Help with abrasive wear

[qmechanics]

Hi,

Recently had an incident where a brass motor casing rubbed against an airframe made of 4130 chromoly. The chromoly is tubing of OD .765 inches. tubing is .047 thickness. The wear mark is approxiametly .006 to .010 inches deep an area about the size of a drop of water. I wonder how much this has weakened the chromoly? I've since removed the motor and there is no additional wear occuring. The load on the airframe at the area of wear is around 60lbs shear. Not constant. Any help would be appreciated. I've searched around but cannot find an answer. Thanks.

 

[iainuts]

Hi qmechanics,
The size and depth of the spot don't seem very significant to me. If I look at how much material was removed, the area of the cross section only reduces by about 1% and wall thickness of a tube can vary by as much as 12% from the mfg. So from a purely static stress standpoint, I can't see the wear you found being signficant.

However, if the wear creates a stress concentration, that might be a different story. If there's a sharp cut in the tube, and if the stress is high enough, there's a chance of a crack initiating at that particular location.

If the spot on the tube is a sharp one, I'd suggest grinding it out very lightly to make it a shallow dip with no sharp corners. A very gradual slope around the ding would be much better than a sharp cut. Also, use a fine grinding stone that leaves a smooth finish. The better the surface finish, the better it's resistance to fatigue.

The second concern is for corrosion. I'd assume the tube has some kind of corrosion protection which should also be repaired following any grinding done on the spot.

 

[berkshire]

qmechanics (Computer)
You are double posting. Since you indicated in your other post, that you thought this part came from the factory like that. Why don't you check with the manufacturer to see if there was a disposition report for a QAR on that part.
B.E.

 

[qmechanics]

Thanks for the information iainuts!