Piston seals are crucial to the performance of a hydraulic cylinder. This seal is located in the groove on the outer diameter of the piston and must be able to resist high-pressure hydraulic fluid without impeding the movement of the piston along the hydraulic cylinder. Since most hydraulic cylinders move in both directions, the seal must be able to withstand pressure from either side. The design of piston seals requires engineers to strike a balance between friction and sealing performance. If too much fluid leaks from the high-pressure side to the low-pressure side of the hydraulic cylinder, the performance of the seal will be reduced. On the other hand, excessive friction on the cylinder wall will lead to increased wear and a shorter operating life. A complete failure of the piston seal, that is, internal leakage, will cause obvious operation and safety hazards, such as causing construction or material handling equipment to lose control of the load.
If the basic parameters of the cylinder are not strict enough, the piston seal will also face huge material engineering challenges. Under high temperature and high pressure, the hydraulic fluid places strict requirements on the sealing material, and it may also cause other components of the cylinder to expand and contract during operation.
PTFE material can meet these requirements. It has many properties suitable for this function, including excellent low dynamic and static friction properties and good chemical resistance. However, PTFE seals also have their limitations. In particular, the limited elasticity of PTFE seals makes it difficult to achieve damage-free installation. Before installation, the PTFE seal needs to be stretched with a special expansion sleeve, and then it must be calibrated back to the correct diameter. For equipment manufacturers and maintenance teams, this process requires more time and cost. In addition, the plasticity of PTFE means that this material is difficult to return to its original shape under continuous torsional load, which limits its sealing performance.
In order to overcome the disadvantages of PTFE seals, the engineering team of SIAIF attempted to develop an alternative sealing technology based on the company's mature ECOPUR polyurethane material.
Manufacturing a polyurethane seal that can match the good performance of PTFE seals is far from just theoretical talk. Wolfgang Swete, the R&D director of the strategic product line of Jiangsu Luyue Fluid Seals, explained, "The gap between the piston and the cylinder barrel means that you need a material with good enough anti-extrusion performance to maintain its shape under all operating conditions."
Jiangsu Luyue tested the prototype seals made from its previously hardest Ecopur polyurethane and found that it could not achieve the goal. However, Jiangsu Luyue faced the difficulties bravely and used its advantages in material development and manufacturing capabilities to produce a special grade of Ecopur material, which is specifically suitable for piston seals. This new material - X-Ecopur PS – is currently the hardest polyurethane manufactured by Jiangsu Luyue.
To test the durability of the new material, Jiangsu Luyue carried out a series of static anti-extrusion tests. The material samples were installed in a fixture under a pressure of 500bar, and the extrusion gap was adjusted from 0.15mm to 0.7mm according to the seal size. Then, at a temperature of 60ºC to 100ºC, the samples were kept in the fixture for two weeks, and the permanent deformation of the samples was measured at the end of the test. "Throughout the test process, X-Ecopur PS performed far better than other commercial materials," Swete said.
With the appropriate material, the Jiangsu Luyue team began to design a reasonable seal cross-section. This is a familiar field for Jiangsu Luyue engineers. Through a fairly mature product development process, combined with a large number of computer simulations achieved using finite element techniques, they quickly developed product prototypes using CNC equipment and then carried out physical tests on customized static and dynamic test equipment.
Through the repeated R&D and testing process, the company obtained the final result. There are two optimized sealing lips in a shallow "M" shape on the outer surface of the polyurethane sliding ring. "Compared with flat seals, the seal shape with obvious sealing points provides a more effective sealing force. While improving the sealing performance, it reduces the frictional resistance generated by the seal," Swete said. "Concentrating the stress on the two sealing surfaces instead of one side means that the seal will not tilt during use, and tilting will cause premature failure."
The further fine-tuning of the design of the new generation of seals is related to the elastomer of the seal. It is a softer rubber component located below the polyurethane sliding ring in the groove, pushing the sliding ring towards the inner wall of the hydraulic cylinder. For light-duty seals, the elastomer is a simple O-ring, while for medium to heavy-duty designs, a special-shaped nitrile rubber elastomer is used.
In the new design of Jiangsu Luyue, oil grooves are added to the side of the sliding ring to ensure that the pressure is quickly applied to the elastomer. This design enables the seal to switch positions according to the rapid change of the pressure direction, reducing the possibility of internal leakage and the loss of the function of the hydraulic cylinder. The oil grooves on the side can also reduce the occurrence of trapped pressure between the sliding ring and the elastomer, which can also lead to the loss of the function of the hydraulic cylinder.
To verify the design, the prototype seals underwent further rigorous tests, including running a test cylinder with a stroke of 400mm for more than 200km under a pressure of up to 250bar and a temperature of 80ºC. Friction and leakage were measured during the test, and the seal was measured and inspected after the test was completed to check for extrusion and surface wear. "Our tests show that the new generation of seals has better performance compared with other commercial seals," Swete said. "In fact, some of the standard seals we used for comparison failed before the end of the test."
Jiangsu Luyue has currently developed a full range of innovative piston seals suitable for a wide range of hydraulic applications. The light-duty LPV series with an O-ring elastomer is suitable for manufacturing machinery in indoor and fixed applications. This series of seals is suitable for a maximum pressure of 250bar, a maximum speed of 0.5m/s, and a temperature range of -20 to 100ºC. The medium to heavy-duty MPV series is suitable for more demanding applications, such as mobile equipment in agriculture and construction machinery. MPV seals are suitable for metric grooves, suitable for a maximum pressure of 400bar, a maximum speed of 1m/s, and a temperature range of -20 to 110ºC. The third seal series is the DPV model, which provides the same operating parameters as the MPV series and is suitable for imperial grooves.
"What satisfies me is not only being able to create piston seals that stand out from any other polyurethane products," Swete concluded. "Moreover, its performance is at least on a par with excellent PTFE seals, and it also has the additional advantages of fast and simple installation, bringing benefits to OEMs and maintenance personnel."