Bearings. On an Olympic barbell, the shaft and bar ends are independent pieces, and the end must spin on the shaft to dampen the centrifugal force of the plates spinning on the bar. To allow the bar to spin more freely and reduce wear, a third piece sits in between the shaft and bar end.

Why do barbell collars spin?

The rotating sleeves on an Olympic barbell allow the weight plates to rotate as you move the bar. This reduces the amount of torque created by the plates, which provides a more secure grip and reduces stress on your wrists and elbows.

Why do barbells not flip?

As long as the net torque is negative, the bar tries to rotate anti-clockwise, and a little bit of force from the left hand support will keep the whole thing in equilibrium. But the moment the clockwise torque becomes too big, the bar will tip.