Is it a Belleville Washer or a Precision Disc Spring?
Unlike the conventional helical compression spring that has a linear force/deflection characteristic, the same characteristic in a disc spring is a function of the variable cone-height to thickness ratio. This is often a useful feature: for example, the seal on a spherical ball valve requires a spring force back-up to achieve sufficient seating stress which must be maintained throughout a significant amount of seal face wear. A disc spring with a cone height approximately 1.5 times the material thickness, offers a near constant force for a range of deflections. A constant force characteristic is also desirable in joints or bolting systems subject to thermal expansion and contraction.
While the variation of cone height to thickness ratio can be employed to produce characteristics of a progressive, linear, or regressive nature, the full realisation of the versatility of the disc spring emerges with ‘stacking’ into spring columns.
A disc spring ‘nested’ inside another – i.e., convex face placed to concave face (same way up) – is described as two disc springs assembled in parallel. This arrangement results in a force capacity double that of a single disc spring, yet the deflection capability remains that of a single disc.
It is a misconception that the resultant force from two disc springs assembled in parallel is equal to the force of a disc spring that is twice the thickness of the single disc. Not so, due to the fact that disc spring thickness is raised to the power of three in the force calculation formula. For example, for identical inside and outside diameters, the force to flat of a single 2mm thick disc may be 5,423N and the force to flat of a single 4mm thick disc is 26,697N.
Having multiplied the force of a single disc spring by nesting further disc springs of the same dimensions in parallel, it is possible to multiply the deflection by adding further nested units to the column which are apposed to one another, - i.e. ‘back-to-back’. Single disc springs of the same dimensions can also be stacked back-to-back, termed stacking in series.
When disc springs are employed in series or parallel stacks, or a combination of both, Figure 1, friction occurs at the bearing points of opposed springs in series stacking, and to a greater degree across the interfaces of parallel nested discs. Often the hysteresis due to friction is advantageous for those applications requiring a damping effect. A ‘rule of thumb’ for guidance on length of disc spring stacks to minimise friction, is that, in the unloaded condition, the stack length should not exceed 2.5 times the disc springs outside diameter.
Disc spring selection
Precision disc springs, produced to the DIN standards 2092 and DIN 2093, are pre-stressed (scragged); machining and radiusing of edges also removes stress raisers, which could otherwise reduce fatigue life.
The term Belleville washer generally applies to the lower cost, less precise form of conical spring disc, normally used on applications of a less critical nature. Scragging is omitted and the inside and outside diameters are not machined. Select therefore disc springs to DIN 2092/2093 for critical loading or dynamic applications and the Belleville washer for static applications where some setting and /or loss of force can be tolerated.
Belleville Springs Ltd, Tel: 01527 500500
Published in Valve User Magazine Issue 13
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