Transmission

To keep the costs low and the strength and reliablility high, we planned to use standard bicycle parts where ever possible but, in the end it was easir to get sprockets and gears from Technobots . I've over-engineered things a bit though and have bought chunkier gears than I planned.
This picture shows the gears mounted in the MDF 'test' chassis to check positioning and alignment. The red lines represent the chains.

Drive Sprockets

The output shaft on my Bosch motors are 12mm in diameter with two opposing flats and a 10mm locking thread.

Technobots  sell a range of sprockets to fit various sizes of output shafts and their datasheet  (PDF) lists a number of suitable sprockets. The smallest one they do for a 12mm shaft has 17 teeth but, I want a much smaller sprocket so I've bought their version with 8 teeth, which is designed for a 10mm diameter shaft. I will drill it out to 12mm, to fit my motor shafts.

Chain

The sprocket teeth are 5/16" wide and a ½" chain pitch. Because the wheels axles are fixed spacing, I might need a chain tensioner on some of them.

Gearing

The basic rule of thumb is to gear down for each inch in diameter of the wheels, e.g. for 8" wheels you need a gear ratio of 8:1. This would still give a rather fast robot in my view though. It's better to do this with one pair of gears but, many use a two-stage gearing system and that's what I'm going to have to do, to stay within the confines of the chassis space.

The 8-tooth motor sprocket drives a 40-tooth sprocket on the the same gear axle as some more 8-tooth sprockets. Each of these drives a 25-tooth sprocket connected to the wheel axles. This gives a gear ratio of about 16:1 and a theoretical top speed under no load of about 6mph according to my calculations (Microsoft Excel spreadsheet).

Gears

Having bought my sprockets/gears from this source I've found that these things are a bit heavy but, they will be made lighter by machining away some of the unwanted metal. A smaller pitch and narrower teeth would have been better. They have a plain bore so work is required to key them to the motor output shaft and axles.

40-Tooth

As supplied, the 40-tooth gear is 166.7mm in diameter, has a boss of 90mm diameter has a 16mm bore. The depth is 35mm and it weighs a whopping 2400g.

The plan is to machine the boss down to 28mm and the depth to 25mm. I'm also tempted to drill some other holes in these to lighten them.

25-Tooth

As supplied, the 25-tooth gear is 106.2mm in diameter, has a boss of 70mm diameter has a 14mm bore. The depth is 28mm and it weighs and it weighs 1010g.

The plan is to machine the boss down to 26mm and the depth to 25mm. They will also be bored out to 15mm and keyed to fit the axle.

8-Tooth

As supplied, the 8-tooth gear is 38.0mm in diameter, has a boss of 20mm diameter has a 10mm bore. The depth is 25mm and it weighs 65g.

The plan is to bore these our and key them to fit a 12mm axle and the 12mm shaft on the motors.

The final gears on the wheel axles have a 14mm bore but, this will be bored out to 15mm as bearing and axles of this diameter are more readily available.

Bearings

The bearings are fixed to the chassis using bespoke bearing plates, which I'm going to have to fabricate.

Wheel Bearings

I'm going to need twelve 15mm bearings to fix the wheel axles to the chassis.

Primary Gear Axle Bearings

The primary gear axle uses 10mm bearings.

Axles

Primary Gear Axle Bearings

The motor shaft has an 8-tooth cog, driving the 40-tooth cog for the primary reduction. On this axle is also a second 8-tooth cog to drive the wheel axles. The bores of these cogs requires two (one each side) bespoke axles to be fabricated with varying diameters:

[image to come ...]