1. The surface roughness of shaft is unreasonable

The surface roughness of the shaft matched with the oil seal should not be too low or too high. Generally, ra0.8-ra3.2 is recommended. If the shaft surface is too rough and the oil seal lip is worn too fast, it is easy to cause leakage; if the shaft surface is too smooth, the oil is easy to be squeezed out from the seal contact surface, and the oil film will become thin or even disappear, resulting in lip heating or burning. In the practical application process of oil seal, many designers tend to go into the design misunderstanding and think that the smoother the shaft surface, the better. In fact, in the process of shaft design, the roughness of oil seal fitting surface should be reasonably selected. If the surface is too smooth, it can not seal the oil, and the processing cost will increase sharply.

2. Damage caused by assembly process

The sealing function of oil seal is mainly realized by the main sealing lip. The main lip is made of rubber, so it is easy to be scratched by sharp metal on the production site, and the small wound is not easy to be found by the operator, resulting in oil leakage fault of the transmission factory, causing customer dissatisfaction. In view of this failure mode, the following measures are often taken:

(1) Design: in order to make the oil seal easy to install and avoid damage during installation, the shaft should be chamfered 15 ° to 30 °.

(2) On site material storage and transportation: in order to protect the oil seal, reasonable packaging should be maintained during storage and material transportation, and the packaging should be removed when using on site; the oil seal should be placed in a clean material box.

(3) On site assembly: parts matching with oil seal shall avoid collision during transportation, and shall be cleaned before assembly; special tooling shall be used for oil seal assembly, and working surface of tooling shall be protected to ensure no damage to oil seal during assembly, and coaxiality of oil seal and matching parts shall be ensured; oil shall be applied on shaft surface during oil seal press mounting.

3. Poor working environment of oil seal

The working environment of the output shaft oil seal mainly includes the cleanliness of the transmission assembly, the air environment at the lip side of the oil seal dust-proof pair, and the working temperature of the oil seal.

3.1 cleanliness of transmission assembly

In the process of transmission production, according to the different manufacturers' equipment and field management ability, the requirements of transmission assembly cleanliness standards are not the same. For the oil seal, the higher the cleanliness, the better the working environment of the oil seal, and the longer the sealing effect and service life of the oil seal.

Transmission assembly cleanliness limit g (mg) is calculated as follows: G ≤ Kle

Where: Le is the rated volume of transmission assembly lubricating oil, l; K is the objective evaluation coefficient of cleanliness, that is, the impurity content of each liter of lubricating oil, mg / L, and K of general medium and heavy transmission assembly is 45 mg / L-70 mg / L.

Transmission manufacturers should clearly inform users of the model and replacement cycle of transmission lubricating oil, because additives in lubricating oil, such as oil soluble organic compounds such as phosphorus, sulfur and chlorine, will have adverse effects on oil seals. Oil soluble organic compounds such as phosphorus, sulfur, chlorine and other oil soluble organic compounds in the oil will decompose and produce gas when heated, which can cross link with the rubber and double chain, so that the rubber will harden and the oil seal will lose its elasticity and leak; moreover, when the lubricating oil is coked at high temperature, it will produce mucus, which will accumulate in the oil seal lip, which will make the lip fail and lead to leakage. Therefore, reasonable use of lubricating oil and timely replacement can provide a better working environment for oil seal.

3.2 air environment at lip side of oil seal dust-proof pair

The working environment of commercial vehicles (especially engineering vehicles) is often very bad. Transmission is an important part of automobile chassis. It has been in high dust and high pollution environment for a long time. A large amount of dust is easy to adhere to the oil seal edge. When the dust enters the oil seal edge, it will wear with the oil seal edge and joint surface, causing oil leakage. Therefore, adding a dust cover to the rear bearing seat of the transmission can stop the dust and prevent the large particles from adhering to the oil seal, which greatly improves the working environment of the oil seal and improves the sealing performance and service life of the oil seal.

After adding the dust cover, it is necessary to change the structure of the rear bearing seat and flange of the transmission output shaft. The structure of the output shaft oil seal before and after adding the dust cover is shown in Fig. 3. The dust cover is generally made of 08F material. The inner diameter of the dust cover is installed with the flange. The outer diameter of the dust cover adopts interference fit. A reasonable clearance is reserved between the outer diameter and the inner diameter of the extension part of the rear bearing seat of the output shaft, so that the flange will not interfere with the rear bearing seat of the output shaft when rotating with the dust cover. According to the different machining and assembly accuracy of transmission manufacturers, the general clearance range is 0.35mm ~ 1mm.

3.3 operating temperature

The working temperature of commercial vehicle transmission is generally about 90 ℃. According to the different operating environment of vehicles, the ambient temperature of transmission is also different. For example, the outdoor temperature of Northeast China in winter can reach - 40 ℃. When the inner diameter of the oil seal rotates relative to the shaft, heat will be generated. The higher the speed is, the larger the inner diameter of the oil seal is, the more serious the heating is. When the heating temperature exceeds the allowable temperature of rubber, the oil seal will be aged, cracked and damaged. Different rubber materials have different working temperature range. It is also an important link to ensure the sealing performance of oil seal to select rubber material reasonably to adapt to its working temperature. Nitrile rubber (adapt temperature - 30 ℃ - 120 ℃) and acrylate rubber (adapt temperature - 15 ℃ - 150 ℃) can adapt to a wide range of temperature and have low cost, so they are the preferred rubber materials for oil seal of transmission in China; silicone rubber (suitable temperature - 45 ℃ - 170 ℃) can only be used in special occasions due to its high cost.