Adjusting & Fitting Motor Belts

How To Adjust Motor Belts

Vintage machines were intended to be driven by reinforced Vee belts, which transmit the drive on the inclined sides of the belt under very low tension. Fitting Vee belts too tightly can cause the belt to twist too deeply into the grooves of the pulley. Most importantly a belt that is too tight can cause the motor to be strained and overheat. This can result in machines running slowly, or under extreme conditions it can even cause machines to stall. Over-tightened belts are perhaps the largest cause of reduced motor life.

Because very few replacement Vee belts are available, many retailers sell round section rubber belts. These are elastic or ‘stretch’ belts which run in the bottom of the pulley grooves under high tension. Due to the excess strain these can put on motors, we don't recommend their use. We always remove them from machines we find them on, due to the harm they can cause.

Vee belts should be fitted as loosely as possible, as long as the motor pulley doesn't slip when the motor starts up.

It is essential that the tension adjuster is fully realised before attempting to remove or fit Vee belts. Failure to do this can damage the belt, or often it can split the motor pulley.

What Motor Belt Do I Need ?

It isn’t practical to match drive belts to serial numbers or model numbers, because there can often be variations within the same model. For example they can have different motors, or different pulley diameters on the balance wheel etc. and this means they need different belts.

You need to either match an existing belt, or if that isn't possible, then you need to measure the machine.

To match an existing belt - either measure around the inside of the belt, or note the part number printed on it. It can sometimes be easier to measure around the outside of the belt and then deduct 1" (2.5cm) from that measurement to allow for the effect of the belt thickness.

To measure the machine - remove the existing belt and lower the motor to the bottom of its adjustment range. Wrap a cord around the pulleys and mark where it crosses itself. Unwrap the cord and measure between the 2 marks. Now fully raise the motor adjustment and repeat the measurement. Taking the average of these 2 figures will give you the ideal belt length. Choose the nearest standard belt size to this, making sure it is still between the 2 limits.

Details of the various standard belt sizes are shown near the bottom of the 'motors' information page, or you can see the motor belts for sale page.