The roller is looped on the sleeve. When working, the roller rolls along the sprocket tooth profile, which can reduce the wear of the gear teeth. The main wear of the chain occurs on the contact surface of the pin shaft and sleeve. Therefore, there should be a little gap between the inner and outer chain plates so that the lubricating oil can penetrate between the friction surfaces. The chain plate is generally made into an "8" shape, so that its cross sections have nearly equal tensile strength, and also reduce the mass of the chain and the inertia force during movement.