How does a belt drive work? (sliding & elastic slip easily explained | slack side | tight side)

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.

In belt drives, power is usually transmitted by friction between the belt and the pulley. V-belts or flat belts are most commonly used. An exception to frictional power transmission are tooth'ed 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 sliding slippage.

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.

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.

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.

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.

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.

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.

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 centre of gravity of the whole system designed in such a way that the motor tends to tilt backwards. At an inclination of approximately 15° to 20°, the weight of the motor base ensures a permanent and almost constant belt tension.

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.

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.

The sliding of the belt on the pulleys due to expansion and contraction is generally referred to as elastic slippage. The actual power transmission takes place in these sliding zones and is therefore subject to sliding friction, not static friction.

00:00 Operating principle
00:22 Power transmission
01:26 Tight and slack side of a belt
02:56 Wrapping (wrap angle)
03:49 Increasing the wrap angle with idler pulleys
05:32 Why must the belt tension be maintained?
06:13 Eccentric tensioner pulley
06:44 Hydraulic damping tensioners
07:15 Motor slide base
07:41 Pivoting motor base
08:05 Slippage
09:50 Sliding and adhesive zones
11:39 Why must pulleys not be too rough?
12:49 Belt friction equation
14:02 Loss of speed due to elastic slip
14:59 Power loss due to elastic slip
15:43 Circumferential force
17:32 Transmission ratio, torque and speed
20:03 Advantages of belt drives
22:14 Disadvantages of belt drives