Clutch quickens stops.

A wrap spring clutch, replacing the resistive loads and Belleville washer stacks on earlier Bosch electric hedge trimmers, now stops the reciprocating blade almost instantly once a gardener's hand has let go of the grip. Actually, the clutch performs two functions, according to project engineer

John Kossett of Reell Precision Manufacturing Corp. in St. Paul, Minn., who designed the device. Foremost the wrap sprang clutch must the blades from the transmission--which gears down the 25,000-rpm motor to 1,500 rpm--when an operator lets go of the tool. It once took about a second and a half for the blades to stop and now takes only a tenth of a second, Kossett said.

If the blades somehow catch and jam--on a chain link fence, for instance--the clutch slips at a torque of about 50 N x m (about 37 foot-pounds). To protect the motor shaft pinion gear from shredding under the full momentum of the high-speed motor, the wrap spring clutch permits the output shaft an additional quarter turn, dissipating most of the motor energy, Kossett said.

The clutch assembly consists of an input gear that attaches to a clip holder. Clips in the holder exert a predetermined force (the slip torque) on a hub. The hub itself couples with a shaft collar by way of the wrapped spring. Ordinarily, the spring wraps tightly around the hub and collar, transferring torque between the two elements and onto the blade shaft.

A pin attached to the operator's controls drops into a release plate when the operator lets go of either handle. This allows the spring to relax slightly, loosening its grip and decoupling the hub and collar. When that occurs, the reciprocating blades stop, having little momentum of their own.