Monday, May 14, 2007

Fitting the cranks

I've done a little background reading on fitting cranks and quartering them, such as descriptions of how people align the cranks by eye then "fine tune" them by rotating one wheel fractionally relative to the others.

I'm not sure how relevant this is. Since I've already got the balance weights aligned and quartered (by eye) I want to try to glue the cranks onto the wheels using the connecting rod as a jig. However there is some play between the gears, ie each axle can rotate a few degrees relative to the others, so there is no absolute "correct" alignment, and I expect that for this reason I may end up enlarging the holes in the rods to allow for the play. Time will tell.

Still I must mention one tip that I came across - someone uses a set of "loose muffs" so they can slide the wheels on and off easily for testing during construction. Wish I'd thought of that a few months ago...

"Work slowly and carefully", the instructions advise. I'll check things at each stage by making sure the chassis still runs downhill when I tilt my test-track in both directions.

First I check the fit of the crankpins in the rods, and have to open some of the holes out slightly so that the pins will fit through perpendicular to the rod.

Now I fit the first crank on, opposite the balance weight, making sure the pin is perpendicular. Waiting for the epoxy to set I remember I should have cut back the pins before fitting them.

Managed to cut the pin with wire-cutters. I allowed enough for the coupling rod, the fixing washer and a little extra to work with.

Next the front crank. As the glue sets I make sure I can turn the wheels gently. So far so good.

I've filed the first 2 extended axles back to allow the coupling rod to turn freely.

Then the third crank. The clearances are tight, but once I've removed every scrap of excess glue from the axles it still coasts downhill.

For the other side, I cut back the extended axles and crankpins as much as possible before starting.

Then I fit the cranks as before, with the rod still in place on the other side.

Well I'm happy to say, it didn't go that badly. I still have to fit washers to the crankpins (see the left axle above), but till then I'll keep the rods in place with plastic wiring.

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