ease of installation;
- no damage to the drive when the belt breaks.
Flaws:
- large pulley sizes;
- violation of the gear ratio when the belt slips;
- little power.
Depending on the type, the belt is flat, wedge, round and toothed. This belt drive element can combine the advantages of several types, for example, poly V-belt.
Areas of use
- The flat belt drive is used on machine tools, sawmills, generators, fans, and wherever greater flexibility is required and slippage is tolerated. Used for high speeds synthetic materials, for smaller ones - cord fabric or rubberized.
- Belt drive with V-belts is used for agricultural machinery and cars (fan), in heavily loaded and high-speed drives (narrow and normal section).
- CVTs are needed where the rotation speed industrial machines infinitely adjustable.
- Drives with toothed belts provide best performance gear in industry and household appliances where durability and reliability are required.
- Round belts are used for low power.
materials
Materials are selected to the operating conditions, where load and type are of primary importance. They are as follows:
- flat - leather, rubberized with stitching, whole-fabric wool, cotton or synthetic;
- wedge - a reinforcing layer in the center with a rubber core and a woven tape on the outside;
- toothed - a carrier layer of a metal cable, a polyamide cord or fiberglass in a rubber or plastic base.
The surfaces of the belts are covered with impregnated fabrics to increase wear resistance.
Flat belt drive belts
The transmission types are as follows:
- Open - with parallel axes and rotation of the pulleys in the same direction.
- Pulleys with steps - you can change the speed of the driven shaft, while the drive shaft has constant speed.
- Cross, when the axes are parallel, and the rotation occurs in different directions.
- Semi-cross - the axes of the shafts are crossed.
- WITH tension roller, increasing the angle of wrapping of a pulley of a smaller diameter.
Belting open type used to work with high speed and with a large center distance. High efficiency, load capacity and durability allow it to be used in industry, in particular for agricultural machines.
V-belt transmission
The transmission is characterized by a trapezoidal cross-section of the belt and the surfaces of the pulleys in contact with it. The transmitted efforts in this case can be significant, but its efficiency is small. V-belt transmission is characterized by a small distance between the axles and a high gear ratio.
timing belts
The transmission is used for high speed with a small distance between the axles. It has both the advantages of belt and chain drives: work at high loads and with a constant gear ratio. The power of 100 kW can be provided mainly by a toothed belt drive. In this case, the revolutions are very high - the speed of the belt reaches 50 m / s.
Pulleys
The belt drive pulley can be cast, welded or prefabricated. The material is selected depending on the speed. If it is made of textolite or plastic, the speed is not more than 25 m/s. If it exceeds 5 m / s, static balancing is required, and for high-speed gears - dynamic.
During operation, pulleys with flat belts experience wear of the rim from slippage, breakage, cracks, and breakage of the spokes. IN V-belt drives the grooves on the working surfaces wear out, the shoulders break, and imbalance occurs.
If a hub hole is produced, it is bored, and then the sleeve is pressed in. For greater reliability, it is made simultaneously with internal and external keyways. The thin-walled sleeve is mounted on glue and bolted through the flange.
Cracks and kinks are welded, for which the pulley is first heated to eliminate residual stresses.
When turning the rim for a V-belt, it is allowed that the rotational speed can vary up to 5% of the nominal.
Gear calculation
All calculations for any type of belts are based on the definition geometric parameters, traction and durability.
1. Determination of geometric characteristics and loads. It is convenient to consider the calculation of a belt drive on specific example. Let it be necessary to determine the parameters of a belt drive from an electric motor with a power of 3 kW to a lathe. Shaft speeds are, respectively, n 1 = 1410 min -1 and n 2 = 700 min -1 .
The usually narrow V-belt is selected as the most commonly used. The nominal torque on the drive pulley is:
T1 = 9550P 1: n 1 = 9550 x 3 x 1000: 1410 = 20.3 Nm.
From the reference tables, the diameter of the drive pulley d 1 = 63 mm with the SPZ profile is selected.
Belt speed is defined as follows:
V \u003d 3.14d 1 n 1: (60 x 1000) \u003d 3.14 x 63 x 1410: (60 x 1000) \u003d 4.55 m / s.
It does not exceed the permissible, which is 40 m / s for the selected type. The diameter of the large pulley will be:
d2 \u003d d 1 u x (1 - e y) \u003d 63 x 1410 x (1-0.01): 700 \u003d 125.6 mm.
The result is reduced to the nearest value from the standard series: d 2 = 125 mm.
The distance between the axles and the length of the belt are found from the following formulas:
a \u003d 1.2d 2 \u003d 1.2 x 125 \u003d 150 mm;
L \u003d 2a + 3.14d cp + ∆ 2: a \u003d 2 x 150 + 3.14 x (63 + 125): 2 + (125 - 63) 2: (4 x 150) \u003d 601.7 mm.
After rounding to the nearest value from the standard range, the final result is obtained: L= 630 mm.
The center distance will change, and it can be recalculated again using a more accurate formula:
a \u003d (L - 3.14d cp): 4 + 1: 4 x ((L - 3.14d cp) 2 - 8∆ 2) 1/2 \u003d 164.4 mm.
For typical conditions, the power transmitted by one belt is determined by nomograms and is 1 kW. For a real situation, it must be refined by the formula:
[P] = P 0 K a K p K L K u .
After determining the coefficients according to the tables, it turns out:
[P] = 1 x 0.946 x 1 x 0.856 x 1.13 = 0.92 kW.
The required number of belts is determined by dividing the power of the electric motor by the power that one belt can transmit, but at the same time the coefficient C z \u003d 0.9 is also introduced:
z \u003d P 1: ([P] C z) \u003d 3: (0.92 x 0.9) \u003d 3.62 ≈ 4.
The belt tension force is: F 0 \u003d σ 0 A \u003d 3 x 56 \u003d 168 H, where the cross-sectional area A is according to the reference table.
Finally, the load on the shafts from all four belts will be: F sum = 2F 0 z cos(2∆/a) = 1650 H.
2. Durability. The calculation of the belt drive also includes the determination of durability. It depends on the fatigue resistance, determined by the magnitude of the stresses in the belt and the frequency of their cycles (the number of bends per unit time). From the resulting deformations and friction inside the belt, fatigue destruction occurs - tears and cracks.
One load cycle manifests itself in the form of a fourfold change in stress in the belt. The frequency of runs is determined from the following relationship: U = V: l< U d ,
where V - speed, m/s; l - length, m; U d - allowable frequency (<= 10 - 20 для клиновых ремней).
3. Calculation of toothed belts. The main parameter is the modulus: m = p: n, where p is the circumferential step.
The value of the module depends on the angular velocity and power: m = 1.65 x 10-3 x (P 1: w 1) 1/3.
Since it is standardized, the calculated value is reduced to the nearest value of the series. For high speeds, higher values are taken.
The number of teeth of the driven pulley is determined by the gear ratio: z 2 = uz 1.
The center distance depends on the diameters of the pulleys: a \u003d (0.5 ... 2) x (d 1 + d 2).
The number of teeth on the belt will be: z p = L: (3.14m), where L is the approximate calculated length of the belt.
After choosing the nearest standard number of teeth, then determine the exact length of the belt from the last ratio.
It is also necessary to determine the width of the belt: b = F t: q, where F t is the circumferential force, q is the specific belt tension, selected by the module.
The load on the shafts will be: R = (1...1.2) x F t .
Conclusion
The performance of belt drives depends on the type of belts and their operating conditions. The correct calculation will allow you to choose a reliable and durable drive.
General information about belt drives
Belt transmissions are friction (friction) transmissions, in which power is transferred due to friction forces that arise between the driving, driven and intermediate link - an elastic belt (flexible connection).
The driving and driven links are commonly referred to as pulleys. This type of gear is usually used to connect shafts located at a considerable distance from each other.
For normal operation of the belt drive, it is necessary to pre-tension the belt, which can be carried out by moving one of the pulleys, by means of tension rollers or by installing the engine (mechanism) on a rocking plate.
Belt drive classification
Belt drives are classified according to various criteria - according to the shape of the cross section of the belt, according to the relative position of the shafts and the belt, according to the number and type of pulleys, according to the number of pulleys covered by the belt, according to the method of adjusting the belt tension (with an auxiliary roller or with movable pulleys).
1. According to the shape of the cross section of the belt There are the following types of belt drives:
- flat-belt (the cross section of the belt has the shape of a flat elongated rectangle, Fig. 1a);
- V-belt (cross section of a belt in the form of a trapezoid, Fig. 1b);
- poly-V-belt (the belt outside has a flat surface, and the inner, interacting with the pulleys, the surface of the belt is equipped with longitudinal ridges, made in cross section in the form of a trapezoid, Fig. 1d);
- round-belt (the cross section of the belt has a round or oval shape, Fig. 1c);
- toothed belt (the inner surface of the flat belt in contact with the pulleys is provided with transverse protrusions that enter the corresponding pulley cavities during the operation of the transmission, photo below).
V-belts and V-ribbed belts are most widely used in mechanical engineering. Transmission round rubber belt (diameter 3…12 mm) used in low power drives (desktop machines, appliances, household machines, etc.).
A variation of the belt drive is a toothed belt drive, in which power is transmitted by a toothed belt by meshing the teeth of the belt with the protrusions on the pulleys. This type of gear is intermediate between gear and friction gears. The toothed belt drive does not require significant belt pretensioning and does not have the disadvantage of belt slip that is inherent in all other belt drives.
V-belt transmission is mainly used as an open one. V-belt drives have a greater traction capacity, require less tension, due to which they load the shaft supports less, allow smaller wrap angles, which allows them to be used with large gear ratios and a small distance between the pulleys.
V-belts and V-ribbed belts are endless and rubberized. The load is carried by a cord or fabric folded in several layers.
V-belts are produced in three types: normal section, narrow and wide. Wide belts are used in variators.
V-ribbed belts are flat belts with a high-strength cord and internal longitudinal wedges included in the grooves on the pulleys. They are more flexible than wedges, provide a better gear ratio.
Flat belts have great flexibility but require significant belt pretensioning. In addition, a flat belt is not as stable on a pulley as a V-belt or poly V-belt.
2. According to the mutual arrangement of the shafts and the belt :
- with parallel geometric axes of the shafts and a belt covering the pulleys in one direction - open transmission (pulleys rotate in the same direction, fig. 2a);
- with parallel shafts and a belt covering the pulleys in opposite directions - cross transmission (pulleys rotate in opposite directions, Fig. 2b);
- shaft axes intersect at some angle (most often 90°, Fig. 2c) – semi-cross transmission;
- the transmission shafts intersect, while changing the direction of the transmitted power flow is carried out by means of an intermediate pulley or roller - corner gear(Fig. 2d).
3. By the number and type of pulleys used in transmission: with single-pulley shafts; with a two-pulley shaft, one of the pulleys of which is idle; with shafts carrying stepped pulleys for changing the gear ratio (for stepping the speed of the driven shaft).
4. By the number of shafts covered by one belt : two-shaft, three-, four- and multi-shaft transmission.
5. By the presence of auxiliary rollers : without auxiliary rollers, with tension rollers (Fig. 2d); with guide rollers (Fig. 2d).
Advantages of belt drives
The advantages of belt drives include the following properties:
- Simplicity of design, low cost of manufacture and operation.
- The ability to transmit power over a considerable distance.
- Ability to work with high speeds.
- Smoothness and small noise in work owing to elasticity of a belt.
- Mitigation of vibration and shocks due to the elasticity of the belt.
- Protection of mechanisms from overloads and shocks due to the ability of the belt to slip (this property does not apply to gears with a toothed belt).
- The electrical insulating capacity of the belt is used to protect the driven part of electrically driven machines from the occurrence of dangerous voltages and currents.
Disadvantages of belt drives
The main disadvantages of belt drives:
- Large overall dimensions (especially when transferring significant capacities).
- Low belt durability, especially in high-speed gears.
- High load on the shafts and bearings of the supports due to belt tension (this disadvantage is less pronounced in toothed belt drives).
- The need to use belt tensioning devices that complicate the design of the transmission.
- Sensitivity of the load capacity to contamination of links and air humidity.
- Non-constant gear ratio due to the inevitable elastic sliding of the belt.
Scope of belt drives
Belt drives are used in most cases to transmit movement from an electric motor or an internal combustion engine, 
when, for design reasons, the center distance should be large enough, and the gear ratio may not be strictly constant (conveyors, drives of machine tools, road and agricultural machines, etc.). Gears with a toothed belt can also be used in drives that require a constant gear ratio.
Power transmitted by belt drive, typically up to 50 kW, but can reach 2000 kW and even more. Belt speed v = 5…50 m/s, and in high-speed transmissions - up to 100 m/s and higher.
After gear transmission, belt transmission is the most common of all mechanical transmissions. It is often used in combination with other types of transmissions.
Geometric and kinematic ratios of belt drives
Center distance a of the belt drive determines mainly the design of the drive of the machine. Recommended center distance (see Fig. 3) :
For flat belt drives:
a ≥ 1,5 (d 1 + d 2) ;
For V-belt and poly V-belt drives:
a ≥ 0,55 (d 1 + d 2) + h;
Where:
d 1, d 2 - diameters of the driving and driven pulleys of the transmission;
h is the height of the belt section.
Estimated belt length L p equal to the sum of the lengths of the straight sections and the arcs of the circumference of the pulleys:
L p = 2 a + 0,5 π(d 2 + d 1) + 0,25 (d 2 - d 1) 2 / a.
According to the found value from the standard series, the nearest greater estimated belt length L p is taken. When connecting the ends, the length of the belt is increased by 30…200 mm.
Center distance in belt drive for the finally installed belt length is determined by the formula:
a = [ 2 L p - π(d 2 + d 1)] / 8 + √{[ 2 L p - π(d 2 + d 1)] 2 - 8 π(d 2 - d 1) 2 )/ 8 .
Small pulley belt wrap angle
α 1 = 180 ° - 2 γ .
From a triangle O 1 IN 2(Fig. 3)
sin γ \u003d IN 2 / O 1 O 2 \u003d (d 2 - d 1) /2 a.
In practice, γ does not exceed π/ 6 , therefore, approximately take sin γ = γ (rad), then:
γ \u003d (d 2 - d 1) / 2 a (rad) or γ ° = 180 °(d2 –d1)/ 2 pa.
Hence,
α 1 = 180 ° - 57 ° (d 2 - d 1) / a.
Belt drive ratio:
u \u003d i \u003d d 2 / d 1 ( 1 – ξ) ,
where: ξ is the gear slip coefficient, which during normal operation is equal to ξ = 0.01…0.02.
Approximately, you can take u = d 2 /d 1; ξ \u003d (v 1 -v 2) / v 1.
The transfer of mechanical energy, carried out by a flexible connection due to friction between the belt and the pulley, is called belt. The belt drive consists of a driving and driven pulleys located at some distance from each other and enveloped by a drive belt (Fig. 182). The greater the tension, the angle of the belt around the pulley, and the coefficient of friction, the greater the transmitted load. Depending on the shape of the cross-section of the transmission belt, there are: flat-belt (Fig. 183, I), V-belt (Fig. 183, II) and round-belt (Fig. 183, III). The most widespread in mechanical engineering are flat and wedge-shaped belts. Flat belts experience minimal bending stress on the pulleys, wedge-shaped belts, due to the wedge effect with pulleys, are characterized by increased traction. Round belts are used in small machines, such as sewing and food processing machines, desktop machines and appliances.
Rice. 182
Rice. 183
TO virtues belt drives include: the ability to transmit rotational motion over long distances (up to 15 m): simplicity of design and low cost; smooth running and unstressed work; ease of care and maintenance.
However, belt drives are bulky, short-lived in high-speed mechanisms, do not allow obtaining a constant gear ratio due to belt slippage, create increased loads on shafts and supports (bearings), since the total tension of the belt branches is much greater than the circumferential transmission force. In addition, during the operation of a belt drive, the possibility of slipping and breaking the belt is not ruled out, so these gears need constant supervision.
Types of flat belt drives
Depending on the location of the axes of the pulleys and the purpose, the following types of flat belt drives are distinguished:
- open gear - with parallel axes and rotation of the pulleys in one direction (Fig. 184, I);
- cross transmission - with parallel axes and rotation of the pulleys in opposite directions (Fig. 184, II);
- semi-cross transmission - with intersecting axes (Fig. 184, III);
- angular gear - with intersecting axes (Fig. 184, IV); gear with stepped pulleys (Fig. 184, V), which allows you to change the angular velocity of the driven shaft at a constant speed of the drive. The pulley steps are arranged so that the smaller step of one pulley is opposite the larger step of the other, etc. To change the speed of the driven pulley, the belt is thrown from one pair of steps to another;
- transmission with an idle pulley (Fig. 184, VI), which allows you to stop the driven shaft when the drive rotates. A wide pulley 1 is mounted on the drive shaft, and two pulleys are mounted on the driven shaft: a working pulley 2, which is connected to the shaft with a key, and an idle pulley 3, freely rotating on the shaft. The belt connecting the pulleys can be moved on the go, connecting pulley 1 with pulleys 2 or 3, respectively turning on or off the driven shaft;
- a transmission with a tension roller that provides automatic belt tension and an increase in the angle of the belt around the smaller pulley (Fig. 184, VII).
Rice. 184
The flat-belt transmission is simple in design, used for large center distances (up to 15 m) and high speeds (up to 100 m/s) with reduced durability.
V-belt transmission
In a V-belt transmission, a flexible connection is carried out by a trapezoidal section drive belt with a profile angle? equal to 40° (in the undeformed state). Compared to a flat belt, the V-belt transmits more traction, but the transmission with such a belt has a reduced efficiency.
V-belt transmissions are advisable to use with large gear ratios, small center distances and vertical shaft axes. The speed of V-belt transmission belts should not exceed 30 m/s. Otherwise, the V-belts will vibrate.
V-belts for general purpose drives are standardized by GOST 1284.1-89.
When installing a V-belt drive, special attention is paid to the correct installation of the V-belt III in the groove of the pulley rim (Fig. 185).
Rice. 185
Belt drive parts
Drive belts. Any drive belt serves as a traction body. It must have a certain traction ability (transmit a given load without slipping), have sufficient strength, durability, wear resistance, good grip on the pulley and low cost.
Flat belts are made in different widths, designs and from various materials: cotton, rubberized, woolen fabrics and leather. The choice of material for belts is determined by the working conditions (atmospheric influences, harmful vapors, temperature changes, shock loads, etc.) and traction. Drive belts (rubberized) are standardized.
There are two types of V-belts: fabric cord and cord cord. In cord fabric belts (Fig. 186, I), the cord is made in the form of several layers of cord fabric with a base in the form of twisted cords 0.8-0.9 mm thick. In cord belts (Fig. 186, II), the cord consists of a single layer of cord wound along a helical line and enclosed in a thin layer of rubber to reduce friction. These belts are used in high speed transmissions and are flexible, reliable and durable.
Rice. 186
Note. Cord - a strong twisted thread made of cotton or artificial fiber.
In recent years, toothed (polyamide) belts have been increasingly used in domestic engineering. These belts combine in their design all the advantages of flat belts and toothed gears (Fig. 187). On the working surface of the belts 4 there are protrusions that engage in the protrusions on the pulleys 1,2 and Z. Polyamide belts are suitable for high-speed transmissions, as well as for transmissions with a small center distance. They allow significant overloads, are very reliable and durable.
Rice. 187
The ends of the belts are connected by gluing, stitching and metal connectors. gluing homogeneous belts (leather) are carried out along an oblique cut at a length equal to 20 ... 25 times the thickness of the belt (Fig. 188, I), and ply belts - along a stepped surface with at least three steps (Fig. 188, II) . The joints of rubberized belts are vulcanized after gluing.
stitching used for all types of belts. It is produced by means of sinew strings or rawhide thongs (Fig. 188, III). Butt stitching with vein strings with inclined punctures is considered more perfect and reliable (Fig. 188, IV).
Rice. 188
Mechanical connectors apply to all belts, except for high-speed ones. They allow for a quick connection, but increase its mass (Fig. 188, V). Particularly good work is provided by swivel connections with wire spirals (Fig. 188, VI). The spirals are threaded through a series of holes, and after pressing they compress the belt. The hinge is created as a result of combining the spirals and threading the axis through them.
Pulleys. For flat belts, the most acceptable form of the pulley surface is a smooth cylindrical surface (Fig. 189, I).
Rice. 189
To center the belt, the surface of the driven pulley is made convex, and the leading one is cylindrical (at v<= 25 м/с оба шкива делают выпуклыми).
For V-belts, the working surface is the sides of the V-grooves (Fig. 189, II) in the rim of the pulleys. The number and dimensions of these grooves are determined by the belt profile and the number of belts.
Pulleys are cast from cast iron, aluminum alloys, plastics and welded from steel. Cast iron pulleys are solid and split, consisting of two halves, which are bolted at the rim and bushing. Split pulleys can be easily removed from the shaft without lifting the shaft from the bearings.
