Page 282 - Cam Design Handbook

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270 CAM DESIGN HANDBOOK

Soft metals are bronze, cast iron, and cold-rolled steel. Soft cams require the same
degree of manufactured tolerance and accuracy as hardened cams. Profiles are milled and
good surface finish is required.
The most widely acceptable metal characteristic reducing the tendency to gall is self-
lubrication. Self-lubrication is exemplified by porous powder metals (oil-impregnated),
gray cast iron, Meehanite, ductile cast iron, graphitic steels (graphite-lubricated), and
leaded bronze (lead-lubricated). These materials suffer less damage when galling takes
place. Other materials with much sliding and high loads may have their wear life increased
by the frequent addition of various coatings and electrographite especially during run-in
periods.
9.4.5.1 Plastics. There has always been a need for lightweight, low-cost cam material
for light-duty applications. In the past, cams were made from phenolic-resin–impregnated
cloth. However, in recent years, with the development of new polymers, many cams are
made of various plastic materials. Table 9.4 lists plastic materials used for molded cams.
The most common molded plastic cams are made of acetate and nylon resins. These mate-
rials are limited in strength, temperature resistance, and accuracy. The nylon and acetate
2
resins have a room-temperature yield strength of approximately 10,000lb/in . This is
2
reduced to approximately 4000lb/in at their upper temperature limit of 250°F. Nylon resin
is subject to considerable moisture absorption, which reduces its strength and causes con-
siderable expansion. Plastic cams can operate for long periods in adverse environments,
such as dirt, water, and corrosive fluids, where other materials would tend to wear exces-
sively. They can also operate without lubrication or can be lubricated by the processed
material as in the food industry. Plastic cams are run in combination with a steel roller
follower to give dimensional control and quiet operation.
Polyimide is a more expensive plastic material than nylon or acetate resin, but it has
an operating temperature limit of approximately 600°F. This makes polyimides suitable
for many adverse applications that might otherwise require metal cams. Polyimides can
be used very effectively in combination with a metal follower without lubrication because
of polyimide’s good sliding properties.
9.4.5.2 Bronze Alloys. Several bronze alloys are used for cams. Most of the bronze
alloys contain varying amounts of tin, zinc, manganese, aluminum, phosphorus, silicon,
lead, nickel, and iron. The bronze alloys are most often used in combination with a steel

TABLE 9.4 Properties of Plastic Cam Materials
Property ASTM Acetate Nylon Polyimide
Yield strength, lb/in 2 D 638 10,000 11,800 10,500
Shear strength, lb/in 2 D 732 9,510 9,600 11,900
Impact strength (Izod) D 256 1.4 0.9 0.9
Elongation at yield, percent D 638 15 5 6.5
Modulus of elasticity, lb/in 2 D 790 410,000 410,000 460,000
Coefficient of linear thermal D 696 4.5 ¥ 10 -5 4.5 ¥ 10 -5 2.8 ¥ 10 -5
expansion, in/in◊°F
Water absorption (24h), percent D 570 0.25 1.5 0.32
Specific gravity D 792 1.425 1.14 1.43
Temperature limit, °F — 250250600