Also called the Stop motion clamp washer. This clutch knob washer is used on the older Ultrafeed^® LS and LSZ Sewing Machines, as well as the Yachtsman, SR200 and Thompson Mini-Walker. Modern Sailrite sewing machines use a patented Posi-Pin^® system which won't slip like this older compression device.

This washer has two inner prongs that protrude out one side (away from the machine). These prongs are pressed against by a threaded knob which screws into the end of the sewing machines upper shaft. Under compression the prongs force the balance wheel of the machine to make friction contact with a flange on the upper shaft. Under hard compression the balance wheel and the shaft then turn together to operate the machine. Loosening the threaded knob will release the friction and allow the balance wheel to spin independently of the upper shaft for bobbin winding.

The outer rim prongs of the washer are used to set the maximum tightening and loosening of the threaded knob. Without them the knob would tend to rotate completely free during bobbin winding.