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Motion System Design

Frictionless, noiseless drive controls fitness stepper

An adjustable arrangement of magnets surrounding a drive wheel varies torque to precisely program resistance profile

The popularity of in-home fitness steppers and exercise bikes results from their programmability. By adjusting the equipment’s resistance, an exercise profile can be tailored to an individual’s abilities. Typically, an exercise profile might consist of a slow, low-resistance warm-up segment followed by a gradual build up to maximum effort and concluding with a lower level, cool-down period.

Commonly, the techniques for varying the resistance of exercise equipment rely on adjusting drive friction. For example, one way to adjust drive friction is to change the wrap of a tension-belt on a spinning pulley. The problem with such friction-based resistance techniques is that friction is the main cause of premature equipment failure and increases the need for maintenance and repair.

Eliminating friction

Designers at ProForm Fitness Products Inc., Logan, Utah, have overcome these problems by developing a silent, frictionless magnetic resistance control for fitness steppers and exercise bikes. ProForm’s Silent Drive system consists of an aluminum resistance disc that rotates when the equipment is in use. A Ushaped tension bracket, with permanent magnets mounted on each side, straddles this disc. The magnetic field within the tension bracket is disrupted by the spinning aluminum disc creating a braking effect on the disc. The strength of the braking effect (resistance) is directly proportional to both the rotational speed of the disc and the relative position of the tension bracket to the disc. Other factors affecting resistance include the thickness and composition of the aluminum disc and its distance from each magnet.

Brad Ellis, ProForm project leader, explains, “The resistance disc spins at variable speeds as a result of stepping or pedaling effort. A clutch and flywheel arrangement keeps the disc spinning in one direction. Programming of the exercise segment determines the position of the magnet bracket in relation to the resistance disc. Magnet bracket position is either electrically controlled or it can be manually set through a cable system by turning a knob. The magnets are positioned off the spinning disc for minimum effort, maximum straddle of the disc for maximum effort.”

Reducing drive costs

This simple design has overcome the prohibitive cost of previous electromagnetic techniques for adjusting exercise equipment resistance. Another feature of the drive system contributing to its low Tension bracket Resistance disc Magnets Figure 1 — Stepper drive uses U-shaped tension bracket, with permanent magnets mounted on each side, straddling a spinning aluminum resistance disc to vary resistance. cost, low maintenance, and high performance is the use plastic drive and tension pulleys to drive the system. Through a system of poly-V belts, 10-in. diameter, nylon, fiber-reinforced pulleys, manufactured by Efson Inc., Wilmington, N. C., enables the resistance disc’s fast spinning action.

The drive pulley hub sprocket is driven by chains connected to the equipment pedals. The drive pulley drives the hub of the tension pulley via a poly-V belt. Through a second poly-V belt, the tension pulley drives the hub of the aluminum resistance disc. The system produces a 32:1 increase in resistance-disc rotation for each rotation of the drive pulley. Rotational speed of the stepper drive pulley is determined by an optical encoder sensing bright silver foil strips hot stamped on the ribs of the pulley.

See Associated Figure 2

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