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When the speed is low (≤0.03/s), the stability of the machine operation and whether there is a chance of creeping phenomenon should be considered.
When the speed is high (≥0.8m/s), the lubricating ink may be damaged so that the seals generate heat due to the friction caused by the lack of lubrication, resulting in a significantly reduced service life. It is suggested that the rubber-to-plastic composite seal works at speeds of 0.3m/s to 1.2m/s.
At a low temperature, the elasticity of rubber-to-plastic composite seal will be reduced, resulting in leakage and even a harder and more brittle oil seal. At a high temperature, it will expand and soften, causing a rapid increase in the frictional resistance of the oil seal during operation and a decrease in pressure resistance. It is recommended that the rubber-to-plastic composite seal works at a temperature of -10°C to +80 °C.
3. Operating pressure
The startup pressure is important to oil seals. The oil seals with low friction performance and small aerodynamic resistance should be chosen for the low pressure operation. Rubber-to-plastic composite seal is not suitable for the environment below 2.5Mpa. At high pressure, the seal pressure should be considered, and antiextrusion rings should be adopted. There are also special requirements for groove processing.
4. Operating medium
In addition to selecting the operating medium in strict accordance with the suggestion provided by the manufacturers, it is important to keep it clean. The oil aging or contamination not only causes system component failure, but also accelerates the aging and wear of the oil seal. Moreover, the dirt may scratch the seal or be embedded in the seal, causing seal failure. Therefore, it is necessary to regularly check its quality cleanliness, and replace the oil filter or oil according to the maintenance specifications of the equipment. The residual air in the oil in the cylinder is compressed by high pressure to generate an instantaneous high temperature to burn or even carbonize the oil seal. To avoid this happening, gas should be exhausted when the hydraulic system starts to operate. The hydraulic cylinder should also run at low speed and low pressure for a few minutes so that the residual air in the oil is confirmed to be drained before it can work normally.
5. Lateral load
Generally, the support ring must be installed on the piston to ensure that the cylinder can withstand heavier loads. The seal and support ring have different functions. The seal can’t replace the support ring in load bearing, so the hydraulic cylinder with lateral force must be equipped with a support ring with strong load bearing capacity (or a metal ring for heavy load) to prevent the oil seal from leakage and abnormal wear caused by working under eccentric conditions.
6. Hydraulic shock
There are many factors that would cause a hydraulic shock. For instance, it occurs when an excavator bucket suddenly hits a stone, or when the crane lifts or off-loads a heavy object. Except for the external factors, for high-pressure and high-flow hydraulic systems, when the actuator (hydraulic cylinder or hydraulic motor) is reversed, hydraulic shock may be easily generated if the performance of the reversing valve is not good. The instantaneous high pressure generated by the hydraulic shock may be several times the working pressure of the system, which will tear the oil seal or the oil seal will be partially caught in the gap within seconds, causing serious damage. Generally, a buffer ring and a retaining ring should be installed on the piston rod of cylinders with hydraulic shock with. The buffer ring is installed in the front of the oil seal to absorb most of the impact pressure, while the retaining ring prevents the oil seal from being squeezed into the gap under high pressure so that the root is broken.
7. Processing accuracy of cylinder
Experiment has shown that the leakage and wear value of the oil seal will rise when the surface roughness (Ra) of the moving surface in contact with the oil seal exceeds 0.8μm. To guarantee the sealing performance of rubber-to-plastic composite seal and avoid damage to the seal during assembly, there are some aspects should be considered in the design and installation of hydraulic cylinders:
a: Material: carbon steel; aluminum alloy, bronze, stainless steel, etc. can be adopted for low-pressure system and good friction conditions.
b: Surface quality and roughness: The inner surface generally needs to be honed, polished or rolled. And it is required to achieve a surface roughness of 0.1 to 0.8 μm without vertical and horizontal patterns.
2) Piston rod
a: Material: carbon steel and chrome-plated steel; aluminum alloy, bronze, stainless steel, etc. can be adopted for low-pressure system and good friction conditions.
b: Surface quality and roughness: The roughness (Ra) is required to be 0.2 to 0.4 μm, and the surface is hard chrome plated after heat treatment. The piston rod of the hydraulic cylinder for construction machinery may be scratched by sand and gravel, so its surface hardness should be higher than HRC 60.
c: Installed groove of the oil seal
d: Partial structural design
e: Sliding fit clearance of wear ring f and extrusion gap of oil seal g