Over sized bore suggestions please.

This topic has 17 replies, 7 voices, and was last updated 19 January 2016 at 21:32 by Michael Gilligan.

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19 January 2016 at 11:21 #7971
Rctintin
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@rctintin
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19 January 2016 at 11:21 #221686
Rctintin
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@rctintin
Hi guys, my 'other' hobby is flying RC Helicopters, of which I have a fair few. I have an issue with a manufactured part that I'd like your opinions on please.

On a brand new machine which uses a .91/120 nitro engine (around 18-20cc) It has a poorly machined clutch hub. The diameter of the engine crank is 9.50mm, but the hub is 9.59mm (internal mic used) The hub has a split collar and 2 pinch bolts. So once installed onto the crank, you would normally tighten the crank nut, and then pinch it for extra force on the crank. But it's so loose, there is no way that pinching with both tighten it and hold it centred!! There's the weight of steel clutch shoes installed to the other side of this hub!!

So, seeing as just buying a new one will have the same miss tolerance, I'm thinking about sleeving this one. I have a 9.5mm reamer, so what I would like your advise on is what wall thickness for the sleeve and what material please?

I'm thinking that if I mount the hub in the 4 jaw and clocking it dead centre, I could ream it to say 10.5 – 11.00mm, and then machine a sleeve in brass to match with a reamed 9.50mm bore. This can be pressed into the hub, and slotted along the existing slots, so it all clamps as one unit. What do you think please?

Cheers, Rob
19 January 2016 at 11:44 #221687
Martin Kyte
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@martinkyte99762
I would tend towards machining the sleeve in situ. Machine up the hub to whatever bore size you see fit. Loctite a solid slug in the bore and then machine out to your finished 9.5mm.

regards Martin
19 January 2016 at 12:30 #221690
pgk pgk
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@pgkpgk17461
I wouldn't go with locktight here unless it's just for the machining process.. If you do get a lean run and start to cook the engine then the last thing you need is the sleevng loosening and lose fan. Was your internal measure with the split collar open or closed down? Can you widen the split so it dogs down more?

For a brand new kit I'd expect lots of comments on heli forums if this is a common issue.. which may have been resolved by the makers for a replacement part. I must admit I haven't flown for nearly a year and not kept up with the rc heli debates.
19 January 2016 at 13:03 #221693
Martin Kyte
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@martinkyte99762
The high strength shaft grade should be good for around 360 degree if my memory serves and I thought the item was going to be clamped together anyhow. If you press fit as soon as you split it its going to release is it not.

Martin
19 January 2016 at 13:19 #221696
Rctintin
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@rctintin
Thanks, good idea re the doing it in situ.

It's not a cheap Heli, it's a Gaui NX7, and yes lots of issues with bad vibration and a lot of complaints about the hub tolerances. Gaui have 'supposedly' resolved these issues, but I'd rather try to fix this with tight tolerances.

So assuming I go this way, what material would you suggest?

The use of Locktite etc will be totally fine, there are very low temps that end of the crank, and I've yet to loosen a crank nut with it on in over 25yrs of flying nitro heli's.
19 January 2016 at 13:25 #221698
JasonB
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@jasonb
I would bore it out to teh larger size rather than ream as the slot may throw things off, loctite in a piece of material that is the same as the hub and then drill and finally bore to a nice fit on your crank, cut slot through bush.

Once the clamp screws are nipped up it won't really mattter if teh loctite bond fails
19 January 2016 at 13:35 #221703
Rctintin
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@rctintin
So alloy will be fine to match the hub? Not brass or steel?

Thanks guys, I really appreciate the help.
19 January 2016 at 13:37 #221704
Martin Kyte
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@martinkyte99762
You probably want to go with the same material is the hub I would have thought.

It's not critical though as essentially all you are doing is creating a packing piece. Steel may be a little less slippery than brass if the hub is steel.

Martin
19 January 2016 at 14:13 #221708
Ian S C
Participant
@iansc
Is a piece of shim .04 mm a possibility, even assisted by some Loctite?

Ian S C
19 January 2016 at 14:16 #221710
Rctintin
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@rctintin
It's an alloy hub, so I have some aerospace grade bar stock here, so I'll go that route, thanks fellas

Rob
19 January 2016 at 15:00 #221717
OuBallie
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@ouballie
Send it back for a replacement if still under warranty.

You don't know what else could be wrong and once you fiddle with it, supplies/manufacturer will wipe their hand of any responsibility.

Geoff – Any decent company will replace without question or suffer the bad press.
19 January 2016 at 16:37 #221731
Michael Gilligan
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@michaelgilligan61133
Rob,

Before going to too much effort, you might try using Loctite Bearing Fit … It's designed for that sort of clearance, and for easy removal when required.

MichaelG.
19 January 2016 at 16:43 #221733
Rctintin
Participant
@rctintin
No point in sending it back, I'll just get another miss match. Shame I cannot post a vid on here, the play is way beyond fillable with Bearing fit. Plus there's to much chance of that seeping into the front crank bearing.

I own 5 nitro heli's at the mo, and over 15 heli's in total, and been doing RC for 39 years, so I've a good background in RC and nitro, but I'm a newer member of the model engineering community. I had a Unimat 3 as a teenager, but it's only in the last 5-6 years I have got the ML7 and Vertical Mill. Starting to fill the shelves with tooling now
19 January 2016 at 17:24 #221736
JasonB
Moderator
@jasonb
It was RC heli's that got me into ME too, I needed something to make the Morley Bell 47G so Dad bought me a Unimat 3 too. The clutch was one of the parts that needed to be turned to fit my engine. They certainly were not ARTF in those days.

I think one problem with using bearing fit to fill the gap is how do you centre the hub on the shaft while it goes off, can't clamp it as the clamp will pull it off line?

Edited By JasonB on 19/01/2016 17:29:36
19 January 2016 at 17:30 #221738
Rctintin
Participant
@rctintin
Haha, my first Heli was a MFA 500 Sport, made from steel, alloys and ply wood!!

It cannot be filled and located centrally. There's a 50+MM steel clutch shoe bolted the other end, and the whole lot spun at 17k rpm!!
19 January 2016 at 17:52 #221741
Michael Gilligan
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@michaelgilligan61133
Posted by JasonB on 19/01/2016 17:24:13:

I think one problem with using bearing fit to fill the gap is how do you centre the hub on the shaft while it goes off, can't clamp it as the clamp will pull it off line?

.
- Ajust the clamp to its nominal diameter
- Apply 'Bearing Fit' to shaft and clamp
- Slide the two together and gently rotate to ensure an even film
- … at a film thickness of 0.045mm it should happily 'float' concentric.
MichaelG.
19 January 2016 at 21:32 #221788
Michael Gilligan
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@michaelgilligan61133
Posted by Rctintin on 19/01/2016 16:43:56:

… the play is way beyond fillable with Bearing fit. Plus there's to much chance of that seeping into the front crank bearing.

.

I note, and accept, your caution; but:

[not wishing to rely upon my memory] here is the datasheet.

9.59 – 9.50 = 0.09 … which puts it nicely in-range doesn't it ?

MichaelG.
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