Ken Massett explains how wave springs can significantly improve the lifespan of bearings
Compression springs are, typically, pictured as helical coils of round wire. These coil springs generate their force through torsional moments in the wire as they deform under load. Wave springs, designed using sinusoidal waves that stack along a helical path, use bending moments at their nodes, rather than torsion, to create their axial forces. Each bending moment per wave spring turn increases its spring rate. At an optimal balance of waves and turns, wave springs have the same force and deflection as coil springs in approximately half the space. Bending at the nodes is dependent on support from a coincident node in the next turn. Therefore, the geometry of a wave spring is such that each crest/trough pair is stacked on one another, evenly spacing the bending moments circumferentially about its axis. The result is equal deflection.
Multiple-turn coil springs, wave springs and stamped wavy-washers all expand radially when they are compressed. These springs bind when they expand to the housing walls and no longer deflect at or near their intended spring rate. Wave springs with only one turn are designed for these conditions.
In contrast to a stamped wavy-washer, the free ends of a wave spring, with either gap or overlap geometry, are permitted to move along the spring’s circumference, preventing binding. Single-turn wave springs still make effective use of their space, maintain their equal deflection and function unhampered when designed to cling to a bore or shaft. This makes them a dependable fit for applications that require dynamic force or tolerance stack-up with any form of space constraint. Bearing preload is at the forefront of such applications.
What is bearing preload?
Ball bearings, composed of multiple parts working together as an assembly, are designed with clearances that permit freedom of motion. Clearance is not necessarily a result of manufacturing precision; designs may specify additional clearance to accommodate higher axial loads or minor axial misalignment. As clearance and manufacturing tolerances stack up, a bearing assembly will begin to experience axial and radial play.
Bearing preload is the process of adding a sustained axial load, independent of external loads, to the bearing. An axial preload ensures constant contact between the ball complement and bearing races, reducing or eliminating both modes of play. Spring preload, a versatile execution of bearing preload, utilises single-turn wave springs to add the necessary preload forces despite dimensional variation and thermal expansion.
Properly preloading a bearing can increase its life and eliminate the vibration and noise that results from specified clearance, manufacturing precision and wear.
Wave springs from Smalley offer the advantage of space savings when used to replace coil springs. By reducing spring operating height, wave springs also produce a decrease in the spring cavity. With a smaller assembly size and less material used in the manufacturing process, a cost saving is realised.
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Ken Massett is with Smalley in Lake Zurich, IL, USA.