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Author Topic: Soft, dull, hollow knock sound  (Read 7545 times)

Ian

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Soft, dull, hollow knock sound
« on: August 30, 2007, 04:16:34 PM »
Hi, I have been lurking for about a year as a guest but have only just signed up as a forum member. I am based in UK and have a 12/1 JKSon Listeroid clone coupled to a 7.5 KVA ST generator situated in an engine shed about 20 metres from my house at the bottom of the garden and running on WVO (the liquid stuff) and old chip fat (the solid stuff). Whilst it is still all coming together, the plan is to provide CHP for my house. The power is grid tied and the hot water (thermosyphon engine coolant and exhaust gas heat exchanger) passes through to my house radiator circuit via a flat plate heat exchanger. I have about 200 hours running experience so far.

I have a couple of ST generator heads (only ONE is coupled to the engine at any one time). Both are remarkably similar in materials and construction (even down to the same braid and knot used to tie cables to the rotor spindle. I have a few questions about these ST heads and I hope that you knowledgeable folk will be able to answer them and enlighten and help me… I will ask each question in separate threads to try to keep the topic concentrated ….. so here goes with number 1:

Knock when turning the rotor slowly by hand
One of my 7.5 KVA ST alternators has been in quite hard use for about 200 hours from new. The front (fan) bearing has started to make a whining noise and I reckoned that it was time to put a decent set of bearings in to replace the Chinese originals. Turning the rotor slowly by hand after disconnection from the prime mover displayed a roughness in the bearings but also a kind of soft non-metallic knock as the rotor was either moved slowly continuously or too and fro. I thought this needed investigating too.

The bell housings came off relatively easily (hammer and a block of wood on the fan end bell housing and a large span puller on the rear end bell). Both bearings were left on the rotor shaft. The rear bearing came off OK with a simple puller with a bit of grunt but the front bearing was a pig and required a hydraulic puller and some very fine arms to get behind the bearing but not foul the fan. Both bearings had unacceptable roughness (but very little real play), and were packed with the dark, thick “yak fat” grease resembling axle grease through which the balls cut a track. These bearings have now been replaced with fully sealed pukka SKF bearings.

Each of the 4 windings on the rotor are carried on formers (I would call them bobbins) that resemble a yellow / cream plaster type material. These formers are located between the rotor shaft and the laminated pole cores. Each core appears to be secured to the shaft by 2 large cap screws. The formers, I think, should be held firmly in place by some angled metal brackets at each end of the former, axially in line with the shaft which I assume to be an interference fit between the former and a milled out pocket section of the shaft – although what actually retains them I do not know. These angled brackets appear to be firmly held in place.

The copper windings on the formers themselves appear to be well wound with enough lacquer to stop any movement of the enamelled copper wires; but not enough for my satisfaction so I will apply a lot more to make REALLY sure they cannot move.

There is no obvious movement of individual pole core laminations or the cores as a whole. They appear to be tight and secure.

Each of the 4 formers have about 5 mm (1/4 inch) of axial movement and about 1.5 – 2 mm of side to side movement about the pole core. There is NO vertical movement. I am assuming this movement is what is causing the soft knocking sound. The movement occurs under the weight of the formers and windings when the rotor is rotated. My guess is that when spinning, centrifugal forces stop the axial movement but I would expect the magnetic forces to cause the formers to be constantly chattering and twisting in the side to side lateral movement. Having said this, I see no evidence of powder from the gradual erosion of the plaster like former material caused by chattering that I would expect to see if it was actually happening.

So my questions are…
1.   Should I be concerned about the movement of these formers ?
2.   What should I do to arrest the movement ?
3.   What is supposed to firmly hold the formers and stop them from moving ?
4.   What, if any, damage may have been caused by the movement and how can I check for it ?
5.   Is there anything else that might be causing the dull, soft, hollow knocking sound ?

Many thanks for wading through a very long first post.
Regards,
Ian

Doug

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Re: Soft, dull, hollow knock sound
« Reply #1 on: August 30, 2007, 06:05:15 PM »
By formers I assume you mean the fiberglass, phenolic what ever it is barries around the coil bobbins.

Could you post a pitcure?

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Ian

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Re: Soft, dull, hollow knock sound
« Reply #2 on: August 30, 2007, 06:19:54 PM »
Doug, correct, I do mean those. They have most of their corners on the slip ring end broken or generally mashed - presumably it happened when the rotor was inserted into the stator on initial assembly.

I will take a photo and post.

Regards,
Ian

Ian

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Re: Soft, dull, hollow knock sound
« Reply #3 on: August 31, 2007, 10:20:14 AM »
Hibbo - I am based in the hills to the east of Macclesfield in Cheshire - a good few miles from you in Scotland. (I bet you guys in USA are wondering where Cheshire is!) ;D


Ian

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Re: Soft, dull, hollow knock sound
« Reply #4 on: August 31, 2007, 12:14:14 PM »
Doug, Here is a photo or two





The top one shows the former moved in the direction of the top of the image by about 5mm (1/4 inch) and the bottom photo shows the former moved in the direction of the bottom of the image by about 5mm (1/4 inch)

The images show the yellowy section (what I am calling the former) and is covered in red primer (presumably cheaper than a laquer varnish); and the angular part is the metal angle.

There are a few more images on Coppermine that you can look at which hopefully will make a bit more sense to tie it all together.http://listerengine.com/coppermine/albums/userpics/10134/

I hope this helps.

Regards,
Ian
« Last Edit: September 08, 2007, 08:25:31 AM by Ian »

rmchambers

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Re: Soft, dull, hollow knock sound
« Reply #5 on: August 31, 2007, 12:20:04 PM »
well it explains the grin, and also  the cheese.

hotater

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Re: Soft, dull, hollow knock sound
« Reply #6 on: August 31, 2007, 02:18:20 PM »
I'm not claiming any kinship with whatever THAT was!!
7200 hrs on 6-1/5Kw, FuKing Listeroid,
Currently running PS-Kit 6-1/5Kw...and some MPs and Chanfas and diesel snowplows and trucks and stuff.

Doug

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Re: Soft, dull, hollow knock sound
« Reply #7 on: August 31, 2007, 02:43:28 PM »
Ok I see now....

I'm not sure what to sugest, try some fiber glass resin brushed on the reinforce any week spots I guess. I wouldn;t try and rebuild the weakend areas with glass cloth unless the area was cleaned well to remove much of the red electrical finnish as possible and there is till a chance it could come loose.

A good coat of electrocal finnish over everything after your done wouldalso help to bind things together.

I've never actualy tried to repair something like this, not cost effective so I wouldn't have tried....

Here is my ST and what I've done. Looking at your pictures I'd have to say yours looks well made.
http://www.putfile.com/dougwp/images/102508
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rcavictim

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Re: Soft, dull, hollow knock sound
« Reply #8 on: August 31, 2007, 03:14:24 PM »
If that were mine I might try a simple fix of letting some crazy glue get into the slide zone through capillary action.  It ought to wick right in there.  Put the fiber plate where you want it first as you won`t get a chance to move it almost instantly after the glue is injested.  Give it a day to cure before using it.
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Ian

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Re: Soft, dull, hollow knock sound
« Reply #9 on: August 31, 2007, 03:23:06 PM »
Many thanks. So I think we are agreed they shouldn't move and they need fixing ?

So what SHOULD be holding them in place ? The capscrews are tight as are the angled brackets. It seems strange that ALL 4 windings are loose and sloppy to about the same degree of slack.

If I assume (but I do not know) that the laminated cores trap the windings by the capscrews (BIG assumption) - do you think it could be possible to undo the capscrews to slacken them off and then feed a rubber packing piece between the core laminations and the winding former before tightening the capscrews again ?

Regards,
ian

Doug

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Re: Soft, dull, hollow knock sound
« Reply #10 on: August 31, 2007, 03:32:05 PM »
Yes they shoudn't move....

You can buy some " Glastic board " from a motor shop and glue it down like a splint that may work. A motor shop may also have some electrical epoxy ( same type of polester resin as you would use to fix fiber glass with ) that you could try fi they price is right.

The stuff the motor shop has is made specificaly for this aplication and will resist heat.
It also has an expiry date so you may be alble to buy some old stock for deal.

Not sure how crazy glue will hold up in this aplication, might be better than epoxy for a quick fix or it may not.
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mike90045

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Re: Soft, dull, hollow knock sound
« Reply #11 on: August 31, 2007, 09:14:12 PM »


Glue it in place at the OUTERMOST point, because that's where centrifugal force is going to send it.  When it's spinning, CF holds it steady, but at low rpm, it will slide around and thump.

Ian

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Re: Soft, dull, hollow knock sound
« Reply #12 on: September 08, 2007, 08:22:58 PM »
Once again, thanks for the input guys.

Just to bring things up to date....

I used "superglue" cyanoacrylate adhesive to initially tack-bond the bobbins in the right place and arrest any movement. Then used a 2-part epoxy adhesive around as many interface points as I could find. The bobbins are now as good as welded into position - I have taken the view that I should never, ever, want to take them apart - if the need arose, it would be easier to buy a new generator.

The enamelled copper wires leading to and from the bobbins appeared to have excessive movement (to my mind) allowing them to move under the influence of centrifugal force. I reasoned that this repeated movement would ultimately work harden the copper and eventually lead to a fracture. So I used some spare epoxy to hold as much of the copper to the bobbin as was reasonable and the jointed "tails" are now held much more firmly than before.

Once again, many thanks for your inputs guys.

Regards,
Ian