Correct - the brass shear pin should go LONG before that roll pin could break. (And they do - I know of several that have sheared under more or less normal use. They get tired...)
Van's Air Force
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Tru Trak Servo Arm ? Belt and Suspenders
- Thread starter Ironflight
- Start date
Correct - the brass shear pin should go LONG before that roll pin could break. (And they do - I know of several that have sheared under more or less normal use. They get tired...)
My attempt at a fix
This is my unproven and unverified theory of what's happening and what I've done to try and fix the issue.
When I removed the screw and the arm from the servo, there was quite a bit of locktite dried around the head of the shear screw. When I cleaned that off and reinstalled the arm there was now a small amount of play between the shear screw and the arm. In other words, the dried locktite was acting as a shim. So, my theory is that if the screw that holds the arm on is tightened to some unknown torque and the stars all align, the small back and forth play between the arm and the shear screw will eventually loosen the arm's mounting screw.
So, what I tried to do was come up with a way to apply the standard 15 in-lbs of torque to the screw and still have the arm free to rotate if the shear pin fails. So, I designed a small clutch shown in the pictures below. The aluminum button bears against the end of the servo shaft when the screw is tightened. And, the beleville washers apply 40-50 lbs to the nylatron washer. I'm not sure how much better this is over just a screw with a nylon washer but at least you can put a know and repeatable torque on the screw and the load between the arm and the washer is known. I unfortunately sheared my shear screw trying to remove it to test the drag of the clutch. But, the drag seems fine.
Here's a picture of the parts.
From left to right:
1) Nylatron washer (Mcmaster # 91545A250)
2) AN960-416L washer
3-4) Belleville washers (Mcmaster # 9713K61)
5) AN960-416L washer
6) Aluminum button - drawing shown below
7) AN509-8R8 flat head screw
Here's a drawing of the button.
Non of this has been tested, so YMMV, use at your own risk......
This is my unproven and unverified theory of what's happening and what I've done to try and fix the issue.
When I removed the screw and the arm from the servo, there was quite a bit of locktite dried around the head of the shear screw. When I cleaned that off and reinstalled the arm there was now a small amount of play between the shear screw and the arm. In other words, the dried locktite was acting as a shim. So, my theory is that if the screw that holds the arm on is tightened to some unknown torque and the stars all align, the small back and forth play between the arm and the shear screw will eventually loosen the arm's mounting screw.
So, what I tried to do was come up with a way to apply the standard 15 in-lbs of torque to the screw and still have the arm free to rotate if the shear pin fails. So, I designed a small clutch shown in the pictures below. The aluminum button bears against the end of the servo shaft when the screw is tightened. And, the beleville washers apply 40-50 lbs to the nylatron washer. I'm not sure how much better this is over just a screw with a nylon washer but at least you can put a know and repeatable torque on the screw and the load between the arm and the washer is known. I unfortunately sheared my shear screw trying to remove it to test the drag of the clutch. But, the drag seems fine.
Here's a picture of the parts.
From left to right:
1) Nylatron washer (Mcmaster # 91545A250)
2) AN960-416L washer
3-4) Belleville washers (Mcmaster # 9713K61)
5) AN960-416L washer
6) Aluminum button - drawing shown below
7) AN509-8R8 flat head screw
Here's a drawing of the button.
Non of this has been tested, so YMMV, use at your own risk......
RV7AV8R
Well Known Member
Why not put a head on the brass shear pin to act as an extra arm hold? If it isn't sheared off (which you would know) it will also hold the arm ON.
TrutrakTech
Well Known Member
Service bulletin to be issued soon. Just wanted to give everyone an update. Thanks!
RV7AV8R
Well Known Member
Adding to this idea, if the shear pin "head" has a safety wire hole in it, and the main screw has a hole, the two could be safety wired together. If the pin shears off you will know it, if not the main screw will be held secure. The shear pin head can be designed to fall out of the hole if it shears, so a run away servo will not unscrew the main screw. I must be missing something.
skylor
Well Known Member
Shear Pine/Safety Wire
If the shear pin shears, and the head does fall out as hoped (I don't think any design could truly guarantee that) then the safety wire with a brass screw head attached may stag on something else and still actually cause the shaft screw to back out if there is a runaway servo.
Skylor
If the shear pin shears, and the head does fall out as hoped (I don't think any design could truly guarantee that) then the safety wire with a brass screw head attached may stag on something else and still actually cause the shaft screw to back out if there is a runaway servo.
Skylor
Pilottonny
Well Known Member
Official solution?
I am moving the plane to the airfield on friday, for final assembly. Obviously I want to have this problem sorted out before first flight. Any news about the official TruTrak solution yet?
Regards, Tonny.
I am moving the plane to the airfield on friday, for final assembly. Obviously I want to have this problem sorted out before first flight. Any news about the official TruTrak solution yet?
Regards, Tonny.
TrutrakTech
Well Known Member
Tonny,
We are awaiting some parts to be cut and shipped to us. As soon as they are here we will release an official bulletin. The bulletin will only apply to retrofit applications where the new servo screws were put in place of screws used with chemical threadlocker. Anything manufactured after 1/22/2009 will not be included as it is not affected. Keep checking back, thanks!
WingsOnWheels
Well Known Member
Let me say first that I don't have a TT servo, so this suggestion might not make any sense, but I'll throw it out there anyway.
OK, how about drilling the screw hole in the arm out and put in a spacer that is the thickness of the arm plus slightly less than the thickness of the nylon washer. Use a screw with a large diameter head and a nylon washer with a larger I.D. The washer would fit around the outside of the spacer and bear on the arm to retain it. The screw would bottom on the spacer allowing for the application of proper torque. The thickness difference between the spacer - arm thickness and the washer thckness would determin the load applied to retain the arm.
I'll draw a quick picture in a moment and add it. This would only work if the shaft diameter was great enough to allow for a larger hole in the arm.
Added picture:
OK, how about drilling the screw hole in the arm out and put in a spacer that is the thickness of the arm plus slightly less than the thickness of the nylon washer. Use a screw with a large diameter head and a nylon washer with a larger I.D. The washer would fit around the outside of the spacer and bear on the arm to retain it. The screw would bottom on the spacer allowing for the application of proper torque. The thickness difference between the spacer - arm thickness and the washer thckness would determin the load applied to retain the arm.
I'll draw a quick picture in a moment and add it. This would only work if the shaft diameter was great enough to allow for a larger hole in the arm.
Added picture:
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TrutrakTech
Well Known Member
chrispratt
Well Known Member
Can't log in to send PM
Lucas:
When I try a PM the site asks me to log-in. I'm assuming its the TruTrak site and not VansAirForce.net. I don't have a log-in for the TruTrak site. Did I miss something?
Chris
Lucas:
When I try a PM the site asks me to log-in. I'm assuming its the TruTrak site and not VansAirForce.net. I don't have a log-in for the TruTrak site. Did I miss something?
Chris
erich weaver
Well Known Member
same issue for me, but I think it is related to Dougs new server. Wont let me send a PM and says I am not logged in even though I am
erich
erich
Brantel
Well Known Member
From looking at that new 3D model of the fix, I can't see how that retainer is going to retain the screw???
The retainer is held by the arm that you are trying to keep from falling off. As the screw loosens, the arm that is holding the retainer can move which will allow the retainer to move which will allow the screw to come out.
That screw could spin itself right out and the arm, retainer and screw will fall off.
Also if the shear screw breaks, and the servo goes into a clockwise rotation (looking at the shaft), the thing most likely would just unscrew itself...
EDIT: REVISION HAS BEEN POSTED>>>>>>>Image Removed
The retainer is held by the arm that you are trying to keep from falling off. As the screw loosens, the arm that is holding the retainer can move which will allow the retainer to move which will allow the screw to come out.
That screw could spin itself right out and the arm, retainer and screw will fall off.
Also if the shear screw breaks, and the servo goes into a clockwise rotation (looking at the shaft), the thing most likely would just unscrew itself...
EDIT: REVISION HAS BEEN POSTED>>>>>>>Image Removed
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Loooks like an interesting fix. I had the same question that Bryan did though....how does it keep the arm from coming off? I then read the text on the drawing, and it sounds like the REAL fix is coming along later, with a threaded portion coming out of the shaft - did I get that right?
I logged in to the TT Forums, and wrote a message - looks like it went OK, but no messages showed up in my "Sent Messages" folder there - so not sure if it got to Lucas or not.
Paul
I logged in to the TT Forums, and wrote a message - looks like it went OK, but no messages showed up in my "Sent Messages" folder there - so not sure if it got to Lucas or not.
Paul
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I tried registering on Trutrak's forum, and it keeps saying it could not verify the image due to server issues.
Vic
Vic
I could not register because of "server issues" The message was to try again later.
I'm guessing that the "screw retainer" rests on top of the center screw to keep it from backing out. The hole in the screw retainer is just large enough to get a screwdriver in to tighten the center screw. The new piece is just to keep the center screw from backing out.
I'm guessing that the "screw retainer" rests on top of the center screw to keep it from backing out. The hole in the screw retainer is just large enough to get a screwdriver in to tighten the center screw. The new piece is just to keep the center screw from backing out.
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tomhanaway
Well Known Member
If the future TruTrak solution is to have a threaded post with locknut, couldn't we just screw a thread into the screw hole with permanent red loctite and use a locknut on the end to hold the servo arm?
Tom Hanaway
Boynton Beach, FL
RV-10
Tom Hanaway
Boynton Beach, FL
RV-10
Brantel
Well Known Member
That method has been mentioned before in this thread. Using a long hex setscrew....
TrutrakTech
Well Known Member
Server issues? Argh. I don't even see any new usernames in the queue. So, my "new method" may fail!
Correct. The retainer is simply there to keep the screw from backing out. It puts a small amount of pressure against the screw to prevent it from turning on its own. It takes a miniscule amount of force to keep it from turning, therefore the screw won't back out, therefore the arm won't come loose.
TrutrakTech
Well Known Member
It is a link to our forum. Since I can control the number of PMs I'm allowed to receive on there, it's much easier to do it through our site. Of course if it won't let people register, it's a moot point. Go ahead and try registering and let me know if it doesn't let you. Thanks!
Clamping force
It appears to me from the drawing that the hole in the "screw retainer" is smaller than the head of the "center screw." Therefore, the idea is that the "screw retainer" when installed will apply some clamping force around the outside diameter of the head of the "center screw" to prevent backing out.
Could be wrong!
It appears to me from the drawing that the hole in the "screw retainer" is smaller than the head of the "center screw." Therefore, the idea is that the "screw retainer" when installed will apply some clamping force around the outside diameter of the head of the "center screw" to prevent backing out.
Could be wrong!
Brantel
Well Known Member
I have no rat in the race other than I do not want to see any of my RV freinds get killed by jammed controls!
My money says this fix will fail....the same arm that you are trying to prevent from falling off should not be used for trying to hold a screw in place.
If that shear screw breaks and the servo turns clockwise (looking at the shaft) this thing most likely will screw itself right apart.
My money says this fix will fail....the same arm that you are trying to prevent from falling off should not be used for trying to hold a screw in place.
If that shear screw breaks and the servo turns clockwise (looking at the shaft) this thing most likely will screw itself right apart.
Brantel
Well Known Member
That same clamping force may just unscrew the screw if the shear screw breaks.
As the arm works back and forth against the normal clearances built into the shear screw holes, most likely this thing will walk itself right out of the hole....
As the arm works back and forth against the normal clearances built into the shear screw holes, most likely this thing will walk itself right out of the hole....
TrutrakTech
Well Known Member
I have disabled the image verification for now. Hopefully we won't be flooded by spam. That should let everyone register.
Brian,
I can understand your concerns. Maybe you're misunderstanding the amount of force the retainer is applying. It is very small. Enough to keep it from backing out, but still allow it to turn with the shaft if the shear screw breaks. Also, our servo shaft and shear screw fitment on the arm is actually pretty tight. There isn't inherent slop between them. The "play" you may feel in one of our servos is gear backlash, which MUST be present to keep the residual torque of the servo at a manageable level.
I hope this helps to clear it up.
Brian,
I can understand your concerns. Maybe you're misunderstanding the amount of force the retainer is applying. It is very small. Enough to keep it from backing out, but still allow it to turn with the shaft if the shear screw breaks. Also, our servo shaft and shear screw fitment on the arm is actually pretty tight. There isn't inherent slop between them. The "play" you may feel in one of our servos is gear backlash, which MUST be present to keep the residual torque of the servo at a manageable level.
I hope this helps to clear it up.
N8RV
Well Known Member
I get Brian's point, and don't really want to scroll back through all of the previous posts to see if this idea has already been suggested.
Sorry for the crummy picture, but it was all I had of my roll servo installation. Couldn't a bracket be made that serves the same purpose as TruTrak's fix, but anchor the bracket to the floor rib instead of to the arm?
Sorry for the crummy picture, but it was all I had of my roll servo installation. Couldn't a bracket be made that serves the same purpose as TruTrak's fix, but anchor the bracket to the floor rib instead of to the arm?
N8RV
Well Known Member
Ironflight;440016 That's essentially what we built and showed in Post #1 Don - and I am still wondering if it is sufficient. The question is how hard hat screw might try to back itself out. If the force is very small said:LOL! No wonder it sounded familar. I should've guessed that I'm not capable of original thought.
Good thing I didn't waste my time scrolling back through the pages -- it wouldn't have hit me till POST #1!!
RVbySDI
Well Known Member
I have sent my PM to Lucas for this update and will see what the retainer looks like when I receive them for my two servos. Having said that, I do have a question on the design of this shear screw.
Ok, I am not an engineer (nor did I sleep in a Holiday Inn last night
), but is there something wrong with using a safety wired bolt as the shearing fastener in place of this screw? The bolt head could be safety wired to the arm to prevent it from backing out. It seems to me this would eliminate the backing out issue without any undue additional problems associated with constructing some other bracket, etc. to keep the screw in place.
I am sure there is a very logical and simple explanation for why this would not work but I sure would like to hear what it is.
Ok, I am not an engineer (nor did I sleep in a Holiday Inn last night
), but is there something wrong with using a safety wired bolt as the shearing fastener in place of this screw? The bolt head could be safety wired to the arm to prevent it from backing out. It seems to me this would eliminate the backing out issue without any undue additional problems associated with constructing some other bracket, etc. to keep the screw in place.I am sure there is a very logical and simple explanation for why this would not work but I sure would like to hear what it is.
That's been brought up before Steve (but I admit, this is getting to be a LONG thread!)....the shaft needs to be free to rotate independently of the arm if the shear screw breaks - that is the purpose of the shear screw. if the servo "runs away", then the shaft has to turn "underneath" the arm. The screw is theoretically fixed to the shaft, so you can't tie it to the arm, or you have tied the two together.
Paul
RVbySDI
Well Known Member
See, I knew there was a logical explanation. I guess the length of this thread is my excuse for not reading about this idea before. I must admit I initially followed this thread at its inception but have subsequently gotten absorbed in the final stages of building my aircraft lately. This has caused me to skip over a great many of the posts here.
The final fix...
...proposed by TruTrak is, IMHO, close to the best. Utilizing a shaft design with a male threaded portion is a lot closer to a foolproof design. The only thing which could make it better (again, IMHO) would be a cross drilled hole to allow for use with a castle nut and a cotter pin.
With due respect to the fine folks at TruTrak, I have my doubts about the effectiveness of the little add-on clip.
Is there any possiblity of sending servos back for retrofit with the new shaft design? What, if any, would be the cost to the user to have the shaft changed out?
Thanks!
...proposed by TruTrak is, IMHO, close to the best. Utilizing a shaft design with a male threaded portion is a lot closer to a foolproof design. The only thing which could make it better (again, IMHO) would be a cross drilled hole to allow for use with a castle nut and a cotter pin.
With due respect to the fine folks at TruTrak, I have my doubts about the effectiveness of the little add-on clip.
Is there any possiblity of sending servos back for retrofit with the new shaft design? What, if any, would be the cost to the user to have the shaft changed out?
Thanks!
erich weaver
Well Known Member
Guess Im an idiot. The service bulletin indicates to return the servos to TruTrak if they have "torque enhancers" and "capstans". How do I tell if I have them?
erich
erich
Brantel
Well Known Member
TrutrakTech
Well Known Member
Erich,
Check the thread on our website again.
http://trutrakap.com/forum/showthread.php?p=897#post897
William Slaughter
Well Known Member
PM successful
Ok, got registered and successfully sent the PM.
I'm trying to keep an open mind about this, but the setscew-installed-with-red-locktite and self-locking nut combo certainly seems to more closely emulate the proposed shaft redesign. Why is this not an approved fix? Of course, I like the idea of having my servos retrofitted with the new shafts the best of all! Sure hope that will become a choice.
Ok, got registered and successfully sent the PM.
I'm trying to keep an open mind about this, but the setscew-installed-with-red-locktite and self-locking nut combo certainly seems to more closely emulate the proposed shaft redesign. Why is this not an approved fix? Of course, I like the idea of having my servos retrofitted with the new shafts the best of all!
Sure hope that will become a choice.
Walt
Well Known Member
I'm trying to keep an open mind about this, but the setscew-installed-with-red-locktite and self-locking nut combo certainly seems to more closely emulate the proposed shaft redesign. Why is this not an approved fix? Of course, I like the idea of having my servos retrofitted with the new shafts the best of all!
I agree with the above, think I'd rather use the set screw method myself as it looks to be the best solution so far.
Brantel
Well Known Member
If my butt was riding in a plane with these servos, I would use the setscrew with red loctite and a locknut (properly installed and cured of course). That clip would not cut it for me.
Then I would swap em out with the redesigned shaft that uses the castle nut and cotter pin.....when TT gets em...
Then I would swap em out with the redesigned shaft that uses the castle nut and cotter pin.....when TT gets em...
TrutrakTech
Well Known Member
Update
Quick update. I received the wrong drawing from engineering. Figures, I'm always the last to know. Updated bulletin with CORRECT drawing is now attached to the first post. The first drawing was a preliminary idea that was tossed around. Imagine my pleasant surprise when the boss looked in on the post and saw the wrong part posted. Oops!
http://trutrakap.com/forum/showthread.php?p=895#post895
Quick update. I received the wrong drawing from engineering. Figures, I'm always the last to know. Updated bulletin with CORRECT drawing is now attached to the first post. The first drawing was a preliminary idea that was tossed around. Imagine my pleasant surprise when the boss looked in on the post and saw the wrong part posted. Oops!
http://trutrakap.com/forum/showthread.php?p=895#post895
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Bill Dicus
Well Known Member
Thanks Lucas. This looks good as a fix. Will request when I can get on your server. Bill
TrutrakTech
Well Known Member
Everyone should be able to register now. I removed the image verification for now.
N8RV
Well Known Member
Aha! Wrap the keeper around the arm to keep it from being pushed out by the screw. Looks like a perfect solution.
Are the brackets made of steel or aluminum?
Well done. I'll be checking the part numbers on my servos at condition inspection and send you a PM (on the TT forum) if I need the retainers.
Thanks, Lucas!
Are the brackets made of steel or aluminum?
Well done. I'll be checking the part numbers on my servos at condition inspection and send you a PM (on the TT forum) if I need the retainers.
Thanks, Lucas!
TrutrakTech
Well Known Member
Aluminum. Your servos fall in the category that apply. Thanks!
William Slaughter
Well Known Member
That looks better
Still like to know if TruTrak is planning on an upgrade path so that people can eventually get the new style shaft, and if you're considering a castellated nut/cotter pin in place of the elastic stop nut. I'm not phobic about elastic stop nuts, but if you're re-designing anyway and it's a rotary motion, well ...
Still like to know if TruTrak is planning on an upgrade path so that people can eventually get the new style shaft, and if you're considering a castellated nut/cotter pin in place of the elastic stop nut. I'm not phobic about elastic stop nuts, but if you're re-designing anyway and it's a rotary motion, well ...
N8RV
Well Known Member
Umm ... how do you know that? Did you look me up on your computer, or are those pesky black helicopters still following me around?
TrutrakTech
Well Known Member
Umm ... how do you know that? Did you look me up on your computer, or are those pesky black helicopters still following me around?
Oh you mean the....oops, I've said too much.
You have an RV8, which uses our DSB/P-B servos. They fall into the applicable group.
TrutrakTech
Well Known Member
Hi Lucas, I've registered and I thought I'd sent you a PM requesting this fix, but there's nothing in the sent items folder. Did you get my msg?
Sure did. The default for copying messages to the sent folder is disabled. You can enable it in the User CP under "Edit Options". Thanks!
