Massive DC Forklift Motor

by Ben N on December 1, 2020

Forklift motors have been popular for a long time in DIY Electric Vehicle Conversions. They tend to be Series-Wound DC motors, which offer tremendous torque at low speeds and are common enough that that can often be found at junk yards.

A few years ago, my friend, Tom, and I scrapped out a 6,000 lb capacity forklift. At the heart of that lift was a hefty DC motor. We pulled it out and set it off to the side for a future project.

The motor weighs about 250 lbs. It’s a 12.5″ diameter beast with a 1-3/8″ 21 spline driveshaft and a 1 to 1-1/8″ tapered tail shaft.



The brushes are a half inch wide by 1 and a quarter inch across. There is a PAIR of those brushes at each of the four locations around the commutator.

The commutator itself has 100 bars. It looks to be in good condition, with minimal corrosion or scorching. With the brushes pulled out, the rotor spun smoothly with little resistance.

I decided to test the speed of the motor, and I did so by connecting it up to a common 12V automotive style battery. The motor easily spun up. Using an optical tachometer, (https://amzn.to/3msENxM) I measured the speed at 1200 RPM. The only time a series-wound motor should be powered up without a load is only at low voltage for testing. Doing so at a higher voltage can make the motor spin incredibly fast and destroy itself.

Speed is proportional to voltage on a motor like this, so in theory, it might spin at up to 3600 RPM on the forklift’s original 36V system. (But that would only be in a low-torque/low resistance situation) The reason why I mention this is that I’ve been slowing working on converting a tractor to electric. Tractors like that typically run at low engine speeds, about 2,000 – 2,500 RPM. Under load, this electric motor at 36 or 48V is probably a very good match for the speeds required by that tractor.

Having a splined driveshaft could be a problem. It’s usually easiest to mate up a motor with a round driveshaft of a common size diameter. Splines might mean some custom machining, which can get expensive fast. After measuring the shaft and counting the splines, I did a web search. It turns out that size is NOT and uncommon PTO connection on a tractor. I wasn’t sure if I had the measurements exactly right or not though. I went to my local farm store and bought the one part they had with a female 21 spline connection. When I tested it on the driveshaft, IT FIT! It looks like I should be able to find a standard component to connect the driveshaft to whatever else I want to connect to.

The DOWNSIDE of this motor is that it’s perhaps TOO big! Measuring the front of the tractor’s transmission, there’s as little as only 13 inches across between two frame members. Measuring from the transmission shaft center down to a support at the BOTTOM of the transmission is only 6 INCHES. The edge of the motor might hit there. On the other hand, if it’s spaced back from the transmission a bit, perhaps it won’t.

It looks like the only way to tell for sure would be to actually hang the motor from an engine hoist and push it into the tractor to check for mechanical interference.

I’d love it if this motor could work for the tractor project. The motor could easily run at 48V. That’s a VERY common system voltage, so it would be easy to get motor controllers, chargers, DC/DC converters, and anything else I would need. 48V is also easy to achieve with lithium batteries such as Nissan Leaf cell modules in a 7S configuration.

Lastly, the tail shaft offers a simple way to power the tractor’s hydraulic pump.

Check out future updates for if this motor actually fits the tractor or not.

Until then, stay charged up!
-Ben Nelson

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