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DIY Drive Gearboxes

Arrangement Drawings     Paid-For Plans

Double Motor Gearbox with Band Brakes

In my experience one of the hardest things to get right when building an electric (or other) buggy is getting the mechanical drive from the motors to the drive wheels right. Over the years I've tried all sorts of novel mechanical power transmission methods from friction drives (Tandem Buggy) and inverted toothed belt drives (Sporty) to sprocket on chain-rim drives (Mini Tractor). Some of these work better than others, some are more reliable than others.  Perhaps not surprisingly however, if you can make them, the drives that work best are those most commonly used in the engineering industries - involute gear, chain & sprocket or belt & pulley drives.

 
For the DIY-er or home-builder there's a big problem here however - usually gears or sprockets are keyed or splined to shafts, shafts are stepped in diameter to fit into bearings, metal bearing housings are bored-out to accept the bearings and the housings are precision machined in the walls of steel or aluminium gearbox casings. How do you do any of this if you don't have access to the range of specialised metal cutting machine tools usually found in an engineering works?
 
 
The answer is to avoid doing it - through some intelligent design of the gearbox. The gearboxes we have developed for use in our most recent designs use carefully selected combinations of common engineering components, adhesive bonding methods and particular build constructions and procedures that allow them to be built with much less sophisticated tooling.
 
There are three boxes shown on the site - one single motor unit and two double motor units, one of which has built in band-brakes (as used in the Go Kart ). Some of their design features are:
  • Self contained - multiple drive components don't need to be separately attached to the buggy main frame or chassis. Drive wheels are attached directly to the output shafts.Rear view on Racer showing double motor gearbox
  • Timber cases - easier to make and easy to secure to.
  • A combination of roller chain and gear stages to obtain overall speed reduction ratios from about 10:1 to 27:1 from the motor output shaft and wheel axle speeds. A range of different final stage gears can be used at the build stage to set the reduction ratio needed for a particular application.
  • 1st stage (motor) chain drive which is more forgiving of slight inaccuracies in the motor positioning than a geared 1st stage - this allows the motor fixing methods to be simple.
  • Adhesive bonding of  gears/sprockets to shafts - no need for splines, keys or stepped shafts. The only machining required is to drill-out gear/sprocket bores to match shaft sizes, no stepping of shaft diameters.
  • Inexpensive needle roller bearings on all shafts.
  • Steel shafts for strength with bench ground end flats for torque attachments..
  • Build procedures to allow accurate positioning of bearing housings in the casing to set correct shaft centre distances and alignments.
Some gearbox parts With the electric scooter motors we use in the gearboxes a range of output shaft speeds are obtained between about 150 rpm and 370 rpm depending on which 2nd stage gears are used. The motors have rated power outputs of 250W giving a total of 500W output for the double motor unit. The braked unit has built-in scooter band brakes which brake directly the final axle - this provides mechanical braking which is independent of the electrical drive. Note however that band-brakes are more effective in one direction than the other so the braked unit has particular "forward" and "reverse" directions.
 
By using gear and roller chain stages and roller bearings throughout, the boxes are efficient and loose little drive energy in transmission. Importantly for buggy use they are both reversible (can be driven in reverse) and back-driveable (can be driven from the output shaft). This later characteristic allows the use of  "4 quadrant" motor controllers to provide some dynamic, re-generative braking for the application. Another practical application of this is to allow the vehicle to be towed relatively easily when the drive motors are isolated from their controller.
 

Follow the links below for .pdf files of the gearbox assemblies.

Single Motor Gearbox             Double Motor Gearbox             Double Motor Gearbox With Brakes
       GBX1.pdf - (single)           GBX3.pdf - (double)        GBX2.pdf(double + brakes)
 
One final note (and a wee admission of failure), try though we might I could not figure out how to fix the final stage gear to a suitably sized final drive shaft without drilling a hole in the gear bigger than most common bench drills could handle. The final shaft is approx 16 mm in diameter to give bending strength required to support the wheel loads. 16 mm in steel is too big for a small bench drill. So this remains the one bit most competent DIY-ers, who don't have or have access to a small lathe, will find problems with. The solution is straightforward however - get the gear manufacturer to do it for you - many will happily, for a small additional charge, modify the bores on their gears to suit customers.
 
See the assembly sheets here for the gearbox with brakes! 
 

 

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