11427 forum posts
In the past I have bought a snake which consists of a plastic outer sheave and what appears to be a solid, PTFE type material.
I am contemplating using it.
You know I have not the slightest idea what can be fastened to the inner. Never mind in a secure manner.
|John Wagg||21/11/2019 20:58:00|
|13 forum posts|
We used to just screw metal quick links straight onto the core as per something like these :-
There are various on the Slec website.
Just seen these which may be better ?
Edited By John Wagg on 21/11/2019 20:59:56
Edited By John Wagg on 21/11/2019 21:06:19
|Robin Colbourne||21/11/2019 21:08:55|
443 forum posts
If they're anything like these: Solid Snake inner
Use these: M2 Solder Extender
They have a blind hole in the end that's not shown. Roughen and clean (with propanol or meths) the end of the snake inner you are going to bond in. Like wise with the inside ot the solder extender. Use epoxy, and gently warm the solder extender once you put it on, to ensure the epoxy flows into the roughend surfaces.
I wouldn't recommend these for a fast or heavy model as this sort of plastic is never going to bond really well.
Edited By Robin Colbourne on 21/11/2019 21:10:10
11427 forum posts
I have tried the method that John has suggested. It certainly works, although the torque required to get the metal link on his quite high. I will look tomorrow to see if I have a 2 mm die and try that to reduce the force required.
Robin I have contemplated a similar method to that you put forward, not as structured as yours, Which I think would work, other than the nagging doubt that will always be at the back of my mind. There is an irony, in that i am happy with lots of glued joints, the difference being is that they have stood the test of time, tested by thousands of others.
Many thanks for your replies, as I can see a way forward now, based on others experiences and knowledge. I will do some experimenting in the morning.
167 forum posts
The ones I have come with a solder extender crimped onto one end. I think they intend you to crimp something to the other end. You can get things similar to the extenders with a z-bend instead of the thread. I think I got some from Nexus at a show, not used them yet.
The photo on 4-max
Edited By Dad_flyer on 21/11/2019 22:03:05
|Andrew Calcutt||21/11/2019 22:10:34|
|16 forum posts|
The inner has a hole in it,what I do is hold that in vice put a 2mm threaded rod in my cordless drill and just spin it in.
|Stephen Jones||21/11/2019 23:12:15|
2737 forum posts
Here you can see i use those type of snakes a lot for small fast light weight models and never had one fail on me.
You can use the crimp on adapters but i find them too long when the clevis is added.
So i put my own thread onto the hard snake inner by holding the inner in some pliers i open up a metal clevis and use that to put the thread onto the snake.
I then put a thread into a plastic clevis with a threaded rod .
Not Sure if i have some pictures on here of how i do it . I know i did take some a while back.
Edited By Stephen Jones on 21/11/2019 23:28:59
8803 forum posts
Depending on how long the run is at the clevis end, I put an m2 push rod down the snake and secure with CA . Only for small planes though.
|Engine Doctor||22/11/2019 11:16:33|
2328 forum posts
I haven't seen those solid snakes around for years . The inners are very handy for threading bits through wings or fuselages etc . Do you need that particular type ? Newer types with a hollow inner are easier to secure linkages and are easier to find . Golden rods do a lighter weight snake for lighter models in a red outer yellow inner combination .
PS just read the post with pics link showing a crimped end which is probably best way to secure .
SJ Do you use a dedicated crimping tool or will a wire/plug crimping tool work ?
Edited By Engine Doctor on 22/11/2019 11:21:23
|6076 forum posts|
To screw those SLEC brass adaptors into the inner of SLEC 'snakes' I use a little gadget shown here on the tailplane -its an opened out clevis ( to give something to hold ) with another threaded piece screwed in behind to 'locknut' the brass adaptor. The locking part has a soldered on part to use when unscrewing. When the adaptor is screwed into the snake inner far enough the locking part is unscrewed and the whole clevis part spins off easily. Saves fiddling with holding the adaptor with pliers etc and mangling the thread.
Edited By kc on 22/11/2019 12:10:22
|Stuart C||22/11/2019 15:03:43|
|122 forum posts|
My experience with threaded rods into plastic tube inners, has been that on high loads, the induced hoop stress due to the thread wedging effect, is greater than the yield stress of the plastic. This results in a longitudinal split to the inner and catastrophic failure. My solution was to apply binding over the plastic inner for the threaded length, thereby adding a negative preload to the hoop stress. Ferrules would be a nicer way to achieve this - if you can find any!
|Steve Balaam||22/11/2019 15:37:46|
110 forum posts
I've used one of these for years I believe metric ones used to be available
11427 forum posts
Oh, err, takes me back, probably +50 years now to pressure on thin walled vessels. Although I almost remember that the ASME code used something a bit more complex than stress= pressure *radius/thickness of wall.
Given we are dealing with a situation where we do not know the internal pressure, then the practical issue of achieving a sleeve that is at best interference fit, to reduce the possibility of splitting and that we are dealing with a polymer, a pretty simple solution to splitting after threading is to simply warm the plastic, to a temperature before it starts to melt. The Mers will relax (or poly mers if you like).
Seeing all the solutions provided, I now know, how little i actually know.
The example I will be dealing with is a solid inner, which is going on the rudder. I have bought some (4 off) SLEC snakes, similar to Sullivan, I will use one for the elevator.
Incidentally I much prefer the golden snake (metal wire inner) made by Sullivan, virtually no sticktion
It may be of interest that when I retired, I undertook some simple experiments involving snakes and pushrods. Snakes were far better than pushrods for transferring accurately end loads. With a short unsupported ends, to the inner and outer, the inner or outer did not cripple at all. Pushrods were a different kettle of fish, due to the ends being the locations, and the rod itself having a high slenderness ratio, it would cripple very easily. Beefing up the rod itself considerably (increasing the mass, a lot) highlighted that the wire ends were plainly seen to deflect.
I came to the conclusion that a really stiff pushrod of low mass was difficult to make.. Better to stick the servo at the back as many ARTF models were then doing, as the moments about the CG, affected the CG more favorably (that is less).
The best pushrod i made used a aluminum arrow shaft as the pushrod with very short wires at each end.
I thought it might interest you.
Now it takes me to long to build to worry about detail.
|Robin Colbourne||22/11/2019 17:26:29|
443 forum posts
Erfolg, if you pre-heat the metal thread you are going to wind into the plastic, it only softens the immediate area of the snake inner that you want to deform. Heating the snake inner runs the risk of it twisting as you try to wind the joining piece in.
The issue Stuart C decribed was particularly bad on Flitecraft models (Cessna 152, 177 & Piper Cherokee, notable for the R/C gear being installed through the cockpit door). On two of these I had throttle snake inners fail at the wound-in clevis joiner. Fortunately, flying around inverted soon stopped the engine and a dead stick landing saved the day.
11427 forum posts
I was only considering heating the snake as an after threading solution. Not part of the process. At the end of the day, what matters is that you develop or use a method that works for you.
The problem I would have with ferrules is that the fit needs to be precise, where there are many suppliers/manufacturers producing their own external diameter extrusions.
With the standard Sullivan snake, I have never had any real problems. If I thought the screwed rod was to tight, when screwing in, i would look to find a slightly smaller screwed rod etc. Then the issue switches to the clevis. Yet for me this is normal, nothing seems to go together without a fight.
The solid inner is a new situation for me and I am grateful for all the examples provided.
|Bruce Collinson||22/11/2019 20:04:27|
|409 forum posts|
Very interesting thread re the crippling (?), bending etc. I fabricated some carbon tube pushrods for a big (Zenoah) 20 Maxford Mentor by threading the inside of the tube with a tap, binding with Kevlar fly-tying thread (Lagartun, really useful stuff, terrific tensile strength) then covering with heatshrink to keep the Kevlar in place. You’re not supposed to be able to cut any sort of thread in c/f tube; you can, sufficient at least to convince me that there was an initial mechanical connection between the tube and the threaded insert, which was simple M2 studding (might have been M3, size to suit).
Bind, then heatshrink, then thin cyano, then tap. Clean up everything with meths or acetone, glue threads. Can’t think why this wouldn’t work on snake inners. Choose glue to suit. The mechanical principles are the same, aren’t they?
11427 forum posts
With materials and sections now available, it is probable that a stiff light weight push rod main body can be produced.
In the past when dowel and similar materials were used they were both heavy and lacked stiffness through crippling. They had to be chunky to be stiff, then they tended to be heavy. Potentially affecting the CG adversely, at least theoretically, inertia issues arising, particularly if crashing.
The issue with modern materials used for the main rod then switches to the end fittings. Where wire is used, if it is of any length, this then becomes the weak point.
I can see the benefits from moving the servo to the back with a very short push rod.
I now principally use snakes, with the ends anchored, as if they are not, that becomes the weak point in the system.
I essence I see no one perfect solution, particularly if attention is not paid to the detail.
Edited By Erfolg on 22/11/2019 20:44:53
|Bruce Collinson||22/11/2019 21:02:51|
|409 forum posts|
Makes sense. My very amateur grasp of mechanics suggests that the shorter and stiffer the better as there’s less to deflect. Rear mounted servos are great provided the fus width and c of g will accommodate. That was what attracted me to the c f tube, I’m not aware of a commonly available material with similar weight to stiffness ratio, which I guess is why it has been the default material for fishing rods, particularly fly rods with their peculiar demands, for c. 40 years.
there were lots of hybrids tried in the early days, boron in particular but the only meaningful improvements of which I’m aware are higher modulus c f and lower ratios of resin in the tube manufacture.
Taking your point re wire ends becoming the weak spot, my Heath Robinson pushrods lacked wire. With careful length judgement of the tube, there’s a minimal length of studding inside and outside the end of the tube then a clevis.
I have seen commercial versions which were too short and where the connection to the tube was an external metal sleeve which seemed to me to rely entirely on the glue joint with no mechanical fixing at all. I didn’t really want to rely on that with 10 kg servos and about £600+ in the air. Also, it was satisfying to cobble them together.
By the way, what is crippling in this context?
11427 forum posts
The term crippling has often been used to describe the failure of a strut or column.
You can calculate with a degree of accuracy at what end load the item will deflect and collapse. Dependent on the aspect ratio, in this case, the relationship between the width and length, which is the best expression to use. The ones that immediately come to mind are Rankin and Euler. The relationships are derived by considering the ends being fixed relative to each other, that the column has one or more hinges along its length, with a of centre axis. As you can imagine, compared to the real world, these models are not absolutely representing the real situation, there are also a number of caveats and assumptions that emphasis that there are limits.
In essence at the scales we are working, with so little real knowns regarding our struts and its detail, the theory is essentially irrelevant, in that common sense predicts what is relevant and what will happen at the extreme.
Probably the real boundaries in our hobby has been the dogmatism of some, who think there is a right way, Particularly when some seem determined to rubbish alternatives. Particularly by those who have poor intuitive appreciation with respect to what is desirable, why and what is the converse. But that is life in general I guess. Now where is my hand cranked calculator.
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