The power that can be transmitted by a wedge Rubber Timing Belt

The power that can be transmitted by a wedge Rubber Timing Belt depends upon a number of factors, namely:

The angle of contact

The greater this is, the more torque and power can be transmitted. If both pulleys are not of the same diameter, then the smaller pulley is the one that determines the maximum torque and power that can be transmitted. Not only does the smaller pulley have a shorter length of contact, but it also has a smaller angle of contact than the larger pulley, so it will always be the first to slip. In cases where the pulleys are not the same diameter, the angle of contact will depend also upon the centre distance between the shafts. The greater the centre distance, the greater the angle of contact. For this reason, centre distances should not be below the recommended minimum value (sum of the pulley pitch diameters) unless there are special circumstances.

The wedge angle of the belt (and groove).

Because you will be selecting a standard belt you are not able to change this angle which is usually 34° or 38° depending upon the size of the belt and the size of the pulleys.

The coefficient of friction.

You have little control over this because it is determined by the belt material and the pulley material (and finish). In practice it is important to keep oil and grease off the belt and pulleys because this will reduce the friction (and could deteriorate the belt). Why would it not be a good idea to use a rough surface on the pulley to increase the coefficient of friction?

The pulley diameters.

The larger the diameters, the greater the torque and power. This is simply because for a given belt tension force, the larger the pulley, the larger the torque (torque = force x radius) and hence the greater the power for a given speed. This is why it is not a good idea to choose pulleys that are too small. On the other hand if the pulleys are too large, the belt speed increases, centrifugal tension increases and the drive takes up more space. So a reasonable compromise is needed when choosing pulley pitch diameters.

Initial belt tensions.

The higher the initial tension in the belts, the greater the torque and power that can be transmitted. At rest, when no power is being transmitted, the belt tensions are equal on both sides of the pulleys. As the pulleys rotate and transmit power, the belt tension rises in one side (tight side) and reduces in the other (slack side). However, the sum still stays the same. When the ratio of the belt tensions reaches a certain limiting value, slipping will occur. Hence the greater the initial tension in the belts, the greater the torque and power that can be transmitted before slipping occurs.

However, it is not a good idea to have too much tension because this will place high radial loads on the shaft and bearings and also will reduce the belt life considerably. For this reason, initial belt tensions should be set carefully. In their catalogue, Penner detail the practical method for pre-setting the belt tensions. This method requires a belt tension indicator (which is really just a force gauge) that measures the force at the centre of the belt required to cause a standard deflection (16 inn per m of span). There is a recommended value for this force that should be adhered to when the belt drive is initially installed.

As the belt wears, the initial tension needs to be re-set, so the designer should allow for an adjustment method. Adjustment can be provided by moving one shaft further away from the other or by means of an adjustable jockey pulley. This pulley should be located on the inside of the drive on the slack side as close as possible to the larger pulley and should have a diameter at least equal to the smaller pulley.

The size of the belt.

The larger the belt section, the greater the tension that can be carried by the belt and the greater the torque and power. In the Data Manual, four sizes of wedge belt are given, namely: SPZ, SPA, SPB, and SPC. These are listed in increasing size, with the SPZ being the smallest and the SPC being the largest.

The number of belts.

Belt drives with a single belt are the most common but belt drives are often used with 2 to 6 belts in parallel on multi-grooved pulleys. In the larger sizes up to 8 belts may be used. Clearly, the torque and power increase in direct proportion to the number of Rubber Drive Belts.