Pushrod connectors - hazard or not?

[PatMc]

Posted by Martin McIntosh on 17/03/2016 20:00:33:

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.

If I was a member of your club I'd challenge your authority to make such a high handed decision based on nothing more than your personal opinion.

[IanR]

Peter - where do you buy your Great Planes screw connectors?

Thanks, Ian.

[Martin McIntosh]

Sorry Pat, it it not a high handed decision but based on the safety of the models and anyone who happens to be in the way when, not if, one of these comes loose. Can you imagine someone building, for instance, a 20kg jet and relying on a grub screw to hold the elevator linkage together? Get real.

 

It is not a personal opinion, just fact.

Edited By Martin McIntosh on 17/03/2016 21:10:20

[PatMc]

Martin, there's a heck of a lot of different sized models between "the lightest electric models" & "a 20kg jet". It would be absurd for you to be able to decide what this particular piece of hardware wasn't suitable in many of them.
What "fact" do you claim you're acting on ?

[leccyflyer]

Posted by PatMc on 17/03/2016 20:19:11:

Posted by Martin McIntosh on 17/03/2016 20:00:33:

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.

If I was a member of your club I'd challenge your authority to make such a high handed decision based on nothing more than your personal opinion.

^^^^^^What he said^^^^^^^

[Martin McIntosh]

Just for the benefit of novices to the hobby a hardened grub screw will bed into an often softer wire. When stresses are introduced this will cause the screw to wriggle around and subsequently further squash the wire and loosen thus inviting a failure of the linkage. Loctite will only prevent the screw from undoing but will have no effect on the metal fatigue taking place.

Fine on an indoor craft or similar outdoor types but anything which vibrates at all, including electric, will result in a failure. Many who are new to the hobby such as I suspect some of the above contributers may be have been brought up with these because `They are what are supplied so must be OK`. You live and learn and would do well to heed the advice of others who have learned the hard way.

Don`t forget to take a black bag with you when you next go flying.

[leccyflyer]

Posted by Martin McIntosh on 17/03/2016 22:54:19:

Just for the benefit of novices to the hobby a hardened grub screw will bed into an often softer wire. When stresses are introduced this will cause the screw to wriggle around and subsequently further squash the wire and loosen thus inviting a failure of the linkage. Loctite will only prevent the screw from undoing but will have no effect on the metal fatigue taking place.

Fine on an indoor craft or similar outdoor types but anything which vibrates at all, including electric, will result in a failure. Many who are new to the hobby such as I suspect some of the above contributers may be have been brought up with these because `They are what are supplied so must be OK`. You live and learn and would do well to heed the advice of others who have learned the hard way.

Don`t forget to take a black bag with you when you next go flying.

Haven't needed a black bag yet, a a result of control linkage failure, despite having used dozens of these sort of connections. Then again, personally I do try to avoid my models vibrating their %$£K off and haven't been brought up with this sort of connector either. Not to mention that I'm not attempting to dictate that something which thousands of modellers having been using successfully warrants an unqualified ban.

[Bob Cotsford]

While I can think of many reasons why they shouldn't be used on medium/large models (e.g. offset load twisting the servo arm, poor quality screws that can't be tightened in many examples, insecure fixing in the servo arm etc.) I have a few electric models that use them successfully and have done for several years - Multiplex Cularis, FunFighter, Dogfighter. I also use them on IC throttles on occasion, though once set up I coat them in CA and believe me, they do not like to come apart again and do not slide on the rod. Would I use them on a control surface for a model of any size? Not a chance, Z bend or clevis every time!

Do I think they should be banned - nope, but I would suggest to newcomers that there are much better ways of doing things. If I saw someone using them on larger models I would be looking a lot deeper than just the suitability of those connectors - for instance, does that person have the experience to be flying a large model in the first place.

[Martin McIntosh]

We try to ensure that everything is done as safely as possible in our club. Z bends, quick links and proper ball joints cannot come off. Please tell me what is stopping your grub screws from coming loose, even on a glider.

[john stones 1 - Moderator]

Opens up a fair old debate when it gets like this

What is a suitable connector on a large petrol jobbie ???? opinions may vary

John

[leccyflyer]

Posted by Martin McIntosh on 17/03/2016 23:12:43:

We try to ensure that everything is done as safely as possible in our club. Z bends, quick links and proper ball joints cannot come off. Please tell me what is stopping your grub screws from coming loose, even on a glider.

It must be witchcraft, since none of them have ever come off. Or maybe Keil Kraft, rather than witchcraft?

[PatMc]

Posted by Martin McIntosh on 17/03/2016 22:54:19:

Just for the benefit of novices to the hobby a hardened grub screw will bed into an often softer wire. When stresses are introduced this will cause the screw to wriggle around and subsequently further squash the wire and loosen thus inviting a failure of the linkage.

That's just scaremongering fantasy.
These fittings are often more suitable than the some snap clevises supplied with many kits. Personaly I prefer all metal clevises but even some of them don't close securely.

Why on earth should the grub screw in these fittings come loose ?

[leccyflyer]

The most catastrophic control surface failure that I have witnessed was on the maiden of a Wot-4, built to order, equipped with a metal clevis (my own preference as it happens) on a threaded coupler. Of course one was metric, the other imperial and the loss of eleator resulted in the model entering a shallow dive into the embankment which was non-retreviable. Does that mean metal clevises are inherently dangerous, sufficient o warrant an unqualified ban? No, of course it doesn't.

[Don Fry]

Posted by Martin McIntosh on 17/03/2016 22:54:19:

Just for the benefit of novices to the hobby a hardened grub screw will bed into an often softer wire. When stresses are introduced this will cause the screw to wriggle around and subsequently further squash the wire and loosen thus inviting a failure of the linkage. Loctite will only prevent the screw from undoing but will have no effect on the metal fatigue taking place.

Fine on an indoor craft or similar outdoor types but anything which vibrates at all, including electric, will result in a failure. Many who are new to the hobby such as I suspect some of the above contributers may be have been brought up with these because `They are what are supplied so must be OK`. You live and learn and would do well to heed the advice of others who have learned the hard way.

Don`t forget to take a black bag with you when you next go flying.

Any engineers on this forum, the above explanation strikes me as belonging to the same group as adverts for slimming products. A few percent right, and then a load of pseudoscience. I don't care either way, but as the tread is claiming to educate newcomers, reasoned peer reviewed argument should be proffered. Any engineers present to examine the above explanation.

[bouncebounce crunch]

I have had servo horn splines fail more than once (long time respectable brands too) but none of those connectors that are on my models have ever failed.

[Kevin Wilson]

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.

[Bob Cotsford]

Posted by Martin McIntosh on 17/03/2016 23:12:43:

We try to ensure that everything is done as safely as possible in our club. Z bends, quick links and proper ball joints cannot come off. Please tell me what is stopping your grub screws from coming loose, even on a glider.

If flight loads are sufficient to cause the grub screw to displace metal in the pushrod allowing thre locking screw to loosen then the load capacity of the fitting is being exceeded. Simple. Seeing as these fittings cope perfectly well with the loads on a flat out FunJet or a 100-odd inch electric power Cularis doing inappropriate aerobatics then I would say these size and type of model are within the connectors load capacity.

[Martin Harris - Moderator]

Posted by Donald Fry on 18/03/2016 07:05:26:

Posted by Martin McIntosh on 17/03/2016 22:54:19:

Just for the benefit of novices to the hobby a hardened grub screw will bed into an often softer wire. When stresses are introduced this will cause the screw to wriggle around and subsequently further squash the wire and loosen thus inviting a failure of the linkage. Loctite will only prevent the screw from undoing but will have no effect on the metal fatigue taking place.

Fine on an indoor craft or similar outdoor types but anything which vibrates at all, including electric, will result in a failure. Many who are new to the hobby such as I suspect some of the above contributers may be have been brought up with these because `They are what are supplied so must be OK`. You live and learn and would do well to heed the advice of others who have learned the hard way.

Don`t forget to take a black bag with you when you next go flying.

Any engineers on this forum, the above explanation strikes me as belonging to the same group as adverts for slimming products. A few percent right, and then a load of pseudoscience. I don't care either way, but as the tread is claiming to educate newcomers, reasoned peer reviewed argument should be proffered. Any engineers present to examine the above explanation.

As a totally amateur engineer, it does make sense to me - much as in the way that copper wire can "relax" after a few years and allow screw terminals on mains sockets to become loose (check your house ones sometime and they will almost certainly take half a turn if they aren't actually loose) - if the screw is no longer under tension and has not been loctited or glued then it will be prone to vibrating loose. Even if secured, there may well be some damage from vibration induced "hammering" even if the indentation is sufficient to prevent significant movement for the short term.

In terms of "banning" such connectors, one of my reasons for posting was that a very senior, experienced and respected member asked me for my opinion of whether we should consider banning these connectors within the club. As someone who is tasked with checking many of the models flown at our club for obvious build errors (we have a requirement for all new or repaired models to be looked over by an experienced member - and before we get comments on over officious clubs, I'd certainly prefer any error I might make to be found by a clubmate before I fly my new ones!), it is not the first time I've come across loose ones which had been missed by the builders.

What concentrated my attention was that the first recent case involved a long term "traditional" modeller (although in this case it was an ARTF) and regular flyer who was aware enough to have substituted hex socket grub screws for the original supplied slotted screws as access was limited for tightening properly with a screwdriver. Prior to the grub screw loosening, I'd questioned him on their tightness and tried to move them with physical pressure. After only a few minutes engine running, the fixing was sufficiently loose that the elevator stayed put while performing a "final" control check prior to taxying out.

[john stones 1 - Moderator]

Grub screws a plenty on my O.S powered Raptor 50, not one ever failed.

John

[Andy48]

Posted by Martin Harris on 18/03/2016 13:51:55:

As a totally amateur engineer, it does make sense to me - much as in the way that copper wire can "relax" after a few years and allow screw terminals on mains sockets to become loose (check your house ones sometime and they will almost certainly take half a turn if they aren't actually loose) -

There's a very simple explanation for this. Any electrical connection introduces resistance. That resistance will heat the wire. As the wire gets warm it expands, and having nowhere to go, the copper is deformed. Gradually over time as the connection works slightly loose due to the heating and cooling, the resistance increases, and so does the deformation.

Absolutely nothing to do with links on a model plane.

[Andy48]

Posted by Martin McIntosh on 17/03/2016 20:00:33:

As club safety officer I have banned these things other than on the lightest electric models.

Wow! Do you have a uniform inspection before everyone flies too?

[Peter Beeney]

Ian, I have say that it’s been a while and a bit since I bought these Great Planes connectors, it was at my LMS and now sadly that’s long gone. In those days they came in packets of about 10 or more maybe, and although they were relatively expensive I’d have have bought at least 2 or even 3 lots; and because they don’t wear or change they can easily transfer to the next model if needed, too. Would a Google be of any assistance? We do have a FAMS, (Further Away Model Shop), but worth the travel, he is well stocked and with many of all the little interesting and useful little oddments. I shall make some enquiries, I’ll be going there shortly; but, of course, it’s also quite possible they are no longer obtainable…

Donald, I’m sure you are correct in what you say about the stiffness of the control surfaces preventing flutter, but I also try and give them best chance by other means, such as using 4 good quality hinges instead of 3 on say a long(ish) aileron; and by utilising the maximum servo travel in the pushrod geometry, as I’ve described. Probably entirely coincidence, but it seems to have worked for me.

Martin Mc, I think this type of pushrod connector has actually been around for a long time. I had a very second hand Flair Patriot as a hack model for many years, all the servos had these connectors installed. Not the Great Planes type, as it happens though, I don’t know what they were. At around 6lb AUW, an Irvine 53 turning an APC 11 x 8 prop at 11k gave it a theoretical top speed of 80+, and indeed it was nice and lively! Nothing about this model ever gave me the slightest cause for concern, least of the pushrod connectors. Eventually it had to be pensioned off because the balsa wood was so ancient it started to crumble away and decay, thus becoming a possible liability.

With regard to the ‘banning’ of various modelling modus operandi, within our small club we have subtly different way of doing things. If these connectors ever became an issue, (and I think that’s very unlikely indeed), as with every other matter arising, we would discuss this as necessary, and resolve it finally with a members vote. I like to think that all my club mates are doddery, daft and democratic enough to make up their own minds… And as Bob Cotsford rightly said, there are other more important areas of model flying that might need attention; but this invariably requires some extended considerate assistance, rather than heavy handedness. - I have never personally seen, or heard about, any reason why these particular fixings should be banned.

Bob, When I did a load test on one of these I got to the point where the servo arm was just beginning to distort, any further and it would have let go. But I didn’t see the point of destroying a perfectly good servo arm for no reason. So considering just the metal pushrod/connector/servo arm combination, I therefore think the weakest link is undoubtably the plastic servo arm. Had there been the usual wooden strip involved I think this would have broken long before anything else! Thus personally I would have no qualms with the 20kg jet, after all, I suspect there is little or no vibration to consider here; also the large petrol jobbie, too; I’ll have a closer look when next at the patch, I’m pretty sure I’ve seen them on some rear mounted servos on at least one petrol model. Having said that, considering the likely cost of especially the 20kg jet kit I’d definitely expect many very suitably engineered bespoke components to be included, including everything to install the servos etc.

Regarding the ‘relaxed’ copper wire, we always knew this as ‘copper creep’ and it happens in exactly the same way to cables and wires with the same terminations but that don’t carry any electrical current. I’ve always assumed that because copper is slightly softer and rather more ductile than steel, under the pressure of the steel holding screw, over time it gently ‘flows’ out from underneath. But I’ve never found this to happen with steel, or perhaps in some cases, harder metal?, pushrods. In fact, if this were so on a regular basis, then surely even just within the aeromodelling fraternity these would have been totally shunned and therefore would have gone out of production?

I’m sure these days there are enough good quality fittings around to satisfy everyone’s requirements. What about the helicopter links, they must get well hammered, and then some; so some sort of captive ball joint maybe? They always look good to me.

I guess there is no one single answer to this one…

PB

[Martin McIntosh]

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.

[PatMc]

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

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.

No it doesn't.

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

That's another analogy that has no bearing on the subject.

Nor does this get away from the fact that your prepared to impose action based on your opinion on other members of your club without any facts to back you up.

[Martin McIntosh]

My `opinion` was backed by the rest of the committee who are also very seasoned modellers and can recognise a potential source of failure when they see it. It is our duty to eliminate problems which novices may not forsee.