The service life of hydraulic valves in hydraulic supports is generally very limited. While foreign countries have focused on improving filtration precision and emulsion quality to address this issue, the results have been modest. This paper examines the development of sealing pairs in stent valves and explores the relationship between seal material and sealing performance. It emphasizes that sealing technology plays a crucial role in extending the service life of hydraulic valves in hydraulic supports.
One of the key trends in the development of general seals for planetary reducers is the increasing demand for seals that can withstand harsh environments. With mechanical products operating under more extreme conditions, the need for high-performance rubber seals has grown significantly. These seals are used in various industries such as aerospace, aviation, oil and mining, construction machinery, transportation, and energy. They must operate effectively across a wide temperature range and in environments with high pressure, sand, and dust. According to the 1984 International Rubber Conference, the operating conditions for rotating shaft lip seals in vehicles have become much more demanding.
As usage conditions evolve, the consumption of specialized rubber materials continues to rise, driving the need for new seal designs and improved reliability. The typical operating pressure for rotating shaft lip seals ranges from 0.1 to 3.5 MPa, with maximum speeds up to 15–20 m/s. Hydrodynamic types can reach speeds of 25–32 m/s, although at higher pressures, the working pressure tends to decrease slightly. The primary cause of seal failure is contamination in the working fluid. Impurities can grind between the sealing surfaces, leading to valve leakage. To mitigate this, foreign countries recommend using filters with appropriate filtration accuracy. For example, a 5μm filter can extend the life of a 10μm seal by ten times. However, in coal mining environments, the conditions are extremely challenging. Hydraulic support systems often have over 1000 meters of piping and more than 4000 joints. The emulsion tank lacks dust protection, resulting in a high level of floating impurities, including coal dust, rock particles, and metal shavings. Frequent opening and closing of the inlet valve plug and seat, combined with high flow rates, causes rapid seal failure. Practical experience shows that reducing contamination in these systems is extremely difficult. Some solutions include installing high-pressure filters at the emulsion pumping station and small filters at each support, but these often get clogged quickly, causing system interruptions.
The operating pressure for reciprocating dynamic seals is typically below 20 MPa, while high-performance combined seals used in aerospace and aviation can handle up to 56 MPa at a speed of 1 m/s. As hydraulic support technology advances, the Shanghai Cycloid Reducer Factory has set higher standards for valve performance and longevity. Currently, valve life is measured based on the total volume of emulsion passing through the valve and the number of cycles it undergoes, using a 120-mesh filter and magnetic filter. However, indoor testing often differs from real-world underground conditions. Many countries now incorporate pollution tests into their formal experiments, either by adding pulverized coal or mechanical impurities to the emulsion. With China's growing economy, there is an increasing demand for seals suitable for extreme environments like cold regions, deserts, and underwater applications.
Another important trend is the development of extrusion seals, especially O-rings, which are widely used in various sealing applications. Other types, such as X-shaped, T-shaped, D-shaped, square, and triangular seals, are also used depending on the application. The improvement direction focuses on enhancing stability, pressure resistance, and reducing friction. Combining low-friction plastics like PTFE or nylon with support rings can improve performance. Some manufacturers already produce seals capable of handling 7.5 m/s at 5.6 MPa.
Reciprocating lip seals, also known as cup seals, are commonly used in hydraulic cylinders. Their lip-shaped contact surface creates a tight seal when installed with slight interference. As pressure increases, the contact force and area expand, improving the seal. During return strokes, lower pressure reduces contact and friction, allowing for lubrication. Leakage between the valve plug and seat occurs due to the intrusion of working fluid molecules. Contact pressure, surface roughness, and pressure differences between the valve core and seat are critical factors affecting sealing performance. When the valve core and seat are pressed together, the working fluid is squeezed, causing elastic deformation of the seal material. If the total resistance of the seal pair exceeds molecular repulsion, the seal remains effective; otherwise, a leak occurs. Early hydraulic supports used direct metal-to-metal contact, requiring precise machining. Otherwise, the contact surfaces needed to be plastically deformed to block the leak path.
There are many types of reciprocating dynamic seals, including J-type, L-type, U-type, V-type, and Y-type seals, as well as various combinations like multi-lip or composite seals. At high pressures, extrusion must be prevented by using materials with sufficient tear strength, hardness, and modulus. Support rings or reinforced elastomers are often used. High-pressure, high-speed, or high-temperature applications frequently use composite seals, such as "Turcon" slip ring seals, "NCF" and "SIMKO" efficient combination seals, Balmaster, and Hallprone. These seals feature two lips in contact with the cylinder wall, with a groove storing the medium to improve lubricity and reduce lip wear. Unlike pneumatic systems, hydraulic lip seals require different structures, often incorporating grease storage, low-friction materials, or designs that minimize contact area and pressure. Some seals also integrate dust wipers for enhanced performance.
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