airframe wear concern

[qmechanics]

Just had an incident where a actuator motor housing had been rubbing against part of an welded (4130 chromoly) frame. I've since moved the motor but it left a wear mark about 1/8 inch wide and 1/4 inch long and .006 to .010 deep. the chromoly had an OD of .765 and is .047 thick. I'm wondering if this has substancially weakened the frame that the point of wear? There's about a 60 pound shear load on the frame at that point but it's intermittant.
I can't find any information on this. Any help would be appreciated.

 

[GBor]

I'm a little confused. You have a chromoly part that is of a circular cross-section, but you have identified the wear rectangular coordinate terms. If you are suggesting that you have worn a groove into your frame, the stiffness reduction is easily calculated. To say whether the strength reduction is "substantial" is a bit difficult.

If all you are concerned about is the shear load through the frame, the stress calculation is still P/A, where P is your load and A is now reduced by the area that is worn away. If it is deflection that you are concerned about, you need to recalculate the area moment of inertia based on the new cross-sectional parameters.

 

[rb1957]

wouldn't it be unusual for a tube to carry a shear load ?

doesn't a chrome-moly tube suggest hot environment, maybe an engine support strut ?

what are you doing to ensure that the wearing doesn't continue ? (maybe adding some tape on the tube to protect it?)

why do you think the wearing started in the first place ? (maybe something else is out of alignment??)

 

[qmechanics]

yes the chromoly is part of the engine mount. This is a helicopter and the engine mount also services as the tail cone mount. The actuator motor had moved due to vibration and had rubbed against the frame in such a manner as to be not immediately detectable. It was moved to eliminate the rubbing. The tailcone produces a moment about the mast. The location of the mount (and spot of the wear) is aft of the center of rotation putting a bending moment on the tubing at the point of wear. My concern is that the wear mark may have weakened the tubing enough to make it unsafe. There is not cracking or corrision...only the wear mark that's about .006 to .010 inches deep. It's a small mark approxiametly the size of an eraser. It's difficult to give exact dimension because of the location. I'm just wondering if this constitutes a significant loss of strenght.
There's only a total of 300 lbs of torque spread over 5 connecting bolts on five seperate chromoly tubing so at first I was not concerned....but it's best to be sure so I thougth I would ask. I did recalculate the forces on that particular spot and the forces are still well within specs but I'm unsure about what else a "wear" may have done to the metal. Again, any help is greatly appreciated.

 

[bf109g]

GM -

As a former airline liaison engineer, here's what I would do:

1. Check the structural repair manual for allowable damage limits on this component. If wear damage on this component is addressed, follow the SRM instructions and your're done.

If there is no SRM or if the component is not addressed within the SRM, accomplish the following:

1. Clean up any sharp edges, taking care not to increase depth.

2. Do a dye penetrant or HFEC inspection of the damaged area to verify it is free of cracks.

3. Look for the weakest part of the damaged component and back out the required bending strength. Compare that to what your damaged component can now withstand. Sounds like you've already done this.

4. Apply corrosion protection.

5. When you put it all back together, make sure the interference condition has been eliminated.

7. Since it sounds like this part is subject to fatigue loads, contact the manufacturer. Report what you've done. They may want repetitive inspections of the area.

 

[rb1957]

in addition to 3, i'd check the effect on the tube's column strength.

i'd suspect that 20% wear (0.01" is about 20% of the tube wall 0.047") is outside any SRM limits. this means you'll need an RDC/RDA to cover the damage.

in your favour the damage is very localised, but helicopter parts tend to have low margins

 

[qmechanics]

Thanks for the info. Can't find an SRM for this ship and FAA 43.13 does not have this either....at least not that I can find. There is no "dent" to repair this is a wear mark.
The material has been removed by rubbing. What's interesting is that I found it by accident. I was sanding the part for painting and wanted to "feather in" the paint so I expanded the region I was sanding. By sanding of the old paint I revealed this wear mark. Appearently it's been there for years. Nonetheless it is a wear mark. It's quite small and the mathematics show it's still well within stress limits. That is the applied force is approx 60 pounds with this part able to take 2654lbs. (accounting for the wear mark). However there are vibration issues not accounted for. Wish I could find an SRM or some "official" source for guidance. The manufacture gave rough guidelines
of around 6 thousands or so? Any other thoughts? Thanks again.

 

[rb1957]

ah, so someone painted over the wear in the past, and you've uncovered it ... poor you !

now that the damage is discovered, something has to be done with it (ie, someone has to be responsible for it). as you say, the SRM doesn't help. you've done some sums and figure it isn't a big deal, but are you in a position to issue an RDC for it (i don't think so). who's the inspector responsible for the work being done ? (possibly you are) if not, make them aware of it (before you paint over it). otherwise, i'd make the owner aware of it. (sometimes they can be reasonable !?)

 

[thruthefence]

I would think a " fishmouth" split tubing weld repair would be appropriate. I have seen several engine mounts repaired this way, over the years, from hot exhaust pipe damage & rubbing generator pulleys. Is this a certificated aircraft? If so, the old "been there for years" justification for no action could get you one day. If it's a homebuilt, well that's another story.

 

[rb1957]

i tought of that too, but thought it might be
1st, a ton of work for what sounds like a minor issue, and
2nd, much harder to justify/analyze, and
3rd, substantiating the welds, and
4th, more work than cutting out the tube (presumably it's part of a welded frame) and putting in a new tube.

 

[bf109g]

Any hope of finding this being previously addressed in a logbook or repair station release?

 

[qmechanics]

Thanks again for all the feedback. Yeah it's certified ship and dang if I had just not tried to be to careful I would never had seen the wear mark. It's real smooth and shiny and was covered up really good. It may have even come from the factory this way cause the paint job looked original....not like it had been touched up. Because of where it's located you have to remove:

Tail cone and TR drive shaft
Cooling fan housing
Cooling fan
Actuator assembly

Just to get to the frame where the wear mark is. You can't spray paint in there without removing all this stuff.....to many parts in the way. Can't put a split tube over it either because the actuator will hit that by making the OD to large. It's in a pretty cramped area. So my options are:

1-do nothing and monitor it
2-dye pen it just to be sure and monitor
3-tear the whole thing apart and cut out the old tube and weld in a new one. This particular piece of chromoly is maybe a foot long.

The tailcone has 5 attach points. The wear mark is about 2-3 inches aft of the attach point on the inside of the tube.
The bending moment is inward towards the wear mark not out ward away from it. There's approx 300 lbs force on the 5 attach points in a hover. That's 60 lbs per attach point.
The math works out OK but it does not account for vibration.
I can say without doubt that this wear mark has been there at least 5 years. It bothers me though. Thanks again for all the input. I do appreciate it.

 

[rb1957]

before embarking on option 3, i'd ...

ask myself if i think there's a real problem (doesn't sound like it)
ask the OEM for an opinion (hey, they might)
point out the issue to the owner and/or pilot
discuss the options with them (pretty much do nothing or rip the guts out and re-build)

 

[qmechanics]

Right. I'll do a dye pen then talk about it. Tearing the guts out for this seems like over kill...especially since this has been there for years with no issues...at least that I'm aware of. Would it help to rosette weld the spot? That would probably make it weaker? Thanks.

 

[rb1957]

i wouldn't weld it

 

[bf109g]

I agree with rb - don't put any heat on it.

 

[qmechanics]

Okey. Just spoke with an expert aircraft welder. He said the same thing. Leave it alone. Since it's smooth no stress riser exists. He said if you can catch your finger nail on it then replace the tube. A crack of notch would be a riser that increases the stress by 4 times! Holy cow...I didn't think it was that much. I'll do the dye pen and keep an eye on it.
Thanks guys!

 

[rb1957]

personally i'd be careful getting stress advise from a welder ... likewise i wouldn't ask a stressor for welding advise !

 

[qmechanics]

I'll dye pen see what the results are. With 75% of the material remaining I suspect it will be ok.
After all it's been there at least 5 years....on the other hand that's 5 years of vibration?? thanks guys!

 

[jetmaker]

One consideration is that if the blend is roughly 1:20 depth:length ratio, then the stress concentration from the blend is very close to 1. Therefore, the only change is stress is due to the reduction in net section.

Obtain the actual part OD and thickness using ultrasonic measurement, then check against the OEM drawing nominal dimensions. Most analysis is done on minimum or nominal dimensions. If your sectiuon area is still larger than that based on the nominal dimensions, then I would not be sweating it. Just document and record.

If the numbers do not work out in your favour, check to see if the stress based on the 60# is below the endurance limit of the steel. Some load factor need be applied for fatigue, but I am not sure what that should be. Maybe someone in this group can provide that info. If the stress is below endurance, then again don't sweat it.

As previously stated, dye penetrant the part at a minimum, and maybe eddy current or mag particle in addition to ensure no cracks exist. Not sure which method has the better accuracy.

jetmaker