Author Topic: Lucas K2F Armature  (Read 761 times)

beezermacc

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Re: Lucas K2F Armature
« Reply #15 on: 24.05. 2020 23:18 »
A mate of mine wondered why his headstock bearings kept failing on his BMW R45. It was because there was no earth wire from the headlamp to the frame so the earth tracked through the headstock bearings. Same applies to magneto armatures. If there is no other earth route the bearings will be shot in no time. Regarding flux and points opening... ideally the points should open just as the polarity switches in the armature. The moment of switching sets up an electrical flux which conducts (I should say inducts actually) into the coil and ramps up through the thousands of turns - that's the only way I can explain it - electrophysicists please don't get picky! If the points open too early there is no flux so no spark. The strongest flux is at the moment of switching, from that point onwards the flux is steadily collapsing but, for a while, still doing enough to conduct into the coil, so if the points open late you still get a spark, but a weaker one. So, timing the points to the internal structure of the magneto is a good thing to do, which is why Lucas provided an eccentric stud under the press fit button on the cam ring housing. By turning the stud with a screwdriver you can rotate the cam ring relative to the points. If you're very brave or a bit cavalier you can play about with this. From the earlier explanation you will deduce that the earlier the points open the better, so you can twiddle with the eccentric screw to make the points open as early as possible and if this hasn't killed the mag it will have made it better! You should only do this on a manual magneto in the full advance position and you will have to reset your engine/magneto timing afterwards as well. The problem with the internal timing is that the flux collapses very quickly so you need to catch it whilst it's falling but before it has fallen very far.  How good your mag is in terms of its consistency i.e. the points open at exactly the same time every revolution, will affect how close to the point of maximum flux you dare set your eccentric screw. If your mag is absolutely top notch you can set the eccentric screw very finely adjusted to point of max flux but if your mag has some wobbly bits like a loose cam ring or worn heel to the points or dirty points etc. your points will open, electrically, at a slightly different time each revolution. In this situation you should not gamble with the eccentric screw because there will be some occasions when the points open earlier than usual and the mag won't spark - hello misfire! I thought I'd throw my two penn'orth in as Groily needs time off for good behaviour! As a p.s. I note somebody mentioned V twins. An interesting case. Normally, one cylinder fires at optimum flux, let's say 0 / 360 degrees on the cam ring, the other cylinder fires 60 degrees later (or 420 degrees later depending on how you look at it but the maths and geometry are the same), but only 30 degrees late on the cam ring because the mag rotates at half engine speed. Typically a V twin will fire at kickstart speed on the cylinder which fires on max flux then generates sufficient engine speed to fire on the cylinder relying on the low flux side of the mag. More mag speed equals more flux (sort of - electrophysicists back off!). This illustrates that having the eccentric screw on a conventional mag set to not quite maximum flux isn't a deal breaker because there's no way you are going to be 30 degrees out! Furthermore it explains why a manual mag, whilst stopping the bike kick back, will be difficult to start if retard the lever/cam ring too much. The closer to max flux on points opening, the easier the bike starts. Of course the strength of flux is directly proportional to magnet strength but don't get me started on that one! Good night!

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Offline Swarfcut

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Re: Lucas K2F Armature
« Reply #16 on: 25.05. 2020 07:56 »
 I would not be tempted to adjust that cam ring, which means that when building a mag from the best bits I have it looks as if I will be either very lucky or definitely on a losing streak depending on the points housing I select from my scraps.

 Is this likely to be of any real consequence? Without electrical workshop test gear, my only test is whether it actually works!

 Thanks to those two Forum Members who have posted the theoretical and practical finer points and aspects of this electrickery.

 Swarfy.
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beezermacc

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Re: Lucas K2F Armature
« Reply #17 on: 25.05. 2020 08:31 »
I would not be tempted to adjust that cam ring, which means that when building a mag from the best bits I have it looks as if I will be either very lucky or definitely on a losing streak depending on the points housing I select from my scraps.

 Is this likely to be of any real consequence? Without electrical workshop test gear, my only test is whether it actually works!

 Thanks to those two Forum Members who have posted the theoretical and practical finer points and aspects of this electrickery.

 Swarfy.
If you don't adjust it the mag will probably work but the cam/points/flux may not be in the optimum position relative to each other. You can test the internal timing if you can rig up a way of turning the mag smoothly and slowly, ideally on a motor with a variable speed. A piece of string wrapped round the armature threads will do, like starting a lawnmower! Start with the eccentric screw in the fully retarded position and advance it until you lose the spark, then give a nudge back into the retarded zone and all should be well. Obviously, if you've built your mag from bits and pieces and some aren't very good you'll need to be a bit more cautious and retard the eccentric screw a bit more to compensate for any inconsistencies in points opening .

To be honest I shouldn't be encouraging people to build mags out of bits and pieces because there are dozens of things which might be contributory factors in the mag not working properly, not all of which the less experienced will spot - sorry if this sounds rude! If lots of people start building mags out of bits and pieces we end up with lots of unreliable mags on the road and the mags then get a bad reputation, undeservedly. Then we get back into that old argument about electronic ignition versus magnetos. A properly built mag should last for many thousands of miles assuming usual routine checking and adjusting.
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Offline Speedy

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Re: Lucas K2F Armature
« Reply #18 on: 25.05. 2020 09:15 »
The Lucas magnetos are from the SIMMS company that Lucas brought  out. The armature is from a design  made by Siemens  comonly refured to as a shuttle  armature.  Invented in the 1870s. So the complete magneto design is over 100 years old. Interestingly on light air craft  a battery powered ignition system is not allowed, instead they have two magnetos per engine . These magnetos are now made by Champion, with  being for aviation use they have to go through stringent testing every so many of hour's of flight. You can also see that magnetos are used in drag racing, so realy it is the way that the magneto are set up that count,and how well they are being g serviced, for happy motoring.
Has anybody got the Eddy Dow servicing sheets  for the A10. Thank you all for helping in this topic.
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Online RDfella

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Re: Lucas K2F Armature
« Reply #19 on: 25.05. 2020 14:51 »
Probably didn't explain my query clearly - not the first time and unlikely to be the last!
I explained that when turning the armature slowly, there's a distinct pull as it passes the magnet, with a 2nd pull slightly later. On the mag in question, the points open a few degrees after that 2nd pull. But my question is - at what point is max flux - after the 2nd 'pull' or between the first and second?
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Re: Lucas K2F Armature
« Reply #21 on: 25.05. 2020 17:12 »
Probably didn't explain my query clearly - not the first time and unlikely to be the last!
I explained that when turning the armature slowly, there's a distinct pull as it passes the magnet, with a 2nd pull slightly later. On the mag in question, the points open a few degrees after that 2nd pull. But my question is - at what point is max flux - after the 2nd 'pull' or between the first and second?

Could your “second pull” be the fibre heel on the moving point touching the cam ring ramp?
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beezermacc

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Re: Lucas K2F Armature
« Reply #22 on: 25.05. 2020 18:31 »
Probably didn't explain my query clearly - not the first time and unlikely to be the last!
I explained that when turning the armature slowly, there's a distinct pull as it passes the magnet, with a 2nd pull slightly later. On the mag in question, the points open a few degrees after that 2nd pull. But my question is - at what point is max flux - after the 2nd 'pull' or between the first and second?
When I'm building a magneto I try to achieve good mechanical 'feel' before fitting the points. Magnetos are invariably old, have been fiddled with, and have had incompatible parts swapped in and out. Before fitting the points there should be no feeling of friction nor any significant resistance in the bearings due to preload nor any endfloat. When the magneto is in this state it should be possible to feel the resistance of the magnetism very distinctly. It feels almost like air pressure resistance which can be burst through. This should happen twice every rotation at 180 degrees apart. The armature and magnet alignment is symmetrical so, unless something really horrible has happened, there will only be two 'bumps' per revolution. Any other 'bumps' must be due to friction, either the earth brush, pickup brushes or the points heel. I suggest you remove all external components including the points to identify what your 'bumps' are!
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Online RDfella

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Re: Lucas K2F Armature
« Reply #23 on: 26.05. 2020 20:42 »
Maybe ‘pull’ is the wrong description. One can feel as the armature approaches the magnet, and again when it’s passed it (around 20* of armature movement). Same 180* later. It’s this window of approx 20* I’m referring to. Whereabouts in that does max flux occur? In the middle? At the end? As currently set up the points for the rear cylinder open very soon after that second ‘pull’ / episode or whatever one wants to call it.
I looked at experimenting with advancing the cam ring, but there’s no adjusting screw – just a rivet (looks rather like those used to fasten engine number plates to cylinder blocks) limiting cam movement, so I’ll leave it alone and concentrate on modifying the distributor I plan to replace the mag with. Machined up a new 60* cam yesterday – just need to get the cylindrical grinder working so I can accurately remove the existing cam lobes ready for the new one to drop on.
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Online Triton Thrasher

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Re: Lucas K2F Armature
« Reply #24 on: 26.05. 2020 21:34 »
The eccentric peg screw is under a cover that looks like a rivet head.  You can ease it up and off with a knife.

Points open at the end of the magnetic pull that you can feel.
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Re: Lucas K2F Armature
« Reply #25 on: 26.05. 2020 22:09 »
Thanks TT. When I looked inside after removing the cam ring, the thread of said screw looked more like a spiral, leading me to believe it was a drive-in rivet.
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Re: Lucas K2F Armature
« Reply #26 on: 26.05. 2020 22:13 »
It’s not intuitive, is it!

As the mag wasn’t supposed to be home-serviceable (or even very BSA agent-serviceable), apart from the points, manuals and service sheets don’t tell you much about them.
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Offline Speedy

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Re: Lucas K2F Armature
« Reply #27 on: 29.05. 2020 18:14 »
Have you looked at the contact breaker plate, the moving contact can wear a groove in the plate from it rubbing it. Also the leaf spring can sit proud of the cam plate and rub the cam ring if the tuffnel block is worn. No adjustment on the leaf spring, only the play in the fixing holes. These leaf springs are not sold with a new set of points, and not sold on their own either. So be carefull as to what you do with them.
Could you please post the drawing for making a new cam points ring. To see how much lift you are giving the contact breaker arm.Also the material that you made it from. I would think EN8.
Thank you.
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Online RDfella

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Re: Lucas K2F Armature
« Reply #28 on: 29.05. 2020 18:18 »
Speedy - I'm not making a cam points 'ring' - I'm making a 60* cam for a distributor to replace the magneto. Material silver steel so it's easily hardenable once fettled.
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Offline Speedy

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Re: Lucas K2F Armature
« Reply #29 on: 29.05. 2020 18:58 »
Thanks for clearing that up. Best of luck with the mods to the mag.
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