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SPRING 2019

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Distributor's Link Magazine Spring 2019 / Vol 42 No2

80 THE DISTRIBUTOR’S

80 THE DISTRIBUTOR’S LINK SPIROL INTERNATIONAL CORP. 30 Rock Avenue, Danielson, CT 06239 TEL 1-860-774-8571 FAX 1-860-774-2048 EMAIL info@spirol.com WEB www.spirol.com HOW TO CALCULATE THE ESTIMATED FATIGUE LIFE OF A DISC SPRING by John Leckfor, Applications Engineer Disc Springs are conically shaped washers designed to provide predictable and repeatable results. This paper focuses on the use of Disc Springs and how to estimate fatigue life in dynamic situations. In discussing fatigue life, it is important to distinguish between Disc Springs and Conical Spring Washers. Disc Springs differ from Conical Spring Washers by design and intended usage. Conical Spring Washers are designed to provide a static thrust load in bolted joints and should not be used in fatigue applications. Conical Spring Washers are specified by DIN 6796. Disc Springs may be used to apply either static or dynamic loads and are specified by DIN EN 16983 (formerly DIN 2093). Typically Disc Springs have a thinner cross section than Conical Spring Washers. Some variation in size is permissible, but calculations only apply to spring steels and when the ratio of the outside diameter to thickness is between 16 and 40 and the ratio of OD to ID between 1.8 and 2.5. Deflection of a Disc Spring at a given load is predictable making it possible to calculate force and stress levels in the Disc. As the Disc Spring flexes, stress levels in the Disc change; the greater the change, the faster the Disc Spring fatigues. Tensile stress at points II and III in Figure 1 are critical in determining fatigue life. These locations are where fatigue cracks originate. Estimation of fatigue life requires evaluation of the maximum stress difference between preload and final load at locations II and III. The location with the highest stress differential is used to estimate fatigue life. Once it is determined which stress values will be used (from location II or III), the fatigue life charts in DIN EN 16983 can be used to estimate the fatigue life of the Disc Spring. FIGURE 1: CRITICAL STRESS POINTS WITHIN A DISC SPRING Stress values may be found in the Disc Spring product catalog, DIN EN 16983 or calculated using formulas in DIN EN 16984. Fatigue life charts are provided in three thickness ranges: < 1.25mm; between 1.25mm and 6mm and between 6mm and 14mm. The examples that follow in this article explain how to interpret the fatigue life charts. TECHNICAL ARTICLE CONTINUED ON PAGE 146

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