Let's Make Robots!

advice on motor

This is off topic for the forum, I apologize, but you guys have the expertise and I am hoping will appreciate the challenge. so...

I need advice on a low speed/rpm high torque electric motor.  it is for my, umm, 'robot oil press' to drive the auger.  it looks a lot like the one on the homepage at piteba.com.  right now it is hand-cranked, but holy hell, it takes a wicked amount of torque.  I had it screwed down into a wooden butcher's table, and even then I had to screw the table to the floor, and then the screws ripped out of the table.  so I am saying it takes a wicked amount of torque, right.  I was wondering if I might get a small low speed/rpm high torque electric motor and get the local machine shop to rig me something to hook the motor shaft to the press auger.  I am just wondering where to get the right motor.  I think it ought to be about 10rpm, as that is about as fast as you could go by hand.  for torque, I don't know, if it takes me practically jumping on the hand crank, does that mean it needs 150lbs torque?  or am I butchering the concept of torque?  I have seen some motors that are in the ballpark...a truck windshield wiper motor can do pretty low rpms but only ~40lbs torque.  McMaster has 5rpm 50lb torque electric motors.  But, unless I am totally butchering the concept of torque (which is quite possible), I am pretty sure that I need something significantly more powerful.

Thanks, and please have mercy.  

p.s. robots are cool.

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thanks all, good stuff to think about and experiment with.  I found some video of people using this thing on YouTube.  they seem to be having an easier time than I am, so I wonder if maybe my seeds are too dry?  most of them are still using 2 hands to crank it, and surely theirs are better bolted down (press to table, table to floor) than mine!


Even without doing the calculations it`s obvious you need a monster of a motor. The easiest and cheapest way to do this might be with an electric winch motor. Something designed to pull a boat onto a trailer or a 4WD out of a hole. You could also try a big starter motor, although these aren`t designed for extended periods of use and might be more hassle than it`s worth.

This might be compicating things here but there is always the option of more gearing. If you take that McMaster unit and stick a sprocket on it, then find another sprocket with 10 times more teeth to attach where the handle went, you just turned that 50lbs motor into a 500lbs motor... just 10 times slower...

My wife is an herbalist and I personally ran into a similar issue but with a hydralic car jack while desigining and welding an herb press for her. --I think we travel in similar circles.

I am really into measuring lately so take this with a grain of salt. Pack that thing full and get it to the point where it takes the most force to turn. Now find a gear-head friend and borrow his "torque wrench". You can put this torque wrench on the nut that holds the handle and then turn the press with it. This device will tell you exactly what force is needed to turn the crank. At this point is just a matter of going through a lot of specs for motor/gearboxes to find one with the strength and speed you need. A bit of advice though, you are not looking for anything "robot" or "hobby' --you should be looking "industrial".

OK, here's some info on torque. Notice that that units for torque are actually distance x force  (e.g. inch-lbs), not just force (e.g., lbs).

So if the crank handle on your press was 18 inches long, and you had to apply 100 lbs of force to make it move, that is:

18 inches x 100 lbs = 1800 inch-lbs of torque (that's quite a lot!)

If you replace the crank with the McMaster motor that provide 50 inch-lbs at 5 rpm, you are not going to be able to move the crank shaft. You have to provide some additional gearing to get more torque at a slower speed.

Let's say the motor shaft has a 1 inch pulley, which you can run a drive belt on to a larger pulley on the crank shaft of your press. To get 1800 inch-lbs of torque, you need a big pulley.

1800 inch-lbs / 50 inch-lbs = a 1:36 gear ratio

Maybe a 36 inch pulley is inconvenient. So you can use several smaller pairs of pulleys to get the same overall ratio. You need a small and a large pulley that are fixed on the same shaft and turn together. Each large pulley is driven by a smaller one from earlier in the gear train. Each smaller pulley drives a larger one on the next gear pair in the train.

  • 1 inch pulley on motor shaft, which drives
  • 12 inch pulley with 1 inch pulley paired to it (1:12 ratio), which drives
  • 3 inch pulley paired to 1 inch pulley (1:36 total ratio)

Now you have a 1:36 gear ratio, but your crank will turn 36 times slower than the motor shaft moves. So your 5 rpm is now only 0.14 rpm. Way too slow. You need a faster motor that still provides 50 inch-lbs of torque.

For example, if you found something that was 50 inch-lbs of torque but spun at 180 to 360 rpm, you could use a 1:36 gear ratio and turn at 5 to 10 rpm.

Get the idea?

So, measure the length of your press' crank shaft. Then if you can find a spring scale or hang some weights on the handle of the crank shaft, determine how much weight (force) is needed to move it.

crank shaft length in inches x weight required to move the crank in lbs = minimum torque in inch-lbs.

This will tell you that you need a motor and gear system that can deliver at least  that much torque. You should aim to provide more than this minimum, to ensure it works OK.

If you happen to have a torque wrench you can connect it to the shaft of your press and measure the required torque that way instead.