Bearings - Fixed vs Floating End 1

[jdogg05]

Are there any instances of gearboxes, pumps, motors, etc. where the concept of a fixed and floating shaft end do not exist/are not implemented? How common is this approach to shaft design and bearing specification?

 

[tbuelna]

jdogg05-

I have seen many examples of electric motor shafts, pump shafts and turbocharger shafts that are mounted using only a pair of angular contact bearings. The angular contact bearings are given a controlled axial preload to remove radial clearance, improve load distribution among the ball complement, and to help prevent skidding of the balls in high dN applications.

As for gearboxes using a bearing system where both shaft ends are constrained axially, the most obvious examples are the R&P gear bearings in a typical RWD auto/truck rear axle. These gear shafts are often mounted using only a pair of tapered roller bearings which are rigged to eliminate virtually all axial free play.

The examples I noted only work when there are no operating conditions that would significantly affect the bearing clearance/preload, such as CTE mismatch between the shaft and housing structures. When bearing systems are tightly constrained axially at installation, even small dimensional changes during operation can easily overstress the bearings.

Hope that helps.
Terry

 

[Tmoose]

Modern rear axle pinion and carrier are set with slight preload (negative clearance). So tight that a torque limit of a dozen or so inch-lb/lb-inch of drag is the measure of proper adjustment.

Even in some of the examples that would appear to have the bearings fully constrained axially with zero clearance or even preload there must be flexibility of the bearing housing to limit preload going sky high. Maintenance procedures for some car/truck axles include using a "spreader" to expand the diff housing significantly, maybe up to 0.015 inch.
Figure 3-2 here -

http://www2.dana.com/pdf/AXSM-0053.pdf

So, sometimes the expansion that would generally require a fixed/floating bearing arrangement is not really ignored, it is accommodated by flexibiity provided other means.