Capacity and Load limits
IntroductionSelection GuideEngineering DataInterchangeCalculate Your SavingsReduce Your inventoryBrochure
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.
continue
on
|
