Mattias Awad explains how mechanical engineering designers can make use of higher-strength specialised piston rod steel to increase buckling resistance in hydraulic cylinders
Hydraulic cylinders control heavy loads with high precision in construction vehicles, lifting equipment, agricultural machinery, wind turbines and other industrial applications. Their 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 from the many thousands of cycles that alternate between compression and tension. These 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 fillets, or at defects in welded joints.
Steel choice in hydraulic cylinder design is crucial
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.
This 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 effective length of a piston rod. The effective length depends on whether the cylinder is just connected at the very ends or whether it has support in the middle at the gland.
Hydraulic cylinder engineers should invest in specialised steel
Specialised piston rod steels have been developed specifically for hydraulics, such as Ovako’s Cromax 180X and Cromax 280X. Cromax 180X comes in the form of hard-chrome-plated bar and is based on a medium-carbon micro-alloyed steel. It has a minimum yield strength of 500N/mm2 thanks to careful control over the alloying mix and processing. This compares with 305N/mm2 for grade C45E.
Higher yield strength means that the piston rod can better withstand buckling. A designer can therefore reduce the diameter of a piston rod, cutting the weight of the entire cylinder.
Alternatively, they could replace a C45E piston rod with an identical one in Cromax 180X to transmit a greater load with the same margin of safety against buckling, provided that the cylinder is dimensioned for the higher force/pressure.
Engineers can compare different grades of piston rod steel using a piston rod predictor that is part of the Steel Navigator tool on Ovako’s website. The piston rod predictor provides insight into how buckling resistance is affected by different grades using both the AISC and ECCS methods.
Engineers can use fatigue modelling to help them design dual-action piston rods. As a general rule, fatigue strength increases with the tensile strength of the rod material. Guaranteed impact toughness might also need to be considered for safety-critical applications.
Other factors being alike, the higher tensile strength of Cromax 180X provides considerably better fatigue performance than C45E. As with buckling resistance, using Cromax 180X could either enable downsizing of a piston rod or improving fatigue life with the same size of rod.
Engineers also have to design for manufacture. Therefore, it’s important to consider the impact on machining and production. The machinability of the Cromax grades has been fully tested in both turning and threading. Even though its strength and hardness are considerably higher, the steel can be processed efficiently compared with materials such as C45E.
Weldability is also important – and particularly friction welding, which is often used on piston rods that will be subjected to heavy loads. Engineers want to avoid brittle constituents forming in the weld heat affected zone (HAZ).
Cromax grades are a good choice thanks to alloy content that was developed to avoid the risk of centre segregation that could lead to embrittlement after friction welding. Together with their higher yield strength, this makes them a good choice to help designers of hydraulic systems improve performance and downsize systems.
Mattias Awad is with Ovako