How much weight can my 270oz-in motor lift?

Using Arduino Motors, Mechanics, Power and CNC
1

I have a stepper motor that is 270oz-in.
This converts to 7.65kg.

This means that it can lift 270oz or 7.65kg 1inch or 25.4mm from the center of the motor shaft, right?

If a length of RHS steel 65x35x3mm thick weighs 4.25kg per meter. And I want to make a robot arm out of this at 1 meter long.

Will that motor lift that 1 meter length of steel RHS that weighs 4.25kg?

2

Depends on how fast. If you add a gearbox the torque goes up by the same ratio that the speed goes down. You probably want to gear the motor down for better resolution and more control.

3

Thinking along the line of what kf2qd has said with a gearbox, you not only want to move the arm but, you probably what to do things with the arm. If you want to pick up objects you will want plenty of extra torque to be able to do so.

Think about what you might do with the arm then, you can work out required torque to do the tasks.

4

Might be worth noting that the 270oz-in rating is only at slow speeds. Try a Google image search for "stepper motor torque curve".

5

You can negate the effect of arm weight for a rotational system if you instal a balancing weight to the other side of the shaft. By that means there is an equal and opposite gravitational force acting to assist in pulling up the original weight. On the basis that you want to lift say 4kg on a 1 metre arm (4kgM) your balance weight can be any mathematical combination that achieves the same torque eg 8kg on a 0.5M arm or 16kg on a 0.25M arm etc.

You also need to take into account the point at which maximum torque takes place. Your 4.25kg 1 metre long weight actually requires 2.125kgM of torque when horizontal (the centre of gravity being 1/2-way along the arm).

Note that weight units under the SI system are actually Newtons and are 9.81 times larger than mass (weight = mass x acceleration due to gravity) So your 4.25kg mass actually weighs in at 41.7Newtons

Torque under the SI system is invariably specified as Newton Metres (NM)

Note that this simple balance rule only applies to relatively slow motion since, if speed is involved the effets of dynamic and inertial forces must also be taken into consideration.