Exercise machine prevents injuries caused by inertia

DN Staff

June 10, 1996

4 Min Read
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 it differently, a user must exert equal force on both ends of the bar or one end will drop. Conventional strength training machines are so designed that the user doesn't need to exert equal force with both arms: "On other machines, your left arm can be getting a free ride," says Husted.

On the movable portion of the Enforcer's frame, a weight counterbalances the exercise bar to make it easier to position. The moving frame member oscillates on needle bearings housed in forgings secured to the vertical portion of the curved frame. Exercises can cover a full stroke range of 52 inches.

Changing over from one exercise to the next requires moving the clevis pin to a new resistance setting. For pull-down exercises, the user places the clevis pin at a position below the "zero" setting on the calibrated vertical frame member; lifting exercises require placing the clevis pin above that setting. If the user wants to stand while exercising, he or she must lift the unit's bench and move it to one side. It pivots on the Enforcer's stationary frame.

Husted expects physical therapists and health clubs to rep-resent the primary markets for the original Enforcer. A lighter version of the machine has been designed for the home market.

Additional details...Royce Husted, Patent Products Corp., Box 764, Forest, VA 24551-0764, (804) 525-1774.

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