Evaluation of Cost Effective Coatings for Automotive Piston and Pin Assemblies

An investigation was conducted to evaluate the elimination of the bronze bushing in the connecting rod of an automotive piston assembly by utilizing a cost effective break in lubricant applied to the piston pin. The typical automotive piston assembly consists of a piston having pin bosses formed with pin bores for the piston pin. The hardened steel piston pin connects the piston through the pin bores to the connecting rod, which also has a pin bore (See Figure 1). Typically, internal to the pin bore in the connecting rod is a press fit bronze bushing. The purpose of the bronze bushing is to reduce friction, material transfer and scuffing by providing a compatible bearing type mating surface with the pin under low lubrication conditions. However, the bushing adds weight ^{1, 2} (with the bushing eliminated the small end of the connecting rod will have a smaller outside diameter which reduces the weight while maintaining the wall thickness of the small end of the rod), additional manufacturing steps and cost to the piston assembly, while not providing any additional structural strength. Material transfer or scuffing at the pin/piston and the pin/bushing interface is a common failure mode especially during the initial start of the engine.

Previous experiments have identified solid film lubricants that will prevent scuffing and wear, but were generally not cost effective relative to the cost of a connecting rod with a bushing. For this experiment six different lubricants were tested on a bench-test rig at Falex Corporation. It consisted of Block on Ring Tests per ASTM G 77. Metrics monitored included coefficient of friction, load, temperature, surface texture, scar width and volumetric material loss. Test results for two of the coatings have lead to engine bench testing and field testing of sixteen engines having connecting rods without bushings.