Stress from Lateral Movement of a Compression Spring

Rockford Spring Co. was asked to review a spring failure issue with a Mil Spec toggle switch. RSC met with the Engineering group from OTTO, the manufacturer of the switch. OTTO designs and manufactures control switches and communication accessories for demanding applications.

OTTO had tried different materials and spring designs with the former supplier with some improvement, but was still experiencing occasional failures in cycle testing for product approval.

The spring design calculations showed low stresses for conventional axial loading of a conical compression spring, but there was a lateral movement in operation as the mating part pivoted that was creating additional stresses. RSC studied the spring failure under magnification, and studied a slow motion video of the spring in operation. RSC was then able to determine the failure mode and make design recommendations for reducing the stresses.

The failure mode was not the normal torsional stress fatigue failure. Instead the fatigue initiation point and the shape and direction of the fatigue crack showed that bending stress, not the typical torsional stresses of a compression spring, caused the fatigue failure. The fatigue location and the slow motion video indicated that the spring was over constrained by the plastic dimple that positioned the large end of the spring.

RSC redesigned the spring using more coils and a straight section of the larger coils before tapering to the smaller end. This reduced the stress in two ways. More coils and additional big coils allowed more sideways motion without over stressing the spring. Additionally, the straight section of larger coils allowed the spring to move laterally without interfering with the dimple in the ID of the spring.

Cycle testing confirmed the reduction in stresses as the springs passed well beyond the required cycle life.
Rockford Spring co. is often able to gain new business opportunities by solving a potential new customer's most difficult problems. We are able to continue those business relationships with our continued engineering assistance, our world class ISO 9001 quality system, competitive pricing and on time delivery.