I got the last drivetrain parts on Tuesday afternoon. So, I guess I’m proud to announce that the power transmission side of LOLrioKart is complete!
With this, work is shifting onto the electrical system and last touches. What does this mean? You’ll probably not hear about this project for the next 8 months, of course, if the scooter build was any indication. However, the sheer level of ridiculousness this project has attained so far, coupled with my lack of usual robot projects, means I’ll most likely keep moving quickly.
Here’s some drivetrain assembly pics.
I tore down all the temporary mounting provisions to await the final parts and hardware. The bolts that temporarily retained the wheels (so we could putz around the hallways under human power) have been swapped with the actual half-shafts. I bought some 3.5″ long socket head cap screws to mount the Etek through one of the transaxle side plates. The only thing I needed to make were longer shafts for the differential so I could use shaft couplers to link them to the wheels.
Making the longer differential shafts mostly involved taking the center shaft section left over after making the halfshafts , splitting that in two, and cutting a retaining ring groove for each half.
Unfortunately, I couldn’t find my Convenient Lathe Bit of Groove Cutting (+1?), so I had to grind one from a blank.With no proper tool grinding provisions, that was quite an interesting feat.
The shafts now poke out another inch, enough to secure the shaft couplers.
Like so. With the “pacmen” secured and all collars, set screws, and whatnot tightened down, the assembly is rock solid.
The couplers are simple 3/4″ bore keyed-plus-set-screw jobbies from McMaster. A long key connects both shafts together. So, under optimal conditions, it is the keyway (not the coupler itself) that will transmit the torque. The coupler’s just there for moral support.
In a similar fashion, in a properly designed bolted joint (Gee, how many times do I actually design my bolted joints?), it’s the compressive friction force between the pieces being tightened together that transmits the load – not the bolts themselves, which are only there to provide the compressive force.
And the chain is mounted.
It’s a single-strand ANSI #40 setup. I didn’t expect that mounting this would be so straightforward, give that I had two odd-toothed sprockets at a non-integer inch center distance. But, to my delight, the chain length required is indeed extremely close to a whole number of links. It’s “droopy”, but not loose.
After the chain stretches from breaking in, it will probably be more on the “loose” side of things.Â In a pinch, I can route the chain over one of the Etek’s lower mounting spacers, and drop a bearing or idler sprocket on said spacer.
So that rounds out the drivetrain. Well, a drivetrain isn’t any good without a power source, so onto the batteries…
Here they are again, the giant nicads of last year. I have no real facilities for taking care of batteries this large. At the same time, I figured batteries this large could take quite the beating before permanent damage occured.
It’s not like they’re healthy after sitting for two or three years before sitting for 8 months, anyway.
To revive the packs again, I first applied the voltmeter on each cell. Most of the cells in each pack still showed a reading above 1.1 volts. Others, however, were pretty much zeroed.
Nickel cadmium batteries, when left sitting a long time, like to grow tiny crystalline filaments of nickel within themselves which cross the electrolyte and separating layers and poke the other electrode. This, of course, internally shorts the cell, causing rapid discharge to zero volts.
To wake those cells up again, there is a scientific process known as zapping, which uses a high voltage capacitor to momentarily dump a large current into the cell. The filaments vaporize, and the cells can be cycled again to get rid of them.
Of course, scientific processes I had not access to, so I made do with a large lead acid battery and some meaty alligator leads. A very short ‘blip’ on each cell released a shower of sparks and put some life into the cell. It was then immediately put on my peak charger to bring it to 1.2-1.3 volts.
This process took a while, since each cell had to be measured, zapped if necessary, then charged after zapping. I think I’ve been babysitting the batteries on and off for the past week.
Eventually, however, all of the cells were at a level close enough together to charge their entire respectiveÂ packs at once. This just involved setting my charger to 7 amps, its maximum current capacity, then leaving the whole thing overnight.
Yes, the building is still standing. I’d come back and see the charger stopped somewhere around 400 to 450 minutes, after the amp-hour counter had fully rolled over. This calculates out to ~40AH put into each pack. My guess is that alot of the charging current went into slightly overcharging the good cells while the weak cells caught up.
For the past two days(!), all4 packs haveÂ been wired in parallel and the whole thing charged at 5 amps, to attempt to equalize all the packs. I had to turn off all the limits on my charger for it to run that long. The 3-digit minutes counter has rolled over several times.Â Each pack is getting about 1.25 amps at this charge rate…which is like C/30. They will probably never peak.
I have no facilities for testing the discharge of cells this large, so I’m just going to lob them on the kart and drive around with a voltmeter. For now, I’ll assume they’re all in decent condition.
What’s next? Oh, designing that battery + electronics mount that I never really got to!
I’m a bit stymied by battery layout. There’s several configurations I can use, each with their upsides and downsides.
This 4-in-a-row layout gives the best possible center of gravity placement vertically, but not horizontally. It puts alot more weight on the rear wheels. With me in the basket, it will be even further back.This means I stand a much higher chance of wheelying instead of launching, and it could affect steering also (heavily rear-weight-biased vehicles tend to suffer from understeer).
Since LOLrioKart is not a real car, I’m not as bothered by this fact. The cheap handcart rims will probably bend first.
Alternate placements include a 2-by-2 layout – that is, two packs stacked on top of eachother, then two of these metapacks side-by-side. Then I can place them either transversely (as pictured) or longitudinally (down the middle). The disadvantage of any ‘stack’ packs is that I don’t have immediate access to all the battery terminals, something that I think I’ll need because of the age of the cells and their demonstrated voltage instability.
Yet another placement is a “3-by-1″ T layout. I can’t fit all 4 packs longitudinally side-by-side, but I can fit 3. Then I can have a single one at the back, mounted transversely. This actually gives the most centered (good!) layout of the batteries, but I’ll have to make a T-shaped mounting basket for them, which makes that issue more complicated.
In the end, I think I’ll just go with the 4-in-a-row. It’s the simplest to design a mount for. I can build a wheelie bar or add something else up front to compensate. I have access to all the cells (electronics and wiring can actually go on the back side of the basket to keep this space clear).
Another advantage of the “3 by 1″ layout is the ability to shift the whole pack forward, past the motor. Right now, I can only elevate the packs off the ground by about 2.75 inches before I hit the motor mount. This means at most, I can have 2.75 inches of ground clearance – throw in 1/4″ for the mounting provisions, and I can probably have 2.5″ of ground clearance, maximum.
Go-karts tend to be pretty low machines, so this may actually be acceptable. After all the sprocket in the back is 6″ in diameter, on 9″ wheels – giving a maximum clearance at the back of 1.5 inches. The front steering brackets hang pretty low also.
But if I ever suffer from ground clearance issues, it’s always an excuse to move to bigger wheels…
Stay tuned for the next episode of Charles plays with large batteries in an inappropriate fashion!