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[FAQ]

Preload in springs can be defined in two way : through initial free length or through initial load. Let’s consider a longitudinal spring defined between two parallel faces that are 1000 mm appart and with a longitudinal stiffness of K=10 N/mm. One end of the spring is fixed and the other end is submitted to u1=10mm longitudinal displacement. If no preload is considered, the resulting force in the spring is F = K x u1 = 100 N. – If a “load” preload of F0=10000 N is considered, the resulting force in the spring is F = F0 + F1 = F0 + K x u1 = 10 100 N. – If a “free length” preload is considered : First, we have to evaluate the initial elongation of the spring. If the free length of the spring is 25 mm and the initial length of the spring is 1000 mm, the initial elongation is u0=1000-25=975 mm. This initial elongation leads to and initial preload of F0 = K x u0 = 9750 N. After displacement u1 is applied, the total force in the spring is F = F0 + F1 = K x u0 + K x u1 = 9850 N.