Galling

Paper Number

Contribution of Oil Traction to Diesel Engine Cam Galling
Mandeep Saini and Frances E. Lockwood
The Valvoline Company

Jerry C. Wang and Carl F. Musolff
Cummins, Inc.
Copyright © 2001 Society of Automotive Engineers, Inc.

ABSTRACT
Heavy Duty diesel engines typically use roller followers in contact with the cam to reduce friction and accommodate high Hertzian stresses.When the rolling contact slips into sliding, cam galling can occur that may lead to major cam failures. Oil traction has been identified as a possible source to cause slipping. In this study, oil traction was first measured in a Mini Traction Machine (MTM). The results were then validated by a series of engine tests to show that the measured oil traction correlated with the occurrence of camgalling. Finally, the MTM was used to evaluate various engine oil formulations. It is concluded that some advanced base oils, if not properly compensated by the additive package, exhibit dangerously low oil traction. Oil traction needs to be part of the oil formulation considerations.

Many other factors can contribute to the slipping at the camshaft- follower interface. For example, the corrosion ofthe bronze pin may increase the friction of the pin3 follower interface to cause the roller to “stick” . The focus of this paper is to study if the lubricating oil can affect the contact friction to cause slipping.

INTRODUCTION
Heavy-duty diesel engines use camshaft rotation to control the timing of valve and injector events. Figure 1 shows a typical valve train assembly in relation to thecamshaft. As the cam rotates, the interfacial friction causes the cam follower roller to rotate while lifting the push tubes to activate the valve or the injector. The 1 profile of the lobe dictates the timing of such events . The friction between the cam lobe and the follower comes from the normal loading due to the combined cylinder pressure and the valve spring force. A rolling contact is necessaryto minimize friction and sustain the high Hertzian stresses in modern engines. However, the normal load varies rapidly from maximum to almost no load as the camshaft rotates through the tip of the profile under high speed. Under certain conditions of rapidly changing loads and high temperature, sliding can occur in the rolling contact. The design of the lobe profile requires the cam and thefollowers to sustain repeated high loading while minimizing slipping for the longevity of 2 the system .

Figure 1 A typical setup of the camshaft and the valve train assembly. The arrows indicate the direction of motion.

EXPERIMENTAL DESIGN
Oil Traction Measurement – The cam and roller follower contact is designed to operate under elastohydrodynamic condition if a pure rolling contact isrealized. The friction of the lubricated contact is the driving force for roller rotation. The oil traction is assumed to be related to the friction measured when a cam-and-follower contact is lubricated by the oil. However, it is very difficult to measure an actual contact reliably. In order to provide a consistent measurement of the traction coefficient, and to make the measurement easily applicable tofuture studies, a Mini Traction

Machine (MTM) is selected to simulate such a contact and provide traction coefficient measurement. The test contact is formed between a polished ¾ inch ball and a 46-mm diameter disc, each independently driven to produce a sliding/rolling contact. To perform a test, a small sample of fluid is placed in the test reservoir and the system steps through a series ofloads, speeds, slide/roll ratios and temperatures following a custom program predefined by the operator.

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heavy duty diesel engine in a relatively short period of time. This is accomplished by subjecting the engine to repeated fast acceleration and deceleration under high loads. The operating condition is illustrated in Figure 4. A visual inspection of the cam is performed every 50 hours...