Spring into action

Paul Boughton

Ken Massett reveals everything you need to know about springs 

At its most basic level, a spring is a device used to store mechanical energy. Although often out of sight, springs play an important role in many motion control applications. They are used in gear assemblies, actuators, rotary unions and different kinds of clutches, among other applications. There are many different varieties of springs, making it virtually impossible to cover them all in great length.

Compression springs are some of the most common types of springs in use. The compression style wave spring is made of coiled flat wire with waves added along the coils to give it a spring effect. They can be used in place of conventional round wire compression springs in applications that require minimal space yet similar deflection. Generally, they occupy one-third to one-half of the space of comparable round wire springs offering more deflection with the same load specifications.

Wave springs are one alternative to coil springs. Wave springs are typically manufactured from a single filament of flat wire formed in continuous precise coils with uniform diameters and waves. They are manufactured with either plain ends (wavy) or squared-flat ends (shim ends). Generally, 17-7 PH is used as the standard material for wave springs.

One advantage of wave springs is that they save space in many applications. By reducing spring operating height, these springs also decrease the spring cavity. A smaller assembly size and less material used in the manufacturing process can lower cost.

Wave springs operate as load bearing devices. They take up play and compensate for dimensional variations within assemblies. A range of forces can be produced whereby loads build either gradually or abruptly to reach a predetermined working height. This establishes a precise spring rate in which load is proportional to deflection.

Some manufacturers offer single, nested and multi-turn wave style springs. A single-turn wave spring with overlapping ends saves axial space so that more space is given for travel. The spring clings to the bore, which saves more radial space. The overlapping ends prevent radial jamming because a circumferential movement is allowed. The spring ends could move against each other so that the specification load at work height is always given.

Nested wave springs suit applications requiring higher forces to meet safety regulations, such as those in government or military applications. A nested wave spring provides a higher load than a single-turn wave spring (or alternatively, a stamped wavy washer) and uses the same radial space as a single-turn design.

Multiple-turn wave springs do not cling to the bore, because radial jamming affects the specified load at work height. If the design of the multi-turn wave spring results in peripheral movement of the turns against each other, this could render the spring unstable.

Compared with a single-turn design, bigger travel/deflection is possible because the deflection in total is split. Every turn has to tolerate less deflection compared to a single-turn design. Use of a multi-turn wave spring could also save 50% in axial space compared to a traditional coil spring. There is also no concern about torsional movements during the compression to work height as there is with a coil spring; a wave spring always provides its load in an axial direction.

Very similar loads without big tolerances are provided at different work heights; in that way, the application could be easily adjusted to meet given requirements.

The basic rules

Lastly, although wave spring applications can be diverse, there is a basic set of rules for defining spring requirements. Those requirements are used to select a stock/standard spring or design a special spring to meet the specifications.

When selecting a wave spring, the load requirement is probably the main factor to consider. The load requirement is defined as the amount of axial force the spring must produce when installed at its work height.

Some applications require multiple working heights, where loads at two or more operating heights are critical. Often minimum and maximum loads are satisfactory solutions, particularly where tolerance stack-ups are inherent in the application.

High temperature, dynamic loading (fatigue), a corrosive media or other unusual operating conditions are also important considerations in spring applications. Solutions to various environmental conditions typically call for the selection of the optimal raw material and operating stress.

If the work height per turn is less than twice the wire thickness, the spring will operate in a 'non-linear' range and actual loads may be higher than calculated. It is not recommended to compress a wave spring to solid. Radial expansion and diameter tolerance must be taken into account while designing a spring to fit in a bore and/or over a shaft.

For more information, visit www.engineerlive.com/ede

Ken Massett is with Smalley