Pushrod connectors - hazard or not?

[Martin Harris - Moderator]

I've always hated these things on anything other than throttle linkages and my standard advice has always been to replace them with conventional Z bends, clevices or swing keepers wherever possible.

A couple of days ago, I made an observation about these little devices on another thread...and rather spookily, just 6 hours later, found myself checking over a clubmate's new Piper Cub which had them on all of his servo arms. I took special care to check that I couldn't move the rods in the connectors and asked him to confirm that he'd done them up as tightly as possible.

We ran the engine for a while in order to iron out some flow problems and once satisfied that all should be well for the maiden, carried it out to the strip. As per my normal practice, I did a final control check for direction and movement before taxying out - and the elevator failed to move.

I stopped the engine and we took it back to the pits for examination. You're probably ahead of me by now - yes - the servo arm was moving freely up and down the pushrod..

That was bad enough - thankfully, there was no harm done but at the field today, a member was flying his foam Wot4 and started encountering pitch problems, having to progressively apply down elevator to avoid it pitching up, culminating in a broken model as he lost his battle to get it onto the ground under control.

Yes - it uses screw connectors and the first thing that greeted me when I looked at it was a detached elevator pushrod...

I appreciate they are easy to adjust but I feel he little extra effort to ensure a reliable connection is well worth the effort.

So - have any of you encountered similar occurrences and does anyone have any observations on the subject?

[Former Member]

[This posting has been removed]

[Don Fry]

You are right, connectors designed by the devil. I don't know the answer. I used to refuse to use, but the conversion to good connectors took hours, then, with loktite and care, I used them , and they work, but even so, only to a point trusted.

And not trusted with summut you care about.

[Peter Miller]

I only ever use them on the throttle run. I refuse to use them anywhere else.

Relyinh on nothing but friction to hold something is not mechanically sound.

Lets put it this way. I have never seen anything relying on friction to hold it on a full size aircraft.

[Don Fry]

And so speaks God, and so to bed. Full of leeks, cooked and chewed, and full of fine wines. Night.

[Denis Watkins]

Please send them to me, we all use them (correctly) at my club. They are so convenient to set up centre throw on servo arm and pushrod, to control service. Send the dreadful devices to me thanks

[Geoff S]

I sit here nodding wisely and more or less agreeing that they are indeed the work of the devil then I glance at the fuselage of my Multiplex Blizzard stored nose down in a wellington boot beside my desk. Both elevators of the V tail are secured by similar devices. It would be almost impossible to replace them because of the unique double horns specially designed to fit in the foam. Obviously, they've never failed or the fuselage would be in land fill rather than by my desk.

I'm pretty sure my foamie Acrowot also has them as well. They seem almost to be standard fittings on small electric foamies and so far I've never had a problem. Perhaps not so good on liquid fuelled models with the extra vibration. Has anyone had a problem with on the throttle? Like everything else they need to be properly tightened and checked.

Geoff

[Capt Kremen]

Surprising comments ... Top German fliers use them extensively. (I have examined their models first hand at many German model airshows down the years. Multiplex kits are a current exponent of their use too).

The observation I would make though is that they use the type of fastener (Graupner usually), which requires an Allen key to securely tightened a grub screw onto the steel push rod/snake. The fixing itself being secured by a 'lock' nut to the control horn or servo arm.

Since visiting Germany from the 1980s onwards for model shows, I have used these extensively and with due regard to regular safety checks as required for any control rod assembly, have not experienced any issues.

As a previous post suggests, a dab of locktite plus you can add a blob of epoxy fore 'n aft of the fixing as extra security has never failed. I do fly all electric though so maybe engine vibration is less of a potential issue.

[PatMc]

Used them for donkeys years in gliders, IC & electric never any problem, often the most convenient method to use. I've even made my own using electric barrier connectors, I think before there were any purpose made ones available.

[IanN]

I have never used screw type connectors on any control surface and doubt I ever will

However, I routinely use these to connect thin piano wire pushrods to throttle arms. So they are subject to quite a bit of vibration, yet I never use threadlock or any other "assistance" to help them grip, and I have never had one even move - never mind fail

But I intuitively don't trust them in any other application. We're a funny lot, aren't we .....

[Ronaldo]

I've had these come loose so now I only ever use them on the servo end of throttle linkages as they are easier to adjust in tight spaces, and once the throttle is set I put a drop of thread lock on the screw/grub screw and re-tighten to make sure they don't vibrate loose. On the bottom threaded part which goes through the servo arm I put the tiniest drop of cyano on the nut to lock it.

Ok they don't always come loose on control surfaces, but they are more likely to loosen than a clevis or ball link as there is more force applied when operating a surface then there is when opening and closing a throttle ... so why take a chance ?

Ronaldo

[Adrian Smith 1]

Filed a flat on the rod and permanent (red) thread locked them. Never had a problem. That said only use them on throttle runs.

[leccyflyer]

I've used them on a number of small models, and even smaller ones than those on indoor and parkflyer stuff. The Multiplex version comes in at least two different sizes and make for a very simple set-up, being supplied as standard in many of their kits. The smaller sizes don;t even have a locknut to fix them to the servo arm, they are just a friction fit and work fine for those small models. I prefer a Z-bend and threaded rod with a clevis for most control runs, but that said I must have used dozens of similar connectors without issue.

Not sure that I'd trust them on an IC powered model with higher levels of vibration.

[Jon H]

I have used them for years without issue. I do tend to Loctite the grub screw though, that or do it up so tight I almost round off the allen key.

I would not touch swing keepers with a barge pole. They are hopeless

Edited By Jon Harper - Laser Engines on 13/03/2016 09:26:36

[cymaz]

Z or U bends on the servo ends will not fail. I use metal clevises, lock nut and fuel tube or zip tie as a securing clamp over the clevis arms. It has be faultless.

[Cuban8]

I think the problem is that the pushrod material that these connectors are screwed down onto is usually very soft metal. As an experiment, I've almost severed a 2mm pushrod by really leaning on the grub screw of the connector - not very encouraging as you get very little indication as to whether the screw is tight enough, because of the yielding of the pushrod material.

They're probably OK on decent piano wire where you can file a small flat and loctite the threads.

[Martin Harris - Moderator]

Thanks for the various comments and thoughts - many of which align with my general feelings about these connectors. I had accepted their use on small electric models to a certain degree but the Wot4 incident yesterday made me question this attitude.

I have the opposite view on pushrod hardness to C8's above though...my feeling is that they are often too hard for the relatively soft screws supplied with many versions of these - particularly with ARTFs and result in deformation of the screw end rather than creating a small indentation for the screw to lock into. Better versions using hard socket headed grubscrews should be superior in this respect.

When I have used these myself on throttles, my preference is to use a Bowden cable which allows a very positive bite from the screw.

Worst of the lot, in my opinion, are the plastic versions with star washers to retain them in the servo arm...the locating shafts seem to have a very hard time when fitting the retainers and usually end up strained and distorted!

Interesting comment on the swing keepers, Jon - I don't use them myself but I have yet to see a failure - what was your experience? There seems little load on the retainer and they could conceivably resist flutter loads better than a clevis - one of which I know to have detached in flight despite a fuel tube "keeper" being fitted...while testing a clubmate's model the elevator encountered flutter but I was lucky enough to be able to control it sufficiently with throttle to get it back on the ground in one piece.

Edited By Martin Harris on 13/03/2016 15:02:07

[John Privett]

I'll admit I'm not too keen on them, but I've used them on small electrics. I guess my 'turning point' was getting my first Sebart Katana (30 size) which was supplied with them for all surfaces. I figured as Sebart is decent quality, then if it's good enough for them...

[Stuart C]

For these things to work effectively, they must be free to swivel in the servo arm. Two design faults arise here.

1} Use of a threaded shaft for a bearing.

2) No suitable locking nut provision (thread too short)

The solution I found was to make a thin tube spacer, bore out the servo arm to suit, and Loctite the nut.

The locking grub for the snake end is much improved by grinding a flat on the snake wire and grinding the screw end square, since these are rolled threads and have a raised irregular lip at the end.

They certainly would appear to be designed by the Devil, though I suspect the original design to have been much compromised in order to aid (cheapen) mass production.

[Bernard Koussoulos]

I am now getting a tad nervouse i have one on the servo end of the throttle The others are curciet contro;ed elevater has some very strong fishing lin from both sides of the servo arm and go direct to the elevator or rudder . i don/t trust anything that,s attached to my control sefaces by a bit of plastic and or screw .thats my way of dealing with it but then i don;t fly fast 3D or missilles i go plodder old timer planes like me lolI'm sur3e every one has his or her own way of doing things lol

[Peter Beeney]

Just to add a another view in favour, I’d always use these connectors in preference to any other, with Z bends being the last in line. Although to be honest, I’ve never really had any trouble with any connector when they are installed correctly. I like the Great Planes screw connectors, and I always start by trying to the geometrics as accurate as I can, creating the shortest servo arm and longest control surface horn, but of course that’s not always possible if you want maximum surface movement. This means I have to decide which servo arm hole hole I’m going to use and I have a set of small drills to make the hole exactly the right size to accept the connector spigot. The Great Planes connectors have both a metal star washer fixing and a separate plastic washer, I use the plastic washer. As always, I’ve done my own primitive security checks on these, basically with a tommy bar and a pint of Guinness for extra beef, and I soon concluded that even if the holding screw is only pinched up it will hold sufficiently well enough without the pushrod moving to start to distort a standard Futaba servo arm; and then from there I also concluded that if there are strong enough forces going on within the model to start to bend said arm a slipping connector is the least of my worries! And as Denis Watkins remarked, they are so convenient to use into the bargain…

Along the same sort of lines, once upon a time I did have a bit of a meddle with the ‘keepers’ on the quick links, with some quite surprising results here, too. I can certainly remember David Boddington making some very cryptic comments (from experience) about using fuel tube keepers on metal (or plastic) clevises years ago, something I also implicitly agreed with at the time.

But then I always was a bit on the perverse side, I also use choc. blocks to make electrical connections in model planes…

So for and against; as usual in the modelling world, probably best to just use the methods you are entirely happy with.

PB

[Martin Harris - Moderator]

I certainly wouldn't argue against the convenience factor but the fact remains that in my personal experience, I have seen one clevis failure (discounting thread mismatch issues between 2mm and 2-56 items) which was due to flutter which is incredibly destructive), no z bend or swing keeper failures but at least two loose choc-block type connectors and several of these screw connectors either failing in flight or during the checks which our club does on all new models.

I do feel that this is more of an issue with IC powered models but out of balance props and spinners will cause meaningful vibration whatever the power source.

[Peter Beeney]

Regarding the control surface flutter, Martin, the is main reason I spend a bit of time on the control geometry first is to try and make the movement of the surface as secure and as rigid as possible in an attempt to help prevent it getting into a wobble in the first instance. In very general terms the types of model that might be likely to suffer control surface flutter, such as fast WW2 fighters etc. fortunately lend themselves to this sort of tinkering, in terms of generally requiring reduced throws. I’ve personally never owned a model that has experienced any flutter, but I have seen, (and heard!) it happen a few times, and indeed I was flying someone else's model once, the first flight of a Spitfire, when the dreaded buzzing started and within seconds it had taken the complete rear half of the fuselage right off! On another occasion, an Acrowot this time, the fin and tailplane were found to still be hanging by the (plastic) clevises, themselves still intact and attached to the servos, when it finally came to rest in the top of a small apple tree.

With respect to the screw type connectors I do admit there are some types I probably wouldn’t use from choice, they seem to require some careful assembly to make them work properly; as I said, for me the correct size servo arm hole is a good start. Also I don’t use choc. blocks as push rod connectors, just electrical connections; I use a good brand, after all a strip of each size will last a very long time. But again, I will agree that you have be careful, some of these are made of what appears to be a ‘brass lookalike’ material, about one grade up from hard plasticine, and if you are a bit over enthusiastic when tightening the screws it’s possible to split the body underneath.

Re: your OP, I do find it quite remarkable that these retaining screws can slacken off in such a short period of time. Although, as you say, if this does happen frequently then an alternative fixing is certainly the best idea, as this would always be an area that a pilot was always wondering about, to say the very least! Also I learnt a long time ago not to rely on anything someone else says anyway, particularly beginners. Way back a new learner assured me he was on a particular frequency, but as it happened he wasn’t; later we had a near miss, another mate at the patch came to me with a little glitching problem which was the frequency clash, but no harm done fortunately; since then I’ve never bothered to even ask, I just check everything myself first…

And just to add a rider to that, in those days my tx, a MPX, had a built in frequency scanner so I also made a point of checking transmissions with that as well. On one occasion I did find one tx xtal that was clearly marked as 62, when in fact it was transmitting on 61. We only discovered this because the owner couldn’t get his radio to work, though. But a pair of incorrectly labelled crystals could eventually indeed have some interesting repercussions…

For me there are other rather more important areas of safety; receiver battery integrity and particularly their electrical connections for a start. I’ve nothing personal against the fuel tube clevis keepers at all, either, but I’ve never been that convinced that in the event they make a lot of difference to the security of the clevis anyhow; I prefer to just use a good quality clevis and insure that it’s properly snapped shut.

Fly Safe..

PB

[Don Fry]

I tend to find flutter to be a function of control surface stiffness. At the end of the day it is a harmonic issue, complex, without a single cause. But they are still devil fasteners. OK in an ARTF, when they are the supplied bits, but not if you have first choice in your own machine.

[Martin McIntosh]

As club safety officer I have banned these things other than on the lightest electric models. The grub screw will eventually, or sooner, dent the rod to the extent that no matter how much Loctite you use it will work loose.

Anyone who has experience of retracts will testify to this.

They were introduced by ARTF manufacturers to make it simple for a novice to connect everything up without hassel. I wonder how many models have gone in because of these?

Learn how to do it properly and just do not use them.

A clubmate once gave me a 91 powered Mustang to sort out because the retracts were iffy. It was an ARTF which had been assembled by a local model shop proprietor who should have known better. Those hideous things were simply pushed into the servo arm and rocked about so much that most of the movement was lost.

Edited By Martin McIntosh on 17/03/2016 20:09:57