Addressing piston seal failure

Published:  28 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.

As 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 polyurethane, and an inner ring – the energiser – made from softer rubber. The latter’s function is to push the glide ring outwards against the cylinder’s inner wall. For light duties, a simple O-ring works well. For medium and heavy duties, there is a specially shaped energiser made from nitrile rubber.

Side vents in the slide ring’s radial side walls ensure pressure activation of the energiser, in response to changes in the pressure’s direction, for rapid seal repositioning. By adapting in this way, cylinder performance is maximised and the potential for a blow-by is lowered. The vents also reduce the chances of producing a pressure trap when the slide ring seals against the radial walls of the seal gland, addressing yet another cause of diminished cylinder functionality.

Burge says prototype seals with these design features were rigorously tested against the market’s existing alternatives. This involved over 200 km of up-and-down movement in a 400 mm test cylinder, under 250 bar of pressure and at a temperature of 80°C. Measurements of friction, leakage, extrusion and surface wear confirmed the superior performance of the new seals. In fact, some of the seals against which they were benchmarked failed during the tests.

A new class of seals

The outcome of this research and development, explains Burge, has been a new class of hydraulic piston seals with tailor-made materials and designs specifically suited to their role. Within industry, the applications of hydraulic machinery are many and varied, so SKF’s engineers have produced a choice of three X-Ecopur PS ranges.

The light-duty LPV range serves static indoor equipment such as production machinery. It will cope with pressures up to 250 bar, speeds up to 0.5 m/s and temperatures between -20 and +100°C. More demanding mobile equipment, as used in materials handling, agriculture and construction, for example, will need the medium-to-heavy MPV or DPV ranges. These extend the operating limits to maximums of 400 bar and 1 m/s, with a -20 to +110°C temperature range. LPV and MPV are sized to fit metric housings while DPV is for inch-sized.

Given that piston seals lie at the heart of hydraulic cylinder performance, there will undoubtedly be continued research and development in this area. From SKF’s work it is clear that PTFE can be improved upon in terms of durability and flexibility. As a result, users can look forward to greater safety and more reliable, productive and profitable operations.

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