AUTOMOTIVE DESIGN
PREVENTING PISTON ROD BUCKLING
Mattias Awad explains how mechanical engineering designers can make use of higher-strength specialised piston rod steel to increase buckling resistance in hydraulic cylinders
H
ydraulic cylinders control heavy loads with high precision in construction vehicles, lifting equipment, agricultural machinery, wind turbines and other industrial applications. T eir designers must take care to ensure that piston rods resist buckling failure when the hydraulic cylinders are under compressive forces. Buckling is a sudden and unpredictable form of failure with serious consequences. It arises from excessive loading in push mode – that’s why engineers pay close attention to compressive stresses when designing piston rods. In single-acting cylinders that provide a pushing force, the piston rod is subjected to compression. It should be designed to keep axial stress below a critical buckling threshold. Double-acting cylinders must also be able to resist fatigue that may arise
Specialist steels
are a good choice for hydraulic
systems designers
from the many thousands of cycles that alternate between compression and tension. T ese cycles create elevated stress around microscopic imperfections, leading to propagation of cracks and eventual failure. Fatigue usually happens at locations with reduced cross section, such as the thread roots or fi llets, or at defects in welded joints.
Piston rods must be designed to resist buckling failure
HIGHER-STRENGTH CLEAN STEEL Careful choice of steel can reduce the risk of failure from buckling and fatigue. When designing cylinders with slender
rods, engineers can apply Euler theory, which is a model of elastic buckling. However, for less slender rods, Euler theory greatly overestimates buckling resistance and engineers can protect against buckling by turning to materials with higher yield strength. T is practice has been integrated into design codes for columns in the construction and building industry through methodologies in the American Institute for Steel Construction (AISC) and European Convention for Constructional Steelwork (ECCS). In addition, the crane standard prEN 13001-3-6A is a useful resource. It includes a process for assessing buckling strength for hydraulic cylinders and a method for calculating the eff ective length of a piston rod. T e eff ective length depends on whether the cylinder is just connected at
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