Click to view
larger image

SCIFIT produces and markets a product line of commercial fitness equipment which bridges the converging fitness and medical rehabilitation markets. SCIFIT uses igus® plastic bearings on three different exercise machines, all with stellar results.

In the first machine, an all body exerciser, rotational bearings were needed for the chain idler. The machine has a drive train consisting of a set of upper and lower pedal cranks for arms and legs. The pedal cranks are connected and drive a resistive load via a roller chain.

To ensure alignment between the sprockets, the axial location was adjusted and it varied from machine to machine. This increased assembly time and made field service difficult. If the sprockets ran out it caused a dynamically varying chain line during operation resulting in excessive noise and possible chain derailment.

Using iglide® L280 bearings in the chain idler enables both axial and rotational movement of the idler. This ability eliminates the need for time-consuming alignment during assembly. The result is faster assembly, easier service, quieter operation and elimination of chain derailment.

Four iglide L280 sleeve bearings also used on the exerciser are to adjust the crank arms on the upper body portion of the exerciser. The previous design required a high level of precision which could not be machined, so hand matching had to be done to mate the crank arm bars to their appropriate dovetail slots. Any slight dent or misalignment due to an impact adversely affected the performance of the adjustment feature.

Maintenance was extremely difficult and adjustment-locking slots in the back of the crank reduced the strength of the crank arms causing frequent failure in high-load applications.

The solution was to design parallel guide rods which provide a torsion-resistant linear motion. Iglide L280 bearings were used in pairs spaced apart for each rod to provide low friction linear motion, durability and minimal clearance between bearing and rod.

Low clearance provides rigidity, noise is minimized and the steel rods are less susceptible to damage. Machining enables any crank arm to fit any hub and eliminates the need to hand match.

The third application was on a brand new exercise machine that entailed a dynamically variable-length crank arm for use on an exercise bicycle. It varies during a cycle according to the position of the user’s foot via a cam and follower arrangement.

A previous prototype optimized a costly, custom-made single row linear follower bearing, but it was prone to failure with constant misalignment, flexing and noise. Resizing the design proved to be too costly. The key was to identify a sufficiently durable, rigid and tooling free design.

A design was created consisting of a crank arm block with a set of igus L280 sleeve bearings sliding linearly along a pair of parallel steel rods It also incorporates an igubal® spherical bearing engaging a steel pin attached to the crank arm block. The crank arms are now sufficiently rigid and misalignment due to bike frame and assembly inaccuracies, as well as user loading, is compensated for without inducing extreme stresses into the machine and crank mechanism as occurred with the earlier prototypes.

To learn more about SCIFIT, visit www.scifit.com.