Tech Briefs

An important advantage over other exercise machines would be light weight.

The vacuum-assisted, constant-force exercise device (VACFED) has been proposed to fill a need for a safe, reliable exercise machine that would provide constant loads that could range from 20 to 250 lb (0.09 to 1.12 kN)

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Atmospheric/Vacuum Differential Pressure on the piston would be utilized to generate an adjustable, nearly constant tension in the output cable.

with strokes that could range from 6 to 36 in. (0.15 to 0.91 m). The VAC-FED was originally intended to enable astronauts in microgravity to simulate the lifting of free weights, but it could just as well be used on Earth for simulated weight lifting and other constant-force exercises. Because the VAC-FED would utilize atmospheric/ vacuum differential pressure instead of weights to generate force, it could weigh considerably less than either a set of free weights or a typical conventional exercise machine based on weights. Also, the use of atmospheric/vacuum differential pressure to generate force would render the VAC-FED inherently safer, relative to free weights and to conventional exercise machines that utilize springs to generate forces.

The overall function of the VAC-FED would be to generate a constant tensile force in an output cable, which would be attached to a bar, handle, or other exercise interface. The primary force generator in the VAC-FED would be a piston in a cylinder. The piston would separate a volume vented to atmosphere at one end of the cylinder from an evacuated volume at the other end of the cylinder (see figure). Hence, neglecting friction at the piston seals, the force generated would be nearly constant — equal to the area of the piston multiplied by the atmospheric/vacuum differential pressure.

In the vented volume in the cylinder, a direct-force cable would be looped around a pulley on the piston, doubling the stroke and halving the tension. One end of the direct-force cable would be anchored to a cylinder cap; the other end of the direct-force cable would be wrapped around a variable-ratio pulley that would couple tension to the output cable. As its name suggests, the variable-ratio pulley would contain a mechanism that could be used to vary the ratio between the tension in the direct-force cable and the tension in the output cable. This mechanism could contain gears, pulleys, and/or levers, for example. By use of this mechanism, the tension in the output cable would be set to a desired fraction of the force generated by the pulley and the stroke would be multiplied by the reciprocal of that fraction.

A vacuum could be generated in several alternative ways. The way that would involve the least equipment would involve the use of a one-way valve in an outlet at the vacuum end of the cylinder (the lower end in the figure). At first, the piston would be forced all the way down in the cylinder to push out most of the air from the lower cylinder volume. Thereafter, the one-way valve would keep air from re-entering the lower cylinder volume, and the device could be used to provide nearly constant tension on the cable during exercise. Of course, air would gradually leak past the piston seals into the lower cylinder volume, so that it would eventually be necessary to repeat the initial bottoming of the piston to restore the atmospheric/vacuum differential pressure.

Alternatively, a vacuum could be generated and maintained by use of a small manual or electric vacuum pump. Still another alternative is to connect the lower cylinder volume to the combination of a low-pressure storage tank, pressure regulator, and vacuum pump. This combination could be used to maintain the lower cylinder volume at a subatmospheric pressure (partial vacuum) that could be controlled to set the differential pressure and thus the output-cable tension at a desired level.

This work was done by Christopher P. Hansen of Johnson Space Center and Scott Jensen of Lockheed Martin Corp. For further information, contact the Johnson Commercial Technology Office at (281) 483- 3809.

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