Sunday, December 17, 2006

Testing the gears

When I read the instructions for this kit and found that there were several ways of putting the gears together my first thought was that I’d go for the simplest layout. But this would mean two of the axles being driven by the coupling rods, and since I’ve had problems with this before (my Farish conversion), I’m coming round to the idea of driving all the axles via gears.

The 18 tooth gears with muffs

The muffs turned out to vary in length from 5.95mm to 6.11mm. I decided to use the shortest one for the worm wheel, as by filing the inside of the corresponding axle bearings I could make the muff a perfect fit.

Worm wheel bearing

Wondering what effect this difference in lengths will make when the muffs are in place. Perhaps I should get some small washers to act as equalisers for the shorter muffs.

The gears can be pushed onto the muffs, but the hole through the muff is (deliberately) too small for the axles, and needs to be opened out until the axle will slide in tightly.

I want to put the gear wheels together on the chassis to check that it all runs freely, but I’m feeling a bit nervous - If I don’t open the holes enough it’ll be difficult to remove the axles. But if I open the holes too much the muffs may rotate on the axles.

Some smaller, say 1.4mm axle steel would be ideal to try the gears in the first place. I can’t find any rod of that size, but 1mm square rod should serve the same purpose.

Putting the gears in and spinning them round, I find they move smoothly in one direction, but get stuck going the opposite way. Trying the gears one by one, turns out one gear is brushing against the underside of the spacer tab. Think it may be because I’m using thin rod which allows more movement, but find the same thing with the axle steel.

I gently file around the outside of the gear teeth and also reduce the underside of the brass tab. Needed to remove quite a bit before the gearwheel turns freely.

Now the whole mechanism turns smoothly. Very satisfying.
[I subsequently decided that I must have bent the tab by more than a right-angle, so that it was below the intended level and hence interfered with the gearwheel. Would have been much easier to remedy if I'd checked before soldering the spacer on.]

underside view

from above

This shows how the shorter muffs can move along the axle, meaning the gears slop around a bit. Not enough to disengage, but a couple of slices of the right diameter plastic tube would fill those gaps neatly.

** January 2007 **

An alternative gear arrangement, in response to a possible difficulty with attaching the brake rigging.

I was concerned that wires coming through the chassis may interfere with the gears, but apparently that won't be a problem.

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