Capacity and Load limits

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Evaluation of the expected service life and limit loads of cam followers is more complex than with housed bearings. In addition to the static and dynamic capacity of the rolling elements, outer ring deformation, track capacity, and cam follower stud bending stress must be considered. In yoke rollers, the pin shear stress must be considered.

Although RBC lists the static bearing capacity for comparative purposes, in most cases the applied loads will be limited by stud strength, pin strength, or track strength.

2.1 Capacity of Rolling Element Bearing

Equations for static and dynamic capacities of roller bearings are given in ANSI/ABMA Standard 11. The more recent revisions leave it up to the manufacturer to introduce factors which account for internal design features and operating conditions. For cam followers and yoke rollers RBC has chosen to apply a conservative rating system, so a direct comparison with capacity figures of competitive products may not be possible.

2.2 Track Capacity

Track capacity is that load which a track subject to a uniform contact stress can withstand without excessive plastic deformation. It is directly related to track hardness. The published track capacity is based on a hardness of HRc 40. For other track hardness values the track capacity must be modified with factors from Table 1.

Track Hardness
[HRc]
Material Strength
[psi]
Modification
[Factor]
26
128
.45
32
146
.61
36
165
.76
40
182
1.00
44
204
1.24
47
229
1.50
50
247
1.78
53
266
2.09
56
281
2.42
58
298
2.78

Table 1: Track Capacity Adjustment Factors

Alternatively, contact stress can be easily calculated and compared directly to the strength of material. The equation for the Hertz contact stress between a cyldrical cam follower outer ring and a flat steel track is given by "Roark, Formulas for Stress and Strain" as:

[psi]

(1)
where   

F = radial load [lbf]
leff = length of outer ring contact [in]
D = outer ring diameter [in]

It can be shown that for infinite life the ultimate tensile strength of track and roller must be at least equal to the maximum contact stress .

Example 1:
Determine the required minimum track hardness for an RBC cam follower S 56 L operating under a 3000 lbf radial load.

Solution:

Referring to Table 1, 149,800 psi is between 146,00 psi (HRc 32) and 165,000 psi (HRc 36). Interpolation yields minimum track hardness of HRc 33.

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