I am now writing about bearing fatigue life. It is what really determines what size bearing is required in most cases since that is what normally causes a bearing to fail (excluding unusulal causes such as lube failure, crashes etc).
As I recall when I was at university the mechanical design office had a SKF bearing manual where there was the formula for calculating the effective load on a bearing subject to both axial & radial load. In fact my recollection was that it also applied for simple radial loads too. That formula included a factor to account for whether it was the inner or outer race that was stationary. We were also issued with personal FAG bearing catalogues which I still have. It also includes the bearing life calulation method. I was surprised that it makes no mention about which race is rotating. So I went to Expensive's SKF link expecting to find the formula as i recall from my varsity days - but the on-line formula is the same as the FAG formula. http://www.skf.com/portal/skf/home/prod ... ink=1_0_25
Radial bearings are often subjected to simultaneously acting radial and axial loads. If the resultant load is constant in magnitude and direction, the equivalent dynamic bearing load P can be obtained from the general equation
P = XFr + YFa
P = equivalent dynamic bearing load [kN]
Fr = actual radial bearing load [kN]
Fa = actual axial bearing load [kN]
X = radial load factor for the bearing
Y = axial load factor for the bearing
An additional axial load only influences the equivalent dynamic load P for a single row radial bearing if the ratio Fa/Fr exceeds a certain limiting factor e. With double row bearings even light axial loads are generally significant.
So I did a bit of Googling and found the formaul that i recall from my varsity days at manchester University. http://cfd.mace.manchester.ac.uk/twiki/ ... ngs%29.pdf
Equivalent combined radial load
For combined radial and thrust loads
P = V X R + Y Ft
P = equivalent radial load (lb)
R = actual radial load (lb)
Ft = actual thrust load (lb)
X = radial factor (usually 0.56)
V = 1.0 for inner race rotating
= 1.2 for outer race rotating
Thrust factor Y is obtained from Table 20.2
Emphasis adeed by me.
I stress that that is the fatigue life calculation but it does include a factor which accounts for the extra fatigue caused by external race rotation causing the balls to travel further per rev thus causing more fatigue loads.
I am very surprised that this seems to be news here & it is even doubted that the distance travelled by the balls is defferent depending on which race rotates.