airframe wear concern
airframe wear concern
(OP)
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.
I can't find any information on this. Any help would be appreciated.





RE: airframe wear concern
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.
RE: airframe wear concern
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??)
RE: airframe wear concern
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.
RE: airframe wear concern
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.
RE: airframe wear concern
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
RE: airframe wear concern
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.
RE: airframe wear concern
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 !?)
RE: airframe wear concern
RE: airframe wear concern
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.
RE: airframe wear concern
RE: airframe wear concern
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.
RE: airframe wear concern
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)
RE: airframe wear concern
RE: airframe wear concern
RE: airframe wear concern
RE: airframe wear concern
Thanks guys!
RE: airframe wear concern
RE: airframe wear concern
After all it's been there at least 5 years....on the other hand that's 5 years of vibration?? thanks guys!
RE: airframe wear concern
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
RE: airframe wear concern
The depth:length ratio is 1:31.5 where would that put the stress concentration? Last night I went over it very carefully....it's does look like someone had been trying to blend the spot in. It's very shallow only about .006 or less. the spot is elliptical major axis about 3/16 of an inch...maybe a bit longer. It's very difficult to measure because of the location. OD is .764 and thickness .047 inches.
No info from OEM available. Sure looks like someone was determined to fill it in and paint over it. The sanded spot came off in one piece....left rough edges of paint around the wear mark. I'm thinking this happened at the factory during initial installation. There's no evidence the frame has ever been repainted...Hmmm. Interesting. Thanks guys.
RE: airframe wear concern
jetmaker
RE: airframe wear concern
RE: airframe wear concern
As for Peterson, it is in the notched bar section. It is a chart which shows that the variation in angle of the notch (v-notch vs U-notch). Unfortunately, I do not have a copy of Peterson available at this time. If you can not find it, I'll try and get the book.
jetmaker
RE: airframe wear concern
RE: airframe wear concern
RE: airframe wear concern
I computed the angle to be 178.645...
which corresponds to a stress concentration factor
of 1.05
RE: airframe wear concern
Can't get the manufactures specs jetmaker.
Did all the steps b109g suggested in an earlier post. The manufacture is not forthcoming with data.
Not much more I can do short of replacement. I have to add that if I do that it's likely to do more damage. If the attachment member is removed and a new one welded in place the new part will have to be exactly where the original one was. Off by as little as a few mils and the tailcone will not go back on. It's that tight. That would require a whole new tailcone or the replacement of the last bay then re-rigging the drive shafts etc. Could take months.
RE: airframe wear concern
i don't think you need to install a s/gauge and collect real data.
you think the damage won't get worse with continued ops ('cause you think it is original); that'd be my main concern.
RE: airframe wear concern
wear mark could have been made. There's nothing there to interfere...not on this helicopter. I think this frame may have come from a different ship that was at the factory being overhauled. Who knows? This isn't the first time something strange like this has happened. They had a crew grinding the shot peen off grips because the crew thought it was a defect. Thanks again for all your help!