Page 283 - Cam Design Handbook

THB9 9/19/03 7:26 PM Page 271

CAM MATERIALS AND LUBRICATION 271

TABLE 9.5 Properties of Bronze Alloy Cam Materials

Material Modulus of elasticity, Yield strength, Ultimate strength,
6
10 lb/in 2 lb/in 2 lb/in 2
Phosphor bronze 15 40,000 75,000
Aluminum bronze 19 50,000 100,000
Manganese bronze 16 45,000 80,000
Silicon bronze 15 30,000 60,000
Nickel-tin bronze 15 25,000 50,000
Leaded bronze — — —

roller follower to give good wear and load capacity, especially where there is a high sliding
component.
Bronze cams are also used where corrosion and water are a problem. The bronze
alloys listed in Table 9.5 are aluminum bronze, manganese bronze, silicon bronze, leaded
bronze, or phosphorous bronze. These bronze alloys have yield strengths ranging from
2
20,000 to 60,000lb/in and all have good machinability. Bronze cams are excellent
for shock, noise reduction, inaccuracies in the fabricated contour, excessive inertia and
wear due to frequent start and stop actions, and special contaminants in the lubricated
operating system.

9.4.5.3 Cast Iron. Cast iron is used for cams because of its low cost, good machin-
ability, and moderate mechanical properties. Many cam applications use cast iron because
of its good sliding and wear properties, which are in part a result of the free graphite and
porosity. The three basic cast irons are distinguished by the structure of graphite in the
matrix of ferrite: (1) gray cast iron, where the graphite is in flake form; (2) malleable cast
iron, where the graphite consists of uniformly dispersed, fine, free-carbon particles or
nodules; and (3) ductile iron, where the graphite is in the form of tiny balls or spherules.
The malleable iron and ductile iron have more shock and impact resistance. The cast irons
can be heat treated to give improved mechanical properties. The bending strength of cast
2
iron ranges from 5000 to 25,000lb/in and the surface fatigue strength ranges from 50,000
2
to 115,000lb/in . Nodular iron, chilled cast iron (heat treatable) and Meehanite are mate-
rials in the categories indicated. Both chilled cast iron and Meehanite have good shock
resistance and good vibratory damping; both are attributes that help reduce noise. In many
drives, a cast iron cam can be used to replace a bronze cam at a lower cost because of the
better sliding properties of the cast iron.
Although the class 20 gray irons have a low endurance limit, classes 30, 45, and others
have shown considerable advantages over free-machining steel. Austempering of Mee-
hanite or class 30 iron approximately doubles surface endurance strength over untreated
iron of similar hardness and thus is well suited for cams, probably because of the fine grain
structure, excellent dispersion of graphite, and conversion of retained austenite.
The cam is supported by bearings on a frame generally of steel or cast iron. A frame
of cast iron is usually a first choice because it provides greater rigidity and vibratory
damping in the performance of the moving parts. The frame should be as heavy and rigid
as possible.

9.4.5.4 Hardened Steels. A large variety of iron or steel alloys are used for cams. When
operating conditions are moderate, such as medium loads and ambient temperatures, a