LUBRICATION FILM THICKNESS BEHAVIOR UNDER TRANSIENT SPEED AND LOAD CONDITIONS

In engineering practice no machine parts are working under ideal steady-state conditions. The transient conditions involved in particular cases of real machine assemblies like cam follower contacts, ball bearings etc. represent a really complex sequence of movements. In these applications high sliding conditions are often replaced by zero entrainment speed connected with squeezing action and followed by opposite sliding. In general the variation in speed of contact surfaces strongly affects the film thickness and it can result in a lubricant film breakdown and in consequence also damage of contact surfaces. Experimental study will also consider the effect of surface topography and dynamic change of load. A new concept of experimental apparatus was used for experimental study of cam and tappet contact under simplified operational conditions. To simplify the problem the concept of cam and tappet contact was replaced by EHD contact of the flat surface of chromium coated glass disk and a steel ball. Also only transient speed and load were taken into account. The method used to measure film thickness was thin film colorimetric interferometry. A high-speed digital camera was used in combination with a high power xenon lamp to capture rapid changes of film thickness in the contact area. Servomotors and piezo-actuator were used to control the speed of contact surfaces and also the load of the contact during the experiment. The experimental results show that the change of speed has a great impact on film behavior during sequences where opposite sliding or zero entrainment speed is involved. It was found that shallow grooves can behave as a reservoir of lubricant which can supply the contact area and increase the film thickness during the above described transient phenomena.