Author Topic: Shaft straightening...  (Read 11013 times)

AdeV73

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Shaft straightening...
« on: September 10, 2009, 09:15:09 PM »
I'm not sure where this thread best belongs... In the bin probably... but, here goes:

As you may recall from earlier writings, I have a bent shaft on my alternator. In fact, it's bent at the bearing housing, so far as I can tell, probably as a result of being dropped. Runout at the pulley edge was around 3/4" as I recall.

You may also recall that it transpires my alternator is in need of a rewind, which may be expensive (maybe not, feelers are out).

At any rate, if I'm going to straighten this shaft, I have two options:

1) Continue disassembling the core, It may require unwinding in order to reveal the rest of the shaft, if it was wound in-situ, rather than slipped over on a core. Given it needs to grip the shaft quite tightly, I'm thinking it's probably wound straight on.

2) Use a fettling hammer to straighten the shaft near the bearing.

This is a fettling hammer:



This is what the alternator core looks like right now:


Although it's not totally clear, there's a threaded section outboard of the roller bearing. This is where the bend begins. My plan is to straighten the shaft as well as possible, without damaging the bearing. Here's how:

1) I'll make a steel sleeve, internally threaded, which screws onto that section of the shaft. The sleeve will be a moderately tight fit in a much bigger lump of steel, which will serve to hold everything in place, The back of the alternator will be supported in a wooden "bearing", like it is now.

2) I'll make another sleeve, with an ID as close as possible to the OD of the shaft, but loose enough I can slide it on & off without any binding, probably 3mm thick steel.

3) The alternator will be rotated so the bend in the shaft is upwards (a dial indicator will help me to position it), whereupon it will be locked in place with some form of grub screw device, and fettled with the Big Fuck Off Hammer. To put that in English, I'll repeatedly wang the end of the shaft, in a vertical direction, with the hammer. After a couple of shots, I'll stop & re-check it all with a dial gauge to see if a) anything moved and, if it did, b) what direction it moved in and c) how much more it needs. Rinse & repeat.

4) Once fettled, the various protective devices will be removed, leaving a now straight shaft ready to be re-installed :)


Anyone see any drawbacks with the plan, as outlined?
« Last Edit: September 10, 2009, 09:17:33 PM by AdeV »

tymbo

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Re: Shaft straightening...
« Reply #1 on: September 10, 2009, 10:32:10 PM »
I straightened a go kart shaft like that, but it was only 3/4" dia.  and had access to the whole shaft.  I wish you luck trying to straighten that puppy! :o

mobile_bob

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Re: Shaft straightening...
« Reply #2 on: September 10, 2009, 10:37:30 PM »
have you considered taking it to a shop with a large hydraulic press,

with V blocks and and careful hand and good eyes, a guy that is good can straighten shafts quite effectively.

i am not a fan of using the fettling/sledge hammer (we call em sledge hammers here, don't ask me why)

one missed blow and you have a bigger mess on your hands, a press is infinitely more refined for this project.

too bad we don't live closer, i would really get a kick out of straightening that puppy for you, and help in
sorting out rewinding it as well.

as for rewinding

was it here that there was a link to a utube from somewhere in the orient showing guys taking apart a hermetically sealed
fridge compressor, so they could strip and rewind the motor?

maybe you can find those dudes, and see if they would rewind your gennie?

bob g
otherpower.com, microcogen.info, practicalmachinist.com
(useful forums), utterpower.com for all sorts of diy info

compig

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Re: Shaft straightening...
« Reply #3 on: September 10, 2009, 10:39:02 PM »
Well it's going to stress the shaft big time , especially considering it's already been bent severly.  Is there any way you can use a long  lever to slide over the shaft then heat with O & A and bend ?  Cold bending a shaft that big is savage !
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mobile_bob

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Re: Shaft straightening...
« Reply #4 on: September 10, 2009, 11:28:17 PM »
i have staighten hydralic ram shafts up to 3" in diameter in a 100 ton press, some were bent horribly and it
takes some doing, but can be done.

i would be concerned with using heat, because of colateral damage to the core fit,

seriously there are folks that straighten this sort of thing everyday, and are very good at it.

sometimes its better to go find a brain surgeon rather than doing it yourself?

bob g
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apogee_man

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Re: Shaft straightening...
« Reply #5 on: September 11, 2009, 12:15:35 AM »
I would just take it apart and see what it could cost for a new shaft.

That way there is not chance of future breakage...

I concur with Bob regarding using a press.  That is the only way to do it imho.  Even then, it's still tricky to get it perfect.

Just my $.02,

Steve

AdeV73

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Re: Shaft straightening...
« Reply #6 on: September 11, 2009, 12:19:07 AM »
As usual, this forum comes up trumps, thanks again for the useful advice & lateral alternatives...

Compig: I'd be leery of using heat; as Bob points out, the sorts of heat I'd need is likely to damage the core even more than it already is. In this case, I think the shaft will have to take the stress (or snap, of course).

Bob,

I'll admit, I'd not considered a press, but - as usual - your thinking makes excellent sense. My only question would be: As the bend is on, or near, the threads, wouldn't a press destroy the threaded bit? Even with a protective sleeve?

Hmm, I'll have to check to see exactly where the bend is. I've got a couple of Vee blocks, some aluminium, and a quiet weekend... if the bend is outboard of the threads, then a press seems like the obvious answer. But if it's on, or inboard of, the threads - Plan B?

BTW, I'm trying to spend as little cash as possible on this... partly because money's tight ATM, but also - from a strictly mercenary PoV - any money I spend on the alternator is money I won't get back if I sell it. But mainly because I'm a tightwad, if I'm honest...  ::) On the other hand, I'm a total sucker for tools... do you reckon a 30 tonne (or less?) press would do the job - I can buy one of those relatively cheaply, lots of them on eBay for example. I think the shaft is 1.500", or maybe 1.750". Will check tomorrow.


mobile_bob

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Re: Shaft straightening...
« Reply #7 on: September 11, 2009, 01:25:48 AM »
a 30 ton will certainly bend a 2" or better shaft, without any problem
its just locating the center of the bend and then supporting the shaft properly

i would think some brass or aluminum (whats that? aluminium?) over the treads as a press block might
save them from much damage?

you need to buddy up with a truck (there i go again, whats that,, a lorrie or lorry??) mechanic and see
if he can tweak it back for you in a press at work? maybe a heavy transmission shop? they will have huge presses
there for all sorts of work, and the guys are familiar enough with them to do a good job

maybe a few pints and you got a straight shaft again?

just a thought

bob g
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(useful forums), utterpower.com for all sorts of diy info

apogee_man

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Re: Shaft straightening...
« Reply #8 on: September 12, 2009, 04:55:55 AM »
Does he supply the pints before or after the work has been done?

 8)

AdeV73

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Re: Shaft straightening...
« Reply #9 on: September 12, 2009, 09:18:18 AM »
Does he supply the pints before or after the work has been done?

 8)

Or during....  :o :o  ;)

Spark_Chaser

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Re: Shaft straightening...
« Reply #10 on: October 04, 2009, 04:09:59 PM »
Any progress / update on the shaft straightening?

Got to say as much as I hate paying someone else to fix things for me,
sometimes hiring out (or exchange) can end up with better quality and at a reasonable price.

ie bad cv boots on a fwd car,
at $59 to $110 (US) its better to get an rebuilt exchange than replace the boots.
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past
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AdeV73

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Re: Shaft straightening...
« Reply #11 on: October 04, 2009, 09:49:34 PM »
No progress yet - I've been learning to use a milling machine instead.... Most recent engine work was to finish off the engine loop of the cooling system with some flexi hose, so now I can start working on the "domestic hot water" side of the bargain.