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The basic knowledge and selection method of oil seal
Nov 15,2023
Hydraulic oil seal:
Oil seal refers to the sealing of lubricating oil, which is used on the rotating shaft, to seal the oil inside, and to prevent sand and dust outside.
Main advantages of oil seal:
The structure is simple, easy to manufacture, the oil seal installation position is small, the axial size is small, easy to process, and the mechanism is compact; Has good wear resistance and oil resistance, good sealing performance, long life; It has certain adaptability to shaft vibration, eccentricity and high-speed rotation. Easy assembly and disassembly, convenient maintenance, cheap price.
Main disadvantages of oil seal:
It can not withstand high pressure, so it can only be used as a seal such as bearing oil, and occasionally used as other seals.
Structure of oil seal:
It can be divided into two kinds of oil seal with skeleton and oil seal without skeleton. The oil seal with a skeleton is composed of a rubber sealing ring, a metal skeleton and a clamping spring. The skeleton is used to enhance the rigidity of the sealing ring, which can be glued on the inside of the rubber sealing body, or on the outside, most of which are placed in the middle of the rubber, the latter will not damage the parts during assembly, and is recommended. If there is no skeleton in the structure, it is no skeleton oil seal. The rubber ring without skeleton oil seal is generally made of a rubber inner ring and an outer ring of laminated cotton fabric. Its outstanding advantage is that it can be cut and assembled, and still maintain the sealing performance, especially suitable for occasions that cannot be assembled from the shaft end. There are two kinds of J shape and U shape oil seals commonly used in China.
In the free state, the inner diameter of the oil seal is smaller than the shaft diameter and has a certain amount of interference. When the oil seal is installed on the shaft, the lip of the oil seal exerts a certain radial pressure on the shaft. However, after a period of movement, this radial pressure will decrease and even disappear, so the spring is added to compensate. It is believed that the main reason for oil sealing is the formation of a stable oil film between the lip and the surface of the shaft. If the oil film thickness is too large, the oil seal will leak, and if the oil film is too small, dry friction may occur, causing the oil seal to wear. The optimal sealing film thickness is the "critical oil film", which can both seal and lubricate the oil. The formation of the critical oil film is directly related to the magnitude and distribution of the radial pressure of the oil seal on the shaft. It is generally believed that the distribution of pressure should have a peak, and only at the highest pressure can a critical oil film be formed. The higher the peak, the better the sealing effect (not simply the higher the pressure, the better the sealing effect).
Therefore, the most ideal situation is: low radial pressure to obtain a good sealing performance of the oil seal.
The main materials of oil seal are generally nitrile rubber, acrylic rubber, and silicone rubber, fluororubber and polytetrafluoroethylene resin in special cases.
Choice of oil seal:
① oil seal material selection
In the selection of oil seal materials, it is necessary to pay attention to the compatibility of the material to the working medium, the adaptability to the working temperature, and the ability of the surrounding speed (that is, the resilience of the material and the recovery speed should be able to follow the change in the amount of compression caused by the circular speed). Because the temperature of the oil seal lip is generally 20 to 40 degrees higher than the temperature of the working medium, it should be taken into account when selecting materials.
② Working pressure
Where there is pressure oil chamber, can not use oil seal. Generally, the oil pressure on the standard oil seal should not exceed 0.05MPa. When in the range of 0.05 ~ 0.lMPa (maximum 0.3MPa), it is necessary to use a special pressure oil seal (up to 1mpa internationally).
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