The Secret of BurnoutChibi

Wednesday, March 27th, 2013 @ 5:18 | Chibikart, Project Build Reports

Continuing on my last post, I’ll talk a bit about how Chibikart is getting a radical design departure from my usual silly vehicles, and later on some about what the hell I’ve been up to for the past week or so. These days, like last spring, my time during the day is heavily  devoted towards herding 2.007 students in some way, including my own “victory garden” of 2.00gokart students who have seemingly become accustomed (read: spoiled) to work late into the night like I do. I’m clearly being the best of influences for my impressionable undergraduates. With my spare time being more fragmented and less conducive to sustained building, I’ve been revisiting some things which I’ve mentally tucked away for better days.

Anyways, to recap, BurnoutChibi is my proposed refitting of the Chibikart 1 frame, which is currently sitting derelict waiting for motors I will never regain the patience to rewind, to a form which offers some more excitement. The reason for the upgrade is twofold – first, I’d like to bring it up to “Chibi 2″ standards, but more importantly, to up the power and show how an inexpensive sensorless drivetrain should be executed. The last time I showed the design, it was a simple one-stage design that was basically the DPRC with more power, and pneumatic wheels.

As I was designing the drivetrain reduction ratios, I began thinking more and more that having 1 speed, even in an electric vehicle, is just suboptimal. With the power levels that the selected drives were capable of, I could reach about 18mph and smoke some rear tires. But that would be it – the fastest it would ever go would be said 18mph. Yet just power-for-power alone – power dissipated by air resistance at speed versus the mechanical power produced by the motors at 50% load – the drives have enough punch to get me to nearly 40 miles an hour. If I were to gear for that speed, it would accelerate very… gently. And that’s only in the ideal case of the motors being able to sustain the high current draws needed to produce enough torque to get there.  Regular permanent magnet brushless motors are really quite limited in the ranges of speeds and torques they can achieve alone.

I’ve wanted to build a multi-speed vehicle for a while, because it still just seems like a better idea. You can shift the peak power and efficiency speeds around depending on your load requirements. Recalling Ben’s successful über-trike build pushed me even more towards that direction – that thing actually has 8 speeds using a bicycle hub gearset as the transmission. I was interested in even having two – burnout mode, and do-more-interesting-things mode.

It was easy enough for me to cook up a custom “shifter” design, but what better opportunity again to test out new parts on the market? Once again, I turned to Vex Pro for the answer. I’ve been eyeing their “ball shifter” (which sounds somewhat painful) transmissions since they were put up on the website.

Many a drivetrain in FIRST have been built using the classic AndyMark shifter transmissions, ever since the drill transmissions went out of style, and AM was the first place I went to when I was looking for COTS 2-speed options. What I didn’t like about them (both now, and years ago in FIRST) was how huge they were. The large, open frame sheet metal design is easy to manufacture, I’m sure, but there was no way I would have stuffed one onto this frame.

The new VEX transmissions seemed to be better packaged, and used a much more compact shifting mechanism. Inside its hollow shaft is another shaft with a round lobe on it that can slide axially. It pushes out on one of two sets of steel balls seated in the outer shaft, overall resembling a rachet wrench’s ball detent. So, depending on where the lobed shaft is pulled, a different set of balls is pushed outwards, acting like a 3-point spline. The output gears have little cavities that the balls lock into. If the lobe is not present under that set of gears, the rotation of the gear on the shaft will naturally shove the balls back towards the center. Pretty nifty.

Seeing no obvious complaints on the FIRST grapevine about them, I’m assuming they’re working pretty well for the competition and are robust enough to shuffle some 120 pound wimpy robots around. But can they stand up to moving a Chibikart at inadvisable speeds, while transmitting enough torque to break traction on asphalt?

I had my doubts – the ball shifting solution seemed like a great alternative to the AM Shifters’ dog clutches, but I trusted the metal-on-metal pushing of the dog clutches far more. Primarily because the dog clutches transmit torque at a greater radius – such that the stresses in the materials are much lower for the same level of torque transmission; and that the engagement is extremely binary – either engaged, or not. The ball shifters seemed to have a much greater potential of being caught “between gears” if one or more balls don’t release.  But maybe that is just a problem if I push so much power through them the ball detents deform.  I was already aware that shifting under power was basically out of the question – there’s no synchromesh devices on any of these things, and even in a real manual transmission car you wouldn’t hold the throttle down while shifting gears anyway.

So, with this many questions about whether the part would be worth anything, it was clear that I was going to have to get myself a set of Vexboxen. Because if nobody in FIRST is going to break them, I might as well.

Or, hold on… time to get myself another set of Vexboxen. I already have a VersaPlanetary that I’ve dissected and taken pictures of and am still waiting on a reason to do a Beyond Unboxing post on – they’re quire nice pieces of kit.

I downloaded the CAD model of the whole gearbox off the Vex website and immediately started cutting it up. First off, nothing was constrained – the solids just floated around, the constraints being broken by the export to a generic format. So I spent half an hour “reassembling” all the gearbox parts.

Next, I removed everything I didn’t care about – namely, all the hardware and the encoder mounting stuff. And the pneumatic cylinder that is supposed to run the shifting shaft. I was intending to cook up a mechanical linkage of some sort, since I didn’t want an air system on this build, no matter how air actuated brakes would have been.

I replaced the CIM motor with my NTM 5060 model using an adapter plate that will be machined. Both motors have an 8mm shaft, so interfacing with the supplied gears shouldn’t be an issue. I took the opportunity to raid eBay of some 2mm endmills to make the keyway in the NTM motors.

A quick fit test to the frame and… My goodness these things are huge. Luckily, when mounted upside down, the resultant sprocket spacing was acceptable.

Notice how these gearboxes are all designed for 2 CIM motors. I have no qualms, if this experiment is successful, of dropping 4 NTMs on this thing and upping my motor sprocket size for even more ludicrous performance.

I flowed some virtual metal around the mounting points to generate these two-pronged mounting adapters. The big gap in the middle means I can still access all the important motor mounting screws. Else, these gearboxes did not seem to require any other additional mounting – even the Vex website just recommended using their stock L-bracket mount.

The output shaft being a Hex of Convenience and Marketing Exclusivity, I decided to just get a 22 tooth hex-bore sprocket from Vex. Okay, Vex, you win this time.

The final drive ratios for each speed, then, are 18.17:1 in low gear and 8:1 in high, for a resultant speed of only 14mph in low gear and 31mph in high. I would have preferred a speed spread of less than 2.0x with a wider low gear, but hey, that’s what I get for not designing the thing. But the extra-tall low gear will definitely prove my hypothesis that sensorless drives can be successful if they are highly geared.

To actuate the gearbox, I needed a mechanical hookup. I was concerned with how much force the shifter shaft needed to operate – the installed shifting mechanism is a small pneumatic cylinder that is capable of nearly 30 pounds of static pull. Clearly this was not required for operation, because that would be ridiculous. After studying the mechanism in CAD more, I determined that it really should not take much force to actuate if I am not applying power at the same time. The actuation method would just need to be very fast to prevent the balls from skipping slot to slot.

I briefly entertained electronic shifting using big solenoids. The required travel was only 1/2″, which is well within the range of big open frame solenoids I could find for cheap. What drove me away from that was finding said solenoid that had a continuous power dissipation rating. Generally, industrial solenoids are rated for only 10% or 20% duty cycle – in other words, 1 minute on then 9 minutes off is 10% duty cycle. More frequently, the “maximum on time” is also specified, usually also 1 minute. Even though electric shifting would likely have been quicker and only required a button instead of running linkages or cables, I was more into the visceral mechanical solution and not particularly interested in cooking solenoids.

I elected to use a spring-balanced cable sort of mechanism with 10 pounds of return force to shove the shifter back into the starting gear. Luckily for me, the lowest gear was associated with the innermost position of the shifter shaft. So, it was easy to find a spring on McMaster-Carr which qualified for the needed return force at the needed stroke (about 1/2″).

What I could not do was find one which also worked with the dimensions of their included shifter coupler. The black object in the center of the gearbox between itself and the standoff-mounted plate is my own quickly whipped up coupler design, which let me fit a spring (not shown in image) between it and the rightmost black flanged doobob. This spring will try to keep the gearbox in 1st gear, so long as I am not tugging on the cable to keep it in second gear.

Some cable adjustment will be needed for sure to ensure synchronization of shifting.

On the other end of things, I put together a simple lever using mostly McMasterables. Did you know McMaster sold random knobs and levers? Now you do. Oh, and little ball detents already loaded into threaded bodies. Super simple instant two-click gear shifter!

This and many other reason are why, this coming Dragon*Con, I am hosting a panel session on how to shop on McMaster, among other places.

Time to mince some metal soon! I ordered this stuff last week. I have yet to assemble the gearboxes, but they seem legit. The casing is heavy fiber reinforced nylon (or fiber reinforced something or other), and the gears are… sticky. Seriously, whatever magic coating they put on these things, it straight up sticks to my hand. Repeatedly, even. I can flat-palm a big gear and lift it straight up off the table every time. Hey, aren’t you supposed to make gears slippery?

The picture quality is so lacking compared to my usual ones because after 5 years of nonstop service and over 11,000 pictures, my free-to-me Fuji S9100 has finally bit it. Cause of death? The USB port just straight up fell off inside and likely shorted something. So, back to the phone camera.

Oh, the thing was secondhand, too, so in a prior life it probably took even more pictures. This thing has been such a brick that I am actually eyeing its slightly newer successor, the S100FS, or even the current generation X-S1.



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    One Response to “The Secret of BurnoutChibi”

    1. Polytech Says:

      But the hub motors were the awesomest feature of the original cart! No-o-o-o!

      Well, perhaps I’m just a little impartial to BLDCs …