Are you serious enough about the risk of hydraulic piston seal failure? PWE studied the demanding requirements for these seals and described the development of new solutions to overcome the challenges.
As the industry's drive to increase productivity and safety continues to increase, every plant component must undergo new scrutiny. Piston seals are a good example. Although they are relatively small and relatively cheap relative to the overall capital and operating investment of the factory, if they fail to play a key role, they can cause disaster.
Essentially, the piston seal is a ring that is located in a groove on the outer surface of the piston and seals the gap between the groove and the cylinder bore wall. Its purpose is to prevent hydraulic oil from leaking from one side of the piston to the other. In doing so, it maintains the hydraulic pressure of the moving piston.
If too much fluid leaks through the piston seal, the performance and efficiency of the equipment will be reduced. If the seal fails completely, the resulting "leak" may cause unexpected and uncontrolled movement of the machine it is operating on. For example, heavy objects and structures supported by mobile material handling, construction or agricultural equipment may suddenly fall from a height. In addition, workers may be thrown from the hydraulic lifting platform, and the irregular moving parts of the static manufacturing machinery may also cause injury and damage. The results may include production losses, expensive damage to hydraulic equipment and other assets, and costs associated with injuries.
The requirements for piston seals are high. They must provide an adequate degree of sealing without excessive friction, which hinders the movement of the piston and accelerates wear. Most hydraulic cylinders operate in two directions, so the seal must deal with the influence of pressure from both sides.
Their materials may face extreme pressures and temperatures, and they must be flexible to deal with the expansion and contraction of other components. They also require high crush resistance to avoid being forced into the gap between the piston and the cylinder bore surface.
A popular choice for piston seal materials is polytetrafluoroethylene, which combines good chemical resistance with extremely low static and sliding friction. On the downside, PTFE seals are difficult to install without damage because of their limited elasticity. They must be stretched using a special sleeve before installation, and then recalibrated to the correct diameter. For equipment manufacturers and maintenance teams, this will add additional time and cost. The plasticity of PTFE seals is another problem, because continuous reverse loads tend to deform them into less effective sealing shapes, so they cannot easily spring back.
Phil Burge of SKF explained to PWE that in order to overcome the shortcomings of PTFE, a team of company engineers has turned to its own Ecopur polyurethane material as the basis for development. To cope with the operating conditions encountered in piston seal applications, including large squeeze gaps, they found that they needed a harder Ecopur grade than before. Using internal material development and manufacturing facilities, they created X-Ecopur PS, which is SKF's hardest polyurethane grade to date, specifically designed for piston seals.
In static extrusion tests, they tried to push new materials and their commercially available alternatives through extrusion gaps of 0.15, 0.3, 0.5, and 0.7 mm. The material is subjected to an oil pressure of 500 bar at a temperature of 60 to 100°C for two weeks. In all tests, the permanent deformation of X-Ecopur PS is significantly lower than other products.
Burge explained that the next step for the engineer is to optimize the geometry of the seal through the company's established iterative product development process. This enables widespread use of finite element technology and computer simulation, rapid prototyping aided by CNC machine tools, and custom static and dynamic equipment for physical testing.
In its final design, the outer surface of the polyurethane sliding (or sliding) ring has an M-shaped profile, which can form two lips for optimal sealing. These two obvious sealing points exert a more effective sealing force than a flat surface and reduce the frictional resistance of the seal. In addition, the use of two sealing edges instead of one can avoid tilting of the seal, which may be the source of premature failure.
The structure of the seal is divided into two parts: an outer sliding ring made of polyurethane and an inner ring (energizer) made of softer rubber. The function of the latter is to push the slip ring outwards against the inner wall of the cylinder. For light tasks, a simple O-ring works well. For medium and heavy loads, there is a specially shaped energizer made of nitrile rubber.
The side vents in the radial side wall of the slip ring ensure that the exciter pressure is activated in response to changes in pressure direction to quickly reposition the seal. By making adjustments in this way, cylinder performance can be maximized and the possibility of blowby can be reduced. When the slip ring is sealed against the radial wall of the sealing gland, the vent also reduces the chance of pressure traps, thereby solving another reason for the weakening of the cylinder function.
Burge said that prototype seals with these design features have been rigorously tested against existing alternatives on the market. This involves more than 200 kilometers of up and down movement in a 400 mm test cylinder at a pressure of 250 bar and a temperature of 80°C. Measurements of friction, leakage, extrusion and surface wear confirm the excellent performance of the new seals. In fact, some of the seals used as benchmarks failed during the test.
New type of seal
Burge explained that the result of this research and development is a new type of hydraulic piston seal whose material and design are particularly suitable for its purpose. In the industry, the application of hydraulic machinery is diverse, so SKF engineers provide three X-Ecopur PS series for you to choose from.
The light-duty LPV series serves static indoor equipment, such as production machinery. It can withstand pressures up to 250 bar, speeds up to 0.5 m/s and temperatures from -20 to +100°C. For example, more demanding mobile equipment for material handling, agriculture and construction will require medium and heavy-duty MPV or DPV series. These extend the operating limits to a maximum of 400 bar and 1 m/s, with a temperature range of -20 to +110°C. The sizes of LPV and MPV are suitable for metric housings, while DPV is suitable for imperial sizes.
Given that piston seals are the core of hydraulic cylinder performance, there is no doubt that research and development will continue in this area. It is obvious from SKF's work that PTFE can be improved in terms of durability and flexibility. Therefore, users can expect higher security and more reliable, more efficient and more profitable operations.
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