Rare earth magnet problems

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mayhugh1

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I've spent the past few days on a problem that is driving me crazy. It has to do with my distributor and ignition for my Hodgson 9. When I built my Jerry Howell V-4, I built his distributor as his plans called out. The TIM ignition uses is a Hall effect Kettering-like ignition. The distributor uses a brass disk into which are glued 4 rare earth magnets in a radius about the center axis of the distributor. When the Hall device turns ON the coil is charged and when the Hall device turns OFF the coil fires. The time that the Hall device is ON is the time the coil charges and this is the dwell. My V-4 distributor uses 2mm dia rare earth magnets purchased fom Howell. The radius on which the magnets are mounted determines the actual degrees of dwell. The further from the center they are located, the fewer degrees of dwell I end up with because for the same angle the magnets are further away from the edges of the Hall device. In the V-4, the radius of the magnets is .3". When I when I built the engine I measured 40 degrees (crank) of dwell. (I kept a good set of notes during construction and testing.) When I designed the distributor for my H-9 radial, I used .578" for the radius and used the same 2mm magnets. I doubled this radius because I remembered my dwell on the V-4 as being 40 crank degrees and this engine ran well between 1200 rpm and 6200 rpm. I decided to cut the dwell in half because I expected to run the radial at a max rpm of only 2500-3000 rpm or so. Since I had 5 more cylinders I didn't want to stress the H-9's TIM ignition any more than I did with the V-4. I measure the H-9 dwell at 21 degrees as I expected. But I recently re-checked my V-4 dwell the other day while running it, and I found it now to be only 24 degrees. The only thing I can come up with is that those 2mm rare earth magnets in the V-4 distributor have lost strength during the past 1-1/2 years since it was built and tested. I purchased a number of those magnets from Jerry Howell's website at that time, and have several left over after building the H-9 distributor. They have been stuck to a metal plate 'keeper' since they were purchased. I checked them today in a dummy distributor disk and they give 40 deg of dwell as expected at a .3" radius. The ones in the V-4 distributor, though, have definitely lost strength. I don't think these magnets are supposed to require 'keepers' but now I'm starting to think that when locked into in a repelling configuration like a distributor disk that they may be susceptible to a loss in strength over time. maybe the engine heat adds to the problem. Even though the Howell V-4 has only 4 cylinders it is actually two single pin crank V-twins in series with an irregular firing order that distributes the four magnets as two pairs that are 45 distributor degrees apart. At a .3" radius this puts the centers .2" apart but the edges are only .129" apart, and so the pairs are definitely fighting one another. - Terry
 
Hi, if I can make any suggestion it would be this.
go to a crafting shop and get regular magnet and replace your rare earth
that you have now.
hope it works:)
 
How hot do the magnets get? Neodymium magnets can lose strength over 100 deg celsius. Cheaper ones may even be worse. Samarium cobalt are better at elevated temperatures.
 
Why would replacing rare earth magnets with conventional be advised?
Mosey

At a .3" radius this puts the centers .2" apart but the edges are only .129" apart, and so the pairs are definitely fighting one another. - Terry

Mosey
If I understand the issue properly both magnetic field are fighting each other
by putting regular magnet "weaker" the magnetic flux is reduce and could solve the problem. I've been using all sensors for decade in robot assembly and ignition system and never use a earth magnet
 
While your design is proven to work, I'll share this idea. On motorcycles the timing rotor is a plate with very low magnetism, and square shaped pins that pass the pickup. I'd think that the high magnetism would muddy the magnetic field, for lack of a better word.
 

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