The Drill:
In my searching for an inexpensive motor, I found that some robot builders were recommending a DeWalt cordless drill motor. I later came across a cheap Chinese 24 volt cordless drill that claimed 20 n-m stall torque (~16 ft-lb), so bought it for $39 and an extra battery pack for $16 to try to power a push scooter.
The Motor and Gearhead:
I opened the drill to examine the motor, and speed control unit. The motor is about 1-1/2" diameter and is brushed. The variable speed control uses what appears to be a single silicon controlled rectifier. The gearhead is translucent plastic with a mysterious collection of gears inside. It has 15 progrssive torque settings and a drill setting--all of which slip when their designed torque maximums are reached. I later took the torque dial off to get a closer look and then had to chase ball bearings as they were ejected from the clutch mechanism. I lost one, but the unit still worked once I figured out how to put it back together.( I have read of pins which replace the ball bearings to disable the clutch. I hesitate to do this as it may possibly damage the gearhead or motor.)
The Motor, Chain Drive and Batteries Mounted:
The motor mount is attached to the scooter with three bolts through ordinary 3/4" plywood. There are two u-bolts holding the drill in position on the plywood--one across the motor/gearhead area and the other below the trigger over the handle. The keyless chuck has been removed. A 16 tooth #35 drive sprocket with 1/2" bore is mounted on the gearhead shaft with set screws. The gearhead shaft was slightly less than 1/2" diameter so about one thickness of aluminum can metal was used as a shim between the sprocket and shaft. The rear driven #35 sprocket has 32 teeth for a 2:1 ratio (+ an unknown reduction of the motor-gearhead) and was face mounted on the hub with six bolts. Two 1.2 AH batteries were hot-glued and wired in parallel for 2.4 AH. They can be charged at the same time in the original charger.
Speed Control:
In order to keep this simple and cheap, I fabricated a bracket to hold the original rear brake caliper straddling the drill's variable-speed trigger and handle. Pulling the right brake lever squeezes the trigger for speed control. It actually works quite well. The left brake lever controls the front brake. There is no longer a rear brake. Note that I could have removed the motor/gearhead from the plastic case, but my primary goal was to see if it would work at all--not to optimize space or reinvent the wheel.
Performance:
On a full charge, I rode the scooter about a block and a half before the batteries ran down. I estimate that the speed was about 10 mph. I had to push the scooter off to start and accellerate at the same time to get it up to speed due to initial slipping of the clutch. Once the scooter accellerated, the slipping stopped. I felt that the gear ratio was too high as I had designed it to give the scooter about a 20 mph maximum speed. It did seem to be able to maintain the 10 mph speed just fine. I AM concerned about how much wear the brushes were receiving with that load. A better gear ratio and someone less than my almost 200 lbs. weight would probably get a good ride. Total cost: about $100.00.
Note: I have ordered a 10 tooth drive sprocket to see what difference it makes. I will post the results when I have tried it out.
03/17/03
I installed the 10 tooth sprocket. The scooter has more torque and still goes about 10 mph, but motor rpms are too low at top speed (larger rear sprocket indicated). Flexibility in the motor mounting in the drill handle allows the chain to jump teeth on the sprocket occasionally. That may be the sound I previously identified as the 'clutch slipping'. Adjusting chain tension mostly eliminates this. If necessary, the motor could be mounted outside it's original drill handle, but that opens a can of worms. I want to use the same drive system on the next project--the motorized garden cart, and reduce the ratio from 3.2:1 to about 6:1 by going with a 60 tooth sprocket at the wheel with 10 tooth sprocket at the motor.
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