Help Wanted

This topic has 15 replies, 10 voices, and was last updated 21 September 2017 at 11:52 by John Rudd.

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19 September 2017 at 17:31 #317722
jeffrey cottrell 1
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@jeffreycottrell1
Hi All

Have to say I am not a model engineer, rather a model helicopter pilot in need of a model engineer.
Here's the deal.
Recently snapped the shaft on one of my electric motors. This shaft is a bit odd, in that the protruding part is 5mm, while the length inside the motor is 7mm.
I have changed the bearings inside the stator to 5mm id, so I can use a 5mm shaft right through.
Problem is the rotor has a 7mm hole in it, so I need an adaptor to fit, and take the 5mm shaft.
Looking for someone willing to take on making this adaptor for me, for a fee of course.
Not sure about material. The shaft is steel, and is an interference fit in an aluminium base, so I'm thinking either hard aluminium or brass would do.
What is vital is accuracy. This motor will be spinning at up to 15000 rpm, so anything out of line will show up as vibration.

So, any offers?

Jeff
Advert

19 September 2017 at 17:31 #25509
jeffrey cottrell 1
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@jeffreycottrell1
19 September 2017 at 18:04 #317726
Jeff Dayman
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@jeffdayman43397
Where are you? (country and nearest city)

Distance will likely be a factor in assistance offered.

A good sketch would be advisable. Micrometer measurements of the shaft and accurate measurement of the rotor bore with hole gauge/micrometer or go no-go check with pin gauges will be important to get best fit with least runout / inaccuracy. The adapter itself could be made on just about any lathe in one setup for max accuracy, but sizes must be known.

I think securing the adapter in the rotor without affecting runout may be the real challenge here. Setscrews will not hold it secure enough for acceleration and deceleration during flight, likely, and will shove it off centre. Loctite 609 cylindrical retaining compound may work. How does the shaft normally get fastened to the rotor? is there a cone lock, spline, key plus e-ring etc?

JD (Ontario Canada)
19 September 2017 at 20:51 #317759
Emgee
Participant
@emgee
Hi Jeffrey

If you can obtain some 7mm OD with 1mm wall thickness tube will give you the 5mm bore you could cut a piece to suit the length needed, use high temp Loctite or simiar to secure the tube in place as needed as JD suggests.

But as you state balance may be an issue unless you can set the complete modified rotor between centres and balance it by removing material from the heavy side, should be doable.

Emgee

typos

Edited By Emgee on 19/09/2017 20:55:10
19 September 2017 at 21:02 #317763
not done it yet
Participant
@notdoneityet
I would have thought that high strength is more important than high temperature loctite? How long is this shaft, and is it is coupled by a gear and not direct to the motor?
19 September 2017 at 21:45 #317769
Marcus Bowman
Participant
@marcusbowman28936
Can you post some photos in an album?

Marcus
19 September 2017 at 21:45 #317770
Emgee
Participant
@emgee
ndiy, read the post again and you will perhaps understand the question asked.

Emgee
19 September 2017 at 22:51 #317781
jeffrey cottrell 1
Participant
@jeffreycottrell1
Hi Guys

Emgee

If you can suggest a source of that tubing, I'd be happy to give it a try.

Jeff

Thanks for the quick reply. I think distance might be a problem. I'm in United Kingdom, about a third of the way round the planet. Ordinarily I'd say put the adaptor in the post, but I don't think that's feasible in this case.

Anyway, bit of extra info. The shaft is held in the rotor by virtue of being a very tight fit. Sometimes needs the assistance of a hammer to get it in. There is a flat machined on the end, and a grubscrew fits in from the side, but I think that's only a second line of defence.

Other than that, it's only friction keeping it in. That's the way the manufacturers do it, so good enough for me.

Thanks

Jeff (the other one)
19 September 2017 at 23:14 #317783
Keith Long
Participant
@keithlong89920
Macc Models have 7mm od brass tubing listed with a 0.45mm wall thickness – about £2.68 for a 300mm length. No doubt other model engineering suppliers and model shops stock it as well. I think the K&S metal tubing has a thinner wall but again a possible source.

Edited By Keith Long on 19/09/2017 23:15:29
20 September 2017 at 06:44 #317794
Neil Wyatt
Moderator
@neilwyatt
If this is a brushless motor the length of shaft in the rotor will be very short, I think brass may prove too soft.

It might be better to get someone to make you a new stepped shaft.
20 September 2017 at 07:29 #317798
JasonB
Moderator
@jasonb
Stepped shaft was probably the cause of teh original fracture where it changed from 7mm to 5mm

Whoever makes it really needs the rotor so they can make the parts to fit rather than trying to give measurements when the OP probably has not got anything to measuer with.

Can you revert back to 7mm in the motor and run the whole shaft as 7mm dia
20 September 2017 at 08:12 #317800
Chris Evans 6
Participant
@chrisevans6
Sounds like a job for a piece of 7mm silver steel with the end reduced to 5mm. Maybe leave a small radius for strength and chamfer the mating part. I would most likely grind the piece for good concentricity.
20 September 2017 at 09:05 #317804
jeffrey cottrell 1
Participant
@jeffreycottrell1
Hi All

Thanks everyone for their thoughts.

Keith
7mm od + 0.45 wall gives me an id of 6.1mm. Not really what I'm after. Did find a company in the far east who supply 0.5 wall, but it would still need two layers to get the 5mm id.
Tried that already, not very successful.

Neil
Not sure what you mean by this. Certainly not considering using brass as the shaft itself if that's what you're referring to.

Jason
Agree with you. The break was exactly on the change of diameter. If I can find someone willing to take on the job, I would happily post the rotor to them
Reverting to 7mm is a non-starter. The shaft goes through a pinion which engages with the main gear and these are simply not available with 7mm id.

Chris
Agree with Jason, it was the change of diameter which was the stress point. Also, no-one yet has offered to drill a hole in a piece of brass. Finding someone with the facilities and knowledge to grind the shaft to such degree would be problematical.

Bottom line, guys. Need someone with a lathe who is willing to drill a 5mm hole on a piece of 7mm brass. I can even supply the material to work with.

Any takers?

Jeff
20 September 2017 at 10:59 #317820
Emgee
Participant
@emgee
Hi Jeff

For the concentricity between bore and OD you need for your application it is unlikely just drilling a 5mm hole through the unknown length of 7mm brass rod will be satisfactory, at least it would need drilling and reaming to size, preferably boring to reamer size at the start of the bore before using the reamer.

What length is required ?

Emgee
21 September 2017 at 11:46 #317961
jeffrey cottrell 1
Participant
@jeffreycottrell1
Hi All

Much as I appreciate the level of engineering expertise available here, I do think the job is just getting too complicated.
Also although everyone has been very generous with their advice, no-one has yet said they are willing to undertake this work.
So, I need to go back to the question I asked in my first post.
Surely there must be someone here with a lathe, willing to bore a 5mm hole in a piece of 7mm brass.
I am even able to supply the material, and will take on the responsibility for finishing it off.

How about it, any takers?

Jeff
21 September 2017 at 11:52 #317962
John Rudd
Participant
@johnrudd16576
Jeff, Private message sent…
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