Broken servo arm screw

[Don Fry]

I've got a Yak 55M, with a Laser 80 shoehorned in. I am in the process of harmonising the controls. The recommended aileron deflections are a bit excessive for me. The mid rate is about 4 rolls per second, which is more than enough, so I wanted to move the push rod one hole in along the servo arm, so I can reduce the total throw at full servo deflection, which will give 4 rolls per second at high rate.

Servo is a metal gear digital, 25 spline Futaba type. I locktighted the screw into the output shaft with blue medium tack locktight.

I did the first servo, without removing the arm, but it was a right faff. So being hot and bothered, I decided to remove the arm to make the job easier.

The servo screw, (supplied with servo) sheared off flush the the top of the spline. Oh how I laughed and joked in a gay manner. It is the last of a series, so I have not got a replacement.

1. I can swap both aileron servos out, and put another pair in.

2. I could cut a groove through the sheared servo screw, (and the spline out of necessity), and see if I can extract it. Might work.

Any one got a better idea.

Also, what do folk use to stop screws vibrating free.

Rest assured, I will be using quality screws made of steel, not cheesium in future.

I am fed up. This thing has cost me a lot of grief to sort it out. Just when I have it on the home stretch, and me thinking I think I might get to like this ( lovely stall turns, impressive verticals, etc), I've broken it.

[Rich too]

Not sure how to fix it but I would swap out in the meantime for two new servos. I also use the blue thread lock, but I use it very sparingly.

[Steve Balaam]

Before removing any locktited screws apply heat, heating the end of the screwdriver or allen key will do it and the screw will come out easily or use a solder iron tip.

A little ordinary white glue applied to the threads is usually enough to stop the screw vibrating out, easy to remove as well.

As far as your existing servo's go can you remove the top gear leaving enough screw to withdraw it or replace the top gear itself.

[Rich too]

Nice tip Steve Yep, I've used white glue for securing servos in the past.

[John Tee]

I have never had to use anything and the screws have not come loose (not on as big  engines as the Laser) perhaps I've just been lucky.

John

Edited By John Tee on 24/08/2019 17:30:41

[Anthony Scott 2]

I use Tippex, on exhaust pipe screws it heat hardens but can still be released as required.

[FlyinBrian]

Can you get a replacement gear set?

[alan p]

As previously mentioned gentle heat to screw or gear and patience

[ASH.]

This is an interesting thread.. I have a few high torque metal gear Futaba which I'll be utilising in the near future. Never thought of using any locktight. I suppose for petrol engine models it's important. I will use PTFE plumbers tape, which is what I use on all manifold and silencer threads. It works perfectly.

I'm sure you'll get there Don, just persevere. Dismantling the servo as suggested may be the answer.

[Don Fry]

I am here, but the rugby is on.

[john stones 1 - Moderator]

[Don Fry]

Now awake again. Thank you all for the suggestions. I will try the white glue idea in future, as it's function is only to make sure the screw doesn't shake loose. It's not a very hostile environment, And as I said, I will use better screws. I was not being brutal on it, and it was seeming to turn on normal drag, but it broke. Should have used heat, but didn't know that trick.

I will try to slot it with the Dremel. If it works it will mean I can get going again without trim changes. If not, I have one of the with a damaged case. I will try to swap the final gear/spline unit, but at this point I have to say I'm going in blind as I don't mess with them normally. Thereafter, a pair of new servos, not the end of the world as they will be physically identical, but I will then have to re trim the ailerons. Anyway, that's for after lunch,

I've been teaching a 7 year old to fly this week, and on her 8 th birthday on Wednesday made her first landing. She's gone home now, and I've got some batteries charged, so I might as well fly them to storage.

Again many thanks.

Edited By Don Fry on 25/08/2019 09:32:30

[Cuban8]

How about tiny shake-proof washers instead of threadlocker? All the metal geared servos I've had have come with shake-proof washers and I've never had a problem. Easily lost though. Even low strength loctite can be a so and so to free off and heat is not always appropriate.

Probably aware, but Loctite or similar is not good for certain plastics (makes them brittle) so never use with nylon output gears as belt and braces.

[Allan Bennett]

I removed an 'impossible' sheared screw once by very carefully drilling into it with a 1mm bit to start with, and then tried to ream it out using a very pointy burr which came in a set of Maplin accessories for my Dremel. For some reason I had the drill in reverse when using the burr, and almost immediately it got a grip on the screw and screwed it out. I'm guessing that the heat from the initial action of the burr loosened the blue loctite enough that it didn't need much friction to grip and turn it.

[Don Fry]

Cuban, I've never come across shake proof washers on metal geared servos. Obviously we use different brands. Are they with their teeth on the inside, or the outside face?

Allan, I have a set of proper centre bits. But the break face has a peak near the centre, and I don't fancy my chances that the bit will take still on centre. I think I will try slotting it across the spline. That will also heat it.

[Cuban8]

Suitable small internal shakeproofs are available on Ebay for a few pence. My Hitec MG servos came with shakeproofs as did a couple of cheap but good Hobbyking servos.

External teeth shakeproofs tend to be used to best advantage on larger diameter nuts and screws, so stick with internal types for servo screws. A new washer every time when removing/replacing the screw, ideally. Resist the temptation to really lean on screw when tightening up.

 

Edited By Cuban8 on 25/08/2019 12:55:09

[Don Fry]

Well not my finest hour. Mind like cotton wool. It's hot here, but my flying was OK.

The groove cutting was a disaster, because idiot here forgot I have two thicknesses of cutter, and putting a groove across the 3mm shaft needs the thin ones, and I did the bull in a China shop with the thick ones. So I attempted to replace the output spline. I thought it was right, but on testing, something was not engaged, and I couldn't be bothered to find out what it was, or tempt fate that I am trusting a nice aircraft and bits, to my skills as a servo repairer.

I might learn, as I've collected a boxful of worn out ones over the years.

So I will fit a pair of matching new servos when I have calmed down. Ordered some shake proof washers, then looked in the M3 box and there are hundreds in there. I used to be clever.

[Martin Harris - Moderator]

Posted by Allan Bennett on 25/08/2019 11:57:41:

I removed an 'impossible' sheared screw once by very carefully drilling into it with a 1mm bit to start with, and then tried to ream it out using a very pointy burr which came in a set of Maplin accessories for my Dremel. For some reason I had the drill in reverse when using the burr, and almost immediately it got a grip on the screw and screwed it out. I'm guessing that the heat from the initial action of the burr loosened the blue loctite enough that it didn't need much friction to grip and turn it.

Left handed drills are available (no, I'm not joking but I'm not sure if they would be easily available at servo screw sizes) primarily for just this sort of job. As you've discovered, a combination of heat and vibration from drilling can often loosen a sheared stud or bolt and the left hand rotation can spin out the broken fastener.

[Brian Cooper]

Another vote here for using white glue to secure servo output arm screws. I have used this method for about 40 years and not had one screw come undone yet, but, crucially, the screw can be released with a firm "jolt" with a screwdriver (or Allen key depending on the type of screw/bolt).

It doesn't need much -- just smear the thread.

B.C.

[Former Member]

[This posting has been removed]

[Don Fry]

Anyway, to close, the job is done, a pair of servos installed, arm screw kept attached by a smear of white glue, and a shake washer.

The servos, Corona HV units, which I like very much, except the fragile cases. I swapped the first out, and the second, had exactly the same problem, screw would not shift, and I think they have an odd head, it looks posidrive, but not quite, or Phillips, but not quite. Anyway, my (new) Bosch drivers are not tight on the cheesium.

And then, leaning on only slightly, a crack, and the servo tray detached. I am totally out of love with ARTF airframes. Now all sorted, with holes for fine adjustment.

More work than a hack deserves.

Doc, I've agree, never seen a 1 mm extractor. Given how hard they are, pretty fragile.

[jrman]

Don, They could be JIS (Japanese industrial standard) heads. Similar to Phillips and Pozi but different enough to give the type of problem you have encountered in the driver not fitting correctly.

Edited By jrman on 27/08/2019 18:15:10

[Martin Harris - Moderator]

Posted by Don Fry on 27/08/2019 17:54:36:

The servos, Corona HV units, which I like very much, except the fragile cases.

Doc, I've agree, never seen a 1 mm extractor. Given how hard they are, pretty fragile.

Doc was talking about left handed drills - which prompted me to check and they are indeed available down to 1mm. I have a small selection but nothing that small.

I'm glad I'm not the only one to have noticed the brittle mounting flanges which spoil the Corona metal geared servos - previous comments haven't elicited any other people who'd encountered the same problem.

This was my solution:

The aluminium flanges are epoxied to the case tops.

P.S. I've always used threadlock on metal output gears and have never had any difficulty breaking its grip.  Different grades are available - perhaps yours was intended for permanent or semi-permanent applications?

Edited By Martin Harris on 27/08/2019 18:45:51

[Don Fry]

Nice photo. My solution has been a bit more agricultural, a bit of birch ply. Mind, I have a secret plan. I will not hit the ground. I have noticed they only break if you hit the ground.

And regarding extractors ( left hand taper bits), I once got a 3 litre Austin engine, 5 Quid, in bits, with a broken head stud. Very nice engine. It had a sheared off extractor in the sheared off head stud. With a load of off centre attempts to sort it out. My dad took me to a different power station, 40 miles from home, and the head got mounted on the biggest mill I've ever seen. And it bored the lot out. It destroyed a mill bit. I suspect the mill bit was worth more than the engine. Such is the worth to an organisation of the blokes who have the skillls to fix machinery.

Edited By Don Fry on 27/08/2019 18:56:25

Edited By Don Fry on 27/08/2019 19:27:46

[Martin McIntosh]

I have several MG servos with shakeproof washers supplied but the easy answer is a dab of Evostick. Mending servos these days is a waste of time since they are generally so cheap to replace. Balance the cost of one against the rest of the model.