Re: Almost like gears…
Cat is out of the bag so to speak…
No Tony you are not crazy, I have for the past two month been working on such a
transmission. The protype turned out to be to heavy (read that as monentum). Let
me explain…
The first prototype, an octogan piece of steel stock with two pieces of the
octogan cut to form slides or linear bearings. I used rollerblade bearings
bolted to the slides so they contacted the octagon stock at the correct
angles the assembly was driven by a lead screw turned with a battery
operated screwdriver which was mounted to the back of the crank arm. This
helped in balancing the arm. The battery was mounted to the seat post the
switch being hand held. Two copper slip rings transmitted power to the motor
on command from the switch. The screw drive assembly would change the “crank
arm” length at will.
So much for the mechanism, now for reality. Take John Foss or any of the other
Muni’ers and you realize that the crank arms undergo alot of torsion. This being
the twisting action of the arm as us unicyclist crank away. I hooked my Zephyr
crank arms to an Instron machine( a machine which gives you a plot of load
versus extension) They started to deflect at 50 lbs of downward force on the
pedal. Not much force needed to twist the arm. Granted the deflectioin was small
but it was enough to effect the movement of a linear slide ie… throw the
bearings off of their track and dramatically increase the amount of torque
needed to move the slide. Okay enough of the the Mechanical Engineering for now.
I planned for this deflection in the first prototype. Or so I thought, turns
out that the octagon piece of stock needed to be hardened. The bearings would
cut a path into the octogan slide. Which further increased the torque required
from the poor screwdriver motor. Ah the trials of life. I never rode this
prototype. Throw in my 215 lbs, soft steel and you have disaster waiting to
take place. Back to the drawing board. Oh for you who are still reading this
and just need to know. My octagon crank started to deflect a 102 lbs of force
on the pedal. Throw in the slide mechanism and the deflection started a 30 lbs.
Can you say lever!?
Fortune smiled on me one Friday afternoon. I was at a salvage facility picking
through some electronic goodies, looking for a high voltage diode (I have a
fondness for high voltage) when I saw some beautiful linear bearing over in
another pile. Of course light bulbs went off and I realized I had the torsion
problems solved with these bad boys. They were 3/4 inch thick hardened rails
with ground groves for the linear bearings. Beautifull pieces and cheap. I
bought them for 25 dollars a set. The catalog price for the same set is 450
dollars!!! Needless to say I was in heaven, I rushed back to the lab, bearings
in hand and started in on the task of mounting them to the hub.
Ah such is life once again. As Rick Bissell our resident Mechanical Engineer
will attest to, hardened steel does not machine very easily. I would go so far
as to say that the hardened rails just stood there and laughed at my meager
efforts to cut a square hole in them to fit to the hub. I could have sent them
off to be EDM’ed, but I am not a patient fellow. So I drilled two mounting holes
in each Zephyr crank arm bolted the linear slides to them. machined two pieces
to hold the pedal on to the bearings attached the lead screws and set out for a
test ride.
Whoa nellie, we now get to the momentum part. Did I mention how much this
monster crank weighed. They tipped the scales at 25 lbs for the set. Not much
you say well start turning them at several RPMs and try to stop them. They
literally lifted me off of the seat. But they were rugged you could not get them
to twist. I dare say a whole crew of Muni’ers could not budge them. The screw
drive system worked perfectly, here is where I found out that you really do not
need alot of crank arm retraction before the uni becomes hard to move. Also to
much extension of the crank and you are singing soprano (read that as seat
height to pedal travel). A happy travel distance is 1 and a half inches to 2
inches total travel with this you go from really cranking hard with great speed.
To going really slow but crap loads of torque. Good for hill climbing. At the
small crank arm length is where I was reminded of the flywheel principal of
energy storage!!
Okay the end finally, In the works in a calmer slide, well more like a train
wheel than a slide. This helps cut weight, either a worm drive system (god those
are expensive) or a screw drive similar to your garage door opener to turn the
pedal through its arc.
Your should stick to electronics engineering, fellow unicyclist Joe