Conical middle hole motor belt wheel - to control the speed of motor

Some motors in the market use this kind of wheel. 50mm-120mm.

The middle hole is conical, front diameter is 15mm, and back diameter is 10mm.

The smaller the speed is slower. How to calculate the working speed for different wheels:

For example, your sewing machine motor wheel diameter is 7.5cm, and its lowest speed is 100RPM, you want to lower the speed more, and you choose a 4cm wheel.

4/7.5=0.53, the lowest speed of your motor will become 53RPM.

Conversely for 801 skiver machine, it needs quicker speed, a larger wheel can offer it.