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With regards to accessory for our Unimat replacement motors, it is important to understand how we measure our pulleys, and what particular dimensions really mean.

Let's assume that we want to use a round drive belt in a vee shaped groove pulley, as shown on the diagram below:

HOW MEASURE PULLEYSx.jpg

where:

 

A - internal diameter

B - external diameter

C - approximate effective pulley diameter (EPD), or pulley pitch diameter

D - belt touch and friction area 

E - groove external width

F - groove internal width

Z - 5mm diam belt length pulley diameter (value to be used to calculate round belt length)

belt length math form.jpg

PJTOOLMAKERS specify their motor pulleys diameter as "A" from the above diagram. However, if true speed for a given pulley is to be calculated, value "C" should be used. Value "C" changes in time due to wear and tear of drive belt (wear and tear decreases belt's diameter). Value "Z" is to be used in place of value "C" if round belt is used in vee shaped pulley groove from the following formula:

This formula allows to calculate reference belt lengths, and is elaborated here:

 How easy you can make your own Unimat belts