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Prop adaptors: How tight is just tight enough?

I lost a prop - keen not to do it again!

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The Wright Stuff25/04/2018 10:21:50
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I have an embarrassingly simple question about tightening collet type prop retainers: the type where the spinner is the nut. How tight to do it up?

Last week, I was flying my vintage glider with electric power pod, when opening the throttle to gain height, when I heard a thud and lost power. Upon landing the prop, prop adaptor and spinner assembly were all missing. Luckily it happened in the air away from the pits. Now, I don't have the missing parts to examine them, but I would imagine a failure unlikely, hence I conclude I didn't tighten the collet enough.

So I have a new one. How do I prevent this from happening again? Do it up tighter, but how tight? Physically tightening isn't the issue - I could put a strap wrench on the motor and a 2 foot steel rod through the hole in the spinner, and tighten merrily until something soft and aluminium strips threads or snaps. Not what I want to do.

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I could file the shaft to roughen it? Use thread-locker and hope I don't need to take it off (the prop being on an over-wing pod is extremely unlikely to get broken)?

It seems that this is an unusual problem - most issues that people seem to have with collets is not being able to get them off again.

Edited By The Wright Stuff on 25/04/2018 10:23:17

J D 825/04/2018 10:41:44
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873 forum posts
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I would say that the bar needed to tighten the adapter should be no longer than one prop blade.To prevent turning hold the prop not the motor.

Thread lock will not prevent you undoing it.

Today just about every bolt on anything has a torque setting.Mechanics of old were expected to learn how much they should lean on different sizes of fastening to secure them.

J D 825/04/2018 10:43:55
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Is that a pusher or tractor set up ?

Nigel R25/04/2018 10:47:42
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I am not mad keen on these nut-is-the-spinner aluminium adaptors. Aluminium is soft, etc.

For starters, I would switch to a larger thread adaptor (M8?) and use a bigger spinner. Or at least thread lock. This assumes the problem was that the nut came undone - not that the collet simply let go of the shaft.

Second set the ESC to soft start the motor. A big prop and instant torque from zero revs are perhaps not friendly bedfellows. If not on this ESC, perhaps another is called for. Or maybe you have a fancy TX that can slow the throttle response.

David Mellor25/04/2018 11:10:55
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757 forum posts
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I use Donkey motors a lot and really like them. I also use the same collet prop adapter as you. I have never had one come off.

I use an Allen key as the Tommy bar, but only tighten up gently. You don't need to tighten it up so much that you can't make the prop slip a tiny on the shat bit with a bit of effort. Don't over tighten.

I wouldn't sand anything - the shallow taper does all the work for you - roughen it up and you may never get it off.  

Are the mating faces clean and smooth? Any bit of swarf or "pick-up" (raised scratches) in the mating surfaces will work against the collet tightening correctly. If the adapter has been cranked up too tight in the past it may have deformed enough to no longer grip on the taper.

An old friend of mine, sadly no longer with us, always used a trace of Vaseline on the mating faces of the collet. They needed only a touch of tightening and always came apart when needed. Very counterintuitive, I know.

 

 

JD8 is asking a key question.... if you've got a reverse rotation then the prop adapter will want to unscrew itself, especially on "snatching" (instantaneous opening/shutting) on throttle.

 

Edited By David Mellor on 25/04/2018 11:17:46

Bob Cotsford25/04/2018 11:15:08
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Whenever I've had a problem it's been with the collet type prop adaptors. Often they've been drilled either off-cemtre or oversize leading to vibration and noise or a prop that can't be tightened. Make sure to use an adaptor that doesn't have any slop before tightening the nut. Abrading the motor shaft and using threadlock definitely helps.

David Mellor25/04/2018 11:24:51
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757 forum posts
230 photos

As Bob says, not every collet type prop adapter is as well made as it should be. I've had an off-centre one too and it was useless.

Donkey motor shafts are very smooth, and the temptation is to sand it slightly. I've never found it necessary, though a touch with very fine wet and dry (used damp) may help.

Just be aware that if you abrade it dry some of the ferrous powder from the abrasion may get attracted to the magnets in the motor can..... you don't want tiny bits of steel in your motor........especially on a Donkey because they use oiled bronze bushes for bearings.

Denis Watkins25/04/2018 11:52:19
2899 forum posts
141 photos

It is annoying to loose the assembly in the air, but dont alter the fit with abrasives

Metal to metal does work, as we know

Degrease the collet and shaft and tighten as normal

Nail polish remover or cellulose thinners

Countless collets are in use and flying reliably

Trevor25/04/2018 12:03:54
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One other thing to watch is to ensure you aren’t fitting a 1/8in adapter to a 3mm shaft!

The Wright Stuff25/04/2018 12:11:13
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1253 forum posts
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Thanks for the replies. Like most of you, I'm a bit mystified too. It is just a standard tractor arrangement with a 'normal' 6"x4" prop. 3 mm taper and 3 mm shaft.

Perhaps I was just unlucky, or there was a quality issue with the adaptor...

Bob Cotsford25/04/2018 12:15:31
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7320 forum posts
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Abrasives needn't alter the fit, if you are 'abrading' to that extent then you are perhaps being a little heavy handed. No, forget the 'perhaps', you are being heavy handed. All you need is to mark the shaft surface to provide some grip, not remove material so use a course abrasive but only give it a quick wipe. I like the idea of greasing the taper on the collet, I've had a few weld themselves together over time.

Don't rely on big brand names as a guarantee of quality, the worst adaptor I've had was the one supplied in an E-Flite Carbon-Z Cub. It sounded like it was running a four stroke until I eventually swapped the adaptor after balancing props and eliminating the spinner failed to cure it. No wonder so many of these throw magnets in the motors!.

Allan Bennett25/04/2018 12:23:58
1347 forum posts
37 photos

Another thing to watch out for is that the hole in the spinner nut is deep enough that the shaft is not bottoming out before it's properly tightened onto the prop. If that's the case, a washer between the spinner nut and prop fixes the problem.

The Wright Stuff25/04/2018 15:14:53
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1253 forum posts
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Allan, that's the most plausible suggestion I can think of as well. It's not obviously bottoming out (i.e. it bites) but a washer sounds like a good precaution.

Biggles' Elder Brother - Moderator25/04/2018 16:20:00
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Posted by Allan Bennett on 25/04/2018 12:23:58:

Another thing to watch out for is that the hole in the spinner nut is deep enough that the shaft is not bottoming out before it's properly tightened onto the prop. If that's the case, a washer between the spinner nut and prop fixes the problem.

Yeap, I've had that happen at least twice! On one occassion it was obvious in that the spinner was fully tightened, but I could still spin the prop wit my finger! The other one was more insidious, the prop seemed tight, but the thread had in fact bottomed out and was just pinching it rather than clamping it.

BEB

Andy4825/04/2018 19:46:53
1333 forum posts
1 photos

I had one fly off whilst testing a new model, thankfully it was in the garden where it clattered against the garden shed, but scared the life out of me. Thank goodness I was behind the model. I try and avoid them now, and use motors which have a bolt-on prop adaptor.

Simon Chaddock25/04/2018 21:36:42
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Just to endorse the danger of the collet 'bottoming' before it has fully gripped the shaft. As long as it is properly gripping the shaft aluminium on steel actually 'binds' at a molecular level so the smoother the surfaces the better. Indeed a good collet is likely to be the very devil to get off again after it has be left on for any length of time.

However once a prop adaptor has been over strained the collet surfaces no longer make intimate contact with most of the shaft so are at risk of coming off.

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