Servo Grommet Inserts

[Skippers Walker]

Please settle a friendly discussion about whether the little metal inserts provided with some servos, should be inserted from the base of the 'rubber' mount or the top (screw entry end) All comments welcome?

SW

[Nick Cripps]

From underneath.

[Andy Stephenson]

+1 for underneath, as it prevents the sharp end of the eyelet from sinking into a wooden bearer, which would lead to the rubber grommet being compressed too much.

[Peter Jenkins]

The reason for inserting from underneath is that when you tighten the mounting screws they squeeze the rubber grommet till they bottom on the ferrule and that gives a repeatable force provided by the rubber grommet for mounting the servo.  Once the screw has bottomed, you will not increase the servo mounting force no matter how tightly you turn the screw.  That's the role of that little ferrules that are supplied.

 

If you put them the other way up, the bottom of the ferrule just pushes into the mounting plate and the more you tighten the screw the more you squeeze the rubber grommet and the higher the force you apply to the servo mounting lugs.  Don't do it that way!

[Martin Harris - Moderator]

Absolutely - for documentary evidence check an older Futaba manual but think logically (see replies above which came in as I was typing). 
 

One of the first jobs on a second hand model I do is check the “top hats” - and they are usually the wrong way up so there are many who haven’t understood their function as spacers rather than “pretty” washers. 

[Robin Colbourne]

See below to get it from the horse's, or rather, Futaba's, mouth (Page 33).  I guess this is aimed at cars or helicopters, hence the machine screws and nuts. 
In the days when most servos were the same size and they lived near the centre of gravity, I liked the supplied plastic servo trays for rudder, elevator and throttle servos (sometimes the switch too).

[Skippers Walker]

Brilliant comments thanks. That settles my friendly argument with a colleague.....I had supported the 'flange at the bearer' idea, thus preventing the over tightening issue discussed.

Happy flying

SW

[leccyflyer]

If you think about the wee brass top hat having one of the two flanges to support the grommet, the flange on the servo screw supplies the second flange, so that makes it very easy to see the point and remember which way they go. I usually insert them from beneath, using the shaft of a small jeweller's screwdriver to get them in place.

[Geoff S]

Do builders still use a vibration free mounting method (as illustrated above) in small electric models?  I sometimes don't bother and have never had any problems.

[leccyflyer]

If the servos are packed with grommets and the servos are mounted conventionally on rails then I still use them, but many micro, nano and pico servos do not have grommets and vibration really shouldn't be an issue with an electric model unless something is significantly unbalanced.

[Robin Colbourne]

14 minutes ago, Geoff S said:

Do builders still use a vibration free mounting method (as illustrated above) in small electric models?  I sometimes don't bother and have never had any problems.

It does seem common these days not to receive grommets with small servos.  The crazy thing is that from my experience with environmental testing on industrial shakers, it is the higher frequencies of vibration that kills electronic components, whereas the lower frequencies are what destroy mechanical (gears, cases, bearings) and electrical parts (motors, wiring, connectors).  Electric models still produce the higher frequencies from out of balance props at higher rpms whereas the torque reversal as an engine at low revs comes up against compression, then fires produce the lower frequencies.
I use the little bits of hardish foam that come between glazing panels to make pads to put either side of the servo mounting flange on servos supplied without grommets.

[John Stainforth]

2 hours ago, Skippers Walker said:

Please settle a friendly discussion about whether the little metal inserts provided with some servos, should be inserted from the base of the 'rubber' mount or the top (screw entry end) All comments welcome?

SW

They should be inserted on the underside, i.e., between the servo tray and the lug (as per instructions!)

 

[Philip Lewis 3]

Personally I always insert them the "wrong" way round in my models BUT they are all electric, electric planes really don't need to isolate the vibration from the servo's and given all the trouble I've gone to in order to make the linkage 100% slop free the last thing I want is the servo moving slightly in the rubber grommet.

 

That is though a special use scenario, IC definitely from the bottom.

[Martin Harris - Moderator]

Isn’t it likely that as the spacer bites into the mounting due to transmitted loads, it will effectively loosen the screw and introduce the slop that you’re trying to avoid?

 

If you really can’t tolerate the designed movement then solid mountings rather than using the rubber grommets might be preferable. 

[Geoff S]

2 hours ago, Robin Colbourne said:

It does seem common these days not to receive grommets with small servos.  The crazy thing is that from my experience with environmental testing on industrial shakers, it is the higher frequencies of vibration that kills electronic components, whereas the lower frequencies are what destroy mechanical (gears, cases, bearings) and electrical parts (motors, wiring, connectors).  Electric models still produce the higher frequencies from out of balance props at higher rpms whereas the torque reversal as an engine at low revs comes up against compression, then fires produce the lower frequencies.
I use the little bits of hardish foam that come between glazing panels to make pads to put either side of the servo mounting flange on servos supplied without grommets.

My experience of using electronics in a harsh environment was when I was involved in the design of a short range (about a mm!) radio telemetry system to make measurements on gas turbines under test.  All the electronics was potted in epoxy and we had very few failures despite their being subject to around 20k g and 125 deg C.  This was over 30 years ago and, although solid state, had nothing like the degree of integration we get now. In fact the transmitters were a simple 2 transistor circuit I designed, built in 'mid-air', and potted.  Most failures were down to wiring to thermocouples and strain gauges becoming progressively detached during an engine run. 

 

Servos rarely suffer`high g loading except in terminal situations 🙂 and certainly not super high temperatures and the vibrations they suffer with a reasonably balanced electrical power propulsion system is relatively low even if the frequencies are in the right range.  I'm amazed how robust and reliable even relatively cheap servos are.  The most common failure in my experience is stripped gears caused as often as not by knocking a control surface accidentally in transit.

[Simon Chaddock]

Servos in electric foam planes simply do not need vibration damping. The foam structure of the plane is doing that already.

[Robin Colbourne]

1 hour ago, Geoff S said:

My experience of using electronics in a harsh environment was when I was involved in the design of a short range (about a mm!) radio telemetry system to make measurements on gas turbines under test.  All the electronics was potted in epoxy and we had very few failures despite their being subject to around 20k g and 125 deg C.  This was over 30 years ago and, although solid state, had nothing like the degree of integration we get now. In fact the transmitters were a simple 2 transistor circuit I designed, built in 'mid-air', and potted.  Most failures were down to wiring to thermocouples and strain gauges becoming progressively detached during an engine run. 

 

Servos rarely suffer`high g loading except in terminal situations 🙂 and certainly not super high temperatures and the vibrations they suffer with a reasonably balanced electrical power propulsion system is relatively low even if the frequencies are in the right range.  I'm amazed how robust and reliable even relatively cheap servos are.  The most common failure in my experience is stripped gears caused as often as not by knocking a control surface accidentally in transit.

Geoff S, yes, you're right its amazing what the servos will tolerate, and transport and hangar rash probably account for most mechanical failures.  Crashes and heavy landings aside, I expect flutter-induced vibration is probably the next biggest servo life reducer.

Strain relief is an area which gets very little attention.  I do wonder whether tiny receivers are better off sitting in a bundle of servo wires than being more rigidly mounted these days.
 

[Philip Lewis 3]

2 hours ago, Martin Harris - Moderator said:

Isn’t it likely that as the spacer bites into the mounting due to transmitted loads, it will effectively loosen the screw and introduce the slop that you’re trying to avoid?

 

If you really can’t tolerate the designed movement then solid mountings rather than using the rubber grommets might be preferable. 

I put a washer over the grommet and then use a self taping screw to screw to screw it down tight, the unshouldered spacer sinks into the mounting plate and makes it pretty secure and tight, but on the last few builds I have done then I did solid mount them.

[Nigel R]

32 minutes ago, Robin Colbourne said:

I do wonder whether tiny receivers are better off sitting in a bundle of servo wires than being more rigidly mounted these days.

 

I still use the 'old school' method of foam wrap. It does not take long, and seems to cover all bases.

 

[Geoff S]

45 minutes ago, Robin Colbourne said:

Geoff S, yes, you're right its amazing what the servos will tolerate, and transport and hangar rash probably account for most mechanical failures.  Crashes and heavy landings aside, I expect flutter-induced vibration is probably the next biggest servo life reducer.

Strain relief is an area which gets very little attention.  I do wonder whether tiny receivers are better off sitting in a bundle of servo wires than being more rigidly mounted these days.
 

 

Strain relief seems to be often neglected. At our indoor flying session last week a Night Vapor failed because the wire from the receiver board to the battery connector broke close to the board itself.  There was no strain relief and all the mechanical support was taken by the tiny soldered joint.  An eventual failure was almost inevitable.

 

Still, that's a bit off-topic.