Understanding Belt Drive Mechanics

How does a belt drive work? In this video, we look at the basics of belt drives. The drive pulley is connected to a motor and drives the belt. This causes the output pulley to rotate. Belt drives often have a power ratio, which means that the smaller of the two pulleys is the drive pulley. In belt drives, power is usually transmitted by friction between the belt and the pulley. V-belts or flat belt are most commonly used. An exception to frictional power transmission are toothed belts, where power is transmitted positively by the teeth on the belt engaging with the pulley. These belts are also known as timing belts. With friction-locked power transmission, the belt must be pressed against the pulley with a certain contact pressure. This is the only way to ensure that the resulting frictional force is high enough to prevent the belt from slipping. The maximum force to be transmitted is equal to the maximum frictional force between the belt and pulley. If the force to be transmitted is greater than the frictional force, the drive pulley will slip under the belt or the belt will slip over the driven pulley. This is known as slippage. Due to the relative movement between the belt and the pulley, the belt will wear out very quickly and become unusable. It is therefore essential to prevent the belt from slipping over the pulley. The section of the belt where the belt is strongly pulled towards the drive pulley and is therefore subject to a high load is also known as the tight side. On the opposite side of the belt, the belt moves away from the drive pulley and is slightly relieved. This section of the belt is called the slack side. It should be noted, however, that the slack side of the belt does not run unloaded as there are also tensile forces acting inside the belt. If there were no forces, there would be no belt tension. However, belt tension is essential to allow the belt to press against the pulleys and generate the frictional force required to transmit power. Special tensioning systems ensure that belt tension is maintained during operation. These belt tensioner systems will be discussed in more detail later. The high tensile forces cause relatively high stress on the tight side of the belt. As a result, the belt is stretched slightly. On the slack side, however, The tensile forces are lower and the belt loosens slightly. It is only when the belt drive is unloaded that the belt forces are equal and the slack disappears. Note that depending on the direction of rotation of the belt drive, the tight side and the slack side are reversed. In the case just shown, the tight side was at the top and the slack side at the bottom when the direction of rotation was counterclockwise, whereas now the tight side is at the bottom and the slack side at the top when the direction of rotation is clockwise. In addition to the belt tension required to generate the contact pressure, the pulley must also be sufficiently wrapped by the belt to provide the necessary frictional forces. The degree of wrapping is described by the wrap angle phi. The greater the wrap angle, the more contact surface the belt has and the greater the frictional force and therefore the maximum transmittable belt force. It should be noted, however, that if the input and output pulleys have different diameters, the belt will wrap around the pulleys to different degrees. The maximum belt force that can be transmitted is usually limited by the smaller of the two pulleys. This is usually the drive pulley as it has a smaller wrap angle compared to the larger driven pulley. In addition, due to the stronger curvature of the smaller pulley, there are greater bending stresses in the belt, which also limit the transmittable belt force. Idler pulleys can be used to increase the wrap angle. These are usually positioned close to the pulleys in order to achieve the greatest possible wrapping effect. If these idlers are also used to tension the belt, they are also referred to as tension pulleys. It should also be noted that the deflection of the slack side of the belt under load will result in a different wrap angle than when unloaded. The arrangement of the tight side and slack side also affects the wrap angle. If the slack side is above the tight side, the wrap angle will increase due to deflection. whereas in the opposite case the wrap angle will decrease. However, the change in wrap angle under load plays a minor role in practice and can often be neglected, especially with large coefficients of friction. The picture shows the drive of a dryer drum with a V-ribbed belt, in which an idler pulley is used to increase the wrap angle. The drive shaft around which the V-ribbed belt is wrapped can be seen. The dryer drum acts as the output pulley, around which the belt is also wrapped. However, due to the large difference in diameter, the belt would not wrap sufficiently around the drive shaft without the idler pulley. Therefore, the idler pulley is used in this case to increase the wrap angle. As well as increasing the wrap angle, idler pulleys are also used in multiple belt drives, where one input pulley drives several output pulleys. Such idler pulleys can also act as a guide to prevent the belt from coming off the pulley. They are then known as guiding pulleys or guide rollers. Guiding pulleys often have protrusions to the left and right, called flanged pulleys. between which the belt is held on track. As already explained, the belt must be kept under constant tension to generate sufficient frictional force to prevent the belt from slipping over the output pulley or the input pulley from slipping under the belt. However, despite pre-tensioning, belt tension can change during operation due to plastic expansion or temperature effects. For this reason, belt drives often need to be kept under tension by means of special devices. It should also be noted that a belt also needs to be serviced over time and removed from the pulley and replaced. This is difficult to do under tension, so for maintenance reasons alone, the belt must be loosened for replacement and re-tensioned by tensioning systems. Tension rollers can be used to maintain belt tension during operation. They are also used to absorb heavy load changes and, in the case of very long belts, to prevent excessive vibration. One way of generating belt tension is to mount the tensioner pulley eccentrically. The desired belt tension can then be set by rotating the pulley to a specific position. Torsion springs can also be fitted to the pulley to allow dynamic adjustment of belt tension during load changes. Another way of generating the required belt tension is to use hydraulic damping tensioners. These consist of a lever arm to one end of which is attached the tensioner pulley. which is held under tension by a spring. These systems are hydraulically damped to prevent vibration during heavy load changes. This is achieved by means of a piston in an oil bath, the viscosity of which provides the necessary damping. The principle is the same as for hydraulic door closers. Another way to achieve the required belt tension is to adjust the drive pulley itself. In the case of a motor slide base, The entire motor is mounted on a movable slide. The position of the slide can be changed using screws, allowing the belt tension to be adjusted. However, if the belt tension drops or the load changes significantly, the slide base will not adapt to the changed conditions and must be readjusted manually. Dynamic adjustment of the belt tension can be achieved by using a self-tightening motor base. The motor is mounted on a pivoting motor base, with the center of gravity of the whole system. designed in such a way that the motor tends to tilt backwards. At an inclination of approximately 15 degrees to 20 degrees, the weight of the motor base ensures a permanent and almost constant belt tension. The effects of the different belt forces as the belt rotates around the pulleys are examined in more detail in the following. These different belt tensions also cause different elongations due to the elasticity of the belt. If marker lines are attached to the belt at equal intervals in the unloaded state, the line spacing on the tight side will increase during operation due to the high belt tension and correspondingly decrease on the slack side due to the lower tension. As the belt turns around the pulleys, the line spacing will gradually adjust to the new belt tensions. For better illustration, the elongations in the animation are greatly exaggerated. First, let's take a closer look at the belt as it runs around the output pulley. The belt is pulled over the output pulley and stretched by the force acting on the tight side. This results in relative motion between the expanding belt and the output pulley, causing the belt to slide over the pulley. As you can see from the animation, the speed at which the belt moves around the pulley is greater than the circumferential speed of the output pulley. Now let us take a closer look at the belt as it runs around the drive pulley. The decreasing force on the slack side now causes a decrease in the elongation or shrinkage of the belt as it runs off the drive pulley. This results in relative motion between the contracting belt and the drive pulley, causing the belt to slide over the pulley. As a result, the belt runs slower around the pulley than the circumferential speed of the drive pulley. So note, the speed of the belt on the tight side is higher than the circumferential speed of the driven pulley. On the slack side, however, the speed of the belt is slower than the circumferential speed of the drive pulley. Note that the belt does not slide on the pulleys over the entire wrap angle, but only in a certain area as it runs off the pulley. This area is known as the sliding zone. In the remaining area, the belt simply rests on the pulleys without relative movement as it runs onto the pulleys. This area is also referred to as the adhesion zone. At this point, a crucial note on what is probably the biggest misconception about belt drives. The actual power transmission by friction takes place only in the sliding zones, not in the adhesion zones. The term adhesion zone is somewhat unfortunate. because it suggests that the belt adheres to the pulley by static friction. In the adhesion zone, however, the belt is in contact with the pulley without the influence of friction. The fact that there is no effective power transmission between the pulley and the belt in the adhesion zones is evident from the fact that the belt sections in these adhesion zones do not experience any change in elongation. Only when a circumferential force is applied to the belt, such as in the sliding zone of the drive pulley. or a circumferential force is transferred from the belt to the pulley, such as in the sliding zone of the driven pulley, does a change in force occur in the belt and thus a change in elongation. In the adhesion zones, however, there is no change in elongation and therefore no power transmission, since no circumferential force is transmitted. So note, the actual power transmission takes place in the sliding zones and is therefore subject to sliding friction, not static friction. This is why the coefficient of sliding friction, not the coefficient of static friction, plays a central role in power transmission in belt drives. In another video, we take a closer look at power transmission in belt drives. The described sliding of the belt on the pulleys due to expansion and contraction is generally referred to as elastic slippage. Elastic slippage is a direct result of the elasticity of the belt and is basically unavoidable. as the belt must inevitably expand and contract on the pulleys. In contrast to sliding slippage under overload, where the belt slides over the pulley as a whole, there are always adhesion zones where the belt rests on the pulley without relative movement. As already mentioned, sliding slip should be avoided at all costs. Elastic slip, on the other hand, cannot be avoided, but should be kept as low as possible, otherwise the strong relative movement will cause enormous wear on the belt. The surfaces of the pulleys must therefore not be too rough, as one might think at first glance due to the increased friction with rough surfaces. Excessively rough surfaces combined with the unavoidable elastic sliding on the pulleys would result in high belt wear. When selecting belt and pulley materials, it is important to ensure that the coefficient of friction is as high as possible for high-power transmission, but that the surfaces themselves are as smooth as possible. It is important to note that the sliding zones marked in green in the animation always cover the same angular range on both pulleys. This depends on the ratio of the belt force on the tight side to the force on the slack side. The basis for this, and for all power transmission in belt drives, is the so-called belt friction equation, which we will discuss in more detail in another video. If the force on the tight side increases sharply, for example because a heavy load is being moved, the sliding zones on the pulleys will increase at the expense of the adhesion zones. Since the smaller drive pulley has a smaller wrap angle, the adhesion zone is consumed first by the sliding zone. In this case, there is a relative movement between the belt and the drive pulley from the very beginning and the limit from elastic slip to sliding slip is exceeded and the drive pulley slides completely under the belt. The aforementioned idler pulleys can be used here to increase the wrap and thus the adhesion zone. The adhesion zone provides a safety margin against sliding slippage, so to speak. It should be noted, however, that the actual transmission of power from the pulley to the belt and vice versa takes place in the sliding zones, not in the adhesion zones. The elasticity of the belt is not only responsible for the elastic slip, but also means that the belt no longer moves at a constant speed. This can be seen in the animation. by comparing the speed of the belt on the tight side with the speed on the slack side. The different belt speeds are ultimately a direct consequence of the conservation of mass, which states that the same belt mass must be moved over an imaginary point on both the tight side and the slack side within a given time. Otherwise, belt mass would be miraculously accumulated or destroyed between the two imaginary points, as more or less mass is moved over one point than is moved over the other. The different belt speeds adjust to each other by the aforementioned sliding processes as the belt passes around the pulleys. As already explained, the speed of the belt on the tight side is higher than the circumferential speed of the driven pulley, while on the slack side it is lower than the circumferential speed of the driving pulley. In the figure, V1 is the circumferential speed of the drive pulley. V2 is the circumferential speed of the driven pulley and VB is the belt speed. However, if the drive pulley is generally moving faster than the belt and the driven pulley is moving slower, the circumferential speeds of the pulleys will no longer be the same. This ultimately results in a loss of speed between the input pulley, which rotates faster than the belt, and the output pulley, which rotates slower than the belt. This inevitably results in a loss of power between the input and output. since power is directly related to the product of circumferential force and circumferential speed. For more information, watch the video on the basics of power transmission. Note that elastic slip has no effect on the circumferential force Fc on the pulleys. The circumferential force depends only on the difference between the tight side force Ft and the slack side force Fs and is therefore the same on both pulleys. This can be seen very quickly using a free body diagram. On the input side, the tight side force is in equilibrium with the slack side force and the circumferential force transmitted by friction from the drive pulley to the belt. The circumferential force is therefore equal to the difference between the tight side force and the slack side force, as shown. On the output side, the tight side force is also in equilibrium with the slack side force and the circumferential force transmitted by friction from the belt to the output pulley. From the belt's point of view, the circumferential force corresponds to the resistance caused by the load to be driven on the output pulley and is therefore directed against the direction of rotation. On the output side, the circumferential force is also the difference between the tight side force and the slack side force. Therefore, the circumferential force on both pulleys is always the same, regardless of the elastic slip, because the belt forces are the same in both cases. Due to the identical circumferential forces but different circumferential speeds, there is a loss of power between the input and output. Note that without elastic slip, the circumferential speeds of the pulleys would be identical and the power of the input pulley would equal the power of the output pulley. This is no longer the case because of elastic slip. The greater the elasticity of the belt, the greater the elastic slip and thus the loss of speed and power. The elastic slip and the associated power losses are in the order of approximately 1 to 2 percent. Let us briefly discuss the conversion of torque and speed in belt drives. The following relationships are explained in detail in the video on the basics of transmissions. The transmission ratio, which is the quotient of the diameters of the output and input pulleys, is used to describe the conversion of speed and torque. Strictly speaking, it is not the diameter of the pulley that is to be taken as the basis here. but rather the outer diameter of the rotating belt in the case of flat belts and the so-called pulley pitch diameter in the case of V-belts, which is the diameter of the neutral axis of the belt as it runs around the pulleys. Ideally, the torque conversion then results as shown from the product of the input torque m1 and the transmission ratio I. It should be noted that, by definition, the torque is the product of the circumferential force and the radius of the pulleys. Since, as already explained, The elastic slip has no influence on the circumferential force, it also has no influence on the torque. As the output torque increases with the transmission ratio, the rotational speed decreases accordingly. However, this formula only applies to the ideal case, without taking elastic slip into account. Since the rotational speed is directly related to the circumferential speed as shown, but this is reduced by the elastic slip, there is ultimately a loss of rotational speed. Elastic slip must therefore be taken into account in the formula for calculating the output speed. The previous considerations were based on the ideal case where there is no friction loss in the bearings of the belt drive. In practice, however, bearing friction does occur. This can be taken into account by using an efficiency factor. Bearing friction reduces the theoretically calculated output torque and must therefore be taken into account. The efficiency factor has no direct influence on the output speed. However, the reduction in output torque due to bearing friction will affect the output power. It should be noted that the power is dependent on the rotational speed and torque as stated. Reducing the output torque therefore reduces the power to the same extent. For this reason, the efficiency factor must also be taken into account in the formula for calculating the output power. As can be seen from the formula, the output power is affected by both the elastic slip, due to the reduction in rotational speed, and the efficiency factor, due to the reduction in torque. Finally, we will look at the advantages and disadvantages of belt drives compared to other types of transmission. We have already explained these in more detail in the video on the basics of transmissions. One advantage of belt drives over a conventional gearbox is that they can easily cover greater distances between two shafts. Friction-locked belts also provide the overload function already described. In the worst-case scenario, only the belt needs to be replaced. rather than all the gears and shafts as with a damaged conventional gearbox. Another advantage of belt drives is the elasticity of the belts used compared to rigid gears. This provides good damping characteristics, especially in the event of sudden torque changes. This is why belt drives are used, for example, in mills or stone crushers. The starting and stopping characteristics are also damped accordingly and are not as jerky as with rigid gears. Another advantage of belt drives over gears is their insensitivity to angular misalignment within wide limits. In many cases, such misalignment is even deliberate. In extreme cases, it is even possible to reverse the direction of rotation. By turning the output shaft 180 degrees or simply crossing the belt, the original direction of rotation can be easily reversed. In contrast to an open belt drive, This is called a crossed belt drive. Unlike conventional gearboxes, belt drives do not require lubrication. This reduces maintenance costs. In addition, belt drives are generally quieter than gearboxes because there are no metal teeth to mesh with. The relatively light weight of the belt also means that high speeds can be achieved. The efficiency of belt drives is also relatively high at over 95%. In addition, Pulleys are generally not solid wheels like gears. Pulleys have recesses to reduce weight and manufacturing costs. As a result, belt drives are generally lighter than conventional gearboxes. But belt drives also have disadvantages. Depending on the environmental conditions, belts age to a greater or lesser extent, which means that they lose their elastic properties over time and need to be replaced. For this reason, Belts can only be used within a certain temperature range. In addition, the belts will plastically stretch over time, which means that they need to be re-tensioned at regular intervals. Another disadvantage of some types of belt, such as flat or V-belts, is the aforementioned elastic slip, which leads not only to a loss of power, but also to a loss of speed. Such belts are therefore not suitable for precise positioning. Slippage can only be avoided with timing belts. due to their positively locking power transmission. Belt drives usually take up more space than conventional gearboxes, which can be a disadvantage in some cases. This is due to the fact that the pulleys cannot be placed directly next to each other, whereas in conventional gearboxes the gears actually mesh with each other and can therefore be arranged in a more space-saving manner. In addition, as the center distance decreases, the wrap angle decreases and can become unacceptably small. This can be compensated for by the use of idler pulleys, but this not only increases the design effort, but can also increase the space requirement. When using idler pulleys, it should be noted that the belt is also bent around the idler pulley during rotation. This increases the so-called bending frequency and can lead to premature fatigue of the belt. Another disadvantage of belt drives compared to conventional gearboxes is the relatively high bearing forces due to the need to pre-tension the belts. Pretensioning the belt often requires increased design effort. I hope you enjoyed the video and found it helpful. Thanks for watching.

How does a belt drive work? In this video, we look at the basics of belt drives. The drive pulley is connected to a motor and drives the belt.

This causes the output pulley to rotate. Belt drives often have a power ratio, which means that the smaller of the two pulleys is the drive pulley. In belt drives, power is usually transmitted by friction between the belt and the pulley.

V-belts or flat belt are most commonly used. An exception to frictional power transmission are toothed belts, where power is transmitted positively by the teeth on the belt engaging with the pulley. These belts are also known as timing belts. With friction-locked power transmission, the belt must be pressed against the pulley with a certain contact pressure.

This is the only way to ensure that the resulting frictional force is high enough to prevent the belt from slipping. The maximum force to be transmitted is equal to the maximum frictional force between the belt and pulley. If the force to be transmitted is greater than the frictional force, the drive pulley will slip under the belt or the belt will slip over the driven pulley. This is known as slippage.

Due to the relative movement between the belt and the pulley, the belt will wear out very quickly and become unusable. It is therefore essential to prevent the belt from slipping over the pulley. The section of the belt where the belt is strongly pulled towards the drive pulley and is therefore subject to a high load is also known as the tight side.

On the opposite side of the belt, the belt moves away from the drive pulley and is slightly relieved. This section of the belt is called the slack side. It should be noted, however, that the slack side of the belt does not run unloaded as there are also tensile forces acting inside the belt.

If there were no forces, there would be no belt tension. However, belt tension is essential to allow the belt to press against the pulleys and generate the frictional force required to transmit power. Special tensioning systems ensure that belt tension is maintained during operation.

These belt tensioner systems will be discussed in more detail later. The high tensile forces cause relatively high stress on the tight side of the belt. As a result, the belt is stretched slightly.

On the slack side, however, The tensile forces are lower and the belt loosens slightly. It is only when the belt drive is unloaded that the belt forces are equal and the slack disappears. Note that depending on the direction of rotation of the belt drive, the tight side and the slack side are reversed. In the case just shown, the tight side was at the top and the slack side at the bottom when the direction of rotation was counterclockwise, whereas now the tight side is at the bottom and the slack side at the top when the direction of rotation is clockwise. In addition to the belt tension required to generate the contact pressure, the pulley must also be sufficiently wrapped by the belt to provide the necessary frictional forces.

The degree of wrapping is described by the wrap angle phi. The greater the wrap angle, the more contact surface the belt has and the greater the frictional force and therefore the maximum transmittable belt force. It should be noted, however, that if the input and output pulleys have different diameters, the belt will wrap around the pulleys to different degrees. The maximum belt force that can be transmitted is usually limited by the smaller of the two pulleys. This is usually the drive pulley as it has a smaller wrap angle compared to the larger driven pulley.

In addition, due to the stronger curvature of the smaller pulley, there are greater bending stresses in the belt, which also limit the transmittable belt force. Idler pulleys can be used to increase the wrap angle. These are usually positioned close to the pulleys in order to achieve the greatest possible wrapping effect. If these idlers are also used to tension the belt, they are also referred to as tension pulleys.

It should also be noted that the deflection of the slack side of the belt under load will result in a different wrap angle than when unloaded. The arrangement of the tight side and slack side also affects the wrap angle. If the slack side is above the tight side, the wrap angle will increase due to deflection.

whereas in the opposite case the wrap angle will decrease. However, the change in wrap angle under load plays a minor role in practice and can often be neglected, especially with large coefficients of friction. The picture shows the drive of a dryer drum with a V-ribbed belt, in which an idler pulley is used to increase the wrap angle.

The drive shaft around which the V-ribbed belt is wrapped can be seen. The dryer drum acts as the output pulley, around which the belt is also wrapped. However, due to the large difference in diameter, the belt would not wrap sufficiently around the drive shaft without the idler pulley. Therefore, the idler pulley is used in this case to increase the wrap angle.

As well as increasing the wrap angle, idler pulleys are also used in multiple belt drives, where one input pulley drives several output pulleys. Such idler pulleys can also act as a guide to prevent the belt from coming off the pulley. They are then known as guiding pulleys or guide rollers.

Guiding pulleys often have protrusions to the left and right, called flanged pulleys. between which the belt is held on track. As already explained, the belt must be kept under constant tension to generate sufficient frictional force to prevent the belt from slipping over the output pulley or the input pulley from slipping under the belt.

However, despite pre-tensioning, belt tension can change during operation due to plastic expansion or temperature effects. For this reason, belt drives often need to be kept under tension by means of special devices. It should also be noted that a belt also needs to be serviced over time and removed from the pulley and replaced.

This is difficult to do under tension, so for maintenance reasons alone, the belt must be loosened for replacement and re-tensioned by tensioning systems. Tension rollers can be used to maintain belt tension during operation. They are also used to absorb heavy load changes and, in the case of very long belts, to prevent excessive vibration.

One way of generating belt tension is to mount the tensioner pulley eccentrically. The desired belt tension can then be set by rotating the pulley to a specific position. Torsion springs can also be fitted to the pulley to allow dynamic adjustment of belt tension during load changes.

Another way of generating the required belt tension is to use hydraulic damping tensioners. These consist of a lever arm to one end of which is attached the tensioner pulley. which is held under tension by a spring.

These systems are hydraulically damped to prevent vibration during heavy load changes. This is achieved by means of a piston in an oil bath, the viscosity of which provides the necessary damping. The principle is the same as for hydraulic door closers.

Another way to achieve the required belt tension is to adjust the drive pulley itself. In the case of a motor slide base, The entire motor is mounted on a movable slide. The position of the slide can be changed using screws, allowing the belt tension to be adjusted.

However, if the belt tension drops or the load changes significantly, the slide base will not adapt to the changed conditions and must be readjusted manually. Dynamic adjustment of the belt tension can be achieved by using a self-tightening motor base. The motor is mounted on a pivoting motor base, with the center of gravity of the whole system. designed in such a way that the motor tends to tilt backwards.

At an inclination of approximately 15 degrees to 20 degrees, the weight of the motor base ensures a permanent and almost constant belt tension. The effects of the different belt forces as the belt rotates around the pulleys are examined in more detail in the following. These different belt tensions also cause different elongations due to the elasticity of the belt. If marker lines are attached to the belt at equal intervals in the unloaded state, the line spacing on the tight side will increase during operation due to the high belt tension and correspondingly decrease on the slack side due to the lower tension. As the belt turns around the pulleys, the line spacing will gradually adjust to the new belt tensions.

For better illustration, the elongations in the animation are greatly exaggerated. First, let's take a closer look at the belt as it runs around the output pulley. The belt is pulled over the output pulley and stretched by the force acting on the tight side.

This results in relative motion between the expanding belt and the output pulley, causing the belt to slide over the pulley. As you can see from the animation, the speed at which the belt moves around the pulley is greater than the circumferential speed of the output pulley. Now let us take a closer look at the belt as it runs around the drive pulley. The decreasing force on the slack side now causes a decrease in the elongation or shrinkage of the belt as it runs off the drive pulley.

This results in relative motion between the contracting belt and the drive pulley, causing the belt to slide over the pulley. As a result, the belt runs slower around the pulley than the circumferential speed of the drive pulley. So note, the speed of the belt on the tight side is higher than the circumferential speed of the driven pulley. On the slack side, however, the speed of the belt is slower than the circumferential speed of the drive pulley. Note that the belt does not slide on the pulleys over the entire wrap angle, but only in a certain area as it runs off the pulley.

This area is known as the sliding zone. In the remaining area, the belt simply rests on the pulleys without relative movement as it runs onto the pulleys. This area is also referred to as the adhesion zone.

At this point, a crucial note on what is probably the biggest misconception about belt drives. The actual power transmission by friction takes place only in the sliding zones, not in the adhesion zones. The term adhesion zone is somewhat unfortunate.

because it suggests that the belt adheres to the pulley by static friction. In the adhesion zone, however, the belt is in contact with the pulley without the influence of friction. The fact that there is no effective power transmission between the pulley and the belt in the adhesion zones is evident from the fact that the belt sections in these adhesion zones do not experience any change in elongation.

Only when a circumferential force is applied to the belt, such as in the sliding zone of the drive pulley. or a circumferential force is transferred from the belt to the pulley, such as in the sliding zone of the driven pulley, does a change in force occur in the belt and thus a change in elongation. In the adhesion zones, however, there is no change in elongation and therefore no power transmission, since no circumferential force is transmitted. So note, the actual power transmission takes place in the sliding zones and is therefore subject to sliding friction, not static friction. This is why the coefficient of sliding friction, not the coefficient of static friction, plays a central role in power transmission in belt drives.

In another video, we take a closer look at power transmission in belt drives. The described sliding of the belt on the pulleys due to expansion and contraction is generally referred to as elastic slippage. Elastic slippage is a direct result of the elasticity of the belt and is basically unavoidable. as the belt must inevitably expand and contract on the pulleys. In contrast to sliding slippage under overload, where the belt slides over the pulley as a whole, there are always adhesion zones where the belt rests on the pulley without relative movement.

As already mentioned, sliding slip should be avoided at all costs. Elastic slip, on the other hand, cannot be avoided, but should be kept as low as possible, otherwise the strong relative movement will cause enormous wear on the belt. The surfaces of the pulleys must therefore not be too rough, as one might think at first glance due to the increased friction with rough surfaces.

Excessively rough surfaces combined with the unavoidable elastic sliding on the pulleys would result in high belt wear. When selecting belt and pulley materials, it is important to ensure that the coefficient of friction is as high as possible for high-power transmission, but that the surfaces themselves are as smooth as possible. It is important to note that the sliding zones marked in green in the animation always cover the same angular range on both pulleys. This depends on the ratio of the belt force on the tight side to the force on the slack side. The basis for this, and for all power transmission in belt drives, is the so-called belt friction equation, which we will discuss in more detail in another video.

If the force on the tight side increases sharply, for example because a heavy load is being moved, the sliding zones on the pulleys will increase at the expense of the adhesion zones. Since the smaller drive pulley has a smaller wrap angle, the adhesion zone is consumed first by the sliding zone. In this case, there is a relative movement between the belt and the drive pulley from the very beginning and the limit from elastic slip to sliding slip is exceeded and the drive pulley slides completely under the belt.

The aforementioned idler pulleys can be used here to increase the wrap and thus the adhesion zone. The adhesion zone provides a safety margin against sliding slippage, so to speak. It should be noted, however, that the actual transmission of power from the pulley to the belt and vice versa takes place in the sliding zones, not in the adhesion zones. The elasticity of the belt is not only responsible for the elastic slip, but also means that the belt no longer moves at a constant speed. This can be seen in the animation.

by comparing the speed of the belt on the tight side with the speed on the slack side. The different belt speeds are ultimately a direct consequence of the conservation of mass, which states that the same belt mass must be moved over an imaginary point on both the tight side and the slack side within a given time. Otherwise, belt mass would be miraculously accumulated or destroyed between the two imaginary points, as more or less mass is moved over one point than is moved over the other.

The different belt speeds adjust to each other by the aforementioned sliding processes as the belt passes around the pulleys. As already explained, the speed of the belt on the tight side is higher than the circumferential speed of the driven pulley, while on the slack side it is lower than the circumferential speed of the driving pulley. In the figure, V1 is the circumferential speed of the drive pulley. V2 is the circumferential speed of the driven pulley and VB is the belt speed. However, if the drive pulley is generally moving faster than the belt and the driven pulley is moving slower, the circumferential speeds of the pulleys will no longer be the same.

This ultimately results in a loss of speed between the input pulley, which rotates faster than the belt, and the output pulley, which rotates slower than the belt. This inevitably results in a loss of power between the input and output. since power is directly related to the product of circumferential force and circumferential speed. For more information, watch the video on the basics of power transmission.

Note that elastic slip has no effect on the circumferential force Fc on the pulleys. The circumferential force depends only on the difference between the tight side force Ft and the slack side force Fs and is therefore the same on both pulleys. This can be seen very quickly using a free body diagram.

On the input side, the tight side force is in equilibrium with the slack side force and the circumferential force transmitted by friction from the drive pulley to the belt. The circumferential force is therefore equal to the difference between the tight side force and the slack side force, as shown. On the output side, the tight side force is also in equilibrium with the slack side force and the circumferential force transmitted by friction from the belt to the output pulley. From the belt's point of view, the circumferential force corresponds to the resistance caused by the load to be driven on the output pulley and is therefore directed against the direction of rotation. On the output side, the circumferential force is also the difference between the tight side force and the slack side force.

Therefore, the circumferential force on both pulleys is always the same, regardless of the elastic slip, because the belt forces are the same in both cases. Due to the identical circumferential forces but different circumferential speeds, there is a loss of power between the input and output. Note that without elastic slip, the circumferential speeds of the pulleys would be identical and the power of the input pulley would equal the power of the output pulley.

This is no longer the case because of elastic slip. The greater the elasticity of the belt, the greater the elastic slip and thus the loss of speed and power. The elastic slip and the associated power losses are in the order of approximately 1 to 2 percent.

Let us briefly discuss the conversion of torque and speed in belt drives. The following relationships are explained in detail in the video on the basics of transmissions. The transmission ratio, which is the quotient of the diameters of the output and input pulleys, is used to describe the conversion of speed and torque. Strictly speaking, it is not the diameter of the pulley that is to be taken as the basis here. but rather the outer diameter of the rotating belt in the case of flat belts and the so-called pulley pitch diameter in the case of V-belts, which is the diameter of the neutral axis of the belt as it runs around the pulleys.

Ideally, the torque conversion then results as shown from the product of the input torque m1 and the transmission ratio I. It should be noted that, by definition, the torque is the product of the circumferential force and the radius of the pulleys. Since, as already explained, The elastic slip has no influence on the circumferential force, it also has no influence on the torque. As the output torque increases with the transmission ratio, the rotational speed decreases accordingly. However, this formula only applies to the ideal case, without taking elastic slip into account.

Since the rotational speed is directly related to the circumferential speed as shown, but this is reduced by the elastic slip, there is ultimately a loss of rotational speed. Elastic slip must therefore be taken into account in the formula for calculating the output speed. The previous considerations were based on the ideal case where there is no friction loss in the bearings of the belt drive.

In practice, however, bearing friction does occur. This can be taken into account by using an efficiency factor. Bearing friction reduces the theoretically calculated output torque and must therefore be taken into account. The efficiency factor has no direct influence on the output speed.

However, the reduction in output torque due to bearing friction will affect the output power. It should be noted that the power is dependent on the rotational speed and torque as stated. Reducing the output torque therefore reduces the power to the same extent. For this reason, the efficiency factor must also be taken into account in the formula for calculating the output power.

As can be seen from the formula, the output power is affected by both the elastic slip, due to the reduction in rotational speed, and the efficiency factor, due to the reduction in torque. Finally, we will look at the advantages and disadvantages of belt drives compared to other types of transmission. We have already explained these in more detail in the video on the basics of transmissions.

One advantage of belt drives over a conventional gearbox is that they can easily cover greater distances between two shafts. Friction-locked belts also provide the overload function already described. In the worst-case scenario, only the belt needs to be replaced. rather than all the gears and shafts as with a damaged conventional gearbox.

Another advantage of belt drives is the elasticity of the belts used compared to rigid gears. This provides good damping characteristics, especially in the event of sudden torque changes. This is why belt drives are used, for example, in mills or stone crushers. The starting and stopping characteristics are also damped accordingly and are not as jerky as with rigid gears. Another advantage of belt drives over gears is their insensitivity to angular misalignment within wide limits.

In many cases, such misalignment is even deliberate. In extreme cases, it is even possible to reverse the direction of rotation. By turning the output shaft 180 degrees or simply crossing the belt, the original direction of rotation can be easily reversed.

In contrast to an open belt drive, This is called a crossed belt drive. Unlike conventional gearboxes, belt drives do not require lubrication. This reduces maintenance costs.

In addition, belt drives are generally quieter than gearboxes because there are no metal teeth to mesh with. The relatively light weight of the belt also means that high speeds can be achieved. The efficiency of belt drives is also relatively high at over 95%. In addition, Pulleys are generally not solid wheels like gears. Pulleys have recesses to reduce weight and manufacturing costs.

As a result, belt drives are generally lighter than conventional gearboxes. But belt drives also have disadvantages. Depending on the environmental conditions, belts age to a greater or lesser extent, which means that they lose their elastic properties over time and need to be replaced.

For this reason, Belts can only be used within a certain temperature range. In addition, the belts will plastically stretch over time, which means that they need to be re-tensioned at regular intervals. Another disadvantage of some types of belt, such as flat or V-belts, is the aforementioned elastic slip, which leads not only to a loss of power, but also to a loss of speed. Such belts are therefore not suitable for precise positioning.

Slippage can only be avoided with timing belts. due to their positively locking power transmission. Belt drives usually take up more space than conventional gearboxes, which can be a disadvantage in some cases.

This is due to the fact that the pulleys cannot be placed directly next to each other, whereas in conventional gearboxes the gears actually mesh with each other and can therefore be arranged in a more space-saving manner. In addition, as the center distance decreases, the wrap angle decreases and can become unacceptably small. This can be compensated for by the use of idler pulleys, but this not only increases the design effort, but can also increase the space requirement. When using idler pulleys, it should be noted that the belt is also bent around the idler pulley during rotation. This increases the so-called bending frequency and can lead to premature fatigue of the belt.

Another disadvantage of belt drives compared to conventional gearboxes is the relatively high bearing forces due to the need to pre-tension the belts. Pretensioning the belt often requires increased design effort. I hope you enjoyed the video and found it helpful. Thanks for watching.

Transcript for:Understanding Belt Drive Mechanics

Transcript for:
Understanding Belt Drive Mechanics