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Schnorr Disc Springs

Disc Spring Engineering
Maximum load - minimum space - maximum flexibility

checklist for disc spring design

Due to the relatively simple geometrical shape the complexity of disc springs in production and application is very often underrated. There are possibilities for mistakes in outlining a disc spring solution, which inevitably cause faulty design or even failures later on. Then it is very difficult to find better substitutes, because most of the times the installation space is fixed.

With a correct design these problems are easy to avoid. The main difficulty is to realise these in the design stage to get an optimum disc spring solution. 

Since for most of the designers the disc spring is not daily bread and to many the rules for disc spring design are little known, the most important aspects are summarised here.

Spring Force

The calculation of the force of a disc spring is based on a model by Almen and Laszlo. Its accuracy in the usable range of the character line of the spring is very good. Yet there is a slow rise at the beginning of the measured load/reflection curve, because disc springs never are perfectly symmetrical. They so to speak have to be pressed even. Also the spring force rises in the last part of the load/deflection curve more than calculated, when the spring is loaded in between two parallel planes, since the leverage changes due to the never ideally even surfaces.

Static Loading

In the design of a new disc spring a certain stress level should not be surpassed for static loading. The maximum allowable limit is given by the reference stress σom. Its value should not exceed the value of the tensile strength Rm of the material to avoid plastic deformations of the spring, i.e. setting losses.

Dynamic Loading

Most of the disc springs only can bear a limited dynamic load. The life time depends on the load span as well as on the load level. The number of cycles, which is to be expected under a certain load condition, can be estimated from fatigue diagrams. It is also necessary to reload disc springs in a dynamic application to at least 15% to 20% of their maximum deflection, to avoid compression-tension alternating stresses in the beginning of the deflection range of the spring.

Stacking

Disc springs can be stacked “face to face” (series arrangement), which means their deflections add up, or they can be stacked in the same sense (parallel arrangement), then their forces add up. The latter induces increased friction and a stronger hysteresis effect. Thus the force in loading direction is higher and in unloading direction lower than the calculated force. Applying suitable lubrication (MoS2 containing grease) can reduce the hysteresis effect. The various possibilities of stacking disc springs can be combined in a stack. Different types of stacking in one spring stack can be used to generate a progressive character line. It is necessary to pay attention to   the weaker parts in a combined stacking though, because these normally are pressed flat quite fast, which is not allowed in dynamic loading. If necessary a deflection limitation has to be provided.

Guide

The surface of guide elements is dynamic disc spring applications always has to be harder than the disc springs themselves. A minimum of 55 HRC is advisable, otherwise the surfaces can be damaged. This again causes uneven movement during the deflection of the spring. The characteristics will be changed and even fatigue cracks can occur. Wrong guide clearance also can change the dynamics of loading in a detrimental way.

Stack Length

Friction and other influences make a spring stack move unevenly. It deflects more on the side of the loading. This effect usually can be neglected for a “normal” spring stack, but not for long stacks. Therefore the length of a spring stack should not exceed three times the value of the outer diameter. If it is longer, the stack can be stabilised by dividing it with guide washers, which as a rule of thumb should have a thickness of at least one and a half times the guide diameter.

Material

The best material for disc springs is standard spring steel. It is always used as long as there are no particular circumstances, which may necessitate a special material. In general special materials have a lower tensile strength and most of the times a different Young’s modulus compared to the standard spring steels. Therefore springs out of these materials generally cannot be designed with the same free height, which means that the spring forces are lower.

Temperature

The different materials have different temperature ranges. Too high temperatures may have a tempering offset, which again results in a loss of force and, in extreme cases, in plastic deformation (setting losses).

Corrosion

Disc springs can be protected against corrosion either by suitable coatings or by using corrosion resistant materials. Such materials are only available in a limited variety of thicknesses. Also almost all high alloy steels may show stress corrosion cracking at high working stresses.

Hydrogen Embrittlement

During the application of certain chemical or electrochemical processes (such as galvanic coating) hydrogen can get in to the material and can cause delayed brittle fractures. This cannot be avoided entirely by thermal treatment. Thus processes, which do not bear this risk, are to be preferred.

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