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Hydraulic cylinders provide both power and precision in construction and agricultural vehicles, as well as wind turbine pitch control and lifting equipment. However, being long and thin, they are vulnerable to buckling when acting under compressive forces. Buckling is a particularly risky mode of failure due to its sudden and unpredictable nature. In addition, with large mechanical forces at play, buckling can lead to additional risks for machine operators. 

Often, when designing hydraulic cylinders, engineers use Euler theory. This predicts the critical buckling threshold for slender rods under compression. It is used widely in structural engineering, where it is used in design codes and methods for engineers designing beams and columns for buildings and bridges. 

However, when applied to hydraulic cylinders, Euler theory has been known to overestimate buckling resistance. Therefore, engineers can also call on design guidance developed by the American Institute for Steel Construction (AISC) and the European Convention for Constructional Steel (ECCS). These provide ways to assess buckling strength. 

Another potential source of guidance is the design standard for cranes. It covers a process that engineers can use to assess the effective length of a piston rod. This varies, depending on the location of mechanical support, for example, whether the rod is only connected at either end or whether it has an additional mechanical support in the middle. 

Metal fatigue is another notable failure mode, and it affects double-acting hydraulic cylinders. These experience both compressive and tensile forces over many thousands of loading cycles. 

The fatigue develops due to tiny non-metallic particles in the steel, called inclusions. Steel around these inclusions experiences elevated stress under loading, and this leads to the formation of microscopic cracks. These further elevate the stress in the surrounding material, which causes the cracks to grow and ultimately causing sudden catastrophic failure. 

Metal fatigue most commonly happens in high-stress areas, for example at a welded joint or where a cross section is reduced to make way for a fillet or a thread root. 

Usually, a designer will increase the factor of safety to reduce potential risk of failure from buckling and fatigue. However, an alternative tactic is to use a high-performance clean steel with high yield strength that reduces risk of buckling, as well as high fatigue strength to provide better resistance to cyclic loading. As a result, it’s possible to reduce the size and weight of a piston rod. 

This opens up design opportunities, for example to fit a system inside a size or weight limit. It also reduces the carbon footprint as it uses less material and less energy at every stage from manufacture to logistics and handling. 

The mechanical properties of steel, such as yield strength and tensile strength vary, depending on several factors. These include the production process and the quantity and type of elements used in the alloy. 

Typically, fatigue strength increases in proportion with tensile strength – but it can be further improved by using special measures during production to reduce the number and size of non-metallic inclusions. This creates a clean steel with less potential for areas of elevated stress around inclusions – and a result is less prone to fatigue failure.   

To achieve the right performance, engineers often use grades of steel such as C45E, which has yield strength of 305N/mm2. However, alternative steels are also available that have been specially formulated for hydraulics. For example, Ovako’s Cromax 280X has significantly higher yield strength of 520N/mm2, as well as having high fatigue performance thanks to a high tensile strength and being a clean steel. 

Upgrading to a high-performance steel like this provides the option to either increase a cylinder’s load-carrying capacity, or to reduce its size and save weight and material. 

To help designers quantify this potential with regards to buckling resistance, we’ve developed a piston rod predictor in our steel navigator, an online resource that includes tools for design and production engineers. 

The piston rod predictor takes account of the AISC and ECCS methodologies to help engineering designers compare and contrast materials when developing hydraulic systems. They can also evaluate the potential for a particular steel grade to save weight or increase load-carrying performance without affecting the design factor of safety. 

Machinability is another important consideration for design engineers. The ability to carry out machining operations such as turning can have a big impact on a workshop’s efficiency. 

It’s also important to know about a material’s weldability – and in particular, its suitability for friction welding, as this is often used on hydraulic cylinders. A poorly chosen steel can be affected by a phenomenon called centre segregation in the heat affected zone (HAZ) of a weld and forming brittle constituents. In addition, welding is typically more successful when the two alloys to be joined are compatible. 

Therefore, to support production efficiency, Ovako performs extensive laboratory testing on Cromax grades to ensure they are suitable for efficient machining and that their performance is not affected by the welding process. 

Ultimately, choice of material has a big potential impact on the performance and lifetime of hydraulic systems, so it’s worth evaluating the options carefully. 

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