Tuesday, August 14, 2018

Insulated Driving Wheels: An Experiment

While the build of Ivor might be on hiatus, I now have four wheels which are effectively scrap (given that they are no no longer the same size) on which it is safe to experiment. The final two pieces of the wheel design I needed to finalise were the insulation from the axle and the crank pin.

Let's start with the insulation first. It's difficult to see in the photos but what I've done is to simply take the test axle from a few posts back and fit this to one of the wheels. This involved opening out the axle hole in the wheel to 2.8mm which is a nice tight fit for the insulating bush I took from a pair of hornby disc wheels. I'm not sure this is the final way I'll go (I might try turning my own insulating bushes) but it is nice and straightforward and seems to adequately insulate the wheels from the axle at little effort or expense. Obviously on the real wheel I'd trim the axle back to the face of the wheel.

When I was originally working on the 3D model for the wheels my plan was to use a 14BA screw as the crankpin with a hole through the wheel tapped for 14BA so the screw would actually screw into the wheel. I had wanted to cast a recess into the back of the wheel to seat the cheesehead bolt but the wall thickness requirements meant that wasn't possible. So my plan was to screw the bolt into the wheel and then cut the head flush with the back. A little drop of loctite helping to hold the bolt in place. Unfortunately while I managed to open out the hole big enough to tap, I then broke one of my 14BA taps and stripped the cutting threads from another. With no taps left I opened the hole up slightly further so that the bolt was a tight sliding fit. Rather than holding it in place just with loctite on the body of the bolt I decided to drill a recess for the bolt head anyway as there seemed to be enough material. With that done the bolt sits nicely in the back of the wheel and the bolt head helps to keep the bolt perpendicular to the wheel while also providing more surface area for loctite to hold it in place. Once the loctite had set I did try pushing the crankpin out and couldn't do it by hand. Even with a set of pliers it took a lot of force to break the seal, so this definitely looks like a workable solution.

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