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Servo Shear Screws (Read 2406 times)
Jun 29th, 2016 at 8:07am

wmince   Offline
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Question for Dynon:

Is there any plans on beefing up the shear screws a little? Roll Eyes
 
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Reply #1 - Jun 29th, 2016 at 5:21pm

Dynon Avionics   Offline
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No firm plans at the moment. You want it strong enough to not break during normal use, but you definitely want it to yield if something goes wrong mechanically. All of that said, we're seeing more screws shearing than we'd expect, and we're looking into that.
 

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Reply #2 - Jun 30th, 2016 at 5:11pm

wmince   Offline
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Dynon Avionics wrote on Jun 29th, 2016 at 5:21pm:
No firm plans at the moment. You want it strong enough to not break during normal use, but you definitely want it to yield if something goes wrong mechanically. All of that said, we're seeing more screws shearing than we'd expect, and we're looking into that.

That is good news.
 
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Reply #3 - Jun 30th, 2016 at 6:07pm

jnmeade   Offline
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I assume that the different servos have different strength shear pins?  I'm looking at putting a small one on my Rans S7S build.  If I went with the next stronger servo, the shear pin would take more force?
 
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Reply #4 - Jul 1st, 2016 at 5:35pm

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The shear screw is the same on all of the servos.
 

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Reply #5 - Jul 17th, 2016 at 8:50pm

Doug1   Offline
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Hi
Dynon

I have just had my shear screw break on the SV42 roll servo, factory installed in a Pipistrel Virus SW100.
A couple of weeks ago, a friend was showing some kids the plane and were sitting in the cockpit. I caught them exercising the control column left to right vigorously for about 30sec until I kicked them out.
The very next flight with AP (TRK+ALT) engaged, I noticed the track would wander from side to side by about 6deg each side with about a 30 second period.
Up until then it has been perfect, more like 1 or 2 deg. and rock steady.
I checked the roll slop using test mode and it appeared OK to me. The Pipistrel install is very robust.
I increased the roll sensy from 10 to about 20 and this helped considerably with track oscillations and made it just acceptable to use, but still not happy as I knew it worked perfectly before with a roll sensy of 10.
Last flight just gone, had really bad turbulence, so bad I aborted my flight. During the turbulence with AP on, it had trouble maintaining direction then suddenly rolled excessively and didn't recover. Although the turbulence was pretty bad, the AP has performed really well in turbulence in the past. I disconnected AP and headed home.

I have since discovered the roll shear screw has broken, consistent with the sudden lack of roll control mentioned above.
Is it possible/likely the kids somehow damaged the shear screw, causing the sudden onset of the track oscillation and then the turbulence finally broke it?

Can you tell me how to go about ordering another shear screw and a replacement cotter pin?

Doug1
 
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Reply #6 - Jul 17th, 2016 at 10:06pm

wmince   Offline
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In my humble opinion, those shear screws need to be beefed up a little.
 
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Reply #7 - Jul 18th, 2016 at 3:16pm

Dynon Avionics   Offline
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Hi Doug. It's possible, but it could also be coincidence. You can request a replacement by going to www.dynonavionics.com/shearscrew.
 

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Reply #8 - Aug 6th, 2016 at 8:19pm

Doug1   Offline
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Hi
All

Just an update on my roll servo shear screw.
Contacted Dynon who promptly posted 2 shear screws and 2 cotter pins to me. Took about 6 days from Dynon to Melbourne, Aust., Great service.
Replaced shear screw using recommended method and went for a fly. Back to normal!
Ground track was maintained to within about 1 degree with the original sensitivity setting of 10.
Did an in-flight roll servo tweak, involving seeing how well the AP handles an abrupt 90deg change in heading. AP responded promptly and banked neatly to the new direction. Overshoot was about 5 deg, but quickly settled to new direction. Tried increasing sensitivity from 10, 15, 20 to see if could reduce overshoot - didn't make hardly a difference.
Have now set sensitivity back to 10 with AP performing as well if not better than when first installed.

My conclusion is that the shear screw may have started life compromised and the kids (see last post) made it worse, introducing a little bit of slop. The turbulence then finished off the compromised shear screw.
Now a happy camper!
Doug
 
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Reply #9 - Aug 16th, 2016 at 3:39pm

Robski   Offline
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From your comments above I'd blame the kids. Period.
 

Rob Swain
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Van's RV-6

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Reply #10 - Oct 16th, 2016 at 9:39pm

Doug1   Offline
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Hi
All

Bad News!
Roll servo shear screw broken again! Fortunately Dynon sent 2 shear screws last time, so I was able to get back into action without too much delay.
The first time the shear screw broke was in severe turbulence, with the shear screw possibly weakened by some kids. (see previous post)

It has only been 35 hours since the shear screw broke the first time. Haven't had any real turbulence to speak of and certainly no little kids mucking around. Just flying along and noticed the AP wasn't maintaining direction!

I was careful the first time to make sure the Capstan bolt was only finger tight, lining up in the previous position with the cotter pin hole. This means the load is carried all by the shear screw, as designed. I also backed off the maximum torque that can be applied to the roll servo from 100%, as installed to about 60%, which is as low as I can go prior to slipping.

I cannot help but think there may be a design issue with the shear screw as other people are also suffering broken shear screws. Maybe although the static load the shear screw can handle is adequate, it may be that short, sharp impulsive loads from say the ailerons in turbulence breaks or weakens the screw due in part to the inertia of the servo itself. This could happen even if the AP was off. I would think that the sensitivity of the shear screw to breakage would be greatly influenced by how tight the capstan screw was done up. Dynon says less than 4.5 inch pounds - not every one can measure this easily.

I got my LAME to replace the shear screw this time as a reference, but I not hopeful it will last any longer.
The SV32 servo allows the shear screw to be replaced easily twice - I have used this up and will keep my fingers crossed.

Doug


 
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Reply #11 - Oct 17th, 2016 at 9:16am

wmince   Offline
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See reply #6.
That still stands.
 
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Reply #12 - Oct 17th, 2016 at 5:24pm

Dynon Avionics   Offline
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While most aircraft never break shear screws (there are literally thousands of Dynon autopilots installed in the world), there are some that are able to break them, sometimes more than once as you've experienced. We're actually doing a pretty deep engineering investigation of the phenomenon right now. While I don't have results to report just yet, I do want to communicate that we're taking a hard look at what's going on in these cases and hope to have more to report soon.
 

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Reply #13 - Oct 17th, 2016 at 8:04pm

wmince   Offline
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Dynon Avionics wrote on Oct 17th, 2016 at 5:24pm:
While most aircraft never break shear screws (there are literally thousands of Dynon autopilots installed in the world), there are some that are able to break them, sometimes more than once as you've experienced. We're actually doing a pretty deep engineering investigation of the phenomenon right now. While I don't have results to report just yet, I do want to communicate that we're taking a hard look at what's going on in these cases and hope to have more to report soon.

Outstanding.
Thank you Dynon for looking into this.
Hope we hear the results soon.
 
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Reply #14 - Nov 25th, 2016 at 2:49pm

wmince   Offline
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wmince wrote on Jul 17th, 2016 at 10:06pm:
In my humble opinion, those shear screws need to be beefed up a little.

Attention Dynon Tech Support:

Any news about the above? Is Dynon still looking at this or have they abandoned any further consideration?
 
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Reply #15 - Nov 28th, 2016 at 5:57pm

Dynon Avionics   Offline
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Yes, this is still a very active project here. Nothing specific to report out at the moment.
« Last Edit: Nov 28th, 2016 at 5:57pm by Dynon Avionics »  

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Reply #16 - Nov 28th, 2016 at 7:30pm

wmince   Offline
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Dynon Avionics wrote on Nov 28th, 2016 at 5:57pm:
Yes, this is still a very active project here. Nothing specific to report out at the moment.

Thank you.
Very interested.
« Last Edit: Nov 28th, 2016 at 7:30pm by wmince »  
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Reply #17 - Mar 10th, 2017 at 5:41pm

Dynon Avionics   Offline
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An update here: Our updated shear screw design is shipping now, but we haven't made any big announcements yet. If you've had past issues with shear screws breaking (particularly more than once) do fill out a request at http://www.dynonavionics.com/shearscrew. If you haven't had issues with broken shear screws and yours isn't broken now, there is no need to replace yours.
« Last Edit: Mar 11th, 2017 at 10:14am by Dynon Support »  

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Reply #18 - Mar 11th, 2017 at 1:17am

wmince   Offline
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Dynon Avionics wrote on Mar 10th, 2017 at 5:41pm:
An update here: Our updated shear screw design is shipping now, but we haven't made any big announcements yet. If you've had past issues with shear screws breaking (particularly more than once) do fill out a request at http://www.dynonavionics.com/shearscrew. If you haven't had issues with broken shear screws and yours isn't broken now, there is need to replace yours.


Thank you very much for the update.
Can you give us an idea on how the shear screw was changed?
 
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Reply #19 - Mar 11th, 2017 at 8:44am

DHeal   Offline
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Dynon, read the last line of your above Reply #17 message.  Is there a "need" or "no need"?  Please clarify.
 

EAA #23982 -- EAA Technical Counselor and Flight Advisor -- CFI-A&I
RV-12 E-LSA (SV w/ AP & ADS-B)   N124DH   Based at STS (Santa Rosa, CA)
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Reply #20 - Mar 11th, 2017 at 10:12am

Dynon Support   Offline
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No need. Sorry for the typo. Post fixed.
 

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Reply #21 - Mar 11th, 2017 at 10:19am

Dynon Support   Offline
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wmince wrote on Mar 11th, 2017 at 1:17am:
Can you give us an idea on how the shear screw was changed?


There is no change the safety aspect of the screw- it breaks at the same force as before. We just improved the fatigue characteristics which sometimes lead to them breaking at lower forces than required.
« Last Edit: Mar 13th, 2017 at 5:37pm by Dynon Avionics »  

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