Progressive Spring Characteristics
When a disc spring is assembled with single, double, and triple parallel sets in series, the discs will flatten consecutively when loaded. The characteristic of such a spring stack changes progressively. The same result is obtained when disc springs of various material thicknesses are stacked in one column. It becomes obvious that single sets, or discs made of thicker material, will be subject to considerable stress when disc springs of standard design are used. This excess stress can be eliminated; however, by decreasing the formed height of a disc or by using space washers or rings to limit deflection.
Spring Stacks and their Characteristics
It is important when stacking disc springs to keep the stacks as short as possible. Friction increases with length and therefore causes the deflection of individual discs to become unequal, with the greatest deflection at the moving ends. As a guide, it is recommended that the stack length be restricted to approximately three times the outside diameter while utilizing the largest possible disc diameter.
Not more than two or three discs should be used in parallel with multiple stacks, especially under dynamic conditions, as considerable heat is generated.
To ensure stack stability, the end springs should have their outside diameters in contact with the abutment faces. This is only possible by using an even number of disc springs.
It is necessary to have the inside diameter in contact when the abutment faces are just a little larger than the inside diameter of the disc springs.
Disc springs arranged in stacks require a guide to prevent lateral movement under load. Guides may be located on the inside diameter (Di) or on the outside diameter (De) of the springs, but the internal guide is preferred. Using close tolerances for the guides, the clearance can be lowered since the inside diameter does not reduce during the stroke when the edges are at right angles to the flank. If possible, particularly on dynamic applications, the guide rod and end thrust faces should be case hardened to a minimum of RC 55. The guide surface should be turned to a fine finish or for better results, ground. Additionally, lubrication with a molybdenum based high pressure grease is recommended.
Alignment of Spring Stacks
Individual discs in a stack always deflect slightly unequally, but some adjustments can be made when conditions are critical. Where possible, the stack should be lightly preloaded on the guide then aligned. After alignment, the stack should not be unloaded to help to keep friction to a minimum.
If the design does not permit adjustments of this kind, the individual discs will tend to align themselves when the stack is flattened once or twice after assembly, and friction will be reduced.
Long horizontal stacks should be avoided if minimum friction is important. In dynamic applications, especially with parallel stacks, bedding in alters both the friction and stack height slightly after a certain working period.