Gyroscopic exercise tool

A gyroscopic exercise tool is a device used to exercise the wrist as part of physical therapy or in order to build hand and finger strength. It can also be used as a clever demonstration of some aspects of rotational dynamics. The device consists of a tennis ball-sized plastic or metal shell around a free-spinning mass, which can be started with a short rip string or by a snap of the thumb. Once the gyroscope inside is going fast enough, a person holding the device can accelerate the spinning mass to high revolution rates by moving the wrist in a circular motion.

How it works

The device essentially consists of a spinning mass inside an outer shell. The shell almost completely covers the mass inside, with only a small round opening allowing the gyroscope to be manually started. The spinning mass is fixed to a thin metal axle, each end of which is trapped in a circular, equatorial groove in the outer shell. A lightweight ring with two notches in it for the ends of the axle rests in the groove. This ring can slip in the groove; it holds the spinning gyroscope centered in the shell, preventing the two from coming into contact (which would slow the gyro down), but still allowing the orientation of the axle to change. Once the gyroscope is spinning, tipping the device causes the gyroscope to precess, with its axle slipping around in the groove in a circular fashion. The groove inside the device is a little wider than the axle; an externally applied torque causes one end of the axle to push against the upper rim of the groove, while the other end pushes against the lower rim of the groove. These two effects combine to make the device work: as the gyroscope precesses in response to an external torque, one end of the axle "rolls" along the top edge of the groove while the other end "rolls" along the bottom edge, speeding up the rotation of the spinning mass.

The acceleration of the gyroscope is best when the precession of the gyroscope is followed by wrist motion, so that an accelerating torque is continually applied. The wrist can be rotated either direction (clockwise or counterclockwise)--the result is always to increase the rotation speed. It takes a while before one finds the "rolling" point, but the gyro will also be accelerated to a smaller extent by the slipping friction which occurs if the wrist is rotated too quickly. Since frictional force is essential for the device's operation, the groove must not be lubricated.