# Trying to understand torque here.

Discussion in 'Radio Control & Other Propulsion Methods' started by Millenniumfalsehood, Nov 20, 2008.

1. ### MillenniumfalsehoodActive Member

Can someone give me a plain and simple way to interpret torque ratings? For instance, I want to buy a robot motor that will be used in the legs, so it has to be very high torque, say capable of lifting 400 lbs on the end of a two-foot lever. What should I be looking for as far as foot/pound ratings? Would it need to have about 800 ft/lbs of torque?
2. ### Jim NunnMember

First my definition of torque:

Remember when you were 20 years old and when you got up in the morning and you needed to relive yourself, and when you pushed down on the end of it you rolled up on your toes that was torque. Force applied to a lever arm.

400 lbs applied to a 2 foot arm = 800 Ft/lbs (force * length) or 9600 in/lbs that is a lot of torque. This is assuming you are applying the force at 90 degrees to the perpendicular. If the arm is at 45 degrees then the torque is 400*(2 *.707) or 565.6 Ft Lbs this is because the effective lever arm is now shorter.

HP is basically Torque * RPM so the faster you want to move this machine the more HP you will need. If you making very quick moves you should also take the inertia of the machine into consideration which may also increase the torque requirements.

Assuming that you are rotating this lever arm through 360 degrees 15 times per minute you will need approximately 2.2 HP to produce the 800 Ft/Lbs that’s about 1600 watts and a 12 Volt DC motor would draw about 140 amps. I’m assuming you are building a walking machine. If you are moving the machine on wheels then the torque requirements will be considerably lower since you are applying the force nearly perpendicular and the lever arm is now very short. The HP required moving the machine will be based more on the inertia of the machine and the amount of friction between the wheels and the surface they are running on.

I’m sorry I got so detailed but there is no quick answer to the implied question of what size motor should I use. In the end get the biggest motor you can find put some wheels on it and see what happens.

Jim Nunn
3. ### MillenniumfalsehoodActive Member

No worries, your response was a heck of a lot easier to understand than Wikipedia's page on torque. :thumb: