Servo Shear Screws

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William_Ince

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Question for Dynon:

Is there any plans on beefing up the shear screws a little? ::)
 

Dynon

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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.
 
W

William_Ince

Guest
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.
 

jnmeade

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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?
 

Doug1

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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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William_Ince

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In my humble opinion, those shear screws need to be beefed up a little.
 

Dynon

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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.
 

Doug1

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Messages
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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
 

Doug1

I love flying!
Joined
Jun 4, 2016
Messages
34
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
 

Dynon

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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.
 
W

William_Ince

Guest
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.
 
W

William_Ince

Guest
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?
 

Dynon

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Yes, this is still a very active project here. Nothing specific to report out at the moment.
 

Dynon

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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.
 
W

William_Ince

Guest
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?
 
D

DHeal

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