| Friction is the general term for the study of surface friction,wear and lubrication science.Among them,the study of friction is the basic content of tribology.All along,in the study of friction,most of its normal load are applied by man-made or fixed heavy objects.Under normal circumstances,the effects of the atmosphere on friction are rarely considered,but due to the presence of the atmosphere and changes in the contact area of the frictional subsurface,there may be different pressure differences on the surface.In addition,there are different roughness and deformation,carrying area,lubricants and other differences,these will affect the surface contact performance and friction performance,and ultimately affect the actual friction process.This paper is inspired by the experiment in the hemisphere of Mardelburg,based on the pressure difference formed by atmospheric pressure and vacuum environment to provide a normal load,and by adjusting different vacuum degrees to control the normal load size to study the friction law between contact surfaces under different pressure differential conditions.The main research content is as follows:The friction model of two contact surfaces with or without lubricant under different pressure difference is established,and the influence of contact interface gas on friction is analyzed from the angle of energy.It is concluded that with the increase of vacuum degree,the static friction coefficient decreases first and then increases.The influence of surface roughness on friction under different pressure difference is analyzed based on adhesive friction theory and support surface curve.They are concluded that under the condition of different vacuum degree(pressure difference),the static friction coefficient decreases gradually with the increase of surface roughness in a certain range and for friction pairs with different surface roughness,the growth rate of static friction coefficient increases with the increase of vacuum degree(or normal load).Based on the theory of boundary lubrication,the friction law of different pressure differential is analyzed.They are concluded that with the increase of vacuum degree(pressure difference),the static friction coefficient increases gradually for the friction pair added lubricant and the static friction coefficient decreases with the increase of surface roughness(R_aand R_q)in the same vacuum degree for the friction pair of the same material in which the lubricating oil is involved and under the action of lubricant,with the increase of vacuum degree,the smaller the surface roughness,the greater the growth rate of static friction coefficient.Construction of experimental apparatus.Based on the wire rod transmission mechanism of the hand slide table,the experimental equipment for measuring friction force under different pressure difference is built.The push-pull dynamometer is fixed on the slide table so that it can be pushed along with the slide table.The experiment uses the hemispheres and the cover plate to form a closed space.The dimensions of the hemisphere are designed by the thrust range that the wire rod mechanism can provide.And valves and vacuum meters are installed on the cover plate.The valves are used to control the vacuum degree.And the air in the hemisphere is extracted through the circulating water vacuum pump to form a vacuum environment.Then comparison experiment is conducted with the press expansion device.By applying normal load of any size in the range(1 KN)with press,the friction force between the hemispheres and the cover surface is measured.Based on the established device,the theoretical analysis of Section 2.2 and Section 2.3 are verified.The first experiment is that eight friction pairs were measured and the atmospheric pressure contrast experiment was carried out by using the extended experimental device built by the press.Combined with theoretical analysis,They are concluded that with the increase of vacuum degree,the maximum static friction between contact surfaces varies linearly and with the increase of vacuum degree,the static friction coefficient decreases first and then tends to stabilize for the friction pair with larger hardness,while for the friction pair with smaller hardness,the static friction coefficient decreases first and then increases.The second experiment was carried out with four kinds of friction pairs of steel-steel and steel-aluminum alloy with different surface roughness(R_a and R_q).Combined with theoretical analysis,They are concluded that the static friction coefficient decreases gradually with the increase of surface roughness in a certain range under different vacuum degree(pressure difference)and for friction pairs with small hardness value,when the surface roughness decreases,the growth rate of static friction coefficient increases with the increase of vacuum degree,but for those with large hardness value,the change of static friction coefficient growth rate is not obvious and can be ignored.Based on the established device,the theoretical analysis of friction law under different pressure differential lubricant is verified.The lubrication experiments were carried out respectively by using rapeseed oil as lubricant and using two groups introduced in Chapter 3and three kinds of steel-steel and steel-aluminum alloy friction pairs.And the experimental results in Chapter 3 are compared.Combined with theoretical analysis,They are concluded that with the increase of vacuum degree(pressure difference),the static friction coefficient increases gradually,and slowly approaching to no lubricating oil and When the vacuum degree is small,the influence of surface roughness on the static friction coefficient is small,and when the vacuum degree is large,the static friction coefficient decreases slightly with the increase of surface roughness(R_a and R_q)and for the same kind of metal friction pair in lubricating state,with the increase of vacuum degree,the smaller the surface roughness,the greater the growth rate of static friction coefficient. |
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