Page 284 - Cam Design Handbook

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272 CAM DESIGN HANDBOOK

low-alloy steel can be used without the added cost of heat treatment and additional pro-
cessing. The low-alloy material in the non–heat-treated condition can be used for bending
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stresses in the 20,000lb/in range and surface durability hertzian stresses of approximately
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85,000lb/in . As the operating conditions become more severe, it becomes necessary to
harden the cam for improved strength and to case harden the cam surface by case car-
burizing or case nitriding for longer pitting fatigue life, better scoring resistance, and better
wear resistance. Several medium-alloy steels can be hardened to give good load-carrying
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capacity with bending stresses of 50,000 to 60,000lb/in and contact stresses of 160,000
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to 180,000lb/in .
The higher alloy steels are much stronger and must be used in heavy-duty applications.
AISI 9310, AISI 8620, and Nitralloy N are good materials for these applications and can
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operate with bending stresses of 70,000lb/in and maximum contact (hertzian) stresses
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of 200,000lb/in . These high-alloy steels should be case carburized (AISI 8620 and 9310)
or case nitrided (Nitralloy) for a very hard, wear-resistant surface. Cams that are case
carburized will usually require grinding after the hardening operation because of distor-
tion during the heat-treating process. The nitrided materials offer the advantage of
much less distortion during nitriding and therefore can be used in the as-nitrided condition
without additional finishing. This is very helpful for large cams with small cross
sections where distortion can be a problem. Since case depth for nitriding is limited to
approximately 0.020in, case crushing can occur if the load is too high. Selection
of the nitriding process requires careful consideration of cost because of the long time
involved in the case formation. A very hard case having a hardness about Rockwell
C 70 ensures excellent wear resistance. Nitrided parts have good corrosion resistance
and improved fatigue properties. Nitriding follows the finish-machining and grinding
operations, and many parts can be nitrided without distortion. AISI 8620 steel (die
steel) is an excellent choice for an accurate cam under heavy loads and endurance at a
reasonable cost. With 8620 steel, an inherent core hardness of approximately 36
Rockwell C and a surface hardness of 60-62 Rockwell C or higher is achieved. It is
an oil-quench material that has less heat-treating distortion than a water quench and
less danger of cracking. It is sometimes carburized. In general, the case depth of
case-hardened materials varies between 0.030in and 0.060in to be slightly below the point
of maximum shear stress.
AISI 4140, 4150, 4320 (usually carburized), 4340, and 4615 steels have been chosen
as cam materials. AISI 4140 is particularly selected as it can be (a) flame hardened, (b)
through hardened Rockwell C 52-54, or (c) nitrided. The nitriding process produces a thin,
very hard surface Rockwell C 70 about a few ten thousandths of an inch thick. This
very hard thin case ensures excellent wear resistance. Below that surface is a case depth
0.020 to 0.035in with a hardness Rockwell C 52 to 54. AISI 4140 is plate or bar stock
and provides for good machinability and hardening properties.
The low- and medium-alloy steels have a limited operating temperature above
which they begin to lose their hardness and strength, usually around 300°F. Above this
temperature, the material is tempered and surface pitting failures or scoring will occur.
The generally accepted minimum hardness required at higher operating temperature is
Rockwell C 58.
In rare cases, Tungsten Carbide Stellite and AISI 52100 have been chosen as the
cam material or as an insert to the cam at high points of undesirable wear. Also, although
hardened tool steel is a satisfactory choice as a cam material, here the selection is either
an oil-quench steel or air-hardened tool steel. The air-hardened steel is best, giving less
distortion under heat treatment.
Stainless steel 360 and 440 C have been utilized for medical purposes. Nitriding of
stainless steels is sometimes employed.