Page 293 - Cam Design Handbook

THB9 9/19/03 7:26 PM Page 281

CAM MATERIALS AND LUBRICATION 281

9.5.3.3 Contaminants. Contaminants in both fluid-film and gas-film systems can sig-
nificantly affect cam performance, life, and reliability. Contaminant particles can brinell
the shaft (journal) and bearing surfaces, causing stress raisers that can act as nuclei for
surface pitting (spalling). Hard particles act as an abrasive medium and wear the softer
surface. The particles can also act to clog lubricant orifices and jets, resulting in lubricant
starvation of the bearing surfaces. The particles in a lubricant are the result of (a) manu-
facturing (chips and grit); (b) machine performance (wear particles); (c) environment result
(dust, sand, paper, etc.); and (d) lubricant breakdown products (precipitates). Even in
systems that are initially clean, multiple startups and shutdowns cause metal-to-metal
contact, resulting in wear debris. Over time this debris can cause damage. As a result, good
filtration is required in the lubrication system for long life and reliability.

9.6 SUMMARY

Limited explanation or theory is available for a full understanding of the life of surfaces
in contact. Therefore, it is no surprise that in cam-follower surface actions there is some-
what of a lack of agreement concerning service performance. However, certain basic
thoughts are presented for the proper use of materials for the system.
• Much of the knowledge now available to solve problems in rolling-element bearings
and gearing is relevant to the needs of the cam designer.
• Section 9.4.4.2, Surface Fatigue Design, and Sec. 9.4.5, Materials Selection, provide
information about the cam-follower material choices.
• Selecting materials and lubricants to fulfill the requirements of smooth cam followers
with minimum wear is ultimately governed by experience in the field.
• Improve the cam-follower action by changing materials and lubricant only after mea-
surement of parts, alignment, and cam contour accuracy has been established.
• In cam roller-follower systems, the design basis is for the roller follower to be the first-
destroyed material for easy replacement and least cost.
• Roller-follower and cam surface failures are primarily fatigue failures with some adhe-
sive wear evident. As the percent of rolling increases, the amount of adhesive wear
decreases.
• Gray cast iron in combination with hardened steel is probably the cheapest choice
for high-sliding galling action. Chilled cast iron and alloyed cast iron are among the
best materials for galling. Meehanite in contact with hardened steel is an excellent
choice, giving long life, good internal preparatory damping characteristics, and easy
machining.
• Bronze or nylon in combination with hardened steel has good wear life, especially under
high shock loads. It gives less noise, less vibration, and smoother action and compen-
sates for inaccuracies in the contour surface. Plastics are excellent for noise reduction.
A lubricant coolant is generally necessary.
• Hardened tool steel in contact with hardened steel gives the best combination under
rolling action when shock and sliding action occur under high loads.
• Good surface finish with a lapping and polishing action is essential with hardened mate-
rials. It is not as important with ductile materials.
• Excellent choices for hardened steels are AISI 4140 and 8620. AISI 4140 can be through
hardened, flame hardened, and nitrided, producing a thin, hard skin a few ten thou-