Compression Springs - Operating lengths

The operating lengths L1 and L2 have to stay between Ln and L0.

Ln is the minimal operating length based on geometrical considerations (there is no stress evaluation). Indeed, dispersions during the manufacture of the spring are such that certain coils will be touching before the theoretical length at solid (Lc) involving an increase in the rigidity and loads. The characteristics of the spring is then not linear any more. As calculations are given starting from this assumption, and as it is generally preferable to avoid contacts between the coils during operation, the working length is limited to a value higher than the theoretical length with length at solid. There exist several approaches to determine the value of Ln.

DIN standard requires to respect a minimal space between the coils starting from the value of Sa: Ln = Lc + Sa with Sa = N (0.0015 D2/d + 0.1d).

In case of a dynamic application, when N > 104, Sa is multiplied by 1.5.

IST (Institute of Spring Technology) proposes another approach by considering a percentage of the theoretical maximum travel until the length at solid: Ln = Lc + 0.15 (L0 - Lc).

In some cases where no precision on the maximum effort is required, one can not consider these limitations. In these cases there is simply Ln = Lc. Be careful however, in practice the spring generally reaches its limit at a length slightly higher than Lc.




Manuel Paredes