The original idea was simple, use a dying electric scooter to power a push mower and take some of the work out of trimming my postage stamp yard. It did not work out as intended, things always seem easier of paper. But, since I have managed to make every mistake possible, please benefit from my mistakes!
Step 1: The Plan
Any 24V scooter has two 12V batteries, a motor and a speed control. Why not pull out the parts and attach it to a push mower to make a variable speed cutter that can be push around the yard. It seemed simple enough.
The motor is way to fast, and finding a simple way to connect the drive belt to the mower did not work for me. The mower runs at about 80 rotations of the cutter for every one of the wheel (WAG estimate). Adding the scooter motor would shoot that high enough to make the mower take-off. Unsafe and unwise.
Use a windshield wiper motor (12V) and one scooter battery (using the other as a spare) to power the motor.
Simple and cheap...except that the windshield wiper motor why strong is about half the speed you will need. It cuts, but the process of trimming even a 10' X 12' lawn becomes a chore.
After destroying the batteries by accidentally overcharging them, I found a powered window motor for $12 online. The RPMs were roughly double, and I could always order the other rotation is I completely ruined one side of the motor.
What a plan! Through a little tinkering, this works very well, and cuts just a little slower than I would like.
Step 2: Acquire Materials
mower (mine was $5 on Craigslist)
12V battery (I had to use an alarm battery from Radioshack for $30. A doner scooter would be better)
12V power windows motor 150RPMs or better ($14.99)
Wire and battery connectors
4" angle brackets
10-24 tap kit
1/2 and 1-1/2" 10-24 screws
steel spacer the diameter of your mower shaft
bolt and nuts for your new wheel
steel strapping and pop rivets for battery case
1" rubber spacer
Step 3: Cut Down the Shafts of the Motor and Mower
The motor shaft and mower shaft will be too long using this setup. With my mower, the gear that drives the shaft is attached with a square key through a hole in the shaft. Since I can't drill a perfectly straight hole to save my life, I decided to utilize this slot in the shaft to connect the motor and the mower.
Use an angle grinder or hacksaw to trim the shaft of the mower about 1/8" to 1/4" longer than the slot location, and trim the motor until it is nearly parallel with the side of the mower when touching the shaft. In my case approximately 1" from the side of the mower hence the rubber spacer.
Use the steel spacer as a guide. One side will need to extend beyond the slot, and the other will need enough room to hold onto the motor. As the two shafts are different diameters, half-lapping them isn't the best of ideas (not that I tried that too or anything)
Step 4: Drill and Tap the Spacer
The steel spacer will need five holes drilled in it. Three in a 120 degree angular layout on the motor side, and a straight hole through both sides on the other to hold the spacer onto the shaft through the slot in it.
The mower shaft hole will need to be fairly straight, but the other three don't need to be perfect. The three will combine to hold the motor shaft tightly, so the exact layout is not that important.
Tap all of the holes for the screws you will use to hold both ends. I like 10-24, but anything could be used.
Setup the spacer(now coupler) on the shafts and tighten the screws. Spin the blades by hand to see if the motor spins. There is some slop built in (at least on mine) so the spinning motor looks more line a cam.
Step 5: Run the Wires, Build the Battery Case and Attached the Wheel
Running the wires is fairly simple. I chose to run the wires through the frame of the mower. They could just as easily be attached to the outside with some zip ties, but it was simple enough to route them through. Only one hole (at the bottom) needed to be drilled.
Building the battery case just takes a little elbow grease. I used welding pliers left over from another project. These have flat wide jaws. Channel locks, pliers, a vise, or even just the corner of work bench should do the job. Don't bend it exactly the size you need, give a little room all the way around. For my case, I used two of the 24" steel straps. The setup was two "J's" and one "O" joining them at the top. I added a side bend for the switch.
The steel is thin enough you shouldn't need a hammer. You can see my "90's" are exact, but the whole case seems to work well enough. I used pop-rivets, but screws and nuts would work better and not leave the mushrooms inside the case. I just had time handy. Test fit the battery, then attach it to the frame.
The reason I waited to mount the wheel until now is the motor may rub against it (as it originally did with mine). You will have to move the wheel forward keeping the blade at more or less the same height.
The remaining sheet metal that formerly surrounded the wheel isn't quite strong enough to support the weight. Use the left over strap or part of an angle bracket to hold the wheel securely.
Step 6: Mount the Motor and Rev It Up
Make you battery connects and remember the gear from the other wheel (non-drive side). If you forget, your forward speed will dictated by the speed of the motor. On my mower, that means a slow walking pace. Probably fast enough.
The motor has quite a bit of torque, but the blades can get bogged down when the grass is tall. I wear heavy boots and kick the top blade to start it rotating when it gets into trouble. This is a stupid idea! Just remember that the motor will continue to spin so long as their is power to it. Strongly consider using a momentary push button switch instead of a toggle switch so power is only delivered while you are holding down the switch.
The mower probably isn't powerful enough cut off a finger or toe, but it will give you a raspberry you won't soon forget. Please be careful!