The handwheel on the Kenmore Model 158 sewing machine has a shiny knurled knob in the middle:
Turning the knob clockwise screws the knob inward and clamps a friction clutch that locks the handwheel to the main shaft; the motor belt drives the handwheel, the handwheel drives the shaft, and the shaft drives everything inside the sewing machine.
Remove the small screw, turn the knob counterclockwise to remove it, and you see the clutch:
Yes, the black stamped metal part is the clutch.
Those three projections around the exterior limit the knob’s travel to a bit under 1/3 turn, with the little screw you just removed traveling between two of the projections. When you reinstall the knob:
- Turn it until it’s snug
- Insert and tighten the screw
The two dogs in the middle project outward from the shaft notches: the bases engage the notches, the tips bears on the knob’s inner surface. Tightening the knob compresses the dogs, presses the clutch against the handwheel, and locks everything together.
It’s entirely possible to install the clutch backwards and, while it’ll come pretty close to working, it’s not quite right.