Machined springs from Helical Products Co. Inc., Santa Maria, Calif. (www.heli-cal.com), have features cut from a solid blank of material.
Gears, cams, tangs, flanges, and threads all can be incorporated without the secondary joining operations needed for conventional wirewound springs.
The machining process itself introduces no residual stresses so machined springs tend to have precise linear-deflection rates. In contrast, the winding of wire about a mandrel builds in tensile stresses that must be overcome before a wound spring deflects, which can result in a nonlinear force response.
Another benefit of machining is the ability to make multiple-start springs. Single-start machined springs have one continuous coil element from end to end as do their wire-wound counterparts. Double-start designs, on the other hand, put two independent, continuous helixes spaced 180° apart in the same cylindrical plane, while triple-start springs have three coils spaced 120° apart, and so on.
Multiple starts provide redundancy should an element fail. Remaining elements can trap broken ones, allowing springs to operate at reduced capacity in many cases. The approach also eliminates the unresolved moments inherent to single-start springs under load. Multiple-start compression and extension springs resolve all internal moments within the spring itself. This stops a tendency for springs to squirm when deflected, eliminating external restraints. The ends of machined springs can be made square, especially important for compression types.