1. Pressure difference inside and outside the reducer:
During the operation of the reducer, the friction heating of the moving pair and the influence of the ambient temperature make the temperature of the reducer rise. If there is no vent hole or the vent hole is blocked, the pressure inside the reducer increases gradually. The higher the temperature inside the reducer, the greater the pressure difference with the outside. Under the effect of the pressure difference, the lubricating oil leaks out from the gap.

2. Unreasonable structure design of reducer
1) Check that the cover plate of the hole is too thin and easy to deform after tightening the bolt, making the joint surface uneven and leaking oil from the contact gap;
2) In the process of reducer manufacturing, if the casting is not annealed or aged, and the internal stress is not eliminated, it will inevitably deform and produce gap, leading to leakage;
3) There is no oil return groove on the box, and the lubricating oil accumulates at the shaft seal, end cover, joint surface, etc. under the action of differential pressure, it leaks out from the gap;
4) The structure design of shaft seal is unreasonable. In the early stage of reducer, oil groove and felt ring type shaft seal structure were mostly used, which made the felt compressed and deformed during assembly, and sealed the joint surface gap. If the contact between the journal and the seal is not ideal, the seal will fail in a short time due to the poor compensation performance of the felt. Although there are oil return holes on the oil groove, they are easy to block and the oil return function is difficult to play.
3. Too much oil filling: during the operation of the reducer, the oil pool is stirred badly, and the lubricating oil splashes everywhere in the reducer. If the oil filling is too much, a large amount of lubricating oil will accumulate in the shaft seal, joint surface, etc., causing leakage.
4. Improper maintenance process: during equipment maintenance, oil leakage may also be caused due to incomplete removal of dirt on the joint surface, improper selection of sealant, reversed installation of seal, and untimely replacement of seal.