Using dual motors in Chain-Drivesystem

[Matt Adams]

Okay, this is a post worth replying to, because this is a very common situation. As a matter of fact, we're combining chips and drills to a single shaft in our drive system this year, so I'll share a bit of info.

As it turns out, you don't want the shafts of the individual motors to spin at the same speed- that will be very inefficient if you combine two different motors with different specs. You want to choose a gear ratio (done with either sprockets or gears) such that when these motors are spinning pull their max torque, that a single output shaft will have an equal current contribution.

To simplify...

Let's say you throw a shaft on the end of the drill motor and let it spin with little to no load. It'll spin at around 1521 RPM. Now let's say you put a Chip attached right on to that shaft on the other end, directly connected.

The Chip has a free speed around 5300 RPM... but the motor won't spin at that... the motors will actually convege to a speed somewhere in between 5300 and 1521 RPM... the reason for this is unique for DC motors, and is known as back-emf. Where exactly this converges depends on some coefficients of the motors. The short story is that this is not optimal.

Please take the following as truth: Free speed, with the drills and chips, HAPPENS to yield approximately equal current draw. It's a coincidence.

So, to match the current draw, you'd want to put a 35 tooth sprocket on that shaft from the end of the drill motor, and a 10 tooth on the end of the Chip. Then they'll both spin around 1520 RPM, and HAPPPEN to be pulling about the same amount of current. This is a good situation.

From here, you're going to have to gear it down a bit more to get more torque. Reducing these to somewhere around 340 RPM (for a 6" wheel) is a safe speed / torque trade off for high(er) quality wheels with good traction (1.2 coefficient of friction).

If you have any questions or comments, just let me know.

Seach for recent threads that talk about dual drive motors, combining motors, and shifting... many great contributions.

I even made some graphs in one.

Good luck,
Matt