Causes of shaft 'walking' axially in Bearing

[pmartinson]

First of all: I realize this a bit outside of the main scope of the Bearing design Forum, but couldn't find one that was more fitting. Suggestions are welcome. Now then, onto the good stuff:

We're developing a belt-driven mobile robot chassis, and have run into some problems with one of the prototypes.

There are a total of 6 drive axles which the wheels are coupled to, with 3 per side. Each side is coupled by a pair of timing belts, and a 3rd belt transfers power from the motor & gearbox to the rearmost axle on either side.

We're experiencing a problem in the rearmost axle on 1 side.
When running in the forward direction, the shaft starts to translate axially towards the outside of the chassis. When running in reverse, the shaft translates in the opposite direction.

The axial forces are enough to deflect the plate meant to constrain it axially, and a quick analytical calc got us 260 lbs of force applied at the shaft's centre to cause the deflecction we've seen. At this amount of translation, the shaft is no longer making contact across the full length of the bearing's inner race (in extreme cases it was only making contact with 1/2 the length), and this is causing abrasion inside the bearing, and visible wear on the inner race as well as the axle.

We believe that this is a function of the high tension in the belts, which may cause the shaft to deflect slightly. This deflection results in an axial component of the tension. We have no tools on-hand to accurately measure for shaft misalignment.

If anyone has some suggestions for ways of measuring the misalignment, or some possible other causes for the misalignment or the translation itself, they'd be greatly appreciated.

Thanks,

Patrick

 

[MikeHalloran]

Timing belts shouldn't need static tension to transmit power.
Consider reducing your center distances a bit.
At least work out the tolerances on the assembly.




Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

The bearings are a "slip fit" on the axle? They Shouldn't be.

Does the axle shift if pushing the robot forward and backward, or only under its own power?

Dan T

 

[MikeHalloran]

How about a photo of the assembly?



Mike Halloran
Pembroke Pines, FL, USA

 

[caoimhin1]

The fit on the shaft depends on whether it is outer ring rotation or inner ring rotation. With a wheel it could easily be outer ring rotation meaning a slip or transition fit on the shaft. Could you not design in circlips either side of the bearing inner ring to constrain the shaft axially?

 

[pmartinson]

Thanks for the responses, guys.

Mike:
The belts were not meant to be as tight as they are, and this was due to under-tolerancing their positions (gotta love prototypes...)
Unfortunately, can't share pictures at this time.

Tmoose:
The bearings are slip fit, as they need to be removed sporadically to access some of the innards of the chassis.

caoimhin1:
Bearings sit at both ends of the shaft, which is stepped to constrain them. The shaft had so much axial force that it deflected the plate which housed the outer bearing. Couldn't add circlips to both sides of one bearing.


We did wind up finding a way to stop the walking, by replacing the plate and outer bearings. Some theories are that the outer bearing was misaligned, or possibly the outer plate had actually deformed plastically.
Seems we've put out the fire on this one, but I'm still very curious about this...

 

[MikeHalloran]

Absent photographs, the symptoms suggest that the chassis is deflecting because of the tension in the drive belt from the motor to the end axle. Which might lead me to further conjecture that the chassis is typical of light robot structures, i.e. made of sheet metal in such a way that it's flimsy, even for sheetmetal.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

I picture (maybe incorrectly) the shaft is driven, and the wheels and sprocket9s) are locked to the shaft. If that is so, the axle will creep inside the slip fitted bearings due the tiny diameter difference and directional load, and the shaft and inner race ID will both wear over time. If the service life is short enough, maybe it does not matter.

 

[electricpete]

Below is an excerpt from Machine elements : life and design / Boris M. Klebanov, David M. Barlam, Frederic E. Nystrom which describeS the phenomenon you are seeing, I think

ISBN 0-8493-9563-1 (alk. paper)
2.1.2 Interference Fit Connections (IFCS) LOADED WITH BENDING MOMENT

Deformations and variations of the surface pressure in IFC when the shaft is loaded with a bending
Moment are represented in Figure 2.3a and Figure 2.3b. When the shaft is bent, the pressure between
the shaft and the hub decreases in the tensioned area (from above), with the shaft slipping out of
the hub in this area. In the compressed area at the bottom, the contact pressure rises. The shaft in
this area would like to slip into the hub, but the increased contact pressure (usually) prevents the
slippage. When the connection rotates with respect to the vector of the bending moment, the tensioned
area (that went out of the hub) moves around the circle. During one turn the entire shaft moves out
of the hub by a tiny increment. But as the shaft continues to rotate, the micromovements accumulate
to cause a macrosized shift in its axial position within the hub. This process is called self-pressingout
(without any axial force applied to the connection). The shaft shown in Figure 2.3b tries to move
out of the hub in both directions with the same force, so the entire shaft remains in place. However,
at the ends of the connection, the surface layers of the shaft are eventually stretched outward. This
causes tension stresses in these areas, which bring down the fatigue strength of the shaft.

In a connection with an asymmetric load, in which the bending moment on one side is much
greater than on the other (Figure 2.4), the shaft strives to go out of the hub toward the larger bending moment. If the bending stress doesn’t exceed a certain limit, the shaft is held in place by the friction forces in the connection, so the self-pressing-out is prevented. But if the bending moment is large enough, it is possible for the shaft to “self-press” itself completely out of the hub. Such a failure mechanism can be demonstrated by pulling out a cork from a bottle; if the end of the cork protrudes from the neck of the bottle, you may easily get it out by just bending it from side to side.

You can see the figures/discussion at the bottom of the page here

http://books.google.com/books?id=B0...ations of the surface pressure in IFC&f=false

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(2B)+(2B)' ?

 

[electricpete]

As mentioned by Tmoose, tighter fit would help prevent this effect

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(2B)+(2B)' ?

 

[diskullman]

shaft collar?

Russell Giuliano