In belt drives, the power transmission between the pulleys is usually force-locked by flexible belts. More information on the basics of belt drives can be found in this article.

This article provides answers to the following questions, among others:

  • How does a belt drive work?
  • What is special about timing belts compared to classic flat belts or V-belts?
  • How does the belt tension and the wrapping of the pulley influence the power transmission?
  • What are tigth side and slack side of a belt?
  • How is the wrap angle defined?
  • What design measures can increase the wrap angle?
  • How does the wrap angle change under load?
  • How can belts be kept on tension during operation?
  • What are the pro and cons of belt drives compared to other types of transmission?


In belt drives, power is transmitted between at least two pulleys by a belt. One pulley drives the belt (driving pulley) and the other pulley is driven by the belt (driven pulley). In belt drives, the speed is often reduced, so that in these cases the smaller of the two pulleys is the driving pulley.

Operating principle of a belt drive
Figure: Operating principle of a belt drive

Power transmission

With belt drives, the power transmission is generally friction-locked, i.e. power is transmitted through frictional forces between pulley and belt and vice versa. Flat belts or V-belts are very often used. An exception to friction-locking power transmission are toothed belts (timing belts), in which the power transmission is carried out positively by teeth attached to the belt, which engage in the pulley.

In belt drives, the power transmission is generally friction-locked by frictional forces between the belt and the pulley (exception: toothed belt drives)!

In the case of friction-locking power transmission, the belt must be pressed against the pulley with a certain contact force. This is the only way to ensure that the frictional force is large enough to prevent the belt from slipping over the driven pulley or the driving pulley from rotating faster than the belt.

Ideally, the maximum force to be transmitted corresponds to the maximum static friction force acting between the belt and pulley. If the force to be transmitted is greater than the static friction force, the belt slips over the pulley and only the lower sliding friction force is effective (called sliding slip). As a result of the relative motion between belt and pulley, the belt wears very heavily and becomes unusable in a very short time. Such a sliding slip of the belt or pulley must therefore be absolutely avoided.

The maximum transmittable force with belt drives corresponds to the static friction force between belt and pulley!

Two parameters have a special influence on the static friction force and thus on the maximum transmittable force. Firstly, the belt tension, which ensures that the belt is pressed against the pulley with a certain force and can thus generate the necessary static friction force. On the other hand, the belt must wrap the pulley sufficiently tightly so that the necessary adhesive contact can be made.

To be able to transmit high forces, the belt must be wrapped around the pulley as much as possible and the belt tension must be as high as possible!

Tight and slack side of a belt

When rotating around the pulleys, the belt is exposed to different loads. The section of the belt in which the belt is strongly pulled towards the driving pulley and is thus exposed to a large tensile load is referred to as the tight side. On the opposite section, the belt moves away from the driving pulley and is slightly relieved by its “pushing” effect. This belt section is called slack side.

On the tight side, the belt moves towards the driving pulley; on the slack side, the belt moves towards the driven pulley!

Animation: Tight side and slack side of a belt drive

Note that on the slack side, the belt is also exposed to a tensile load! The acting tensile loads are smaller than on the tight side but still present and they must even be present. Because if no belt forces would act on the slack side, this would mean nothing else than that the belt would have no tension. However, a belt tension is absolutely necessary so that the belt can press against the pulleys and thus generate the necessary static friction for power transmission. Special tensioning systems ensure that the belt tension is maintained during operation (see section Belt tensioning systems).

Forces on the tight side and slack side of the belt
Figure: Forces on the tight side and slack side of the belt

The high belt forces on the tight side cause the elastic belt to be tensioned relatively strongly. The belt is stretched and will then sag slightly in the less stressed belt section on the slack side. Only if the belt drive is not under load the belt forces on both sides are equal and there will be no sagging. Note that depending on the direction of rotation of the belt drive, the tight side and slack side as well as the associated deflection are reversed (see animation above).


In the following, a single-stage belt drive with two pulleys that are wrapped around by a common belt is considered. The magnitude of this wrap is described by the wrap angle \(\varphi\).

The wrap angle is defined as the angle between the run-up and run-off of the belt on the pulley.

Definition of the wrap angle
Figure: Definition of the wrap angle

The larger the wrap angle, the more adhesive surface the belt has and the greater the frictional force or the force that can be transmitted. However, it must be noted that the pulleys of a belt drive are wrapped to different degrees by the belt if the pulleys have different diameters!

The maximum transmittable force is usually limited by the smaller of the two pulleys (usually the driving pulley), as this pulley has a smaller wrap angle compared to the larger driven pulley. In addition, the greater curvature causes greater bending stresses in the belt, which also limit the transmittable belt force.

So called idler pulleys can be used to increase the wrap angle. These are usually placed near the actual pulleys to achieve the greatest possible wrapping effect. If these pulleys are used to tension the belt at the same time, they are also called tensioner pulleys.

To increase the wrap angle and thus the transmittable force, idler pulleys can be used!

Enlarging the wrap angle with an idler pulley
Animation: Enlarging the wrap angle with an idler pulley

The figure below shows a belt drive of a dryer drum as an example of the use of an idler pulley (the drum itself serves as the output pulley). In order to increase the wrap angle on the drive shaft and thus ensure sufficient power transmission, a idler pulley was used.

Belt drive of a dryer drum
Figure: Belt drive of a dryer drum

Furthermore, it should be noted that the deflection of the slack side under load results in different wrap angles compared to the load-free state. The arrangement of the tight side and the slack side also influences the wrap angle. If the slack side is above the tight side, the wrap angle increases due to the deflection, while in the opposite case the wrap angle is reduced.

To ensure that wrapping is not reduced under load, the slack side should be arranged over the tight side!

Influence of the arrangement of the slack side and tight side on the wrap angle
Animation: Influence of the arrangement of the slack side and tight side on the wrap angle

However, the change of the wrap angle under load plays a subordinate role in practice and can often be neglected (especially with high coefficients of friction). See also the article Power transmission of a belt drive.

Belt tensioning systems

The importance of the belt tension for power transmission has already been explained in the previous sections. Despite the pre-tension (initial tension), however, the belt tension will change during operation due to plastic deformation or temperature. For this reason, belt drives must often be kept on tension by so-called tensioning systems.

It must also be borne in mind that a belt will have to be serviced over time and therefore have to be removed from the pulley and remounted. This is hardly possible when the belt is under tension, so that the tension must be removed when the belt is changed and must be tensioned again after installing by means of tensioning devices (belt tensioners).

Tensioning systems are used to generate and maintain the belt tension and thus ensure reliable power transmission!

Tensioner, idler and guiding pulleys

The belt tension can be maintained during operation, for example, by means of tensioner pulleys. Tensioning pulleys also serve to cushion heavy load changes. In addition, the wrap angle can be increased by tensioner pulleys. Tensioner pulleys are also often used for long belt lengths to prevent excessive belt vibration.

Animation: Idler pulley

If such pulleys are merely used to deflect the belt, then these are generally referred to as idler pulleys. Idler pulleys are used, for example, in multiple drives in which one driving pulley drives several other pulleys. Idler pulleys can also be used for long belt lengths to reduce belt vibrations. Such pulleys can also take over the function of a guide at the same time, so that the belt does not jump off the pulley. Such pulleys are called guiding pulleys or guide rollers. Guiding pulleys often have protrusions (called flanged pulleys) on the left and right, between which the belt is held in track.

Multiple belt drive
Figure: Multiple belt drive

Tension pulleys do not yet have a tensioning effect; certain devices are required to achieve a tensioning effect (also known as tensioning systems). Tensioning devices are available in a wide variety of designs. The tensioning systems shown in the figures above were each designed with a simple spring mechanism. The springs offer the advantage that the belt tension can adapt dynamically to the operating state, e.g. strong load changes.

Animation: Multiple belt drive

Eccentric tensioner pulley

Another way of generating a belt tension is by an eccentric mounting of the tensioner pulley. The desired belt tension can then be set by rotating the pulley to a certain position. In addition, torsion springs can be installed in the pulley, which then allow dynamic adjustment of the belt tension during load changes.

Eccentric tensioner pulley
Figure: Eccentric tensioner pulley
Animation: Eccentric belt tensioner pulley

Hydraulic damping tensioners

Another way of tensioning a belt is by means of a lever arm to one end of which the tensioner pulley is attached and held in place by a spring.

Hydraulic damping tensioner
Figure: Hydraulic damping tensioner

However, if frequent load changes occur during operation, this tensioning system can be excited to strong vibrations. Hydraulic damping systems such as those found in door closers, for example, can therefore be used to absorb shocks. A piston is then placed in oil, which provides the necessary damping effect due to its viscosity.

Setup of a hydraulic damping tensioner
Figure: Setup of a hydraulic damping tensioner
Animation: Hydraulic damping tensioner

Motor slide base

Besides the use of tension pulleys, the belt tension can also be applied by adjusting the driving pulley itself. When so-called motor slide bases are used, the entire motor is mounted on a movable slide which runs in a fixed guide.

Motor slide base
Figure: Motor slide base

The position of the carriage on the guide and thus the belt tension can be adjusted. However, if the belt tension decreases or if there are strong load changes, the motor slide base does not automatically adapt to the changed conditions but must be readjusted manually.

Animation: Motor slide base

Pivoting motor base

A dynamic adaptation of the belt tension to the existing load conditions (or to plastic expansion processes) can be achieved by using a self-tightening motor base. The motor is screwed onto a pivoting motor base, whereby the centre of gravity of the entire system is designed in such a way that the motor tends to tilt backwards. At an inclination of about 15° to 20°, the motor base with its weight ensures a permanent and almost constant belt tension.

Pivoting motor base
Figure: Pivoting motor base
Animation: Pivoting motor base

Pro and cons of belt drives


Compared to a gear drive, a belt drive can be used to bridge greater distances between two shafts in a simpler way. Chain drives also offer this advantage and are therefore used for bicycles where a relatively large distance has to be covered between pedal and rear wheel.

Frictionally operating belts such as flat belts or V-belts also offer a natural overload function. In contrast to gear drives, overload simply causes the belt to slip through (sliding slip). This protects the transmission from major damage. In the worst case, only the belt needs to be replaced and not the entire gears and shafts as in the case of a damaged gear drive.

Animation: Overload (sliding slippage)

Another advantage of belt drives is the elasticity of the belts compared to rigid gears. This offers good damping characteristics (shock absorption), especially in the case of sudden torque changes. This is why belt drives are used, for example, in grinding plants or stone crushers. The starting and stopping behaviour is also damped accordingly and is not as jerky as with rigid gear drives. Note, however, that a high elasticity of the belt also results in increased elastic slip. Belts can therefore not be made too elastic, but neither can they be designed too inelastic, as otherwise the positive shock absorption properties would be missing.

An additional advantage of belt drives over gear drives is their insensitivity to angular misalignment as long as the axes continue to run in a parallel plane to each other. In many cases, such a misalignment is even deliberate. This makes it easy to redirect the direction of rotation. If the axis of the output shaft is turned by 180° and the belt is crossed, the original direction of rotation can easily be reversed. In contrast to an open belt drive, this is also referred to as a crossed belt drive.

Crossed belt drive
Figure: Crossed belt drive
Animation: Crossed belt drive for direction reversal

Belt drives do not have to be lubricated in comparison to gear drives. This reduces maintenance costs accordingly. Belt drives also have lower noise emissions than gear drives, since no metallic teeth engage but only relatively soft, elastic belts drive the pulleys. This enables the transmission of high rotational speeds.

In addition, pulleys are usually not complete solid wheels, as is often the case with gears. Pulleys usually have recesses to reduce weight and manufacturing costs. As a result, belt drives are generally lighter than comparable gear drives.


However, the above-mentioned advantages of belt drives are also countered by disadvantages. Depending on the ambient conditions, belts are subject to more or less severe ageing phenomena, i.e. they lose their elastic properties over time and must be replaced. For this reason, belts can only be used within a certain temperature range. In addition, over time the belts become plastically stretched, so that they have to be re-tensioned at regular intervals.

Another disadvantage of some belt types such as flat belts or V-belts is the associated slip, which reduces the efficiency of the transmission accordingly. Slippage can only be prevented with timing belts due to the positive force transmission.

In some cases, the increased space requirement of a belt drive compared to a gear drive can also have a disadvantage. This is due to the fact that the belt pulleys cannot be placed directly against each other, while the toothed wheels of gear drives can even mesh with each other and thus be set up in a more space-saving manner. In addition, the wrap angle decreases with decreasing centre distance, so that wrapping can become unacceptably small. Although this can be compensated by idler pulley, it not only increases the design effort but may also increase the required space again.