Introduction. (by John Self.)
I read Jan Cack’s description of his KISS PMM and was
interested in the possibilities of his design – an elegant solution, using
epicyclic sun and planet gears to remove as much of the fatal magnetic lock
positions as possible, but mechanically challenging. His CAD drawings were also
very helpful in establishing the most favourable number of rotors (8)and
stators (10). That, and Nick Rasmussen’s inspiring magnetic flux and 3D
simulations convinced me that it was worth trying to make one.
It was interesting working out the design: much frantic
sketching and measuring on paper, and thinking while driving to and from work.
Also, making a cardboard model just to figure out the minimum gap between the
sun and planet magnets so as to avoid clash, and the rough dimensions of the
sun magnet holders. I’ve made them with a slotted hole to allow for set up.
To keep the
costs down, I chose to make the sun and planet wheels out of MDF without
running to the expense of cutting gear teeth – just relying on friction.
Risky, I know, because there's the potential for the planets to slip out of
register, but I'll mount the planet wheels against the sun with a fair amount
of friction. I reasoned that it was worth taking that risk because I was trying
to save costs while I overcame my scepticism and the possibility that it
wouldn’t work - let’s face it, the odds are against me. I'm treating this as a
proof principle exercise, the ‘A’ model. Having said all that, if the motor
shows the slightest inclination to work, and even if it destroys itself in the
process, it’ll be worth pushing on with the ‘B’ model,
I have had a
local engineering firm turn some simple ‘top-hat’ bearings out of brass, to be
used between the base plate and the sun and planet wheels. Not ball bearings,
but they spin freely enough for the test.
made the base plate (400 x 400mm) out of MDF and marked it out ready for
drilling the sun and planet bearing holes.
A wood turner
friend has done an excellent job making the sun and planets in the required
profile, and also making the sun magnet holders. The sun has 240mm diameter and
the planets 60mm, maintaining Jan Cack’s design ratio of 4:1.
I now have the
special order planet magnets (50 x 20 x 5mm) with the north and south poles on
the faces rather than the ends, and I've also got the sun magnets
(22 x 10 x 10mm) with north and south poles traditionally on the ends.
The design of
the planet magnet fixing has caused me a bit of sweat, but now I've cracked it
by using slotted pillars, and I'll use the same engineering firm to turn them
out of brass too.
Sun and planets and holders
Bearings and fixing bolt
Sun wheel and bearing