The pulley is a whole made of iron. And the pulley on the motor will vibrate somewhat.

In order to reduce the impact vibration caused by engine combustion, many machines cut the middle ring into inside and outside parts on the ordinary all-iron pulley, and then bury rubber in the gap between the inside and outside parts. Thus, the power of the engine will first be transmitted to the inner ring of the pulley, and then to the damping rubber in the middle layer. Then from the shock absorber rubber to the outer ring. In this way, the vibration of the engine will be reduced outside the rubber. To put it bluntly, it's a layer of rubber about five centimeters thick. It can effectively increase the life of the belt. Of course, this rubber layer will also be damaged. If it breaks, the whole belt will rotate. However, the life of this thing is much longer than that of the belt.