Works on the bulkhead of the electric motor are nearing completion. Getting to the calculation of pulleys belt drive machine. A little bit of belt drive terminology.

We will have three main input data. The first value is the speed of rotation of the rotor (shaft) of the electric motor 2790 revolutions per second. The second and third are the speeds that need to be obtained on the secondary shaft. We are interested in two denominations of 1800 and 3500 rpm. Therefore, we will make a two-stage pulley.

The note! To start a three-phase electric motor, we will use a frequency converter, so the calculated rotation speeds will be reliable. If the engine is started using capacitors, then the values ​​​​of the rotor speed will differ from the nominal one in a smaller direction. And at this stage, it is possible to minimize the error by making adjustments. But for this you have to start the engine, use the tachometer and measure the current speed of rotation of the shaft.

Our goals are defined, we proceed to the choice of the type of belt and to the main calculation. For each of the produced belts, regardless of the type (V-belt, poly-V-belt or other), there is a number key features. Which determine the rationality of the application in a particular design. The ideal option for most projects would be to use a V-ribbed belt. The polywedge-shaped got its name due to its configuration, it is a type of long closed furrows located along the entire length. The name of the belt comes from the Greek word "poly", which means many. These furrows are also called differently - ribs or streams. Their number can be from three to twenty.

A poly-V-belt has a lot of advantages over a V-belt, such as:

• due to good flexibility, work on small pulleys is possible. Depending on the belt, the minimum diameter can start from ten to twelve millimeters;
• high traction capacity of the belt, therefore working speed can reach up to 60 meters per second, against 20, a maximum of 35 meters per second for a V-belt;
• The grip force of a V-ribbed belt with a flat pulley at a wrap angle above 133° is approximately equal to the grip force with a grooved pulley, and as the wrap angle increases, the grip becomes higher. Therefore, for drives with a gear ratio greater than three and a small pulley wrap angle from 120° to 150°, a flat (without grooves) larger pulley can be used;
• due to the light weight of the belt, vibration levels are much lower.

Taking into account all the advantages of poly V-belts, we will use this type in our designs. Below is a table of the five main sections of the most common V-ribbed belts (PH, PJ, PK, PL, PM).

Designation	PH	PJ	PK	PL	PM
Rib pitch, S, mm	1.6	2.34	3.56	4.7	9.4
Belt height, H, mm	2.7	4.0	5.4	9.0	14.2
Neutral layer, h0, mm	0.8	1.2	1.5	3.0	4.0
Distance to the neutral layer, h, mm	1.0	1.1	1.5	1.5	2.0
	13	20	45	75	180
Maximum speed, Vmax, m/s	60	60	50	40	35
Length range, L, mm	1140…2404	356…2489	527…2550	991…2235	2286…16764

Drawing of a schematic designation of the elements of a poly-V-belt in a section.

For both the belt and the counter pulley, there is a corresponding table with the characteristics for the manufacture of pulleys.

cross section	PH	PJ	PK	PL	PM
Distance between grooves, e, mm	1.60±0.03	2.34±0.03	3.56±0.05	4.70±0.05	9.40±0.08
Total dimension error e, mm	±0.3	±0.3	±0.3	±0.3	±0.3
Distance from pulley edge fmin, mm	1.3	1.8	2.5	3.3	6.4
Wedge angle α, °	40±0.5°	40±0.5°	40±0.5°	40±0.5°	40±0.5°
Radius ra, mm	0.15	0.2	0.25	0.4	0.75
Radius ri, mm	0.3	0.4	0.5	0.4	0.75
Minimum pulley diameter, db, mm	13	12	45	75	180

The minimum pulley radius is set for a reason, this parameter regulates the life of the belt. It would be best if you deviate slightly from the minimum diameter to the larger side. For a specific task, we have chosen the most common "RK" type belt. Minimum radius for of this type belts is 45 millimeters. Given this, we will also start from the diameters of the available blanks. In our case, there are blanks with a diameter of 100 and 80 millimeters. Under them, we will adjust the diameters of the pulleys.

We start the calculation. Let’s revisit our initial data and set goals. The speed of rotation of the motor shaft is 2790 rpm. Poly-V-belt type "RK". The minimum diameter of the pulley, which is regulated for it, is 45 millimeters, the height of the neutral layer is 1.5 millimeters. We need to determine the optimal pulley diameters, taking into account the required speeds. First speed output shaft 1800 rpm, second speed 3500 rpm. Therefore, we get two pairs of pulleys: the first is 2790 at 1800 rpm, and the second is 2790 at 3500. First of all, we will find the gear ratio of each of the pairs.

Formula to determine gear ratio:

, where n1 and n2 are shaft rotation speeds, D1 and D2 are pulley diameters.

First pair 2790 / 1800 = 1.55
Second pair 2790 / 3500 = 0.797

, where h0 is the neutral layer of the belt, parameter from the table above.

D2 = 45x1.55 + 2x1.5x(1.55 - 1) = 71.4mm

For the convenience of calculations and selection of the optimal pulley diameters, you can use the online calculator.

Instruction how to use calculator. First, let's define the units of measurement. All parameters except speed are indicated in millimeters, speed is indicated in revolutions per minute. In the "Neutral belt layer" field, enter the parameter from the table above, the "PK" column. We enter the value h0 equal to 1.5 millimeters. In the next field, set the rotation speed of the motor shaft to 2790 rpm. In the electric motor pulley diameter field, enter the minimum value regulated for a particular type of belt, in our case it is 45 millimeters. Next, we enter the speed parameter with which we want the driven shaft to rotate. In our case, this value is 1800 rpm. Now it remains to click the "Calculate" button. We will get the corresponding diameter of the counter pulley in the field, and it is 71.4 millimeters.

Note: If it is necessary to perform an estimated calculation for a flat belt or a V-belt, then the value of the neutral layer of the belt can be neglected by setting the value “0” in the “ho” field.

Now we can (if necessary or required) increase the diameters of the pulleys. For example, this may be needed to increase the life of the drive belt or increase the coefficient of adhesion of the belt-pulley pair. Also, large pulleys are sometimes made intentionally to perform the function of a flywheel. But now we want to fit into the blanks as much as possible (we have blanks with a diameter of 100 and 80 millimeters) and, accordingly, we will select for ourselves optimal dimensions pulleys. After several iterations of values, we settled on the following diameters D1 - 60 millimeters and D2 - 94.5 millimeters for the first pair.

A belt drive for CNC machines is a mechanism that converts the rotational movement of a shaft into movement along a translational axis. The main tool for such a transmission is a toothed belt. Due to its presence, the processing of the workpiece along a given axis is ensured, in order to obtain a higher indicator of accuracy and productivity. Belt driven transmission is one of the most common, due to its purpose.

purpose

The simplest transmission design of this type is represented by pulleys with a belt stretched over them. It fits only part of the pulley, forming a wrap angle. It depends on its indicator how good the clutch will be. The higher the index, the higher the quality of the clutch.

With the help of a pulley-roller, the wrapping angle can be increased. If it is too small, then the machine will be able to fulfill its purpose only partially.

Thanks to the belt drive, rotational movements can be converted into translational ones. The device is able to perform a similar conversion in reverse. The unit provides transmission by friction. The design of the equipment involves the presence of three links:

• leader;
• slave;
• intermediate.

The last element is represented by a rigid belt, which makes it possible to form a flexible connection. Between the links, a friction force is formed, which forms and transmits power.

The gear for the CNC is responsible for the speed of work and the productivity that the machine will have.

This type of transmission is used on units, the configuration of which requires the location of the shafts at a large distance. To connect them, a toothed belt is used. For proper operation of the transmission, it must be well tensioned.

High-quality tension can be obtained in several ways:

• by moving the pulley of the device;
• using tension rollers;
• supplementing the swinging plate with a working motor.

Fixation is carried out using special plates. This type of transmission is used when the movable part is the same large mass. Tension rollers are responsible for the girth of the pulley.

Kinds

There are many types of belt drives. They differ in a number of ways. Classification is made according to characteristics. The main features that divide transmission into different types, are:

• external qualities of the cross section of the belt;
• number and types of pulleys;
• the location of the shafts and the belt in relation to each other;
• the presence of additional videos;
• the number of shafts that the belt covers.

The appearance of the cross section can be: flat belt, V-belt, poly-V-belt, round belt, toothed belt. Wedge and poly-wedge type products are the most common. Used with low power drives.

The arrangement of the shafts in relation to each other can be parallel and intersecting. Parallel spans the pulleys either in one direction or in opposite directions. With an intersecting arrangement, the angle is different.

The number and types of pulleys suggest the presence of shafts: single pulley type, double pulley type, stepped pulley type. The number of shafts that the belt covers is two or more. Auxiliary rollers are divided into: tension, guides, or may be absent.

For the manufacture of flat belts, leather, cotton yarn, rubberized fabric are used. The connection is carried out in several ways: by stitching using small straps, with glue or metal clips. If the belt is loosely tensioned, intermittent slippage is possible. The quality of the product is affected not only by the angle of coverage, but also by its dimensions.

For the manufacture of wedge-shaped options, a rubberized fabric is used. The profile of this type of belt has a trapezoid shape. In one row, several products are stretched. When used, the slippage rate is minimal. Their difference is smooth operation. Together with wedge-shaped options, metal-cutting machines equipped with numerical control are most often used.

An analogue can be a ball screw pair capable of providing a screw transmission.

Advantages and disadvantages

Having provided optimal tension, wrap angle and coefficient of friction, you can create a load sufficient for the CNC machine to work well. The use of a belt drive has both positive sides, as well as negative ones.

Advantages:

• silent and smooth operation;
• no need for high-precision processing;
• resistance to overload and vibration;
• no need for lubrication;
• affordable cost of the mechanism;
• availability of conditions for manual use;
• ease of installation on the machine;
• in the event of a belt break, the drive does not break;
• power is transmitted over a long distance;
• there is the possibility of interaction with a frequency of large rotation;
• the presence of protection systems that reduce the likelihood of breakdowns in the event of a malfunction.

Flaws:

• pulleys are large-sized elements;
• slippage entails a decrease in the transmitted load;
• small power indicator;
• periodic replacement of the belt is necessary;
• risk of malfunction if parts are contaminated or used in an environment with high rate humidity.

The number of advantages outweighs the level of disadvantages. It is possible to reduce the influence of the negative aspects of the equipment by observing the rules for its operation. With periodic maintenance, the likelihood of failure of the device is reduced.

Usage

CNC units equipped with a flat-belt transmission are used as machine tools, sawmills, generators, fans, as well as in other areas where it is necessary to work devices with increased level flexibility and slippage. If the equipment is used at high speeds, synthetic materials are used. For more low speeds Cord fabric and rubberized belts are used.

Wedge-type analogs are used in the agricultural industry. Transmission of various section is able to withstand high loads and great speed. Industrial-grade machines involve the use of CVTs. best performance have toothed belts. They are used in both industrial and domestic areas. Round belt transmission is used for low-power devices.

The main disadvantage of a CNC belt drive is the quality of the belt. Even the highest quality products tend to stretch. Long views stretch the fastest. The tool on stretched belts cannot provide high machining accuracy. The stretching effect can be reduced by attaching two straps on top of each other. Only a certain length is stretched, so this drawback is not so dangerous.

Transmission of this type provides soft movements, in the absence of resonance. Dust and chips are not able to adversely affect its operation. It is possible to tighten the belt.

When using a CNC machine, there are several factors to remember:

• toothed belts provide movement of the moving parts of the unit;
• belts are divided into closed and open;
• polyurethane belts are more wear-resistant;
• on CNC machines, the use of reinforced belts is allowed.

This type of transmission on CNC machines at high speeds can reduce power and accuracy levels. This shortcoming solved by installing special equipment. After installing them, you may need to configure the drivers. This action required in order to smooth the operation of the unit. It is done in the program settings. The value for pulleys that provide correct movement depends on which model of machine or ball screw is selected.

CNC units using a belt drive do not require special software media. The program is compiled and developed depending on the type of work it is needed for. In order for the device to work properly offline, you should periodically check its status. The program cannot solve the problem of faulty hardware.

The transfer of mechanical energy carried out by a flexible connection through friction between the belt and the pulley is called belt. It consists of two pulleys 1 and 2 and a belt 3 (Fig. 15).

Rice. fifteen.

Classification

1. Depending on the shape of the cross section of the belt, the following types of belt drives are distinguished (Fig. 15):

Flat-belt (with a rectangular belt cross-sectional profile);

V-belt (with a trapezoidal belt cross-sectional profile);

Poly-V-belts (with endless flat belts having longitudinal wedge-shaped ledges-ribs on inner surface belts included in the annular wedge grooves of the pulleys);

Round belt;

Serrated.

Rice. 16.

2. According to the mutual arrangement of the axes of the shafts:

With parallel axes (Fig. 17, a, b);

With intersecting axes (Fig. 17, G);

With crossing (Fig. 17, in).

Rice. 17

3. In the direction of rotation of the pulleys:

With the same (Fig. 17, a, in);

With the opposite (Fig. 17, b).

4. According to the method of creating belt tension:

Simple (Fig. 15);

FROM tension roller(Fig. 18);

With tension device.

Rice. eighteen.

Advantages of belt drives:

Ability to transfer energy over considerable distances: up to 12 ... 15 m - flat belts, up to 6 m - V-belts;

Simplicity and low cost of construction;

Smooth and silent running, the ability to soften shocks due to the elasticity of the belt and protect the mechanism from breakage during slipping caused by overload;

Ability to transfer power from fractions of a kilowatt to hundreds of kilowatts (more often up to 50 kW, less often up to 300 kW) at peripheral speeds up to 30 m/s;

Ease of maintenance and care;

Relatively high efficiency: h = 0.91…0.98;

gear ratio i? 7 (usually i?4... 5).

Flaws:

Inconstancy of the gear ratio due to elastic sliding, which varies depending on the load;

Relatively large transmission dimensions and low belt durability (especially in high-speed transmissions);

Pulling the belt during operation of the transmission leads to the need to install additional devices(tension roller);

Large loads on the shafts and their supports (bearings).

Despite these shortcomings, belt drives for use in industry and national economy ranked second after gears. In any branch of mechanical engineering and instrumentation, you can find a flat or V-belt transmission: drives for pumps, fans, conveyors, conveyors, roller tables, etc.

V-belt and poly V-belt drives they are used for relatively large gear ratios, vertical and inclined arrangement of parallel axes of the shafts, the requirement for a small-sized transmission and lower loads on the shaft supports, and energy transfer to several shafts.

Round belt drives are intended mainly for the transmission of low powers and therefore are less common ( sewing machines, appliances, desktop machines, etc.).

Toothed belt drives

Toothed (polyamide) belts combine in their design all the advantages of flat belts and toothed gears. On the working surface of the belts 4 there are protrusions that engage in protrusions on pulleys 1,2 and Z. Polyamide belts are suitable for high-speed transmissions, as well as for transmissions with small center distance. They allow significant overloads, are very reliable and durable.

Gear ratio of belt drives:

i= w1 / w2= n 1 /n 2 =D 2 /D 1 (1- e )

where w1 and w2 - angular velocities on the driving and driven shafts;

n 1 and n 2 - shaft speed;

D 2 and D 1 - diameters of the driving and driven pulleys;

e--=?0.01…0.02 - coefficient of elastic sliding.

stitching used for all types of belts. It is produced by means of sinew strings or rawhide straps. III. Butt stitching with vein strings with inclined punctures IV is considered more perfect and reliable.

Criteria for the performance of belt drives

The main criteria for the performance of belt drives are the traction ability of the belt and its durability. The main calculation is the calculation of traction, which is reduced to determining the cross-sectional area of ​​​​the belt, which ensures the transfer of the necessary force. The durability of a belt, which is mainly determined by its fatigue strength, depends not only on the magnitude of the stresses, but also on the nature and frequency of the cycle of changes in these stresses (or the number of belt runs)

n--=--u/---- l--Ј--,

where u--- circumferential speed, m/s;

l- belt length, m;

[n] - allowed number of belt runs:

For flat belts Ј?5; - for wedge -10 .

Practice shows that if the necessary recommendations are observed, the durability of the belts is 2000 ... 3000 hours.

Designs of the main elements of belt drives

Belt is a traction body, the quality of which determines the durability and normal operation of the transmission. The following requirements are imposed on it: sufficient strength, reliability and durability, low cost and not a shortage of belt material; high traction and elasticity; a sufficiently high coefficient of friction between the belt and the pulley.

flat drive belts are a flexible end or less often an endless tape made of rubberized cotton fabric or leather.

Leather belts possess high traction ability of elasticity and elasticity. Due to scarcity and high cost they are recommended for use only in critical transmissions with frequently changing loads and high speeds up to 40 m/s.

Rubberized belts under calm loads, they have good traction and elasticity, are scarcely scarce, and therefore are widely distributed. They work in wide range power (up to 50 kW) with significant speeds (up to 30 m/s).

are made endless (seamless) in special molds. They consist of a twisted rubberized cotton or synthetic cord (cord) located in the area of ​​the neutral layer of the belt, a rubber-fabric or rubber layer located above the cord and working in tension when the belt is bent, a rubber layer located under the cord and working in compression when bending and a wrap made of rubberized fabric. V-belts are divided into cord-fabric (Fig. 19, a) and cord-cord (Fig. 19, b).

Rice. 19.

The use of a V-belt made it possible to increase the traction ability of the transmission by increasing the friction and adhesion of the belt to the pulley compared to a flat belt transmission.

In poly V-belts(there is no standard) the carrier layer is made in the form of a cord made of chemical fibers (viscose, lavsan, fiberglass).

These belts combine the advantages of flat belts - solidity and flexibility, and wedge - increased grip on the pulley.

timing belts are capable of transmitting energy at a constant gear ratio at high circumferential speeds and power up to hundreds of kilowatts. These belts are made of neoprene reinforced with a metal cable, plastic (polyurethane) is used much less frequently.

Pulleys belt drives are made of steel, aluminum alloys or textolite at u-->?30 m/s. The most common material for the manufacture of pulleys at u--Ј? 30 m / s is gray cast iron SCh 15 and SCh 21, at u--Ј? 25 m / s - SCh 12

Fig.20

The shape of the pulley groove (Fig. 20) in the V-belt drive is performed so that there is a guaranteed gap between the belt and its base, while the side edges of the belt are the workers. At the same time, the belt should not protrude beyond the outer diameter of the pulley, otherwise the groove will quickly destroy the belt with its sharp edges.

Belt transmission is the transmission of mechanical energy using a flexible element (belt) due to friction forces or engagement forces (timing belts). Consists of driving and driven pulleys and a belt (one or more). Belt transmission refers to friction transmissions with a flexible connection.

Belt drive classification

Depending on the shape of the cross section of the belt, belt drives are divided into:
flat-belt (a);
V-belt (with a trapezoidal profile) (b);
round-belt (with a round profile) (c);
poly-V-belt (g);
gears with toothed belts.
In modern engineering greatest application have V-ribbed and poly-V-belts. Gears with a round belt have limited use (sewing machines, desktop machines, appliances).

Depending on the purpose of the transmission and the relative position of the axles:
open with parallel axes of the shafts and rotation of the pulley in one direction;
cross with parallel axes of shafts and rotation of pulleys in opposite directions;
semi-cross with crossing axes;
angular with crossing and intersecting shaft axes.

Advantages and disadvantages of belt drives

Advantages of belt drives:
Simplicity of design and low cost.
Ability to transmit power over long distances (up to 15 meters).
Smooth and quiet operation.
Mitigation of vibration and shocks due to the elastic stretching of the belt.
Disadvantages of belt drives:
Large dimensions, especially when transferring significant power.
Low belt durability in high-speed gears.
Large loads on shafts and bearings from belt tension.
fickle gear ratio due to the inevitable elastic slippage of the belt.
Inapplicability in hazardous areas due to the electrification of the belt.
Disadvantages of belt drives (compared to chain drive):
large dimensions;
low bearing capacity;
slippage (does not apply to toothed belts);
low durability.
Advantages of belt drives (in comparison with a chain drive):
smoothness of work;
noiselessness;
overload compensation;
no need for lubrication;
low cost;
easy installation;
the ability to work at high circumferential speeds;
when out of order, no damage.

Application of belt drives

Belt drives are used in most cases to transmit movement from an electric motor, when, for structural reasons, the center distance a must be large enough, and the gear ratio u is not strictly constant (in the drives of machine tools, conveyors, road and construction machines etc.)
The power transmitted by a belt drive is usually up to 50 kW and in rare cases reaches 1500 kW. The speed of the belt ranges from 5 to 50 m/s, and in high-speed transmissions it can reach 100 m/s.
Power limit lower limit speed is caused by the large dimensions of the transmission.

Belt pulleys

Pulleys are made of steel or cast iron. In high-speed gears, pulleys made of aluminum alloys or textolite are used. The shape of the working surface of the pulley rim depends on the type of belt. For flat belts, the pulleys have a smooth running surface. To center the belt, the surface of the driven pulley is made convex, and the leading one is cylindrical. For v-belts pulley design and rim dimensions depend on the number and size of belt grooves.

belt drive belts

The material of the flat drive belt must have sufficient strength, wear resistance, elasticity and durability, good adhesion to the pulleys and low cost.

For flat belt transmissions, the following types of belts are used:
Leather belts- have good traction ability, tolerate fluctuations and loads well, but they are expensive and scarce.
Rubberized straps- consist of several layers of cotton fabric connected by vulcanized rubber. Rubber ensures the operation of the belt as a whole, protects the fabric from damage and an increased coefficient of friction, but is destroyed by oil, gasoline and alkalis.
Cotton belts- are made as a single fabric with several layers of base, impregnated with a special composition (bitumen, ozakerib). These belts are light and flexible and can be used on small diameter pulleys with high speeds, but have less durability and traction.
Wool belts- fabric with a multi-layered woolen base and cotton weft, impregnated with a special compound (minier on drying oil). They have significant elasticity, are less sensitive to temperature humidity and acids, but have low traction abilities.
Film belts– new type belts made of plastics based on polyamide resins, reinforced with nylon or lavsan cord. They have high static strength and fatigue resistance. Used for transmission high power and speed.
For V-belt transmission, rubberized belts of two designs are used: with a carrier element made of several layers of fabric or a layer of cord wound in a spiral, vulcanized into rubber, with or without a fabric wrap.

Typically, a V-belt drive is an open drive with one or more belts. The working surfaces of the belt are its sides.

Compared to flat belts, V-belt transmissions have greater traction, have a smaller center distance, allow a smaller angle of wrapping of a small pulley and large gear ratios. (and< 10). Однако стандартные клиновые ремни не до­пускают скорость более 30 м/с из-за возможности torsional vibrations of the driven system, associated with the inevitable difference in the width of the belt along its length and, as a result, the variability of the gear ratio for one run of the belt. V-belts have high friction losses and bending stresses, and the pulley design is more complex.

V-belt drives are widely used in individual drives up to 400 kW. V-belt transmission efficiency η = 0.87 ... 0.97.

V-ribbed belt drives do not have most of the disadvantages inherent in V-belt drives, but retain the advantages of the latter. V-ribbed belts have a flexibility comparable to rubber-fabric flat belts, so they run more smoothly, the minimum diameter of the small transmission pulley can be taken smaller, gear ratios can be increased to and< 15, а скорость ремня - до 50 м/с. Передача обладает большой демпфирующей способностью.

V-belts and V-ribbed belts. V-belts are made of endless rubber-fabric construction of trapezoidal section with a wedge angle φ 0 = 40°. Depending on the width ratio b a the greater base of the trapezoid to its height h V-belts come in normal sections ( b 0 /h = 1.6, see); narrow (b 0 /h=1.2); wide (b 0 /h=2.5 and more; used for V-belt variators).

At present, V-belts of normal sections are standardized for drives of machine tools, industrial plants and stationary agricultural machines. The main dimensions and methods of control of such belts are regulated by GOST 1284.1-89. Section belts E0 apply only for operating machines and settings. Standard belts are produced in two types: for temperate and tropical climates, operating at air temperatures from minus 30 to plus 60 °C, and for cold and very cold climates, operating at temperatures from minus 60 to plus 40 °C. To increase flexibility, belts of sections A, B and C can be made with teeth (grooves) on the inner surface, obtained by cutting or molding (Fig. 6.9, c).

V-belts (Fig. 6.9, a, 6) consist of a rubber or rubber-fabric stretching layer 1, carrier layer 2 based on chemical fiber materials (cord fabric or cord cord), compression rubber layer 3 and wrapping layer of rubberized fabric 4. Cross section of cord fabric belt (a), cord-cord (b) structures are shown in fig. 6.9. Cord-cord belts used in high-speed gears are more flexible and durable. Permissible speed for normal section belts v< 30 м/с.

Specifications for V-belts of normal sections are regulated by GOST 1284.2-89, and transmitted power - by GOST 1284.3-89.

In addition to the above-mentioned drive V-belts, the following are standardized: fan V-belts (for engines of cars, tractors and combines) and drive V-belts (for agricultural machines).

If it is necessary to operate a belt with a bend in two directions, hex (double V-belts) belts are used.

Very promising are narrow V-belts, which transmit 1.5-2 times high power than belts of normal sections. Narrow belts allow smaller diameters of the small pulley and operate at speeds up to 50 m/s; gears are more compact. Four sections of these belts U0 (SPZ), UA (SPA), UB (SPB), UV (SPC) replace the seven normal sections.

Narrow belts have increased traction due to better load distribution across the width of the carrier layer, which consists of high-strength synthetic cord. The use of narrow belts significantly reduces the material consumption of belt drives. Narrow belts are not yet standardized and are manufactured in accordance with TU 38 605 205-95.

It should be noted that in V-belt transmissions with several belts, due to different lengths and unequal elastic properties, the load between the belts is unevenly distributed. Therefore, it is not recommended to use more than 8 ... 12 belts in the transmission.

V-ribbed belts (see fig. 6.1, G) are endless flat belts with ribs on the underside, running on pulleys with V-grooves. A high-strength synthetic cord is located across the entire width of the belt; the width of such a belt is 1.5-2 times less than the width of a set of belts of normal sections at the same transmission power.

V-ribbed belts are not yet standardized; on the basis of the normal, three sections of corded multi-ribbed belts are made, designated K, L and M, with the number of ribs from 2 to 50, the length of the belt from 400 to 4000 mm and the wedge angle φ 0 = 40 °.

Compared to flat belts V-belt transmissions have a significantly greater traction capacity due to increased adhesion, due to the reduced coefficient of friction f between belt and pulley.

As is known from the theory of wedge slider friction considered in theoretical mechanics,

f"= f sin( a/2),

where f- coefficient of friction on the plane (for rubberized fabric on cast iron f = 0,3); a- pulley groove profile angle.

Having accepted a= φ 0 = 40°, we get

f" = f sin20°=3 f.

Thus, ceteris paribus, V-belts are capable of transmitting three times more circumferential force than flat belts.

Calculation of transmission with V-belts. The calculation is carried out from the conditions for ensuring the traction capacity and durability of the belts; it is based on the same assumptions as the calculation of flat belt drives.

Belts are calculated using tables containing the rated power transmitted by one belt depending on the belt section, the calculated diameter of the small pulley, its speed and gear ratio (the calculated diameter of the V-belt drive pulley corresponds to the position of the neutral layer of the belt installed in the pulley groove; see Fig. diameter dp in fig. 6.14).

The design calculation of a V-belt transmission begins with the selection of the belt section for a given transmitted power and the speed of a small pulley using graphs (Fig. 6.10). For powers up to 2 kW, section Z is used, and section EO- at capacities over 200 kW.

belt............. Z A B C D E YO UA UB SW

d min , mm......... 63 90 125 200 355 500 63 90 140 224

It should be remembered that the above values ​​of the calculated diameters of the small pulley provide the minimum dimensions of the transmission, but with an increase in this diameter, the traction capacity and transmission efficiency increase, as well as the durability of the belts. In the absence of strict requirements for the dimensions of the transmission, the estimated diameter d1 small pulley should be taken more than the minimum allowable value. Diameter d2 large pulley is determined by the formula

d2=ud1,

where and- transmission ratio; the resulting value is rounded to the nearest standard size.

The calculated diameters of V-belt pulleys are selected from the standard range (mm):

63; 71; 80; 90; 100; 112; 125; 140; 160; 180; 200; 224; 250; 280; 315; 355; 400; 450; 500 etc.

v= πd 1 n 1 / 60 ,

where d 1, n 1 - the estimated diameter and speed of the small pulley.

In the course of further calculation, all geometric parameters transmission.

center distance a pre-determined by the condition

0,55(d1 + d2) + h 2(d1+ d2),

where h- the height of the section of the belt. It should be remembered that with an increase in the center distance, the durability of the belts increases.

Estimated belt length Lp calculated according to the formula given in § 6.1 and rounded up to the nearest standard length from the series (for section B) (mm): 800; 900; 1000; 1120; 1250; 1400; 1600; 1800; 2000; 2120; 2240, etc. up to 6300. Then, using the formula given in § 6.1, determine the final center distance a depending on the accepted standard estimated belt length.

Wrap angle a, on a small pulley is calculated by the formula,

given in § 6.1.

The power P p transmitted by one belt is calculated from

P p \u003d P o C a C L / C p,

where R o is the rated power transmitted by one belt (for belts of section B it is according to Table 6.2; for other sections - according to GOST tables).

C a - wrap angle coefficient:

a° 1 .............. 180 160 140 120 90

S a................ 1.0 0.95 0.89 0.82 0.68

C L - belt length coefficient, depending on the ratio of the accepted length L of the belt to the original length L P specified in the standard:

L/L p .......... 0.3 0.5 0.8 1.0 1.6 2.4

C L .............. 0.79 0.86 0.95 1.0 1.1 1.2

(a detailed table of C L values ​​is given in the standard); C p - coefficient of dynamism and mode of operation; tentatively taken as for flat belt drives, see § 6.2 (a detailed table of C p values ​​\u200b\u200bis given in the standard).

Further calculation of the V-belt transmission is reduced to determining the number of belts z according to the formula

where P is the transmitted power on the drive shaft; C z - coefficient that takes into account the number of belts in the set, is entered at z > 2:

z...................... 2-3 4-6 >6

Cz ................... 0.95 0.90 0.85

In order to avoid significant uneven load distribution between the belts, it is not recommended to use more than 8 belts of normal section and 12 narrow belts in one gear; the number of belts of small sections should not be taken more than 6.

R = 2F0 z sin( a 1/2), where F o- tension of a branch of one belt; a 1- angle of wrapping of the small pulley.

The value F 0 of the tension of the branch of one belt is calculated by the formula

F 0 \u003d (0.85RS p C z) / zνC a +θν 2

where v- circumferential speed of the belt; θ- coefficient taking into account the influence centrifugal forces:

Belt section.... Z A B C D E E0

θ, N*s 2 /m 2 0.06 0.1 0.18 0.3 0.6 0.9 1.5

Gears with narrow and V-ribbed belts are calculated using a similar method. Tables of power transmitted by a single narrow belt and a poly-V-belt with 10 ribs are available in course manuals for the design of machine parts.

When calculating poly V-belts, the number of ribs z is determined by the formula

z=10P/P p

where R- transmitted power on the drive shaft; R p- power transmitted by a belt with 10 ribs.

The calculation of the durability of V-belts of normal sections is established by GOST 1284.2-89. The average resource L h cf belts in operation for an average operating mode is set to 2000 hours. For light, heavy and very heavy operating modes, the calculated resource is calculated by the formula

L hp \u003d L h cf K 1 K 2

where K 1- operating mode coefficient equal to: for light mode- 2.5; for heavy duty - 0.5; for very heavy duty - 0.25; K 2- coefficient taking into account climatic conditions operation, equal to: for areas with a cold and very cold climate - 0.75; for other regions - 1.0.

The operating mode for specific machines is set according to GOST. So, for example, for machines with a continuous cutting process (turning, drilling, grinding), the operating mode is assumed to be light; for milling, gear-cutting machines, the operating mode is assumed to be medium; planing, slotting, gear shaping and woodworking machines operate in heavy duty; very heavy duty work is required for lifts, excavators, hammers, crushers, sawmills, etc.