Page 289 - Cam Design Handbook

THB9 9/19/03 7:26 PM Page 277

CAM MATERIALS AND LUBRICATION 277

< 1, surface smearing or deformation will occur on the rolling surface accompanied by 20
times more wear. For 1 <L< 1.5, surface distress will exist accompanied by superficial
surface pitting with about 4 times more wear than L> 3. For 1.5 <L< 3, some surface
glazing can occur with eventual surface failure by subsurface rolling pitting fatigue with
1
about 1 / 2 times more wear than L> 3. In other words, when L is below 3 mixed film or
boundary lubrication may occur.
The easiest way to raise the L ratio is to change the lubricant necessary, using a higher
viscosity lubricant or raising the lubricant viscosity at operating temperature. Producing a
high-quality surface finish on the rolling elements is, of course, more expensive.
Zaretsky (1997) states that the L ratio Eq. (9.19) can be used as an indicator of gear-
ing and rolling-element bearing life. For aerospace gears, the composite surface finish
is 23m in ms and most gears operate at L< 1.5 with either a mixed or boundary lubrica-
tion regime. The surface distress may have superficial pitting without proper additive in
the lubricant. With proper lubrication, failure will eventually occur by classical subsur-
face-originated rolling (pitting) fatigue.

9.5.3 Lubricant Selection
A liquid lubricant has several functions in cam-follower system performance. It is the
medium for the separating film (a) between the roller follower and its mating cam surface
elastodynamically and also the rolling-element inner bearing and (b) between the slider
and the cam surface of a sliding follower. The lubricant also serves to absorb the shock
of the roller on the cam during the roller acceleration action as it rolls and slides. It should
maintain the best rolling action and reduce the sliding to a minimum. The lubricant also
serves as a coolant to the contacting bodies. A lubricant serves to flush out wear debris
and carry it to a filter where it can be removed from the system. Last, it provides corro-
sion protection.
The best choice of lubricant is difficult to determine depending on the load and speed
conditions of the cam-follower mechanism. A lubricant should be selected along with the
cam follower materials in an early part of the design process to eliminate costly redesign
necessities that may occur otherwise. The equilibrium temperature of a lubricating system
is also an important consideration.
This section presents a study of lubricants in nonrecirculatory systems of (a) grease
(which contains the liquid lubricant) and (b) oil lubricants. Recirculating systems may be
used for cam and follower mating surfaces and other parts of the total machine. It is also
important to decide whether to feed the lubricant on the entering side of the cam or the
exit side. The author has observed early cam fatigue wear in heavily loaded systems using
entering lubricants. Experience is critically important. Note that the Ferguson Company
of St. Louis, Missouri uses Chevron SAE 80W90 for its high-speed, heavily loaded cam
systems, especially the roller-gear drive. This lubricant contains numerous additives for
surface protection and wear.

9.5.3.1 Grease. The least expensive mode of lubrication is grease. In the commercial
roller follower, the bearing (a) is packed with grease or (b) is unsealed and supplied with
a liquid lubricant; see Chap. 10. The use of grease reguires a simplified housing and seals
with a Zerk fitting for feed. Grease is not a liquid, but the liquid or fluid constituent in the
grease is the lubricant. Greases consist of a fluid phase of either a petroleum oil or a syn-
thetic oil and a thickener. The most common thickeners are sodium-, calcium-, or lithium-
based soaps, although thickeners of inorganic materials such as bentonite clay have been
used in synthetic greases. Since there is no recirculating fluid, grease-lubricated bearings
must reject their heat by conduction and convection. The heat transfer requirements of