Audi 100/A6

1990-1997 of release

Repair and car operation

14.8.8.1. As the bearing works

THE GENERAL DATA

Sliding bearings almost exclusively serve in modern automobile engines as support for cranked and camshafts. Bearings качения (ball, roller, needle) apply to the similar purposes only in small motorcycle motors.

Necessary working capacity of bearings of sliding is reached by use of so-called effect of an oil wedge. At rotation of a smooth shaft in a backlash between shaft and an aperture oil moves. As the loading operating on a shaft, causes its excentric displacement, oil is as though tightened in a narrowed part of a backlash and forms the oil wedge interfering contact of a shaft with walls of an aperture. The more pressure and viscosity of oil in a backlash, the the big loading (before contact of surfaces) maintains the sliding bearing.

Actual pressure of oil in a wedge zone reaches 50–80 МПа (500–800 kg/sm²), and in some designs and it is more. It in hundreds times above, than in submitting system. However it is not necessary to think that pressure of giving influences bearing work a little. The it is more, the there is an oil prorolling through the bearing more intensively and the its cooling is better.

Under certain conditions the operating mode with the minimum friction (it also name liquid) can be broken. It happens at понижениее to viscosity of oil (for example, because of its overheat owing to insufficient giving) and decrease in frequency of rotation at loading increase.

Quite often, especially after engine repair, not optimum geometry of knot affects also. At an insignificant deviation of the form of surfaces from cylindrical, at a warp of axes and other defects of details probably local increase of specific loading (that is the loading carried to the area of a surface) above an admissible limit. Then the oil film in these parts becomes thin, and shaft and bearing surfaces start to adjoin on microroughnesses. There is the mode of semiliquid greasing characterised by increase of a friction and a gradual warming up of the bearing. Further it can lead to a so-called boundary friction with full contact of the surfaces the overheat, схватывание (teases), jamming, fusion and bearing destruction will be which consequence.

It is clear that in operation the mode of a boundary friction is unacceptable. Nevertheless it appears at infringement of giving of oil, and it occurs because of its shortage in картере, that is or owing to oversight of the driver, or at pallet damage картера as a result of arrival on an obstacle more often.

The mode of semiliquid greasing is admissible only for short time when he has not time to affect deterioration of the bearing. An example – start-up of the cold engine. However, here there is other danger: at very low temperature oil можеты to be too viscous and its normal giving is restored long (20–30 seconds and more). Here already semiliquid greasing is capable to affect deterioration of details considerably.

Perfection of automobile engines is connected with constant growth of frequency of rotation and capacity increase. Increase of compactness of designs, including reduction of width and diameter of bearings is simultaneously observed. It means that specific pressure grow in knot. And as loading on the bearing at engine work cyclically changes on size and a direction, there is real a so-called fatigue failure of details. To provide working capacity of bearings in such conditions, special designs, materials and technologies are required.