As a Product Engineer, I spend an appreciable amount of time on the phone with Engineers explaining spring parameters and terms. One of the questions I get most frequently is why a spring needs to be pressed.
With all things mechanical, the credibility of a design is told by the stress of any given device when under load. Since springs provide a force, the associated stress created by that force determines if that spring will reach its elastic limit. In the case of a helical spring, reaching that limit means the material will ”set” and not return to its original state. For a compression spring, that translates to free length. If a spring is 5.000″ long and reaches its elastic limit at solid height, this means the spring may only return to a height of 4.800″. This would then make a load requirement on the low side, or possibly completely out of tolerance. Under normal circumstances, the elastic limit is only reached because the spring needs to travel a given deflection to meet a load within the dimensional limits of the application. A spring designer will want to keep stresses low if possible, but the “fit” limits may not make that possible. This means the only design that will produce the required load will take a set if pressed to a given height. If the required height is beyond the boundary of the elastic limit, the spring will take set. The springmaker will then make the spring longer, when coiled, to compensate for the set.
That said, reaching the elastic limit is not a bad thing. If the spring is pressed to solid height and a set occurs, the elastic limit of that spring has now been exceeded and the life of the spring has been increased. This is due to the stress distribution being more uniform after pressing/set. The only true negative aspect is that processing spring will require an extra step of pressing each spring to solid height (sometimes called scragging).
Pressing may tend to vary free lengths more than the coiling and grinding process. Grinding is many times seen as the great equalizer for springs that are not pressed because some coiling free length variations can be reduced by the end grinding. But pressing, which makes the spring shorter, cannot be adjusted- it is at the mercy of a certain lot of material, which contains variations in both modulus and tensile. The tensile strength will have a direct impact on how much the spring takes a set, and this is not predictable. An Engineer can predict if the spring will take set under a given force and tensile, but not specifically to a given set-up when the material varies from job to job. If the spring takes more set than predicted, the grinders cannot simply grind less to make up for the loss of free length and also hold their specified grind requirements. So pressing adds another variable and challenge to the process itself. It is the spring coilers job to produce samples that will predict how much set will occur and make the appropriate adjustment to the spring free length for the correct free length after both pressing and grinding.
Some designs have very high stresses at solid height, but the spring may never deflect to a height remotely close to solid height. In these cases, springs can be pressed to a height to induce set at a point just beyond the use height. This process sometimes requires special pressing stands and will cost more than a step that presses many springs to solid height at the same time. The ultimate point being that a spring that is highly stressed at a given point does not mean the design is not viable. Pressing is a common practice and most manufacturers can easily provide pre-set springs. Just be prepared to pay a bit more for the spring.
By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping