Bogie Spring Fatigue Damage - A Function of Static Displacement

Springs with low static deflection (stiffer springs for a given load) should have a more severe rig test than springs with a higher static deflection.

It was theorized that the alternating to mean strain ratio increases as the spring stiffness for a given load increases. A series of strain measurements were conducted using three different spring stiffnesses for two different bogie load conditions. A total of six different static deflections were measured. The measurements at the truck bogie suspension show a clear trend that the ratio of alternating to mean strain increases as the static deflection decreases. Damage calculations were conducted to compare the relative fatigue damage caused by the road inputs. This data was then used to determine the alternating to mean strain ratio for a 100,000 cycle rig test equivalent to the relative damage caused by road inputs. It was found that the lower the static deflection, the larger this ratio must be.