The future of hydraulic materials
By Steven North, design and project engineer, Apex Hydraulics.
As with all industries, the manufacture of hydraulic cylinders is constantly developing as engineers respond to new markets and competition. As machinery becomes stronger, faster and more durable, the materials and coatings used for hydraulic systems need to match this growth. Industries demand hydraulics that is more robust, with greater longevity and the ability to endure more extreme environments.
Stainless steel is often the material used to make piston rods. In the recent past, the stainless steel of choice for piston rods has been 431 stainless steel, but now hydraulic system manufacturers more commonly use 17-4 PH stainless steel. This is stronger than 431, with better corrosion resistance and mechanical properties, but it can be difficult to weld and has a disadvantageous resistance to chrome adhesion.
The future is likely to be a prevalence of a newer steel such as F-51. F-51 benefits from the positive mechanical properties of 17-4, but with better weldability and chrome adhesion. Currently, F-51 is extremely expensive, but as prices are driven down it will become more commonplace.
Some experts suggest that the coating used is more important than the material underneath. The most resistant of steel can be corroded if suffering from a cracked coating. Hydraulic rods cannot be painted, as the rod has to go through all the seals, so it requires a coating to protect the material from pitting and corrosion.
Traditionally, Hard Chrome Plating (HCP) has been used – but it contains microcracks, making it somewhat porous, reducing the lifespan of the rod. A new product has been developed; Corex, that is 10 times less porous than hard chrome plating. While more expensive than HCP, it benefits from a massive bond strength of more than 10000 PSI, with a hardness of 1400Hv – almost double the hardness of HCP.
These properties give it a resistance to impact as well as resistance to pitting and corrosion in the day to day wear and use, meaning that a less expensive steel can be used, with the potential for greater longevity of the rod in spite of that. In addition, the production of Corex is far better for the environment than HCP is.
Chrome has a limited production capacity, as it is applied by a process which involves utilising positive and negative charges by passing electricity through the material. There is a size limitation of around 8 metres in the UK, over which it has to be transported to subcontractors in Europe for coating. Therefore, for especially oversized cylinders HCPis an expensive option. Corex has been developed to have fewer of the restrictions that HCP carries and can be used to coat rods of almost unlimited lengths.
The future of hydraulic materials should see more and more hard wearing, corrosion resistant coatings developed – particularly with a view to being kind to the environment – both in the way that they are produced and disposed of and also by the benefit of their longevity on the field.
As hydraulic materials and coatings have become more advanced and robust with a greater longevity of piston rods, so seals have had to improve to keep up with this. Specialised seals made from materials such as Zurcon and PTFE are especially durable and long lasting.
Extreme high and low temperatures have driven the development of seals made from materials like Viton, that benefits from an extremely high-temperature resistance; and polyurethane that is less likely to become brittle in extremely low, sub-zero temperatures. Coming years will see a number of new developments in rubberised materials that are increasingly durable and resistant.
Hydraulic fluids and accumulators
New hydraulic fluids are regularly being developed with a particular push towards sustainability and recyclability. However, a number of these special synthetic oils can be damaging to seals and other parts such as bladder accumulators. The oil can be particularly aggressive, causing the usually used nitrile rubber to swell and deteriorate. Bladder accumulators have been developed using ethylene propylene diene monomer (EPDM) rubber which has a greater resistance to the harshness of the oil.
New materials are constantly being developed and tested throughout the industry. The next ten years is likely to see stronger steels that are easier to work with, more resistant coatings, more durable materials for seals and other parts. It is also vital that materials will be produced that have a lower environmental impact.