Torque

[ANTlabs]

Is there any formula that if you put in weight and the speed you want, It will give you torque required?

 

[MrAl]

Hi,


It depends partly on what you are trying to turn. Perhaps you can tell us more about what you are trying to turn.

 

[ANTlabs]

I have an Idea for a butler bot. It will carry around, well, almost anything that fits in the box. It will have two levels. the top level will be a 15.5 x 12 black rectangle with a metal frame around he bottom for support. the box will sit in a hole cut out of the rectangle. this sits on 4 15" aluminum tubes hooked up to the lower level. It has all of the boards, chips, batteries, and wheels. the wheels will go through the lower level holes and will be controlled by a gear box or whatever motor fits. It weighs about 8 pounds.

 

[Triode]

those aren't enough variables to go on, just like velocity in a straight line, torque depends not just on the load but on the acceleration. Also, in the case of rotational forces and accelerations you have something called moment of inertia, it's an exponential average of the mass and its distance from the center of rotation. Because weight at a greater distance from the center takes more force (torque) to accelerate.

This is the formula for moment of inertia. Don't worry if you don't know calculus, they have derived forms for different shapes. r is the distance from center at every point, and dm refers to the mass at each point.
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This is a list of derived moments of innertia for basic shapes
List of moments of inertia - Wikipedia, the free encyclopedia
The units come out in mass*length^2, but this is ok, because torque is moment of inertia times angular acceleration
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which has units of seconds^-2
so when you multiply them you get mass*length^2*seconds^-2

force is mass times acceleration, acceleration is length*seconds^-2 so if we pull force out of the above equation we get, length*force, aka foot lbs, or newton meters, torque.

Edit: I see you posted while I was typing, I guess the first reply was right in asking first. For propelling something you are mainly considering F=ma. but if you want a simple solution just push the thing at the speed you and estimate the force, multiply that by your wheel radius and there's your torque. Any gear ratios will divide the speed and multiply the force, or vise versa.

 

[3v0]

As I was trying to tell you in the other threat there is no easy answer.

It also takes less effort to move a robot over a smooth floor then a carpeted one too.

I suggest you look at what others have used on robots of similar size.

Does that 7 lbs include the battery pack ?

 

[Triode]

I agree with just looking at other robots and machines. There is no easy answer except that this doesn't need to be exact, were not shooting something into space. Depending on gearing almost anything will work, you'll just get different speeds. For example I was amazed when I saw that the motor in a power wheels, which can carry several small children is only about 1.25 inches wide, just a little motor like you would have in a larger RC car, then a really low gear box. The thing moves but it cant go up a hill or go faster than maybe 3 mph. On the other hand the motor we put in our go cart for a summer camp I worked at was the size of a 2 liter bottle, which also worked, but it could tear around a track with a full grown adult.

 

[dknguyen]

I tend to base my torque required on the incline my robot will climb. I have a thread on how to calculate that (you'll actually need more torque than that to actually move up the incline). For me, I don't look at acceleration really. And it involves guesstimation of the roll friction of the surface and wheels. 0.3 is a pretty worst case one.

You'll need to know the wheelsize too. With a zero incline, my process just ends up guesstimating the coefficient of rolling friction of the surface and getting enough torque to overcome that. On flat ground though, I don't usually bother. Just inclines.

https://www.electro-tech-online.com/threads/motor-sizing-for-moving-robots.23264/

 

[Sceadwian]

Using a simple incline equation will not allow you to climb door jams. The torque required depends on the center of balance of the vehicle and the APPLIED weight at the wheel, not the weight of the entire unit. As 3v0 said, there is no easy answer. Experimenting around yourself is the only way to get a hang for it without going into mathematics.