Rod End Bearings - Maintenance?

JonJay said:

An Xray would be the only non destructive way that I know of and you would need an expert to read the result. If you took them to someone who operates a Tig welder professionaly, they probably know what they are doing. It is not an easy skill to aquire.

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Gil Alexander has found that AN490 threaded rod ends are heat treated to Rockwell C25 to C30. While you may not have a Rockwell hardness tester in your shop, they are not that uncommon. Rockwell testing would be considered "destructive" in some cases, depending on the test specimin. During testing a small carbide ball or diamond indenter is pressed into the test sample. I doubt this would be considered destructive in the case of a threaded rod end.




I'm anxious to see the NTSB Material Lab report on the Velux crash because the failed rod end was probably tested for Rockwell hardness. If the part was welded and quenched improperly there could be a significant change in the hardness (brittleness) of the part. Additionally, Rockwell testing only determines surface hardness. Perhaps the NTSB used other testing methods.

AN490's are made from 4130. Rockwell 25C to 30C would put tensile strength in the 120,000 to 135,000 psi range. For comparison, normalized 4130 is about 97,000 psi and 13C Rockwell. Annealed is down around 81,000 psi.

Ron is correct is saying certain changes take place when we weld an AN490. The welding itself is not the big deal; the important point is the rate at which the part is cooled. Slowly is what we want, in warm, still air or warm dry sand. The predictable result is a slightly softer, more ductile part, not quite as strong as the original. The entire range of possibility is the original 125,000 psi down to fully annealed at 81,000. Fully annealed isn't likely.

The ultimate stupid welding trick is to dunk the hot part in water or blow it with compressed air. Rapid cooling is makes the part much harder (and stronger), but very brittle and much more subject to surface flaw.

Here's a strength vs elongation graph (credit Univ. of Vermont) for three 4130 samples, annealed, water quenched, and tempered after water quench.



The quenched (cooled quickly) sample is about 3x stronger than the annealed sample. However, the tight little hook at the top means when it reaches its maximum, it snaps without stretching, bending or deforming. The annealed sampled stretched and stretched before it finally gave up. Nice behavior for an airplane part.

More later, gotta go to work.

There are many commercially available....

az_gila said:

...you are referring to are AN490's (the ones Vans calls out) then your statement is not quite correct.

They come heat treated to Rockwell C25 to C30 which puts them at a strength well above "normalized".

However, in our pushrod application, I'm sure the loads are not that high.
I would think that a Mil-Spec application would call for a heat treat of the finished welded parts assembly to retain full strength.

Is there another part number you are referring to that is "weldable"?

AN490 spec here --

http://www.gen-aircraft-hardware.com/images/pdf/Rod_End_Terminals.pdf

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But your right to stick to the part most relevant. I will edit my post to say "You will lose little strength.....". How's that?

JonJay said:

An Xray would be the only non destructive way that I know of and you would need an expert to read the result. If you took them to someone who operates a Tig welder professionaly, they probably know what they are doing. It is not an easy skill to aquire.

Click to expand...

An X-ray would tell you if the weld joint is good, that it is free of voids, but it won't help to determine if the material properties have changed.

If one is concerned that there is a loss in the load carrying ability of either the welded materials or the threaded portion I would suggest a destructive test. Have a third push rod welded, and then put the assembly in a tensile tester and do a pull test to ultimate failure. This would definitely answer whether or not the material properties of the materials have changed and what the yield and ultimate loads are.

Just my 2 cents.

Wow. Special cooling down process. X-rays?

Instead of the weld why not just use the rivets.

Funny

Ron Lee said:

Wow. Special cooling down process. X-rays?

Instead of the weld why not just use the rivets.

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I think the point was that welding is ok if you know how to weld. Riveting is ok if you know how to rivet. Van's gives you both options. Neither one is wrong but both need to be done right.

Ok, let's assume our builder is conservative and careful, the sort who welds and cools slowly. The result will be AN490 material good for somewhere between the spec 125,000 psi and a reduced 100,000 psi....quite predictable.

The short RV aileron pushrod is a 1/4-28 thread, so stress area at the thread minor diameter is .0364 sq in. Strength of the new AN490 is 125,000 x .0364 = 4550 lbs. The softer welded one will be 3640 lbs. Even if you managed to get material strength all the way down to the annealed level at 81,000 psi, the AN490 would still be good for almost 3000 lbs.

"Ahhh..." you say, "..3000 lbs is much less than 4550! Welding must be bad!"

Ok, fine, let's install the AN490 with rivets. An 1/8" AD rivet is rated at 388 lbs single shear in all the books, so 388 x 4 = 1552 lbs. The actual material spec is 26,000-28,000 psi in shear. Shear area is .01226 sq in, so the calculated strength is not much more than 340 lbs x 4 = 1360 lbs. Hmmmmm....

Now let's look at the pushrod itself, 23-13/16 long, .500" dia, .032" wall. Strength in tension would be about 4850 lbs, but that doesn't matter much because the limiting factor for any pushrod is stiffness in compression. In this case the pushrod will bow and buckle at about 700 lbs.

Of course that isn't a problem because the maximum applied load is 250-300 lbs.

Kinda gives the debate a new perspective, yes?

Back to the original question...

So for those who use TriFlow, is the regular version or the Dry Lube version better for rod end bearings? And, is one more appropriate for rod ends in a dry environment (inside the wing or fuselage) vs. those exposed to the weather and blowing dust (such as the aileron, elevator, and rudder rod ends)? The version that comes in aerosol form seems the most convenient, but is unavailable in the dry lube version

Regular (available in drip or aerosol):
http://triflowlubricants.com/Superior_Aerosol/superior_aerosol.html

Dry Lube Version (available in drip bottle only):
http://triflowlubricants.com/Dry_Lubricant/dry.html

I use spray white lithium grease

I've been using spray white lithium grease from NAPA for years. It dries into a wax like substance, seems to work very well. I think I heard about it on this site a long time ago....