Disc Springs, with all of their variations, are among the most widely used tension generating washers. They are used for spanning alignment holes, distributing bearing loads and for generating and sustaining the tension needed to hold assemblies together. Belleville washers provide a very high spring force for short movement and have a high energy storage capacity. In a true Belleville washer, the ratio of material thickness to rim width is held to about one in five. Crown height actually should not exceed 40% of material thickness. In application, yield strength is not exceeded and the washer returns to its full crown height when compression force is removed. Commercial Disc Springs are not held to the specific O.D./crown height/thickness ratios required of the true Belleville Disc Spring washers. Traditionally, the crown height to thickness ratio is considerably greater for commercial Disc Springs. When loaded to flat, their yield point may be exceeded. These washers, however, are often used in applications where they function entirely within their elastic range. In applications where they are loaded beyond their yield point they will act in a consistent manner over a reduced crown height. Such washers have, in effect, become reformed. By varying thickness and crown height relationships, design engineers meet a wide range of load/deflection requirement with Disc Spring washers. A crown height to thickness ratio ranging from 0.4 to 0.8, for example, produces a fairly constant spring rate (Figure 1). With a crown height to thickness ratio up to 1.4, the washer will show a positive rate of increase in the load up to 100% deflection (Figure 2). At a crown height to thickness ratio of 1.4, the Disc Spring shows a constant load over a fairly large deflection, making it useful in applications where extreme wear conditions must be absorbed (Figure 3). Where the ratio of crown height to thickness exceeds 1.4, yielding will occur and, possibly, oil canning or inverting (Figure 4).
Actual Test Results versus Calculated Disc Spring Characteristics
The example, in the graph on the left, is typical of most disc springs, and underlines the necessity of limiting maximum deflection to 75 per cent to avoid sharply increasing force and stress characteristics.
As the compressed disc spring nears its ‘flattened’ condition, the reducing cone angle results in the movement of bearing point toward the centre, thus effectively shortening the ‘lever’ length and ‘stiffening’ the spring.
The ability to change the force/deflection characteristic, by way of varying the cone height to thickness ratio, is a particularly useful feature of the disc spring.
Shown above are some examples of different cone height to thickness ratios, and up to a ratio of 1.5 the disc springs may safely be taken to ‘flat’ or stacked in columns.