Page 95 - Rollingbearings

Size selection based on rating life
Considerations when Geared transmissions Belt drives
calculating equivalent With geared transmissions, the theoretical When calculating bearing loads for belt driven
dynamic bearing load tooth forces can be calculated from the applications, “belt pull” must be taken into
power transmitted and the design charac- consideration. Belt pull, which is a circum-
For the sake of simpliication, when calculating teristics of the gear teeth. However, there ferential load, depends on the amount of
the load components for bearings supporting are additional dynamic forces, produced torque being transmitted. The belt pull must
a shaft, the shaft is considered as a statically either by the gear, or by the input or output be multiplied by a factor whose value depends
determined beam resting on rigid, moment- shaft. Additional dynamic forces from gears on the type of belt, belt tension and any
free supports. Elastic deformations in the can be the result of pitch or form errors of additional dynamic forces. Belt manufacturers
bearing, the housing or the machine frame the teeth and from unbalanced rotating usually publish the values. However, should
are not considered, nor are the moments components. Gears produced to a high level information not be available, the following
produced in the bearing as a result of shaft of accuracy have negligible additional forces. can be used:
delection. These simpliications are necessary For lower precision gears, use the following
if you are making bearing arrangement cal- gear load factors: • toothed belts = 1,1 to 1,3
culations without the aid of relevant computer • V-belts = 1,2 to 2,5
software. The standardized methods for • pitch and form errors < 0,02 mm: • plain belts = 1,5 to 4,5
calculating basic load ratings and equivalent 1,05 to 1,1
bearing loads are based on similar • pitch and form errors 0,02 to 0,1 mm: The larger values apply:
assumptions. 1,1 to 1,3
It is possible to calculate bearing loads • where the distance between shafts is
based on the theory of elasticity, without Additional forces arising from the type and short
making the above assumptions, but this mode of operation of the machines that are • for heavy or peak load type duty
requires the use of complex computer pro- coupled to the transmission can only be • where belt tension is high
grams (SKF SimPro Quick and SKF SimPro determined when the operating conditions,
Expert). In these programs, the bearings, the inertia of the drive line and the behaviour
shaft and housing are considered as resilient of couplings or other connectors are known.
components of a system. Their inluence on the rating lives of the
If external forces and loads – such as iner- bearings is included by using an “operation”
tial forces or loads resulting from the weight factor that takes into account the dynamic
of a shaft and its components – are not effects of the system.
known, they can be calculated. However,
when determining work forces and loads –
such as rolling forces, moment loads, unbal-
anced loads and impact loads – it may be Bearing size
necessary to rely on estimates based on
experience with similar machines or bearing
B.3
arrangements.

Diagram 6

Rotating load

Diagram 7
f m
1,00
Short time peak load
0,95
0,90
0,85
0,80
F max
0,75
0,70
0 0,2 0,4 0,6 0,8 1,0
F 1 Time
+
F 1 F 2

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