Pushrod connectors - hazard or not?

[PatMc]

10 out of 10 for pomposity, Martin.

[Jack Banner]

I have had one fail. It was on a throttle on my first I.C trainer, and locked the throttle fully open. No damage as I just flew round pylon style until it dead sticked. I still use them but always with threadlock and only on the throttle.

Banning them seems a bit much but will have saved planes.

[Kevin 216]

My Chris Foss Wot 4 Foam has these connectors as standard I presume I would be banned from flying it.

[Andy48]

What about snakes with a small length of 2mm threaded rod screwed into the end of a piece of soft plastic?

[IanR]

Hello Peter,

I came across the Great Planes connectors whilst putting together a GP Avistar Elite artf model and was very impressed. Other pushrod connectors have left me a bit cold for all the reasons mentioned above but the GP ones seemed very good indeed.

The nylon retainer they use would not come off without using a flat screwdriver to prise it and the tightening screw, as you know, is a cap head allen bolt that you can really tighten down.

Anyway, I ordered a packet over the internet from Wheelspin Models but they were not to the same standard. The retaining shoulder which is meant to hold the nylon retainer was barely there - I could pull the retainer off with my fingers. The thread for the tightening screw was not deep enough and the screw would not go deep enough into the hole.

Anyway, after a brief phone call, Wheelspin Models sent me a stamped addressed label to send them back and I'm currently waiting to hear from them. In the meantime I have noticed that these connectors no longer appear on their website.

I have googled but cannot see them listed anywhere else.

Ian

P.S. I would just add that Wheelspin Models have been very fair and did not quibble about my sending the connectors back to them. I would recommend them to anyone - their website is well worth a browse, particularly the spares section which is extremely comprehensive listing things you just don't see on other websites.

[Peter Christy]

The flybar on every helicopter I've come across in the last 40-odd years has been restrained by a simple collet and grub-screw. In all that time, I've only ever seen one come adrift, and that was on a rather ill-constructed model. Even then, the paddles prevented the flybar from - er - flying off! (Although the helicopter did disintegrate in spectacular fashion!)

If a grub-screw and collet can safely restrain the flybar in a high vibration environment like a helicopter rotor-head, I fail to see why it should not be entrusted to restrain a relatively low load push-rod.

Provided it is done up properly, of course!

--

Pete

[Dave Hopkin]

Having used this type of connectors many times, I find the biggest risk with them is not the grub screw grip on the control rod, but the retaining nut/thumbwheel on the main shank where it retains the connector onto the servo arm - Typically the retaining nut is concealed by the servo arm and cant be easily visually checked and repeated movement can cause the nut to come loose and fall off - easily prevented with a dab of CA on the thread or some locktite etc -

I have never had an issue with the grub screw working loose or the control rod "spreading/growing" under the pressure (and as grub screw grip is widely used in industry for many years I frankly find it hard to believe that steel will somehow spread or grow......

Like ANY connector there are good ones and bad ones,,,,,

One wonders why a club would ban this type of connector yet allow grub screw retained electric motor shafts.....

[Don Fry]

Cos they can I suspect Dave.

[Andy48]

Posted by Dave Hopkin on 19/03/2016 10:23:13:

Having used this type of connectors many times, I find the biggest risk with them is not the grub screw grip on the control rod, but the retaining nut/thumbwheel on the main shank where it retains the connector onto the servo arm - Typically the retaining nut is concealed by the servo arm and cant be easily visually checked and repeated movement can cause the nut to come loose and fall off - easily prevented with a dab of CA on the thread or some locktite etc -

I always replace these with 2mm locknuts, and then add a drop of cyano.

[Peter Beeney]

Martin Mc, I’m not really sure that it is always a case of `because it hasn`t happened to me yet so these must be OK`, I think it’s sometimes rather more a case of ‘ I consider this to be a reasonably sound engineered item and I also consider this will do the task for which it was designed in a satisfactory manner’.
The metal of a decent steel rod will not relax out from under the grub screw at all in the same way that copper does and if the grub screw is sufficiently tightened then the rod will not slip or move in the connector body; and certainly not under the normal loads of flying surfaces, as I’ve always considered them to be fairly light anyway.
I would consider the case of the undercarriage legs to be a different entity altogether. I’m sure many of these items could be very much improved but how many ordinary modellers have the necessary knowledge and the resources to be able to do this?
Using the Great Planes connector again as an example, this has a cap head hex drive retaining screw, I have a decent set of hex drivers and I also have a set of the extra long hex (Allen) keys. If I installed a push rod with these connectors I would be prepared to bet a small fortune and my house that this would not slacken off before I got to the runway…..or at any time in the future, either…

So…. this leaves us with the conundrum, why are these things failing? I’don’t know, but if it did happen to me I would quickly be trying to replicate the situation to try and get a handle on it. Perhaps you might like to change your label to Safety Investigative Officer, and then carefully analyse what exactly is going on here… is it the component, or the parent material at fault, the operator or perhaps a combination of all three. Then, instead of a what appears to be a questionable blanket ban some ‘best advice’ could be offered, appropriate to what ever you’ve found to be the reason for it all to fall apart.

I reckon that Dave H hit the proverbial head right on the retaining nuts, too. I’ll go along with this all the way, I’ve found that these infernal devices can in some instances require more than a modicum of patience to get exactly right (for me) …and from many years of sitting watching and helping in the pits, with the greatest respect I think some folks will find it well nigh impossible… I’ve always used a small dollop of contact adhesive as a locking medium, it works well even on those aggravating little screws holding cowls on etc. that will insist on coming loose at every opportunity. But the real benefit is, should you wish to remove the screw/nut/fastener you can grip the glue with a pair of needle nosed pliers and easily pull it of completely cleanly, leaving the metal in pristine condition.

Thanks for your reply, Ian. Regrettably, these rather ominous changes do now appear to be happening with monotonous frequency. However, from what you said, it crossed my mind that could they be counterfeit? It has been stated that just about everything that’s ever been made has been faked at sometime or another; and it does seem unlikely that an outfit like Great Planes would be willing to knowingly provide items that are so obviously unsuitable for purpose. Let’s hope that Wheelspin are able to dive into it and you get some feedback on what they come up with.
As you say, 100% full marks to them, it really can’t be easy with all the different situations they have to deal with, but they certainly do seem to have your best interests at heart at least! I have occasionally dealt with them but it’s always been perfectly straightforward.

Happy Landings..

PB

Edited By Peter Beeney on 19/03/2016 14:10:07

[Stuart C]

"What about snakes with a small length of 2mm threaded rod screwed into the end of a piece of soft plastic?"

These can be quite a liability. Depending on the brand, the thread length can vary enormously - even in the same packet! The inability of the plastic to maintain the hoop stress is something I have needed to sort out in the past. Having made ferrules, crimp bands, all sorts of glues, the simplest was to add whipping. Belt and braces where ever possible with control systems.

[Colin Carpenter]

This type of connector is in use in my 3 Sebarts , Acrowot Foam E , Riot , Multiplex Easyglider , HK S Bach. Zero problems. I do loctite however . About 750 flights. Not sure I would use on a vibrating IC model though . Colin

[leccyflyer]

Posted by Martin McIntosh on 18/03/2016 20:24:38:

Well, I seem to have stirred up a right can of worms here and there have been some very good points and some not quite so `because it hasn`t happened to me yet so these must be OK`. The analogy with the copper wire connectors just about sums up what will happen over time.

As I said before, try using retracts which are almost invariably secured with grub screws and you will see that these will work loose in a very short time.

I think that the matter is best left at that.

What sort of comparative shock loads do you imagine are put on a retract unit, versus the shock loads put on a pushrod connector?

It's not a case of "because it hasn't happened to me yet so these must be OK" at all - it's been completely described by a large number of posters actually using these connectors successfully across a range of applications that they consider them to be a useful and usable alternative. Those posters have qualified their use and identified the circumstances under which they would not use them - borne out by the small number of described instances where they have failed, such as on the throttle connection of an IC motor.

Those posters aren't relying on false analogies with completely different metals or making boldly definitive assertions that cannot be justified.

[leccyflyer]

Posted by Kevin Wilson on 18/03/2016 07:59:08:

I have used the pushrod connectors shown in the first post. They were good quality items from Multiplex.

Decent metal, smooth shaft, decent hex grub screw. Bearing on hard piano wire.

I was very suspicious of their holding ability having been brought up with Z bends and metal clevises.

Securely tightened and a drop of CA adhesive. They remained serviceable for years, surviving many novice 'landings' and are still rock solid.

I wouldn't hesitate to use them again (although probably not by preference) as long as it was an appropriately sized model.

Like much of aeromodeling you have to have an appropriate solution with appropriate equipment.

Spot on - good post

[Martin Harris - Moderator]

Posted by Peter Christy on 19/03/2016 09:53:27:

 

If a grub-screw and collet can safely restrain the flybar in a high vibration environment like a helicopter rotor-head, I fail to see why it should not be entrusted to restrain a relatively low load push-rod.

Provided it is done up properly, of course!

--

Pete

I think this may be the crux of the matter. Helicopters are, in my limited experience, built with the expectation of vibration induced fastener loosening and in the knowledge that they are retaining critical applications. Therefore, great care is/should be taken to fasten components (which are normally of good quality) correctly - and lock them with anti vibration compounds.

It may be that less care is taken with fixed wing (yes - I know that we forumites all take the time and trouble to build our models to exacting standards but not everyone is so conscientious and well informed!) How many times have we seen wheels lost from fixed wing models that were retained by collets? I always grind a small flat and Loctite mine...as I'm sure many others do.

The example earlier of motor shafts is a similar principle - I've yet to see one retained by a grubscrew without a machined flat on the (hardened) shaft for it to lock into.

As the OP who asked the question in order to gain opinions and experiences - and test my innate prejudice against them, it seems that (in general) electric flyers are happy to use these fixings and IC operators are somewhat warier. I'm tempted to put the foam Wot4 incident down to the inexperience of the owner - although there was no sign of looseness when it first appeared at the club, he had done some repairs recently so he might have disturbed it...

The fact remains that I have come across loose examples quite a few times - mostly during pre-flight checks on models newly prepared by their owners (with varying experience) but on a few occasions I have witnessed models which have been operated for some time develop slack connections.

I think I would still discourage their use in IC models and certainly wouldn't contemplate using them in one of my own builds - perhaps retaining (no pun intended) them in an electric ARTF if there was no reasonable alternative and I'd prefer them to have hex socket grubscrews rather then slotted or crosshead (themselves a minefield due to people thinking one driver type fits all) screws. The issue of the threaded shaft retained without any locking mechanism while being subject to twisting moments worries me - again, there are adequate safeguards but they should all be designed with a plain shank to pass through the servo arm at the very least.

Thanks to all who've contributed their thoughts - I think all have made constructive points even if we haven't all come to the same conclusions!

Edited By Martin Harris on 20/03/2016 12:10:25