Jieve
Mechanical
- Jul 16, 2011
- 131
Hello,
A student of ours recently began disassembling a transaxle used in an off-road buggy, and after spending a little time taking a look at how it was designed, I had a few questions about the bearing setup.
As I understand it, when a torque is applied to a rotating shaft, the inner rings of the bearings should be press fit and the outer rings a clearance fit. Additionally, one bearing should be non-locating i.e. its outer ring should not be constrained axially to allow shaft expansion.
In this design, the outer rings of the ball bearings for each of the shafts are pressed fitted into the cast housing. The inner rings, as far as I can tell, are clearance fitted (actually it may be that one inner ring is press fitted also, since I can’t remove one end of each shaft and I don’t see any axial locating devices fastening the bearing to the shaft).
Each of the outer rings of the bearings are butted up against a shoulder in the housing, but nothing is retaining them axially from the insertion side. However, due to the shaft shoulders against which the inner rings of the bearings sit, after the entire thing is assembled, there would be no way for the shaft to move axially, or for the bearings to move axially.
I’m curious however, why the bearings were press fit into the housing and not onto the shaft. Wouldn’t this have a tendency to damage the bearing inner ring surface through that rotating load? Also, I’m not seeing where the expansion of the shafts is accommodated. I guess it is possible that there is slight axial play where the loose end of the shaft fits into the bearing inner ring, but I obviously can’t see through the housing with it assembled to verify. However, this doesn’t seem like an optimal design from what I understand. I had heard from a local machine part supplier that if a shaft is very short, such as these transaxle shafts, that axial thermal expansion is almost negligible and can be taken up by the bearings (in a similar design we calculated shaft expansion to be on the order of fractions of a mm).
What are your opinions about this? Is it common practice to neglect thermal expansion for such short shafts? Or maybe it is assumed that the transaxle fluid removes heat fast enough to keep that expansion minimized? And is this press fit arrangement simply for ease of assembly? It seems they could have just as well pressed the bearings on the shafts and left the outer rings floating.
Thanks for any responses!
A student of ours recently began disassembling a transaxle used in an off-road buggy, and after spending a little time taking a look at how it was designed, I had a few questions about the bearing setup.
As I understand it, when a torque is applied to a rotating shaft, the inner rings of the bearings should be press fit and the outer rings a clearance fit. Additionally, one bearing should be non-locating i.e. its outer ring should not be constrained axially to allow shaft expansion.
In this design, the outer rings of the ball bearings for each of the shafts are pressed fitted into the cast housing. The inner rings, as far as I can tell, are clearance fitted (actually it may be that one inner ring is press fitted also, since I can’t remove one end of each shaft and I don’t see any axial locating devices fastening the bearing to the shaft).
Each of the outer rings of the bearings are butted up against a shoulder in the housing, but nothing is retaining them axially from the insertion side. However, due to the shaft shoulders against which the inner rings of the bearings sit, after the entire thing is assembled, there would be no way for the shaft to move axially, or for the bearings to move axially.
I’m curious however, why the bearings were press fit into the housing and not onto the shaft. Wouldn’t this have a tendency to damage the bearing inner ring surface through that rotating load? Also, I’m not seeing where the expansion of the shafts is accommodated. I guess it is possible that there is slight axial play where the loose end of the shaft fits into the bearing inner ring, but I obviously can’t see through the housing with it assembled to verify. However, this doesn’t seem like an optimal design from what I understand. I had heard from a local machine part supplier that if a shaft is very short, such as these transaxle shafts, that axial thermal expansion is almost negligible and can be taken up by the bearings (in a similar design we calculated shaft expansion to be on the order of fractions of a mm).
What are your opinions about this? Is it common practice to neglect thermal expansion for such short shafts? Or maybe it is assumed that the transaxle fluid removes heat fast enough to keep that expansion minimized? And is this press fit arrangement simply for ease of assembly? It seems they could have just as well pressed the bearings on the shafts and left the outer rings floating.
Thanks for any responses!
