July/August 2019

Are you taking the risks of hydraulic piston seal failure seriously enough? PWE takes a look at the heavy demands placed on these seals and describes the development of new solutions to overcome their challenges. A s industry’s drive to improve both productivity and safety intensifies, every plant component must come under fresh scrutiny. Piston seals are a case in point. Although small and relatively inexpensive when viewed against a plant’s overall capital and operating investment, they can cause catastrophes if they fail in their crucial role. In essence, the piston seal is a ring which sits in a groove on the piston’s outer surface and seals the gap between this and the cylinder bore wall. Its purpose is to prevent leakage of hydraulic fluid from one side of the piston to the other. In doing so, it maintains the hydraulic pressure that moves the piston. If too much fluid leaks past the piston seal, the equipment’s performance and efficiency are reduced. If the seal fails outright, the resulting ‘blow-by’ can produce unexpected and uncontrolled movements of the machinery in which it operates. For example, heavy loads and structures held up by mobile materials handling, construction or agricultural equipment may be suddenly dropped from a height. In addition workers may be thrown from hydraulically lifted platforms and erratically moving parts of static manufacturing machinery may also cause injury and damage. The results can include lost production, expensive damage to hydraulic equipment and other assets, and injury-related costs. Seal challenges Much is demanded of piston seals. They must provide the right degree of sealing without creating so much friction that piston movement is impeded, and wear is accelerated. Most hydraulic cylinders operate in two directions, so the seals must deal with the effects of pressure from both sides. Their materials may face extremes of pressure and temperature, and they must have flexibility to cope with expansion and contraction of other components. They also need high extrusion resistance, to avoid being forced into the clearance gap between the piston and cylinder bore surfaces. Improving on PTFE A popular choice of piston seal material is PTFE, which combines good chemical resistance with exceptionally low static and sliding friction. On the downside, PTFE seals are difficult to install without becoming damaged, as their elasticity is limited. They have to be stretched before installation, using special sleeves, and then recalibrated to the correct diameter. For both equipment manufacturers and maintenance teams, this adds extra time and cost. PTFE seals’ plasticity is another problem, as continually reversing loads tend to deform them into a less effective sealing shape from which they cannot easily spring back. SKF’s Phil Burge explained to PWE that tasked with overcoming PTFE’s disadvantages, a team of the company’s engineers has turned to its own Ecopur polyurethane material as a basis for development. To cope with the operating conditions experienced in piston seal applications, which include large extrusion gaps, they found they needed a harder grade of Ecopur than was previously available. Using their in-house material development and manufacturing facilities, they responded by creating X- Ecopur PS, SKF’s hardest grade of polyurethane yet, which is specifically designed for piston seals. In static extrusion tests, they attempted to push the new material and its commercially available alternatives through extrusion gaps of 0.15, 0.3, 0.5 and 0.7 mm. Materials were subjected to an oil pressure of 500 bar, for two weeks, at temperatures between 60 and 100°C. Across all tests, permanent deformation was significantly lower in X-Ecopur PS than in the others. Optimising seal geometry The engineers’ next step, explains Burge, was to optimise the seal’s geometry, through the company’s established iterative product development process. This makes extensive use of finite element techniques and computer simulation, rapid prototyping aided by CNC machine tools, and custom-built static and dynamic equipment for physical testing. In its final design, the polyurethane slide (or glide) ring’s outer surface has an M-shaped profile which creates two lips for optimal sealing. These two pronounced sealing points apply more effective sealing forces than a flat surface and reduce the seal’s frictional drag. In addition, the use of two sealing edges rather than one avoids tilting of the seal, which can be a source of premature failure. The seal’s construction is in two parts: an outer glide ring, made from Process, Controls & Plant Focus on: Seals, Bearings & Lubrication Addressing piston seal failure 30 | Plant & Works Engineering www.pwemag.co.uk July/August 2019