Rotor

wilko · Post by **wilko** » Fri Jul 25, 2008 9:13 am

Anyone know if i can skim 5 thou off my alternator rotor will cause loss of magnetism. Yeah i know! but there is no way i'm getting the required clearance by shifting the stator. it is almost an interference fit in the outer case, so making up a dummy rotor is going to do Jack Sh%$#&t because it wouldn't budge anyway. Dugless

TommoT · Post by **TommoT** » Fri Jul 25, 2008 10:50 am

...but even if it does you can have the rotor re-magnetized. Yes I know you would like to avoid it, but do you have any other options?

Tony Birkett · Post by **Tony Birkett** » Fri Jul 25, 2008 11:08 am

Have the same problem with a new stator and rotor, interested to see the replies, turning a few thou off the rotor would appear to me to be the only solution.

woster · Post by **woster** » Fri Jul 25, 2008 9:23 pm

IF the stator or rotor or both are new why not take them back to the manufacturer to sort out the problem. If not and you have the equipment to skim the rotor, why not bore (skim) out the stator, and avoid any problems with the magnetism. Keep in mind the greater the gap between the two becomes, the more the generating capicty is reduced.

vnlm · Post by **vnlm** » Fri Jul 25, 2008 10:10 pm

If I remember correctly from my years of learning, magnetism is 'induced'. Thus it is placed into a suitable material as a 'whole' inducing poles in the process, I can't see skimming it will greatly affect it, if at all. However I agree it should not have been paired with a stator that does not give the required tolerances. This is unusual, did you try them before fitting ?? it may not be aligned correctly, as these columns will tell you this is most common.The only thing I can think of by reducing the mass is the proportional 'flux'density reduction. This in itself should not be a problem, however I am not an expert in this field (no pun intended). The principal upon which it works should not be affected, unless theres something I don't know about the construction.

Don Madden · Post by **Don Madden** » Fri Jul 25, 2008 11:53 pm

It is correct that skimming the rotor would reduce the flux very little as long as nothing is done to partially demagnetize the magnets, (shock, heating to curie point, etc). However, the rotating magnets induce a force in the stator across a necessary gap & of all the variables, (velocety, alignment, etc.) the most critical is spacing loss, the distance from the magnet to the pole. While the other variables are linear, the spacing loss is inverse logrithmic. If you double the gap, you reduce the induced field by four, triple gap by nine, etc. The gap rotor & stator must be kept concentric to reduce spacing loss so first determine why the gap is insufficient. If either is off-center, they will rub even if the diameters are correct.

Cheers, Don, (retired magnetic tape/disk drive engineer).

wilko · Post by **wilko** » Sat Jul 26, 2008 2:02 am

I'll go ahead anyway!As it's a seconhand unit from a Triumph TR25 so i dont really mind if things go wrong. I have a lathe.I put some bearing blue on the rotor and it's only slightly scuffing and the rotor has some old rub marks on it so i will just clean it up. I shall be the guinea pig! Dugless

Don Madden · Post by **Don Madden** » Sat Jul 26, 2008 3:22 am

I had a different problem with a stator made in India. The outer diameter was too large to fit into the recess in the primary case outer. I had to skim a little to get it to slide down into place. This would only be a problem with AMC as I have seen no others that mount the stator in the outer half.
Cheers, Don

vnlm · Post by **vnlm** » Sat Jul 26, 2008 8:21 am

Don,
You are correct my old mate, forgot to mention the old 'inverse square law' the bane of my life in other matters. Does anyone know what the gap (or tolerances) should be.
I agree that the engineering principle of putting the units in separate halves so you can't line them up or check them correctly is poor. Some people have spent hours imgeniously correcting this oversight by several methods (most of which in here).Room here for some enterprising person for a conversion/mod kit although it won't be easy, involving the rear casing.
One method I thought of to align them and prevent damage on assembly was to insert a plastic shim that dissolved in the oil when in use. Don't laugh, any fisherman here will tell you this is a common method of bait retention. However I don't know if this stuff dissolves in oil.Thus leave the stator slightly slack, wrap the shim tightly round the rotor. Loosely assemble, in theory they should line up, carefully remove outer cover, tighten stator, then reassemble with shim in place which should dissolve.
Don't tell anyone Don !! we could be sitting on a fortune here.

Eric · Post by **Eric** » Sat Jul 26, 2008 8:34 am

I seem to re-call an article in the Jampot magazine some years ago where someone had changed the fixing location of the stator from outer chaincase to the inner one and it didn't seem to involve too many problems. That's just from memory of course I not given it much more thought than that.