Posted to News on 12th Aug 2014, 00:00
Disc spring travel and fatigue life
Conical spring washers are referred to by a number of terms including disc springs, Belleville washers, cupped spring washers, and other similar terms. They are made according to a variety of methods (stamping, fine-blanking and machining) and series are available in globally aligned standards, such as DIN2093.
>The primary benefit of disc springs is that they provide a solution for significant forces in small spaces. The limited travel offered by traditional DIN 2093 disc springs and Belleville spring washers leaves design engineers with very restricted options for integration. In high performing packages with limited space, such as valves, the limiting factor in working travel is the stress in the material, where beyond key stress levels the parts take a set and/or fail prematurely in application.
>DIN 2093 provides guidance on acceptable stress levels for fatigue life depending on both the pre-stressed condition (pre-load) in the application and the upper-bound of the stress (maximum deflection), for fatigue lives of 100,000, 500,000 and 2,000,000 cycles. Meeting the requirements of this guidance puts extremely stringent limitations on acceptable deflection, approximately 17% of inner free height for longer life.
>Improvement in travel in disc springs, while maintaining acceptable stress levels is a significant step in making disc springs work for other applications. As mentioned according to DIN guidance and general engineering principles the acceptable maximum stress level depends on the minimum stress level in the application (pre-load), but there are other means to change the maximum acceptable stress. Geometry plays a key role in stress levels, and with appropriate designs travel can be extended approximately 50%. CloverDome geometry is an example of optimised geometry to provide increased travel while maintaining the disc spring envelope and load magnitude.
>Extending fatigue life is directly correlated with lower stresses. If a smaller displacement is not possible some change to the geometry must be considered in order to reduce stresses. Maintaining traditional disc spring geometry would require expanding the OD and ID to substantially lower stress levels for the same displacement. Where space is a concern, and it often is for design engineers, expanding the OD and ID to reduce stresses is not possible. In such cases an alternative geometry offers a possible solution to reduce the stress and increase the fatigue life for these designs. The CloverDome geometry provides stress reductions for both minimum and maximum stresses by up to 40%. A significant design feature of disc springs is the ability to stack them in parallel or series, or the two can be combined for infinite variation.
>Modifying the geometry of a disc spring has the potential to eliminate the stacking capability. Under this scenario the possible force and deflection combinations is reduced from infinite possibilities. Therefore any modifications to disc spring geometry must still accommodate stacking of parts. In the CloverDome geometry the stacking capability still remains intact, offering infinite solution possibilities from a small set of tooled parts.
