armature shaft undercut function

[ColinScowen]

Hello everyone, I'm not sure if this is the correct forum for this question or not, but, here goes.

We had a failure recently where one of our armature shafts (small motor for hand held powertool) failed with rotational shear.

In the area of the failure, we have a stress relieving undercut (DIN509-E 0.4 × 0.2)between the thinner part of the shaft, diameter 11.8, where the insulating tube and the laminations are mounted, and the thicker part of the shaft, diameter 22.0 that provides the mounting point for the fan.
In the assembly process, if we get maximum material condition on the length of the insulating tube, and a minimum material condition on the dimension between the shoulder and the end of the shaft, we have the situation where the tube is pressed in to contact with the thicker area of the shaft. This essentially means that the undercut has been covered by the tube.

My question is, if I have that situation in the whole SA, is there still any stress relief happening, or has the connection between the shoulder and the thinner part of the shaft, due to the insulating tube, effectively cancelled any stress relief out?

If you need any further info, please let me know.
Colin.

 

[kingnero]

depending on what material the tube is made off (can it transfer a torque), the geometry of the part, how much (and how exactly) the tube is pressed against the thick part of the shaft, ...

a picture might tell us more but I'd doubt so at first glance.

 

[ColinScowen]

OK, the shaft is made from C45 steel, the insulating tube is 11.91mm ID, 14.40mm OD, fibre reinforced Glass filled nylon.

The assembly method is that we have the tube tapped with a plastic mallet in to one end spider, this sa then tapped, same hammer, in to the stack of laminations.
At this point, the laminations, end spider and tube are put on the base of the press. The armature shaft is then pressedinserted in to the moving head of the pressand then pressed down in to this sub assembly. The press is controlled by depth stops that limit the press depth, with the tube resting on a lip in the tooling on the base of the press. At the end of teh stroke, the meter on the press is reading around 14 tons.
It is because of this set up that we may have the possibility of the lamination tube assembly being pressed with significant force, in to the shoulder of the shaft. (I am reminded of the difference in strength of a cardboard tube when you stand on the side, vs standing on the end.)

Hope this helps a little. And hopefully, there should now be a pdf that helps to explain the question. This shows the state I am querying, not the optimal state.

Colin.

 

[desertfox]

Hi ColinScowen

If the steel armature shaft failed in the undercut area then the insulating tube isn't seeing a great deal of the external torque
applied and would suggest the load path is through the shaft.
I doubt the insulating tube as negated the stress relieve how long was the tool in service? was the tool being abused?
I would calculate the stress in the region of the undercut and consider the stress concentration within that region.
Finally as anyone done a fatigue analysis on the armature shaft?

desertfox

 

[BrianE22]

If the pressed-on tube is producing a tensile axial stress on the shaft you might see an effect on fatigue life.

 

[ColinScowen]

Desertfox,
tool was not in use for long, and the usage was a standard test that we run and which has never seen this failure before.
As I'm not a mechanical engineer by training, I will try to get one of my colleague's to help me with the stress calcs.
We have only gone as far as teh material analysis so far, management baulked at the expense of the fatigue analysis.

Brian,
when we took a detailed look at the manual part of the process, we did see that there may be times when, if the tube is not pushed all the way through the stack, that at the end of the stroke, the press is also pushing the tube through the stack, as well as the shaft through the tube.

We have also sectioned some shafts, and found on 1 of 3 that the undercut radius looked more like a chamfer.

Colin.

 

[dgallup]

Being plastic, I doubt that the insulating tube has any relevance in this failure. I think you are on the right track to look closely at the form and condition of the undercut. In high stress conditions, these are sometimes roll formed to put the material in a compressive stress condition.

 

[dhengr]

Colin:

It seems to me that both of what you call stress relieving undercuts, on the left and the right of the larger shaft diameter, are actually stress raisers. They squeeze the lines of stress down just before they would start to transition up into the larger diameter part of the shaft. I think they should both look like the one on the right, but they should be oriented so that one of the half circle’s side tangents flows from the smaller horiz. shaft surface, and 180° away from that point on the half circle, it will meet the vertical surface on the larger diameter at 90°, with a slight chamfer.

Another way of explaining my thoughts is to rotate the right half circular undercut, 90° counterclockwise, so it is actually cut into the vertical surface on the larger diameter shaft portion. The larger the diameter of this undercut half circle, the more effective it is as a stress reliever, because of its larger radius as it transitions up from the smaller shaft on the right. Alternatively, if this tangent/transition area can be elliptical in shape, again a larger radius leaving the smaller shaft surface, that is an improvement. Rolling this area to induce surface compression stress would be an improvement, as would polishing this region of the shaft.

I don’t think the plastic pressed on sleeve has much to do with your problem. If the tool had only been used for a short time before failure, it certainly wasn’t a high cycle fatigue failure. It might have just been a defect in that one shaft, but I still don’t like the orientation of your undercuts.

 

[dhengr]

Colin:

In my second paragraph above, I actually meant to say rotate the right half circular undercut, 90° clockwise, not counterclockwise, so it is actually cut into the vertical face/surface on the larger diameter shaft portion. And, I have some trouble understanding how you can possibly press that fiber reinforced glass filled nylon tube only 2.5mm wall thickness with 14 tons, without crushing it. If your press is reading 14 tons you are bottoming out, steel on steel, and possibly/likely inducing some shear yielding stresses at the shaft diameter transition. Without running any calcs. 28 kips plus is almost enough force to shear the large dia. portion for the fan right off the smaller shaft dia.