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Heim joint without big washer (1996 RV4 )?

Larry DeCamp

Well Known Member
My RV4 kit ( circa 1996 ) came with Heim joint studs that secure the flap actuator rod in 1/8" doublers on the last rib. There is no way to add a big washer to prevent the race from coming off the ball. The ball simply has a dimple in it like a lathe center locator for threading the stud.

A loose flap could be a problem, right ? Any comments or solutions appreciated.
 
Control linkage hardware can be among the most difficult little bits to fiddle with in cramped quarters. If you can get a bolt, 960 washer and lock nut in there, why not the standard-practice large washer? Maybe a photo would be helpful. I don't believe the dimple in the Heim body has anything to do with securing the race, but is an artifact of the manufacturing/threading process as you said. Others with far more knowledge will doubtless answer today and we'll all learn something.
 
If this is one of those studded ball deals, the stud can be pressed out and then you have a regular heim. I did this on my Rocket for the exact concern you have.
 
CM-4S?

Is it one of these CM-4S affairs, which are still used on the RV-7,8,9 flap attach pushrod ends.

Swap it out for a CM-4M and an AN4-nnnA bolt + washer stack.
 

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You guys saying 1 pulls right out of 2? Was it run dry for a decade? Which part wore enough to allow that?
 

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The stud is simply a close tolerance bolt which is swaged to the ball. No magic here. Apply enough force and the stud will release from the ball. Mine did not come loose in service, I simply wanted a nut and bolt there.

If you want to make it easier, just drill through the stud at the swage countersink and knock it free with a punch.
 
Every RV I've touched has used this stud style rod end on the flap linkage.
I have never encountered one that became loose.
My pre-flight walk around procedure is to visually check & shake the flap with my finger on that bearing before every flight.

If I ever encountered one that rattled even the slightest bit, that plane would be grounded till those linkage bearings were replaced. I'd be having a conversation with the owner/pilot about his maintenance practices too!
 
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Good input, but I will stir the pot a little.

So, for safety, van specifies large washers where they can be applied to keep the outer race from coming off the inner ball.
Yet, the kit is supplied with components that do not support the washer option, and Ralph has never seen one fail.
So, is it possible they are both acceptable, but conform to DIFFERENT material or design specs ?
 
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If you want to make it easier, just drill through the stud at the swage countersink and knock it free with a punch.

This is what I have done on two 4's and my 7. Very easy to do. Only takes a couple minutes to drill off the flare and drive out the stud. install with a bolt and large washer. Might need to enlarge the opening in the fuselage a bit to clear.

All of the RV's built on our field (over a dozen) have been done this way, ever since an MD-RA inspector pointed it out as potential issue years ago. Even though we have never heard of a failure of one of these ball ends we still do it just to be safe.
 
Or just get the correct heim joint

Very simple instead of modifying the ballpoint stud to just order the same one used on top of flap rod from VANS. On advise from a very well respected MDRA inspector that had made a " containment "arm out of aluminum I addressed this issue without molesting the heim joint . Vans designed the GREAT plane but this would be better in double shear and the rod end would need to be contained on a racecar. I don't know how to post pictures ...but Larry ..I sent you a PM
 
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Fair point to order a std. heim joint in a new build, but for those with the "studded" version on hand, it takes less time to press out the stud than to open the Van's website on your computer.
 
Good points made, and easy to accomplish for me on my “new” RV6 build since I’m at that point - and I have the correct replacement bearings that just require me to grind a bigger hole in my fuse for the extra bolt end/washer assembly. But I’m still curious about the company still recommending the screw mount bearing that they do. Is there any record of failure of this joint? It seems like, with more than 11,000 units flying with this configuration, there should be some data on failures if it’s a problem. Does anyone know if Vans has an opinion on this?

I’m not trying to second guess anyone, but trying to not fix something that isn’t broken and possibly never has broken. I’m not an aeronautical or structures engineer, but the folks that designed my airplane are. Why are they still recommending this bearing?
 
Scott Hersha;1579 Does anyone know if Vans has an opinion on this? I’m not trying to second guess anyone said:
Well, its the holiday season, excess food & too much idle time = lets 'Overthink the engineers' on something else & create another preceived issue.
 
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Started with a std rod end, bolt & big washer then switched to Van’s design. On final inspection my MDRA rejected the Van’s design and would only approve a bolt/big washer attachment.
 
Well, its the holiday season, excess food & too much idle time = lets 'Overthink the engineers' on something else & create another preceived issue.

Well, that's a bit harsh Ralph. The Op's question is valid: If the big washer is standard practice on all rod ends to contain the race within the body, and this "studded" unit is metallurgically identical to the other Heim joints that DO require the washer, why is it OK to ignore this containment practice?

And I agree - I have never seen a failed/uncontained race. I simply removed the stud in my Rocket because I changed the flap geometry and the stud would not work.
 
To washer or not to washer

I recall paying a very experienced test pilot to go over my build prior to first flight. I learned a lot from that, but the one thing that seemed out of place was the discussion of the push rod to flap attachment on my RV7.

Unless I missed something, Van's Plans do not call out a washer on that particular Heim Joint. I assume this is due to the reliability of the joint, the need to cut the fuselage for clearance, and the problem generated by a washer hanging up on the aircraft skin.

The inspector shared his desire to see a washer there as personal and not based on any specific knowledge of failures. I can certainly see his point.

However, in that particular assembly I would point to the aluminum tube/pushrod being scored along its inner circumference during threading as being the most likely impeding failure over time. I wouldn't call it likely, or even remotely likely, just much more likely to see fatigue failure in scored aluminum rather than smooth steel.

The inspector's observation bothered me as the consequence of failure weighs heavily on peace of mind, even though the likelihood is so remote at least in my mind to be undefined. How many things can we imagine as going wrong?

I guess I am asking this question again: Does anyone know of a failure mode for these joints other than obvious overload?
 
How many things can we imagine as going wrong?

That's a very long list my friend. For what it's worth, the failure of that particular CM-4MS rod end bearing isn't on mine :)

That particular rod end is loaded in predominately in shear, a failure is likely going to happen when extending the flaps (higher loading). Back of the envelope FMEA tells me that the push rod will bend & bind before the attachment fails -- likely causing the flap motor to stop turning, blowing a fuse/circuit breaker and causing a partial or split/partial flap configuration.

...but that's just me...
 
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