Author Topic: Magneto Magnet Strength  (Read 6012 times)

Online RichardL

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Magneto Magnet Strength
« on: 19.02. 2012 00:15 »
Hi All,

I am working on my magneto with plans to change slipring, add EasyCap from Brightspark Magnetos, replace brushes, check for eccentrcity (a la A10GF's "My Big Magneto Rebuild Project"), etc.  In checking the magnets, their strength seems less than impressive. I'd like to hear some descriptions of how to judge the correct maget strength without requiring a guassmeter or other test equipment that I don't have. Otherwise, maybe offer opinions of what adequate magnet strength should feel like.  As a test, I stuck a ½?-ratchet drive in the magneto body and there was enough strength (and more) to hold the weight of the drive by the square drive peg while horizontal. Does that make sense?


Richard L.
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Offline MG

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Re: Magneto Magnet Strength
« Reply #1 on: 19.02. 2012 08:50 »
Sounds good to me, Richard. They are not really impressively strong as you say, by no means comparable to modern rare earth magnets.
I generally found the AlNiCo magnets used in the 50s/60s would hold their "charge" (as in magnetism) much better than the older ferritic things.

You got a new coil in your mag already? If not, this might be a good chance to get it rewound while you are at it...
From reliable sources I know that Brightspark have successfully tested their K2F remagnetizer just these days.  *grins*
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Offline LJ.

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Re: Magneto Magnet Strength
« Reply #2 on: 19.02. 2012 09:24 »
I would have thought that some hand held luggage weighing scales attached to a bolt and pulled off giving some kind of lbs of reading would have been acceptable. This method although crude, would enable some of us to compare with each others spare magnetos. Just a though...

 Richard, I think your ratchet drive experiment would be more self explaining with pictures. I'm guessing this thread is going to need a lot of pictures!  *lol*
Ride Safely Lads! LJ.
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Online RichardL

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Re: Magneto Magnet Strength
« Reply #3 on: 19.02. 2012 14:08 »
MG,

Thanks for the input. I was not expecting neodymium-like strength, but the ease with which a screw driver pulled off didn't seem quite right. Then, when I used the ratchet drive, there was a better sense of some strength. For one thing, the ratchet drive has more iron to affect, then, it is somewhat radiused, making it a fairly close fit to the radius of the magneto poles.

I think I am going to go ahead with the existing coil, for now. I was not completely without spark and, if a new coil is needed, a second dissassembly of the mag after the install of the EasyCap is but small trouble compared to replacing the coil. I will be using the "Quick Snip Condensectomy" per the Brightspark instructions.

LJ,

OK, I came up with some photos to show the method and applied a scale similar to your suggestion. At this point, I can confirm that the end of the ratchet represents 0.9 Kg. It seems somewhat more tricky to come up with a standardized method to determine the ultimate release strenth. Torturing the idea, I see a rod with a contact lobe on it (all of known dimension and material specification) routed through the magneto and with weight hung from both ends (or a tension scale, like our photos) until it releases.

Richard L.

 
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Offline KenF

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Re: Magneto Magnet Strength
« Reply #4 on: 19.02. 2012 21:52 »
Hi Richard. Thanks for purchasing one of our Brightspark EasyCaps.

I agree 100% with MG. If you have played with these new-fangled rare-earth neodymium magnets, then the alnico magnet in the Lucas K-series mag ain't too impressive, but the good thing is that they don't age quickly and they're fairly insensitive to the temperatures experienced in our mags (unless there is a complete catastrophe).

My pet method for checking the strength of a K-series mag's magnet without any specialised instruments, just a compass, is this:

1. Put the compass on the floor without any metalwork nearby, and line up the scale so that the needle's pointing North.

2. Draw a (pretend) line on the floor going due West for 15 inches from the compass.

3. Put the mag housing on the floor at the end of the (pretend) line, flange downwards, with the magnet in the housing (the bulgy side) facing South.

4. If the mag housing has skewed the magnetic field at the compass by 45 degrees or more, then the magnet is OK (i.e. the needle is now pointing  NW or more westerly than that, or NE or more easterly then that).

5. If the mag housing has skewed the magnetic field at the compass by 60 degrees, then the magnet is very, very good.

6. If the mag housing has skewed the magnetic field at the compass by a lot less than 45 degrees, then it's probably time to have the mag body remagnetised.

There are some limitations on this method. It works fine in Hampshire, UK, where the Earth's magnetic field is about  48 mTesla (http://upload.wikimedia.org/wikipedia/commons/c/c7/WMM2010_F_MERC.pdf) and dip is about 66 degrees down (http://upload.wikimedia.org/wikipedia/commons/3/33/World_Magnetic_Inclination_2010.pdf). It's probably OK for the rest of the UK and Chicago. Elsewhere, people may need to use different limits.

If you'd like to try this method and let me know how much your magneto skews the Earth's magnetic field, I'll gladly give you an opinion about how lively your magnet is.

Ken (Brightspark Magnetos)

Online bsa-bill

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Re: Magneto Magnet Strength
« Reply #5 on: 19.02. 2012 22:31 »
So Ken me lad it's you that's sending all those trucks up country lanes by upsetting their satnavs *lol*
All the best - Bill
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Online RichardL

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Re: Magneto Magnet Strength
« Reply #6 on: 20.02. 2012 00:57 »
Ken,

Thanks for the advice. I will try it as soon as I get a chance.

Bill,

If the magnet in my magneto could upset satellites, I could fire the spark plugs in your bike from here in Illinois (uuuh, maybe).

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline KenF

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Re: Magneto Magnet Strength
« Reply #7 on: 20.02. 2012 08:57 »
By the way, I should have said that the compass test is done without the armature in the body.

With the armature in there, (1) the compass skew is a lot less beacuse less magnetism is escaping, and (2) the thing won't stand up on its drive end  *smile*

Ken (Brightspark)

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Re: Magneto Magnet Strength
« Reply #8 on: 20.02. 2012 10:07 »
As Ken says, got to get the armature out!
It's serendipity really that this Q came up just when we were starting work on our Brightspark measuring (and remagnetising) kit.
Having lost my boy scout shoes with a compass in the heel about 45 years ago, I just scoured the local town for a decent orienteering compass and I've run the identical tests on two K2F boxes, in northern France. Where I guess the magnetic pole is about as strong as in southern England (about 100 miles away).
The first body was a very old one, not used in years and stored with no armature in it (which makes no odds they say, but still). The second was my known-good spare K2FC in tip-top order and ready to roll. It was remagnetised about 3 years ago.
The unknown oldie showed 50° to 55° deflection at 15" distance (which says something for the retentive powers of those old magnets), and the Comp mag showed a good 60° deflection, maybe 62°.
On which purely empirical basis, I agree with Ken that anything much below 45° deflection  is probably a bit feeble, under the identical test.
But dunno at what point they cease to function, maybe we have to demagnetise one by stages to find out what the lower limit is!
Bill
Bill

Offline MG

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Re: Magneto Magnet Strength
« Reply #9 on: 20.02. 2012 10:11 »
Hi Ken!

Welcome to our happy bunch of Brit bike lunatics!
Now all left to do is get yourself a decent pre-unit twin from Birmingham.  ;D *whistle*

Best wishes, Markus
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Online RichardL

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Re: Magneto Magnet Strength
« Reply #10 on: 20.02. 2012 12:17 »
Bu' bu' but, will not the quality of the compass and/or the extent of magnetization of the needle have a big affect on the results of this method?
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Offline LJ.

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Re: Magneto Magnet Strength
« Reply #11 on: 20.02. 2012 12:39 »
Welcome to our forum Ken and thank you for your description on how to test a magnetos magnet. Markus is quite right about the lunacy here as I just had to rush out to test my magnets!  *lol*

Richard beat me to compass quality issues etc and I wondered how exact the layout needs to be. In my pictures below I have drawn exactly 15 inches East to West and placed the compass and magneto housing exactly centred over each end of the line. Looking at the compass I read about 52 degrees. I presume this is the way to get a general idea of the magnetos magnetism?

The magneto I used here is a spare that's just cleaned up. It will likely need a new capacitor and its windings testing. No idea when it was last fitted to a bike. The magneto has always been stored as a complete unit.
Ride Safely Lads! LJ.
**********************
1940 BSA M20 500cc Girder/Rigid- (SOLD)
1947 BSA M21 600cc Girder/Rigid-Green
1949 BSA A7   500cc Girder/Plunger Star Twin-(SOLD)
1953 BSA B33  500cc Teles/Plunger-Maroon
1961 BSA A10  650cc Golden Flash-Blue
1961 BSA A10  650cc Golden Flash-Red

Offline wilko

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Re: Magneto Magnet Strength
« Reply #12 on: 20.02. 2012 21:04 »
Too much spare time lads!

Offline beezermacc

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Re: Magneto Magnet Strength
« Reply #13 on: 20.02. 2012 22:02 »
Just looking at all this compass stuff makes me wonder whether a magneto would work at the north pole.....maybe that's why we've never seen a polar bear on a motorbike.

But seriously, I 've never come across a K2F or a MO1 where the magnetism had become so weak the mag wouldn't work after fitting with a rewound coil, new capacitor and proper assembly. Boosting the magnetism helps at very low rotational speed but even at normal kickstart speed there's sufficient rotational armature speed to achieve the necessary switch of polarity through the armature core to achieve enough voltage in the primary winding. Many of us have experienced the bike 'kicking back' when we are just trying to locate compression. This shows just how slowly it is possible to turn the mag and still get a spark, yet we have to give the bike a good kick to turn the engine over quickly enough for the flywheel momentum to take the piston over TDC whilst harnessing the power from the ignited fuel. What is equally important is the air gap between the armature and the magnet laminates; the build quality of Lucas mags unfortunately often leads to quite large air gaps, therefore poor magnetic induction through the armature core. On the other hand, BTH mags are usually very snug. A further issue is the fit of the alnico bar magnet between the laminates which again, on early Lucas magdynos (covered wagon type), was poor so the magnetism did not get conducted into the laminates because of their uneven fit against the magnet. In spite of this these magnetos still perform perfectly well.
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Offline KenF

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Re: Magneto Magnet Strength
« Reply #14 on: 20.02. 2012 22:06 »
Evenin' all. Thanks for the 'welcomes' Marcus and LJ. Nah, don't need to get myself my own Brummy pre-unit twin cos (1) there's no room in the garage, and (2) luckily I have a mate who lets me ride his whenever I want ... a very very special (unique even) A10 which once had exhaust-assisted suspension ... but that's another story (hah, hah).

What you've shown in your excellent pics, LJ, is spot on.

A very good question, Richard and LJ. Any compass of the sort that you can get at the local camping shop will do. Within reason, it doesn't matter how strongly magnetised the compass needle is. All the needle is doing is aligning itself with the ambient magnetic field at the place where it's sitting on the floor. And the ambient magnetic field is the result of the Earth's magnetic field trying to make it point North-South, and your magneto body's magnetic field trying to make it point East-West. I chose the 15 inch distance because at that distance a K2F with a magnet I would consider borderline will twist the Earth's magnetic field in South Hampshire half way, to NE-SW or NW-SE

Having said that, you don't want a great big bar magnet as a compass needle, and you don't want something so feeble it can't overcome the friction of its bearing. Just a half-decent orienteering compass is fine, strongish or weakish doesn't matter.

Sorry if you find this a bit boring, Wilko. Was just trying to answer a sensible question and come up with something a bit more definitive than "Does the magnet grab the tip of screwdriver, voraciously, so so, or feebly?"

Cheers.

Ken.