Measuring tappet rotation in a valvetrain rig when lubricated in a fully formulated oil containing MoDTC-type friction modifier

In a direct acting valve train configuration, tappet rotation plays a key role in improving lubrication, reducing wear and friction. However, to the best of the authors' knowledge, no studies were found to investigate the rotation of tappet under the effect of different coatings, thicknesses of tappets and formulations with Molybdenum Dialkyl Dithiocarbamate (MoDTC) which has been recently reported to be detrimental to Diamond-Like Carbon (DLC) wear. In this work, a new technique of measuring tappet rotation has been developed. A giant magnetoresistance (GMR) sensor coupled with a split pole ferrite disk magnet was used. The sensor was installed very close to the tappet/bucket while the magnet was mounted into the underside of the tappet. Experiments were performed using standard production steel tappets coated with Mn-phosphate (MnPO4) and diamond-like carbon (DLC) coatings. In general, results showed that the tappet rotation is strongly dependant on oil formulation, clearance, speed/temperature, and surface roughness of the coating. MoDTC promoted the rotation of the tappet under both coatings. In addition, DLC inserts showed an increase in tappet rotation as compared to MnPO4 inserts. Nevertheless, regardless of the type of coating, the thickest tappets showed the highest rotation.