Dry Film Lubricants in automotive applications provide
a unique challenge. In 1995, NPI was involved in a test program conducted by Loyola Marymount
University evaluating the effects of three dry lubricants on automotive valve train wear
resistance. Scuffing wear occurs as the cam slides across the lifter face where the
rotating motion of the camshaft is converted into the linear motion necessary to drive
the cylinder head valves. This scuffing is caused by localized microscopic bonding between
the skidding surfaces. It was determined that this can be minimized by using dry film
lubricant coatings to increase the boundary lubrication depth adjacent to the contact area.
INTRODUCTION
The operating conditions at the contact line between the
rotating camshaft lobe and the reciprocating valve lifter in an automotive engine are extremely
severe. High loading forces, high sliding velocity and high friction between the cam and lifter
make this interface one of the most wear-prone areas in an internal combustion engine. Scuffing
wear is the common wear mechanism. Fortunately, judicious material selection and processing, excellent
cam surface alignment and proper lubrication can minimize scuffing wear and provide long engine life.
Since proper lubrication is essential for minimum wear, virtually all production four-stroke internal
combustion designs provide for the splashing of copious amounts of oil in and around the camshaft during
continuous engine operation. However, during engine start-up the situation is different since splashing
cannot occur until oil is first pumped upward from the oil pan located at the bottom of the engine.
In the seconds before the oil pump reaches full pressure, thereby filling oil galleys and running
clearances, camshaft lubrication is scant. During this time, dry film lubricant coatings can best
protect the engine.
INVESTIGATION
Testing was performed to determine the resistance of various dry lubricant coatings
to scuffing wear at the camshaft-to-follower interface. Mating hydraulic lifters and valve springs from a V-8 engine
was employed. Camshafts used in the tests were made of alloyed gray iron (ASTM A-159 Gr. G4000D, SAE J431C Gr. G4000D),
with a lobe surface hardness of Rc 50. The lifters were made from hard enable iron, having a minimum hardness of Rc 55.
An unlubricated camshaft was tested to serve as a control. Various camshaft surfaces were individually treated in one of
three ways - by Parkerizing (a patented type of manganese phosphate coating), by spray applied graphite coating,
and by molybdenum disulfide coating. Coatings were applied to the camshaft surfaces in accordance with
manufacturers' specifications. The molybdenum disulfide coating was NPI-16 manufactured and applied by National
Process Industries, Temecula, California.
To measure the cam-lobe-to-follower friction coefficient, the camshaft was supported in the lathe on centers coincident
with the camshaft rotation axis (figure 1). Oil was applied to the center contact areas to minimize bearing friction.
The lathe headstock was adjusted to press the follower in its fixture against the cam lobe, which was positioned at the
top dead center.
Next a string was wrapped around the axis of the camshaft and attached to a weight. By varying either the weight or
the follower spring compression, the assembly could be adjusted to equalize the weight and the camshaft geometry, the
follower contact force and the torque could then be calculated.
Figure 1 Experimental Set-up Used to Determine the Cam-Lobe-to-Follower Coefficient of Friction
RESULTS
Coefficients of static friction at the camshaft-to-follower interface, correlating to each
of the various coatings are displayed in Table 1 and Figure 2.
As recorded in Table 1, the coefficient of static friction for the NPI-16 coated interface was considerably lower than that recorded
at the unlubricated interface. Experimental results showed that NPI-16 provided the most significant resistance to scuffing wear
of all the tested coatings. The follower face profile was visibly affected only after 12 hours of testing and no visible effect
was noted at the cam lobe surfaces; this is due to the high thermal resistance of the coating. Even though approximately 40% of
the coating had mixed with the engine oil and worn away from the interface after 4 hours of testing, it was observed that the
remaining coating did not further disappear; it remained entrained at the interface for the duration of the test.
More specifics related to this test program can be obtained by contacting NPI sales personnel.
Table 1 Coefficients of Static Friction for Each Camshaft-to-Lifter Interface, by Lubricant Type
Figure 2 Histogram of Coefficients of Static Friction for Each Camshaft-to-Lifter Interface, by Lubricant Type
NPI RECOMMENDATIONS
NPI coatings provide a back-up and second layer of protection that will lubricate during extreme operating
conditions, high loads/contact stresses and at high temperatures. Not only should Valve Train Components be coated but all internal engine parts
where optimum performance is desired; all Moving Mechanical Assemblies (MMA's) such as:
Bearings
Gears - rear end or transmission
Pistons
Rocker Arms
Fuel Pumps / Bertha Spacers
Crankshafts
Valves and Valve Springs
All should be coated to provide that extra edge during competitive operation. As we have seen with the Automotive Valve Train test results, with
proper lubrication the benefits are unmistakable. You can expect to see:
Less wear
Reduced friction
Increased power
Longer part life
Information on all NPI coatings is available on this web site.
