rotary bobbin case

rotary bobbin case – a type of a sewing machine hook system (which catches the thread below the throat plate to create a stitch).
There are two main types of hooks: rotary (usually in modern machines) and oscillating. In a rotary hook machine the hook rotates in a full circle around a stationary bobbin. Machines with a rotary hook run smoother and are usually more quiet at higher speeds, have less frequent thread jams than oscillating hooks machines.

rotating bobbin case