Page 281 - Cam Design Handbook

THB9 9/19/03 7:26 PM Page 269

CAM MATERIALS AND LUBRICATION 269

Solution Equation 9.13
k
P ¢ =
1 1
+
r 1 r 2
From Table 9.3 K = 5500, r 1 = 0.5in, r 2 = 2.0in
Therefore
5500
P 2 ¢ = 1 1 = 22001b in
+
.
05 2
which gives an allowable load
P ¢ = 2200 ¥ 3 4 16501b.
=
Note that after 100 million cam cycles the roller will wear out first since it has more
cycles than the cam.

9.4.5 Materials Selection
A wide variety of materials are available for the cam designer; see Table 9.3. Depending
on the application, the designer may choose from materials such as plastics; bronze; gray
cast iron; nodular and malleable iron; Meehanite iron; a range of low-, medium-, and high-
alloy steels; and stainless steels. In addition, there are different ways of modifying or pro-
cessing the materials to improve their properties or to reduce the cost of manufacture
(Zaretsky, 1997). These include reinforcements for plastics, processing for titanium, and
hardening for many of the iron-based materials.
When selecting a material for an application, the cam designer will first determine the
requirements for the cams being considered. The design requirements for a cam in a given
application will depend on such things as accuracy, load, speed, material, and noise lim-
itations. The more stringent these requirements are, the more costly the cam will become.
Some of the considerations in the choice of a material include allowable hertzian stress,
wear resistance, impact strength, water and corrosion resistance, manufacturing cost, size,
weight, reliability, and lubrication requirements. Steel, under proper heat treatment, meets
most of these qualifications for roller followers, while cast iron is excellent for sliding
followers.
Furthermore, these tests reveal that the rate of stress cycling has no significant effect
on the surface wear of metals. This is not the case for nonmetallic materials with low heat
conduction properties. Under these conditions, high speed was found to cause blistering
or excessive material in test specimens.
Also, although surface endurance usually increases with hardness in a given material,
hardness alone is a dangerous criterion for comparing different materials. For example,
SAE 6150 steel at 300 BHN shows a surface endurance strength only slightly greater than
the much less expensive and easier to machine SAE 1020 at 130 BHN.
Nevertheless, it is generally an established practice to harden materials to resist
wear. Although the surface hardness of a material is not always the sole criterion for wear
resistance, it is not possible to abandon hardness as an important factor. We shall outline
in the following subsections the commonly used cam materials and their surface treat-
ments, starting with those materials with the least hardness.