Metal fatigue

[Ben B]

Well that's 2 planes out of 3 out of action!

Went for a fly tonight with the Breeze. Went very nicely but the outrunner was a little loose (loose grubs screws) so I landed it after just one circuit.

 

Got it back home and the motor wouldn't rev properly. Took it all apart and the reason became obvious- one of the wires had sheared where the motor wire goes into the bullet connector. A second one broke with minimal force when I pulled the cables through the hole in the bulkhead. At least it means the ESC is okay!

 

I can only imagine the wires were made brittle by the heat of soldering. Has anyone else had this? Are there any easy fixes?

 

Annoyingly I soldered on some new bullet connectors and gave it a test run.Worked perfectly. But.... I'd only gently tightened the grub screws in case I had to take it apart again. And of course the blooming motor flew into pieces at high speed, knocking off one of the cheeks on the plane and ripping all the wires out of the motor Insert rude words here.

 

So I've just ordered a new motor. But any idea about how to stop the same thing happening when I solder up the new bullet connectors? Should I wait until the solder pool melts then let it cool a bit before putting the wire in? This is the first time this has happened after years of soldering!

Edited By Ben B on 11/07/2011 20:37:58

[Biggles' Elder Brother - Moderator]

Hi Ben,

 

bad luck - its never rains, but it pours eh? They say troubles comes armed in threes!

 

I very much doubt that its a thermal effect - copper would have to be heated much higher than soldering temperatures to have any thermal effects and - if I remember my metallurgy correctly - any thermal effect you would get if you overheated it would actually be benificial because the joint cools naturally and so the copper would be annealed which would make it softer and more flexible - not brittle! Anyway I think that's academic unless you got it very hot indeed!

 

Copper does work harden though - within a couple of flexes backwards and forwards it becomes quite brittle. Could you have unknowingly "worked" the copper before soldering it?

 

BEB

[Ben B]

Perhaps yes. I must say I've never experienced anything like it before. Perhaps it did just work harden when I was putting the plane together, I suppose the bullet connector is quite big relative to the wire so the heatshrink around the connector won't have a good grip on the cable, allowing some flexing... Yes, perhaps that's the cause...

[Tim Hooper]

It's not uncommon for the wire to fracture at the end of the soldered area. I suppose it's just the point where the wire takes all the bending forces?

 

tim

[Ernie]

Hi Ben, I reckon the important thing for all wires in our models is to thoroughly support them, so that flexing is kept to a minimum

 

ernie

[Peter Miller]

Any sudden change in section or strength creates a stress point and that is where things break.

 

I don't know how you overcome this in the wire/solder joint apart from suporting it firmly.

 

It is for this reason that all construction should taper in strength. A typical example would be doublers, how many times do you see them end in a straight line. Making them end in a sweeping curve is far better.

[001]

Nearly on the same topic, when I started buying big batteries, brushless motors and ESC's a few years ago, they all had copper multiple strand wire power 'cables'.

 

Some of the recent ones that I have bought without plugs have silver coloured inner wires.

I do not really feel very happy soldering plugs and sockets to them.

But it seems to work.

 

So, what is in this flex? Is it electroplated copper wire? If so, what is it plated with? Or is it made of some other metal or alloy

[Peter Miller]

I always thought that silver coloured wire was pre tinned for beter soldering.

 

At least that is what it is often called. "Tinned copper wire"

[001]

Peter, I have just found a sample of the wire I referred to. It has very fine elements and is silver coloured.I have scraped the conductor and looked at it with an 8x magnifier and it is indeed copper that has been plated or tinned.

 

I probably thought that as the components I am buying are getting cheaper and cheaper, that money was being saved on the wires by using something inferior!

[Djay]

i have just installed 4 brushless motors from 4-max, purple power motors in my TN Lancaster.

They came with the connectors presoldered to the motor wires, but one by one they have all sheared, as I was running up the motors for testing.

There is practically nil solder in the connectors so the wires have broken or just pulled out.

most annoying when you have 12 wires to check after they have all been installed.

 

the other thing was the silicon insulation on the wires coming from the motor case are to short on some of the wires, so the exposed wire is resting on the metal casing of the motor.

 

Test flight at the weekend so hopefullythey will all run ok.

 

Darryl

[Snorbitz]

Soldering = Brittle. Add vibration and you've got an accident waiting to happen. If you must solder then fully support the joint with glue potting or similar and only allow the wire to bend away from the joint.

[leccyflyer]

The better quality motors - Mega Motors for instance - used to come equipped with external solder tags, for you to ad your own cables. Then they started to be supplied with the cables already fitted, nice, flexible, multistrand silicone insulated cables.

 

Some other outrunner motor brands adopted the practice of terminating the cables by just bringing the windings out through the case. These were often of lacquered,very stiff copper wire, with minimal support. The lacquer needed to be scraped off to add plugs. More significantly the very stiff wire had a propensity to work harden and fracture, just outside the case, requiring some careful work to repair and replace these.

 

It's a weak point on some motors - a blob of epoxy for support can help and indeed is absolutely necessary on some small motors, which would otherwise fail, just from minimal vibration or movement of the cables.

[Ben B]

Well there you go, you live and learn. Looks like it's not only me that's had the problem. I think what I'll do is put some epoxy (or maybe hot glue) on top of the solder and try and taper it so it supports the cable better, and then put the heatshrink on.

[Steve Hargreaves - Moderator]

Peter/Richard...you can get copper wire plated in all sorts of things...silver & nickel plating are very common & are "silver-y". The plating helps prevent corrosion & is often used in highly specc'd cables (military/aerospace or medical use).

 

The stuff I really hate is the enamel they often put on motor wires....its the devils own job trying to clean that off.....I have even been known to swear when trying to get it off....

[Tim Mackey]

A bare flame, followed by wire wool is the method I use for enamel wiring.

[Djay]

The worst thing is when you realise you are using an aweful lot of solder to fil the cup up,and then realise you have the connector upside down and you are filling up the socket itself.

takes a lot of solder to fill a long reach 4mm socket.

 

Darryl

[Steve Hargreaves - Moderator]

Thanks Tim....sounds well worth a try...

 

I've used fibre glass pencils, emery paper, scalpel blades to scrape it away & I still look at the joint & think...."Nah!!"

[Biggles' Elder Brother - Moderator]

Posted by Djay on 12/07/2011 12:39:14:

The worst thing is when you realise you are using an aweful lot of solder to fil the cup up,and then realise you have the connector upside down and you are filling up the socket itself.

takes a lot of solder to fill a long reach 4mm socket.

 

Darryl

 

LOL - I'm glad I'm not the only one that's done that!

 

BEB

[Tim Mackey]

Can I jojn that club

[Alan Randall]

I had the motor come loose in a Breeze in mid air. I have never seen a model do a forward roll in its own length before. It did two before the prop caught and I could close the throttle. The motor stayed connected via the wires and I glided the model back to me. I have used thread lock on the grub screws ever since.

[Peter Beeney]

With regard to the copper wire fracturing, I’ve found that what can happen is that when you remove the insulation from solid copper wire such as 1.5 mm household wiring by running a knife around the wire at right angles through the insulation and thus cutting a very tiny groove in the copper conductor, on the odd occasion that conductor then only needs to be bent slightly just once to cause the wire to break cleanly apart. But I do emphasise that although this does happen it’s definitely not every time; in fact, if you wanted to break it, it would most likely hang on like grim death!
So I try and avoid marking or scratching even flexible wires, in some cases it seems that a multi-strand wire can also fall apart very quickly, in other cases they will shake around for a long time and not come to any harm. Crimped wires are exactly the same, it will fracture where the wire leaves the crimp, if it’s given half a chance! I would certainly agree with the other forumites on this thread, some sort of support is needed, to stop the wires waggling about. Twisting together and some small cable ties might be a start.
I tend not to use epoxy glue on any joint reinforcing jobs, I use a tube of Evo Stik impact adhesive. Squeeze out plenty, enough to cover and support where necessary. You need to do this at a time when you don’t have to touch it for a while, such as overnight. It holds and supports nicely, but it always still has a degree of flexibility. It’s also good for items such as cowl screws, which can tend to work loose sometimes. Make sure the screw head head is clean and dry, and a smear over the top will stop any screw from working loose; it’s certainly done so for me, for many years; and then when you want to remove it you can pull the skin off cleanly, using a pair of needle nosed pliers. ’Fraid I can’t lay claim to this idea, Skyleader were doing this way back, impact adhesive on any wires where necessary etc. and a silicone compound, intended for use on electronic components, spread over the bits and bobs in the receivers and servos. They did much work for the military; and I suspect that would be to a military spec.

Djay mentioned the 4 mm connector and the solder, that brought on a little reminiscence of times past for me, if I might waffle on for a moment.
About 50 years ago part of the job was soldering brass connecting lugs onto copper cables, These were from small, 50 - 70 mm squared up to about 300 mm squared. Maybe we did even bigger, I can’t remember, but the next stage up after that was aluminium bus bars. I’ve related elsewhere about tinning these with solder, for the overlapping joints. The lugs were big, too, their weight measured in lb., (not metric then!). A few fire bricks as a crude oven if possible and a large propane gas torch for heat. We used a mixture of solder sticks and (mostly) heavy duty multi-core solder. You’d put the cable end in the lug and heat it until the the solder ran in. Then you just kept filling till forever! The cable were often associated with batteries, so all the lugs and soldering were left uncovered so they could be inspected periodically for signs of corrosion. When solder changes from it’s liquid state to solid it contracts and shrinks quite a lot, so this left holes in the solder around the rim of the lug. This would be unacceptable so you had to carefully keep the heat gently on, and continue to ‘top up’ as the solder continued to cool and shrink; until you finished with a nice even smooth wet covering, with the solder rising well into the cable under capillary action. This stage could be a little delicate, when a chunk of solder is changing state, from liquid to solid say, it all wants to change together; it doesn’t much like being part liquid and part solid. You just kept tickling it until it co-operated. Any pits and holes had to be avoided because of the possible onset of corrosion. Also this would always be an individual site job, this work would be done in situ and the cable had to fit the termination's exactly, so it was always very very carefully cut to size first; there was no room for any adjustments. The connecting bolts and nuts were some pretty hefty items, too, so you can imagine we are talking multiple amps here.
Although invariably the large cable size was more often than not concerned with volts drop, rather than the absolute maximum current capacity. As I’ve said before, the resistance of a piece of wire is proportional to it’s length and inversely proportional to it’s cross sectional area. So that’s why we had to use big fat cables; and why we struggled a bit at times! All just to keep the volts drop to a minimum. Fortunately the big stuff was very infrequent, and the smaller cable sizes were much easier.
By ‘smooth wet covering’ I mean that the solder feathers out to a fine edge, rather than stays in a lump, like a droplet of water. This might indicate a ‘dry’ joint, which can eventually cause a high resistance, or worse.

But for a long time now the same power supplies have been facilitated by multiples of small units all paralleled together, for the maximum speed in assembly and ease of maintenance etc. All standard size small cables and crimped joints. In other words, minimum cost!

Still, at least it was good practise for soldering 4 mm connectors and the like. It’s very important to get the priorities right!

PB

[Tim Hooper]

Posted by Djay on 12/07/2011 12:39:14:

The worst thing is when you realise you are using an aweful lot of solder to fil the cup up,and then realise you have the connector upside down and you are filling up the socket itself.

takes a lot of solder to fill a long reach 4mm socket.

 

Darryl

 

 

Been there, done that.

 

Also managed to pour enough solder into the back end of a male connector that it overflowed and filled up the 'basket' below. Goes without saying that it was the one one I had in stock........

 

tim