A more sophisticated method of presetting torque is with a
calibrated clutch mechanism. At the point where the desired
torque is reached, the clutch slips, signaling the desired
torque and preventing additional tightening. The most common
form uses a ball detent and spring, with the spring
preloaded by an adjustable screw thread, calibrated in
torque units. The ball detent transmits force until the
preset torque is reached, at which point the force exerted
by the spring is overcome and the ball "clicks" out of its
socket. The advantage of this design is greater precision
and a positive action at the set point. A number of
variations of this design exist for different applications
and different torque ranges. A modification of this design
is used in some drills to prevent gouging the heads of
screws while tightening them.
