Servo mounting tips needed
 

I have a hi torque rudder [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXUZ80]servo[/link] I just installed in a used 72" QQ Yak I purchased. In all the time I've been perusing the rcu forums I've never seen detailed information on hi end servo mounting.

The servo came with quality rubber bumper pads for each hole which I installed. It also came with little brass inlets for each hole. I installed these with the lip up. I used my own heavy duty cap head self tapping servo screws and a long metal servo double arm with Hitec insert. Questions:

1.) How tight should the servo screws be tightened down, enough so the rubber pads are compressed about half way?

2.) What's the purpose of the brass inlets?

3. Are there any tips on installing digital hi torque servos that anyone can offer in order to do a quality installation that will last and not come loose nor put unnecessary strain on the servo itself? Pictures would be appreciated.

Thanks in advance.

RE: Servo mounting tips needed
 

Many people make mistakes when mounting servos that can lead to servo damage, premature servo failure, or other problems. When installing the servos the brass eyelets install from the bottom of the rubber grommet, and then the screw is tightened to where it just touches the top of the rubber grommet. The purpose of the rubber grommet is to isolate the servo from the vibration in the airframe during flight. If the screw is over tightened to the point that the rubber grommet is compressed any the isolation characteristics of the grommet are gone and vibrations will travel directly into the servo, which can damage it.

See attached diagram

Ken</p>

RE: Servo mounting tips needed
 

Here is the mounting procedure from the Futaba website. Other brands will be pretty much the same.

Proper Servo Mounting:
The proper way to mount a servo is as follows:

1 Insert a rubber grommet into each of the four servo holes.
2 Insert a metal eyelet from the bottom side of the rubber grommet. This way the wide portion of the eyelet will be in contact with the servo tray when mounted.
3 Test fit the servo in the tray, and enlarge the openings so the servo will not touch the tray. The rubber grommets will isolate the servo from the hard vibration of the airplane's structure.
4 Position the servo, then mark the location of the mounting holes. Drill pilot holes with a 1/16" bit at each mark.
5 Use the servo screws supplied with your radio to mount the servo(s) in the servo tray. Tighten the screws until they just touch the top of the metal eyelet.


Of course, while I was looking for this, Ken shows up with one of his nifty pix.

RE: Servo mounting tips needed
 

Here's a suggestion, I toss the supplied screws, and replace with #2 x 9/16 allen head screws.

RE: Servo mounting tips needed
 

Yes, I did do that already.

RE: Servo mounting tips needed
 

...and after all that was mentioned...pull out servo and hit each screw hole with some thin CA, let dry and then redo mount servos....the screws will loosen the wood if not hardened with CA via vibration

RE: Servo mounting tips needed
 

Thanks, Ken and all. Useful info and diagram, and I'm really glad I asked. I've been mounting those brass inlets wrong for a year now.

RE: Servo mounting tips needed
 

Crossman and Ken appear to be resident experts, but, there is still a disparitry.
Ken - a respected expert " just till the screw touches the rubber" and drawings are killer,
Crossman - a respected fact researcher - "just till the screw touches the top of the metal eyelet.

Both appear to be the best, but which is trully which?
Not trying to be a smart*** but I really use these forums for my bible.
Again, as cappaj1 says thank you very much

RE: Servo mounting tips needed
 

In theory yes you would screw down to the top of the eyelet, but the eyelet should be even with the top of the rubber grommet as it's shown in my diagram. However, in practice it usually doesn't work that way. Usually the eyelet does not come to the top of the eyelet. The problem when you screw down to the top of the eyelet is that you compress the rubber grommet. When the grommet is compress it removes all shock absorbtions properties from the rubber, which then defeats the purpose of the grommet. It is there to isolate the servo from the vibrations of the airframe. If the grommet is compressed it can no longer isolate the servo and all vibrations will translate directly into the servo which can cause the servo to fail.

Ken</p>

RE: Servo mounting tips needed
 

Thanks
It makes sense, if it touches the top the compression is stopped.
Very good wordsmithing and I enjoy stumbling on to some of your replies.
Again, thanks
r
Dennis

RE: Servo mounting tips needed
 

I just reversed the brass inlets, put a drop of ca in the mounting holes, and reinstalled the servo.

I had a difficult time telling when the screw contacted the brass. I could see when the screw heads touched the rubber but not the brass and I can't understand how anyone would know this, especially when the servo is halfway down inside the plane.

There was no noticeable difference in the torque required to tighten the screw as it went down into the hole, and there was resistance all the way down to the rubber. I guess just going to the top of the rubber is enough, but I'm worried that the screws might be too loose or come loose from vibration. Should I not worry about that?

RE: Servo mounting tips needed
 

In your picture it appears that the wood used for the servo mounts is square balsa stock exposing side grain upwards. That means that the screws are separating the grain when tightened - weakening the mount - offering little resistance to hold the screws.

I've been using a layered servo mount - a piece of balsa covered with 1/8" aircraft ply (not lite ply). The ply will maintain the integrity of the "bite" over a long period of time and use. Like others have advised, drill a pilot hole, screw in the servo screw, remove screw and apply thin CA to harden.

Perhaps part of your preflight should include testing the torque on all of your servos. If any are loose, tighten. If they won't tighten - don't fly.

RE: Servo mounting tips needed
 

Thanks for the tips! I purchased this QuiQue Yak used as it is shown and just needed to slap in the rudder servo.

RE: Servo mounting tips needed
 

Are you using the screws used by the previous owner? Or have you replaced 'em? If replacements are they the same size screws?

RE: Servo mounting tips needed
 

Yes, the previous owner took the servo but left me the cap head screws inside the holes. I used those, not the ones that came with the new servo I purchased, as I prefer the cap head type screws anyway.

RE: Servo mounting tips needed
 

I've never had a plane that used hi-torque servos so I can't advise as to frequency of mounts loosening up over time. Maybe someone else can share from their personal experiences.

RE: Servo mounting tips needed
 

Well, I think I'm alright. The torque level to tighten the screws until they contacted the rubber grommets was fairly high to get through the ca dried glue coated holes and I'll just add to my flight checks to look at the servo screws at the beginning of a flight day as someone had suggested.

RE: Servo mounting tips needed
 

All comments have merit especially using the cap head screws and CA hardening of the hole.
I am sure that when I go above 60 size airplanes that this information will be emblazened in my memory as what to do.
Thanks again to RCKEN and appreciate your monitoring and inputs to assist this thunderhead.


corrected copy thought the mention was for RCKEN drawing

RE: Servo mounting tips needed
 

also I try to avoid the"wood screw" type and use "sheet metal" type servo screws....wood screws are usually triangular shape all the way down to a point...meaning once/if it loosens just a bit, I will get looser even quicker because of the triangular shape...while sheet metal screws are more parallel all the the to the point , so there will always be consistent friction against the threads....

...and even still it does not hurt to add a drop of canopy glue to the threads..easily comes undone with a jolt from the screw driver...my feeling is most servos should last the life of the airplane, and you will not be messing with it until maybe 5 years from now, if the plane lasts that long

RE: Servo mounting tips needed
 

Good to know - thx.

RE: Servo mounting tips needed
 

See how the rubber thing twisted and looks squished? I think your too tight. Back the screws up a tad. Nothing wrong with adding a tad of CA to make sure those screws don't back out, but think "Rubber Mounted". The servo may wiggle slightly and thats ok. It's a touchy feely thing.

An old car guy!

RE: Servo mounting tips needed
 

Hi Everyone,
Great thread here. A question I have is when mounting a Futaba servo do I put a rubber gromit on both the top and the bottom of the servo with the brass inlet protruding from the bottom up?
Thank you

Crash...then whiplash

RE: Servo mounting tips needed
 

Yes, top AND bottom, grommet through the bottom hole.

The idea is to have the servo completely isolated from the airframe. Since the screw is directly fastened to the airframe, the servo must be isolated from that, too. Everything should be able to vibrate around the servo, and so you should be able to wiggle the servo once it's installed. Fasten the screw until you just start to see the rubber deform.

RE: Servo mounting tips needed
 

Thanks Joe

AWESOME....thats what I thought....wasnt sure because the Futaba rubber gromets are two pieces, one for top and one for bottom.

RE: Servo mounting tips needed
 

Yes, you need both the top and bottom rubber grommet pieces on a Futaba servo. As I stated above, tighten the screw until is just touchs the rubber. If you see the rubber defrom any then it's too tight, just back it off a little bit so the rubber isn't deformed.

Ken