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The Composition of Surface Layers Formed in a Lubricated Cam/Tappet ContactThe frictional behavior of two oils was measured in a cam/tappet friction apparatus using a direct acting bucket tappet geometry. One oil contained a soluble molybdenum friction modifier. The tappet was equipped with a removable friction disk, which allowed the frictional surface to be easily removed for surface characterization using surface profilometry, reflection infrared and XPS spectroscopies. The films consisted of varying amounts of inorganic phosphates, sulfates and sulfides with zinc, magnesium, and possibly molybdenum as cations. MoS 2 , expected because additives containing molybdenum and sulfur were present, was not detected. The detection limit was less than 0.1 atom percent Mo. The film formed by the friction modified oil was thinner, smoother and contained small amounts of molybdenum. While the smooth surface finish could reduce friction by improving microelastohydrodynamic lubrication, friction reduction could not be tied to any of the chemical structures detected in these experiments. . Paper No. 90-Trib-17. Associate Editor: B. Bhushan. Experimental2.1 Test Apparatus. The cam/tappet friction apparatus is an improved version of equipment described previously (Willermet, Pieprzak and Dailey, 1989;. A cam/tappet module is driven by a variable speed 2 hp. DC motor with an attached flywheel. The torque required to rotate the cam shaft is measured with a torque meter positioned between the cam shaft and flywheel (Fig. 1). Torque Meter Attachment and Mounting.Solid split shell couplings are used to attach the torque meter. These solid couplings have much lower rotational inertia than flexible couplings. This reduction in inertia significantly increases the natural frequency of the system and reduces the amplitude of the natural frequency "noise." To make the use of solid couplings possible, a visco-elastic mount was incorporated to support and stabilize the torque sensor. Cam/Tappet Module.The cam/tappet module is essentially a temperature controlled aluminum box which can accommodate a variety of value train designs (Figs. 2 and 3). The drop-in components, which establish the test configuration, are located within ±0.05 mm via locating holes. The temperature of the module is controlled by circulating a

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The instantaneous tappet rotation velocity and instantaneous friction torque were measured simultaneously for the CVH center pivot rocker arm geometry. The measurements were made in a cam/tappet friction apparatus equipped with a laser reflectance tappet rotation monitor capable of following rapid changes in tappet rotation velocity. The data showed that more rapid tappet rotation resulted in lower friction. The effect was greatest in the region of maximum cam lift. Several tappets were modified to produce higher rotation rates than production tappets. The best results were obtained with tappets incorporating an internal squeeze film bearing to reduce resistance to rotation. The data were analyzed to elucidate the cause for the friction reduction effect. The analysis showed that most important effect was the partial rolling motion between the surfaces, rather than enhancement of the oil film thickness. A model based on that conclusion agreed reasonably well with the experimental data.