Thursday, July 21, 2011

tinyKart: Round 2

Round 2 is all about putting together the steering things:

These things.
But first, armed with many more t-nuts, we finished putting together the lightweight 80/20 frame:

It's more like 0.75-scale kart.
Next up, pressing 1/2" ID bearings into the steering reinforcement wings, a.k.a. the "cool parts":

Easy so far. The moving assembly includes a front wheel, brake disk, brake caliper, and a control arm (not shown in the above picture because it's between the chassis plates). To hold all of this together, there's a little bit of 1/4" plate jigsaw puzzling:

The control arm doubles as a support for the lower thrust bearing. It, and the upper thrust bearing support, slip (press) into what would normally be called the upright, if it wasn't so short not-upright. I'll just call it the shaft mount. Then, three 4-40 screws pass through the shaft mount, the upper thrust bearing support, the shaft mount again, and finally thread into the control arm.

Like so.
I guess I forgot one important step (in real life, too, the first time): First, the 1/4-20 cap screw that threads into the front wheel shaft must be in place, with its head in the relief cut in the top and bottom thrust bearing supports. So far, so good.  To properly align and stiffen the thrust bearing supports, three countersunk 1/4-20 screws with plastic spacers pass from top to bottom:

The image above also shows the brake caliper mount. The brake calipers will need some shimming, but they're at least located to within a few millimeters axially. No major machining or design mistakes there. Finally, we attached the front wheel shaft. This requires removing one of the screw/spacer combos and shoving a hex key in between the thrust bearing supports. It's kind-of a pain, and we don't want it to come loose ever, so we Loctited the shit out of it.

After the two front wheel assemblies were together, we had to ream the kingpin holes since (a) they were undersized to begin with and (b) there's nothing really aligning the top and bottom thrust bearing supports. Luckily, we bought a 0.499" reamer two years ago for reaming BWD's stators, and it hasn't disappeared.

The kingpins were quickly fabricated from 1/2" 7075:

The result is a tight fit to the 0.499" hole, but not so tight that you can't force it back and forth with a rubber mallet. And it's a slip fit in the two "cool part" bearings. Those are the radial bearings that support the bending load on the kingpin, but there are also two needle-roller thrust bearings:

And then the top plates complete the sandwich:

The end result is a nice, stiff steering pivot with radial and thrust bearings on top and bottom.The frame passes the "stand on it" test, though it is a little more flexible than I thought it would be. We'll see how it changes with the added structure from the seat mounts. Total weight of the chassis so far is 21.5lbs. Still have the entire powertrain to add. The good news is that the controllers are a bit lighter than expected, so 50lbs still seems within reach.

Next up: steering linkage, motor mounts, and controller testing.

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