1 The curvature radius of gear teeth is small, because of the relationship of surface geometry, the relative motion of two friction surfaces is not easy to form oil film, so it is difficult to establish a fluid lubrication state between the two gears.
2 Gear lubrication is periodic discontinuity, two gear teeth each time meshing, to re-establish the oil film, so each time to form a complete film conditions are poor.
3 Except the gear teeth of circular arc gear are basically rolling, the other gears such as involute gear, hyperbolic gear and worm gear have sliding, and the sliding direction and size are always in sharp change, so it is not conducive to the formation of oil film.
4 Because the contact stress of the two gear teeth is very high and the gear load is heavy, the equipment is often in the state of boundary lubrication.
5 Because of the special shape of gear teeth, the processing and manufacturing technology is more complex. If the machining accuracy and surface finish are low, when high load, the rough surface is easy to make the lubricating oil film friction will occur between the protruding points to make the gear teeth damaged.