The oil seal of the petrol jack is a seal of the rotating shaft. Its function is to prevent liquid leakage along the rotating axis and external impurities from intruding into the inner part of the petrol jack body. As a rotating shaft sealing device, besides oil seal, there are mechanical seal and labyrinth seal. But in these sealing devices, the oil seal is simple in structure, easy to install, low in cost and widely used. Oil seals are mostly used for rotating shaft seals of hydraulic pumps and motors.

In the free state, the inner diameter of oil seal is smaller than that of rotary shaft (excess). So when the oil is encapsulated on the shaft, the lip of the oil seal has a certain radial pressure on the shaft. It is generally believed that the main principle of oil seal is to form a stable oil film between the lip edge and the shaft. That is to say, when oil seal is sliding, a viscous oil film is formed between the lip and the shaft to protect the sliding surface, avoid damage and wear caused by direct contact between the lip and the shaft, and prevent film leakage. This is the boundary lubrication theory. In order to keep the working performance of oil seal stable for a long time, the existence of lubricating oil film is necessary. However, when the oil film exceeds a certain thickness, oil leakage will occur at the oil seal.

In order to prevent leakage, oil film should be discontinuous, but excessive emphasis on oil film discontinuity can not achieve the desired long-term stable performance. It can be seen that the sealing characteristics of oil seals can only be guaranteed if the two contradictory factors of leakage prevention and oil film formation on sliding surface are unified. Others think that the direct factor affecting the sealing ability of oil seals is the distribution of radial force (the thickness of oil film varies with the radial force). They measure and calculate the radial force distribution of oil seals by photoelastic test. The sharper the radial force distribution is, the better the sealing effect will be. In a word, the structural design of oil seal for petrol jack, the formulation of rubber and the use of rubber must provide conditions for the formation of thin and stable oil film.