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In a hydraulic cylinder, the pressure imparted by hydraulic fluid against a piston is converted to a force, which then performs the useful work. Because this force is transmitted by the piston rod, it must be dimensioned so that the stress of the rod (force/cross-sectional area) is low enough to minimize any risk of failure.
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The forces in piston rods can be "push," corresponding to a compressive stress in the rod, or "pull" (tension), which is associated with rod tensile stress. These forces are usually axial and uniformly distributed over the rod section.
If the rod is side-loaded, either deliberately or inadvertently, then bend forces may occur and the resulting stresses can be very high. Consequently, most designers of hydraulic cylinders will strive, insofar as possible, to avoid side loading. With single-acting cylinders, the piston rod is usually loaded in push-only mode so that fatigue is not an issue. In these instances, the dimension of the rod must be designed whereby its axial stress remains below levels that give rise to buckling.
Buckling is a sudden, large, and unstable lateral deflection. It can be associated with only a small increase in compressive load above a critical level, known as the buckling load. The corresponding stress can be far less than the yield strength of the rod material. Once it occurs, buckling leads to instability and collapse of the piston rod. This is why buckling in "failure mode" is potentially highly dangerous and may result in serious accidents.
Fatigue and Fatigue Testing
Double-acting cylinders feel alternating push-pull (compression-tension) loads. So, in addition to the risk of buckling on the compression part of the load cycle, cylinder designers must also consider the possibility of fatigue. Fatigue is a damage process whereby cracks are initiated and develop under repeated variable stresses that are much less than the tensile strength of the steel in the rod.
Apart from the mechanical characteristics of the steel chosen for a piston rod, other key factors for fatigue strength are surface finish, stress concentrations, and, for welded rods, weld quality. Piston rods that fail by fatigue inevitably do so either at the thread...