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  #1  
Old 03-21-2010, 05:51 PM
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flickroll flickroll is offline
 
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Location: Charlottesville, VA
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Default Tru-Trak Servo Gear Backlash

I am installing a TT DigiFlight II VSVG sytem (AFS branded) in my RV-8. The pitch servo is the B series, and it seems to have excessive gear backlash. The servo arm will 'wiggle' fore/aft about 1/32" which seems excessive to me. This servo has a similar amount of axial play on the output shaft as well. The roll servo (B series as well) is much tighter with very little backlash or axial play, less than 1/64".

Does anyone know if this is too much play for the pitch servo? TT indicates this is OK, but it seems to me that much play in the servo will cause the AP to have a hard time controlling the aircraft in pitch.

Thanks
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Jim Shannon
RV-8 N52VV
Charlottesville, VA

AFS 4500 EFIS & 3400 EFIS/EM
G430W - SL30 - G327 - G696 - G240
TT DigiFlight II VSVG w/pitch autotrim
VP-X Pro

Last edited by flickroll : 03-21-2010 at 08:41 PM.
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  #2  
Old 03-22-2010, 12:18 AM
rv72004 rv72004 is offline
 
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Mine had a lot of axial play [1/8] and upon inspection the oilite bushing that supports the shaft was turning in the housing and had worn the housing. It was totally loose. I contacted trutrak and was told not to worry that it was going nowhere. Maybe im too pedantic?
Well I sucked it up and bought a whole length of oilite and turned out a new oversized bushing . Reamed the housing oversize and did a nice tight press fit. I reduced the axial play to a minimum.
Check if you have this problem [I doubt as yours hasnt flown yet and mine had done 70 hours TT] I also had the whole shaft moving up/down/sideway in the worn bush/housing.
You cannot get rid of the backlash, but this did improve mine.
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Last edited by rv72004 : 03-22-2010 at 12:20 AM.
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  #3  
Old 03-22-2010, 06:07 AM
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Quote:
Originally Posted by rv72004 View Post
You cannot get rid of the backlash, but this did improve mine.
EJ, do you recall how much backlash you have? I'm not as worried about the axial play as I am the backlash. With 1/32" backlash I don't see how this servo will be capable of controlling altitude worth a hoot. The Dynons they are replacing have much less slop.

Thanks
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Jim Shannon
RV-8 N52VV
Charlottesville, VA

AFS 4500 EFIS & 3400 EFIS/EM
G430W - SL30 - G327 - G696 - G240
TT DigiFlight II VSVG w/pitch autotrim
VP-X Pro
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  #4  
Old 03-22-2010, 07:32 AM
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Ironflight Ironflight is offline
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Jm,

I don't recall if the servos we have exhibited any baclash prior to installation - certainly nothign that got my atention - but I am curious if it is there with power on, or just power off? I am by no means an expert on stepper motors, but they maybe be more rigid with power and control signals.

Just a thought to help troubleshoot. I have had no problems with play in-flight (except when a shear-screw sheared!)

Paul
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  #5  
Old 03-22-2010, 09:30 AM
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flickroll flickroll is offline
 
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Thanks for the response Paul. Since the backlash is in the gear train I suspect the 'slop' will be there whether or not there is power applied to the servo. TT says it will be OK so I guess the only thing for me to do is to fly it and see what happens. I am not optimistic however because this slop is essentially the same thing that you would see if there was a loose shear screw. The good news is the pitch servo in the -8 is relatively easy to access so if there is a problem it will be easy to get to.
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Jim Shannon
RV-8 N52VV
Charlottesville, VA

AFS 4500 EFIS & 3400 EFIS/EM
G430W - SL30 - G327 - G696 - G240
TT DigiFlight II VSVG w/pitch autotrim
VP-X Pro

Last edited by flickroll : 03-22-2010 at 09:33 AM.
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  #6  
Old 03-22-2010, 09:55 AM
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Bob Martin Bob Martin is offline
 
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Location: Richmond, Virginia
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Default order more

Jim,
Call TT and order 2 more......
pick the tightest one and send the other two back for credit.
if they are all the same, well, at least you will know it is either normal
or they are all that same way.
It's only money!
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  #7  
Old 03-22-2010, 10:46 AM
rv72004 rv72004 is offline
 
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Quote:
Originally Posted by flickroll View Post
EJ, do you recall how much backlash you have? I'm not as worried about the axial play as I am the backlash. With 1/32" backlash I don't see how this servo will be capable of controlling altitude worth a hoot. The Dynons they are replacing have much less slop.

Thanks
Jim probably 1/32 and less. I also dont like it , but it doesnt seem to hunt too much in still air.
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  #8  
Old 03-22-2010, 02:38 PM
TrutrakTech TrutrakTech is offline
 
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Let me see if I can help a little. There HAS to be some backlash in the geartrain. Without it, the residual torque of the servo would be through the roof. No one would like that. We use a general spec of .010-.050". 1/32" falls right about in the middle of that range.

It will not cause any adverse effects on AP performance. I hope this helps clear things up some. Let me know if there are any other questions. Thanks!
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  #9  
Old 03-22-2010, 05:29 PM
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Quote:
Originally Posted by TrutrakTech View Post
Let me see if I can help a little. There HAS to be some backlash in the geartrain. Without it, the residual torque of the servo would be through the roof. No one would like that. We use a general spec of .010-.050". 1/32" falls right about in the middle of that range.

It will not cause any adverse effects on AP performance. I hope this helps clear things up some. Let me know if there are any other questions. Thanks!
Thanks for the response Lucas. The spec you published definitely clears it up!
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Jim Shannon
RV-8 N52VV
Charlottesville, VA

AFS 4500 EFIS & 3400 EFIS/EM
G430W - SL30 - G327 - G696 - G240
TT DigiFlight II VSVG w/pitch autotrim
VP-X Pro
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