Servo Tray Mounting

[Konrad]

Last night I was helping a forum members program his X9D+ for a 6 servo glider. I noticed that when the glider was assembled that there was a lot of distortion on the wing skin and movement of the wing servo.

This was traced to the servo mounting trays. With today's real narrow servos the servo arm spline boss is often very close in diameter to the servo’s body thickness. This means that it is very easy for the servo output arm’s spline boss to to hit (rest) against the servo tray before the lugs of the servo and tray make contact. If not corrected this will distort the servo tray prior to being glued into the wing. This will cause all sorts of havoc with skin distortion and servo motion. I will try to make a drawing showing this concern.

In the mean time make sure that there is nothing from the cut off arms than can hit the servo tray prior to mounting (glueing in) the servo trays. All the servo mounting pads should be making contact with the servo trays mounting pads and not distorting the tray prior to being glued into the wing.

All the best,
Konrad

 

[Smooth3D]

This is Awesome. Key points identified. Clear descriptions to aid proper implement.

Thank you.

 

[Konrad]

To add functionality to the tray, I want to allow the servo to be replaced should the need arise. To help keep the servo mounting screws from distorting the wing surface with a replacement servo, I like to add clearance from the epoxy that will have filled in the screw holes. To do this I wait for the epoxy to fully cure. I them remove the servo mounting screws and grind the tip back 0,1 mm to 0.2 mm. This gives me the needed clearance should I "over torque" the screws any time during the service life of the model.

 

[Motowncali]

Also watch out for the servo screw length. The screws supplied with the KST’s I was recently installing stick out the backside of the tray.

 

[Konrad]

My screw have, as of late, protruded past the surface. (I wonder if the supplied screws are longer or are the servo flanges thinner)? I grind the screw tip to be below the surface with a Dremel tool prior to installation.

I've learned that after the epoxy has cured that it is a good idea to grind the tips again to get a clearance in the bottom of the hole of about 0.1mm to 0.2mm. All this grinding might mean that the screws are NOT interchangeable. This could be a problem. To help with that issue I use a caliper to measure their lengths. I try to size all their lengths to the shortest. This way should I mix up the screws they are interchangeable.

 

[jvaliensi]

I use washers to increase the clearance. The Nylon ones RTL Fasteners work perfectly.

 

[Konrad]

Why not eliminate the risk and grind down the screw to length? Cutting off the tapered screw tip will actually result in a stronger fastener hold as there will be more of the full diameter screw length engaged in the servo frame. Also less parts to try to get aligned in those small servo pockets! Less parts look like a better solution.

 

[jvaliensi]

It be better if the trays were a little thicker. The screws are not always too long for every type of tray I used.

 

[Konrad]

It be better if the trays were a little thicker. The screws are not always too long for every type of tray I used.

Many wing pockets can't encapsulate the servo as it is. Making the tray thicker will force the servo further outside the airfoil profile. For example the RedShift's aileron pocket can't keep a 10mm servo (KST125/135) inside the airfoil profile. A thicker tray will make the servo bump larger, uglier and dragier. This is a penalty I don't want to suffer just for longer screws. Particularly when 5 seconds with a grinder solves the length problem and give us more thread contact area (larger diameter screw for more of its engagement length).

 

[Konrad]

Here I'm showing a trap that can happen when installing the trays. If one needs to change out the servo it is easy to over torque the scews. This will drive an epoxy plug above the mounting surface and cause a ugly dent (bulge) in the skin.

I like to remove the servo mounting screws after the epoxy has curred and grind them down about 0.15mm. This will keep the screw tip way from any epoxy plug that may have formed in the screw holes.

All the best,
Konrad

 

[Konrad]

I've been suffering a lot of loose servo trays. I degrease and sand all surfaces and still have these trays pop loose. I'm finding that the plastic used does not allow for much of a chemical bond with epoxy.

If one looks close at the servo tray you can see the Servorahmen has supplied a micro ledge to allow the epoxy fillet to flow over the tray. This makes a mechanical bond that holds the servo tray in place.

Please make sure that there is some fillet flowing over this micro ledge before the epoxy fully cures.

All the best,
Konrad

 

[Wayne]

I see a lot of folks use CA on these trays. I have always been shy to use CA for this, but seems to see happy folks with this method. Comments?

 

[jvaliensi]

I have used CA a few times. Some have popped off, but I've had epoxy ones pop too. Of course I sometimes have 300g landings!

 

[Konrad]

Servorahmen's video shows the use of CA. My first attempts at using these trays was with CA and they were an utter failure. All servos broke loose prematurely. True, I didn't incapsulate the micro flange in CA. CA is an inappropriate glue for any shock loads. Flap servos are prone to seeing these if one doesn't raise the flaps soon enough.

Later use of epoxy has proven to be much more durable. I've only had about 12% of the servos pop loose. Again these trays did not have the micro flange encapsulated.

To my way of thinking I'd like a servo gear to strip and not loose the mounting datum for the replacement servo. I can swap out a servo on the slope. I can't properly re-glue a popped servo tray at the slope.

 

[jvaliensi]

I prefer epoxy too, especially the thixotropic stuff. However, some applications are easier to assemble with CA, i.e. the Servoahamn's stuff. I clean it up really well, and put a bead of thin down first. Then a couple rounds of the thicker CA and some kicker.
It isn't the best & strongest, but the way I land sometimes, it don't matter...

 

[Konrad]

That edge dam of thicker CA over the micro ledge is the key.

 

[purview]

That edge dam of thicker CA over the micro ledge is the key.

I will look at mine and secure when needed. Thanks for this advice. Unfortunately till now I only had pooped out control horns due to weak glue spot and mechanical overtorque

 

[Konrad]

I wonder if the "rubberized" CA, often used to bond toy car tiers on to the wheels, would work in this shock load area.

 

[purview]

Sorry, no experience on that.
If the wing skin is stiff and not flexible then the shock loads aren't able to pop of trays? Or am I wrong here? Deformation of the surface is the cause and not the glue??

 

[Konrad]

Actually the stiffer the mount the MORE likely the bond will fail due to impact shock. The flexing will absorb some of the shock.

I'm not say which mount, the flexable one or the ridged one, will fail sooner. I'm just saying the stiffer one will fail sooner due to shock loads.