Exercise machine prevents injuries caused by inertia

Weight training is a fine way to build strength and tone muscles. Inherent
problems, however, can make this method of exercise unattractive. Some persons
find free weights (weights unrestrained by a machine) difficult or awkward to
use. Although easier to work with than free weights, weightlifting machines
produce inertia effects that can double the force experienced by a user. Muscles
and ligaments can be damaged by this sort of inertia-generated overload. To
avoid injuries, coaches and therapists prefer that resistance remain constant
throughout the range of a user's movements.

  • Variable force, zero-rate spring

Conventional weightlifting machines use pulleys and stacked weights, clutches, shock absorbers, or rubber tension elements to produce resistance. By employing a gas spring as a source of uniform resistance, this new exercise machine eliminates inertia problems and provides a workout much like that experienced with free weights, accord-ing to its designer, Royce Husted of Patent Products Corp. "Our machine duplicates gravity," he claims.

Called the Enforcer(TM), the strength training machine has a frame made up mostly by a formed steel tube. The vertical part of the frame has a calibrated scale printed on it. Attached to the vertical member is a 3/8-inch-diameter threaded rod.

One end of the machine's gas spring mates with a stainless-steel clevis pin 5/8 inch in diameter. A half-thread formed on the pin matches the thread on the vertical rod. The pin fits into a molded nylon clevis that surrounds the threaded vertical rod.

Two fasteners extend through holes in the pin and secure the clevis (thus the cylinder) to the hinge. Helical springs trapped between the two fasteners' heads and the clevis pin urge the pin into the vertical rod. Because of this spring loading, threads on the rod and the pin always remain engaged.

To select a resistance, the user releases a jam nut that functions as a mechanical stop on the gas spring's rod. Spinning the nut toward the gas spring allows the rod to move freely inside a steel tube at the end of the rod. The user can then pull the cylinder away from the vertical, thread-ed rod and move the clevis pin to a new location (a new resistance). Next the user tightens the nut, fixing the gas spring's position.

Husted secures the exercise bar to a universal joint made mostly from Du-Pont's SupertoughNylon. The joint is injection-molded in two halves. One half creates a bushing for a pin that's attached to the movable part of the machine's frame (the swing arm), the other creates a bushing for the exercise bar. In both bushings, the properties of the nylon yield a self-lubricating surface.

To join the halves, Husted uses a screw-machine-made, insert-molded steel part that's symmetrical about its center. It consists of three plates on a hub. The two outer plates physically capture the nylon bushings. As for the center plate, it forms a bearing surface. Both the exercise bar bushing and the aforementioned pin bushing turn on that surface.

This joint allows the exercise bar to mimic the behavior of a bar used when lifting free weights. Putting

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