Der Uber Coker was built up for Kris Holm using a modified Chinese prototype geared hub with one of Dave Stockton’s world’s strongest Coker wheels laced around it. It’s been around a bit. Brian MacKenzie, the bowler formerly known as Sofa had it for awhile and then it went to Andrew (HardcoreCokerRider) for some speed work. The hub seized when Andrew was performing a turn at low speed so, fortunately, he wasn’t hurt. He shipped me the wheel and I gutted it leaving the hub laced to Dave’s beautiful wheel.
A segment of one of the planet gears had broken off. It further shredded the bushing it surrounded spredding some fragments of sintered bronze through the mechanism. One of the remaining intact planet gear/bushing assemblies was jammed quite tightly, probably from this detritus.
Some photos of the damage are IN THIS GALLERY. The way REAL gears are supposed to look are in photos on MY WEBPAGE in the sections on gears and bearings and modifications.
My theory is that the cheap, apparently EDM cut gears made by the Chinese firm were not anywhere near the real gear tolerences specified. In fact I know that. The planet gear that broke, as well as all the planet gears, could have their teeth cut too deep so they would fit in the assembly. This thinned out the effective wall thickness between the ID and minimum OD of the gear.
We are presently thinking of what will work and what compromises to make to get the thing going again as soon as possible. I kind of think that I will have to buy a set of gears that are commercially (and correctly) made. Then I may want to make a new axle out of hardenable 17-4PH stainless steel. I would probably need to make a new sun gear bearing cage, too.
You are right. The photos of the broken gears look nothing like the ones on your web page. The ones in the new gallery are all blurry and out of focus. The ones on your web page are sharp clear and look much better. That’s obviously what the problem is. The Chinese gears don’t photograph well.
I predict you’ll be getting your Blue Shift back ASAP. Only a fool would ride that thing now. Maybe we can get Blackwood to ride it on Sunday.
So, I look back on this and see that I received der Uber Coker in May. I just started doing the maching a couple of weeks ago in my spare time. Spring and summer are for playing on unicycles, not making unicycles.
I bought commercial automotive gears for the rebuild and modified them the way I did for BlueShift. I reamed the planet gears to accept press fit, commercially available, sintered-bronze oil-impregnated bushings and reamed the bushings to 0.2495". The Chinese planets ran on mild steel rods. The new ones run on polished, 0.2500", hardened steel dowel pins. I had to machine a new planet cage because the Chinese one was too sloppy, was not built correctly, and had unreachable burrs that could snag moving parts. The dowel holes are 0.0005" undersized for an interference fit. I peened the dowel pins in two places on each side (5 pins, two sides, 20 peen dimples) to ensure that they remain anchored.
I salvaged the sun gear cage and welded a modified, commercial gear onto it. It is a full fusion weld rather than a few spot welds like the Chinese sun assembly.
The ring gear was easy to do. Then I found that the bolt circle in the Chinese aluminum housing was off a few thousandths for one of the dowel pins. I reamed that hole out and shimmed that dowel in the ring gear. The shim is covered with a spacer plate so it can’t work its way out of the ring into the gear teeth.
I machined an axle from 17-4PH stainless steel. I welded the axle to the planet cage and hardened it to Rockwell 42. Now it is hard enough to be usable with steel cranks.
When Kris first got the wheel he drilled a hole in a stock Coker frame. He attached the sun arm (torque arm) to the frame leg with about a 4mm screw and nut with some spacer washers. It was a good anchor with what he had on hand. The drilled hole was not quite straight and the 4mm screw was not to beefy, but Kris doesn’t have a machine shop immediately accessible. I bored a 0.5" hole straight through the leg in the right place, centered over the tapped hole in the sun arm. I machined a brass plug with a lip and counterbored for a 6mm screw which matches the tapped hole in the sun arm. I used 0.100" of brass washers to space the sun arm. The length of the plug is just a few thousandths less than the leg tubing thickness so when the assembly is tightened it doesn’t crush the frame tubing. It’s a good, beefy anchor.
Not enough of my sissy Seattle unicycling buddies wanted to test it before I shipped it back to New York. I rode the unicycle a couple of miles today, tightening the cranks occassionally until they were fully engaged. I freemounted it on my first try. My second go took me six tries. My third attempt, I got the freemount on my second try and then landed it every try from then on. I take a RUNNING start and do a jump mount when the wheel has some inertia. It is a fast rascal.
Oops. Typo. The holes in the planet cage are reamed to 0.2495" for the interference fit. The bushings are reamed to 0.2510" to spin freely on the dowel pins. I also don’t know how to spell “machining” as you can see in my first paragraph.
After riding awhile, I measured the backlash with a dial indicator. It was 0.023" or a little over 0.5mm with 6" (150mm) cranks. I expect 0.040", or 1mm, from these gears with 6" cranks.