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View Full Version : TruTrak Servo Shear Pin...sheared!


Ironflight
11-22-2008, 10:28 AM
Rather than take this (http://www.vansairforce.com/community/showthread.php?t=36393) very important thread on Pat's incident with a jammed elevator linkage in a tangential direction, I'll start a new one with a coincidental failure we had on Louise's roll servo yesterday.

Louise was off on a trip and called to tell me that her roll autopilot wasn't working. All the lights came on as if it were engaged, but there was no resistance to moving the stick in that axis. Pitch worked normally. Since we were going to inspect for the problem noted in Pat's thread anyway, I had her fly it home (recommending no loops or rolls!), and we took it apart last night. Sure enough, you could hear the servo trying to control, but nothing happened with the stick, and sure enough, when we got it out, the shear pin (a screw actually) had failed. The servo appears to be operating completely normally, the center screw was tight - the shear pin had simply failed after time.

I'm curious if anyone else has had these fail, and if they might be a life-limited item? I haven't had a problem with either servo in the -8, and plan to simply put a new pin in this servo. (BTW, anyone in Houston have a spare? I'm sure TT will send me a few next week, but then I have time to install one this weekend!)

The center screw did it's job of keeping the servo arm captured in this case, so it failed exactly as it was supposed to.

Paul

frankh
11-22-2008, 01:25 PM
Mine failed in IMC on a bumpy Xcountry flight...No big deal but it does wake you up..:)

replace screw all well for a year and a half.

Frank

Rosie
10-23-2011, 09:40 PM
Rather than take this (http://www.vansairforce.com/community/showthread.php?t=36393) very important thread on Pat's incident with a jammed elevator linkage in a tangential direction, I'll start a new one with a coincidental failure we had on Louise's roll servo yesterday.

Louise was off on a trip and called to tell me that her roll autopilot wasn't working. All the lights came on as if it were engaged, but there was no resistance to moving the stick in that axis. Pitch worked normally. Since we were going to inspect for the problem noted in Pat's thread anyway, I had her fly it home (recommending no loops or rolls!), and we took it apart last night. Sure enough, you could hear the servo trying to control, but nothing happened with the stick, and sure enough, when we got it out, the shear pin (a screw actually) had failed. The servo appears to be operating completely normally, the center screw was tight - the shear pin had simply failed after time.

I'm curious if anyone else has had these fail, and if they might be a life-limited item? I haven't had a problem with either servo in the -8, and plan to simply put a new pin in this servo. (BTW, anyone in Houston have a spare? I'm sure TT will send me a few next week, but then I have time to install one this weekend!)

The center screw did it's job of keeping the servo arm captured in this case, so it failed exactly as it was supposed to.

Paul

My TruTrak roll A/P servo stopped working on the way home (yesterday) from Copperstate 2011. The plane started wandering off course, and after a few power cycles I just left if off. Got it apart and found that the shear pin (not a screw but a pin) had broken. Same as Ironflight said: "The center screw did it's job of keeping the servo arm captured in this case, so it failed exactly as it was supposed to."

It was a simple fix (new shear pin) and am back to normal ops. I checked the aircraft log and see that I have ~500 hobbs hours since servo installation, and I only use the A/P on long trips. A quick look through the pilot log book shows I have maybe ~220-ish hours on the servo.

No complaints here: Just a data point. I LOVE my TruTrak A/P system :D Rosie

TrutrakTech
10-24-2011, 08:02 AM
I know where you can get spares. :) They're on the way!

Fred.Stucklen
10-24-2011, 02:41 PM
Rather than take this (http://www.vansairforce.com/community/showthread.php?t=36393) very important thread on Pat's incident with a jammed elevator linkage in a tangential direction, I'll start a new one with a coincidental failure we had on Louise's roll servo yesterday.

Louise was off on a trip and called to tell me that her roll autopilot wasn't working. All the lights came on as if it were engaged, but there was no resistance to moving the stick in that axis. Pitch worked normally. Since we were going to inspect for the problem noted in Pat's thread anyway, I had her fly it home (recommending no loops or rolls!), and we took it apart last night. Sure enough, you could hear the servo trying to control, but nothing happened with the stick, and sure enough, when we got it out, the shear pin (a screw actually) had failed. The servo appears to be operating completely normally, the center screw was tight - the shear pin had simply failed after time.

I'm curious if anyone else has had these fail, and if they might be a life-limited item? I haven't had a problem with either servo in the -8, and plan to simply put a new pin in this servo. (BTW, anyone in Houston have a spare? I'm sure TT will send me a few next week, but then I have time to install one this weekend!)

The center screw did it's job of keeping the servo arm captured in this case, so it failed exactly as it was supposed to.

Paul

Mine failed about 1.5 years ago. Replaced it, and now also carry a spare in the tool bag. Hasn't failed since!

gvouga
10-25-2011, 07:13 AM
I had a shear pin fail a few years ago. At the time, I estimate the servo had less than 50hrs of operational time. Lucas sent me a new one and I have almost 800hrs (maybe 300hrs servo time) on the plane now without another shear pin failure.

I'm not 100% sure what caused the failure, but it is possible that I kicked the stick climbing in and out of the plane while working on the interior stuff. A sharp force on the stick could possibly add enough force to the pin to cause it to snap.

Greg
RV-7

Geico266
10-25-2011, 08:22 AM
I had a roll servo go bad and TT repaired it and I'm back up and flying again, no problem with the shear pin in 500 hours.

turbo
10-25-2011, 07:04 PM
after 2,200 TT AP hrs i have had no shear pins fail. numerous servos replaced at no charge. one thing that may prove helpful........ if a servo goes bad, you can go into the settings and 'turn' it off and continue on using the other servo. this has been helpful for me on a prior trip. also, a rebuild of the DG at a slight cost of $200. amazing company and product. love it. coupled to the 430 it does everything i need. http://img232.imageshack.us/img232/4294/lockheedf117nighthawk.gif

Toobuilder
12-09-2012, 08:21 PM
Looks like I have a failed shear pin on the roll servo as well. Was working fine one minute, gone the next. Probably about 250 airframe hours since install.

molson309
12-09-2012, 08:29 PM
Had one shear in the RV7A, put in the replacement and 200 hours later it is still going strong. I'd rather have one shear once in a while than be too strong and not shear with jammed controls as a result...

Wayne Gillispie
12-09-2012, 08:46 PM
When I set up my TT AP, I set the torque very low on roll and pitch, so that I could easily overpower it if necessary. If set too high I would think that it would put more stress on the shear pin. TT guys may chime in on this.

I only had one time this year that I lost roll during cruise and never figured out why. Once landed, refueled, restarted and back up to cruise everything worked fine. I did have a few times where my GRT settings were incorrect, but that was simply operator error.

TrutrakTech
12-10-2012, 08:31 AM
I don't recommend setting your torque below about 10. On our newest Sorcerers we have actually removed the torque adjustment completely, it is simply set at the maximum.

Even at the maximum torque setting you can easily override the servos. Setting it at a low value just reduces the AP's ability to fly the aircraft.

humptybump
12-10-2012, 10:27 AM
Lucas - I almost panicked for a moment knowing my torque is set to 180! then you mentioned "newest" and I remembered I am flying a TruTrak dinosaur :)