Wednesday 25 January 2017

Connecting Thin Gears to Shafts

I have an ongoing project that required me to connect 4 gears to 4 coaxial shafts in the minumum of axial space. To complicate matters the gears needed to be relatively easy to demount.
I Googled for ideas, and didn't really find very much for the sort of miniature gear I am working with (40mm dia)[1]. So I thought I would write a short post about the method I came up with.

I have used Taperlock bushes quite a few times, and they work extremely well, but don't scale to 3mm thickness particularly well. I have also had great results with Trantorque bushes which also work extremely well. We used them to connect the sprockets to the motors of our RobotWars robot, and never had any hint of trouble there. (and that was something that couldn't be said for many other teams). But they need rather more thickness than I had available.

In the end I came up with a sort-of hybrid system with elements from a number of similar taper-mounting systems.

There are two 6-degree tapers, one at 27mm notional diameter and the other at between 15 and 12mm diameter depending on the shaft size to be connected to. Both tapers have a depth of 1.5mm and between the two diamters is a ring of 4 M2.5 tapped holes. Additionally the inner tapered collar is split all the way to the bore at one point and to the inner taper at the opposite point.

I am very happy with the way it worked out. And the grip seems plenty strong enough for my purposes. I was able to use the tapers to mount the gears on a dummy shaft for the hobbing process.

[1] I realise that for many folk a 40mm gear isn't particularly miniature, but my dad used to work for David Brown and they can make gears up to 14 metres diameter.

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