Stepper or Servo and how to figure out needed torque?

I’m trying to figure out which motor would be the best for me (my gut says the servo but I’m not sure) and I need to figure out how much torque I’ll need.

I have two elements mounted to one side of a square tube 71cm long. Motor will be attached to one end of tube and rotate the tube around the center of its axis. The range of motion is no more than 85 degrees. I would like the speed to be around 30 degress/1.5-2 seconds. Combined weight of both elements and tube is 19.3kg. I’ve included an image with measurements to help illustrate what I’m talking about. The crosshairs indicate where the motor axle will be.

I hope I’ve provided enough information but if I haven’t I’ll be glad to provide whatever else is needed.


Image from Original Post so we don’t have to download it. See this Simple Image Guide



What is this thing supposed to launch?

Can you provide a 3D sketch of the machine to make it easier to visualize?


Sure. No problem. Hopefully, these make it easier. It's a soccer ball launcher.

I don’t think I would use either a servo or a stepper motor for this project.

How much of the design is already fixed?

It would be much better if the force from launching the ball passes through the axis of rotation. The way the design is presented the business of launching a ball will put a lot of extra torque on the rotation shaft.

If the design can be arranged so that the force passes through the axis of rotation the moving parts could be balanced which would mean they could be rotated slowly with little effort.

In any case I suggest you arrange a large step-down gearing between the motor and the rotating shaft. 10 or 20 to 1 would not be too much. If you use a worm-gear for reduction it will hold the system in place without any power applied to the motor. But you will need strong components to resist the force of launching a football.

If you put a rotary encoder on the rotating shaft you will be able to identify the angle and could use a simple DC motor for the motion.


The off-axis distance to centre-of mass of the wheel assemblies will be a bit more than 11.5cm perhaps,
the mass is about 9kg per assembly I guess, so torque is about 2 * 9.8 * 9 * 0.125 = 22Nm approx.

You might want to redesign with the ends attached to offsets so the bearings are inline with the centres
of mass. Unbalanced rotating mass can be more trouble than its worth.

22Nm will require significant reduction gearing however you do it, but if the speed requirements on
that axis are modest that's going to be OK.

The tube should be as wide as possible to increase torsional stiffness to reduce kickback-related twisting
which would throw the aim off, and the gearing backlash will also be involved - the shock load of
launch is off-axis and may provide a large torque depending on mass / acceleration of the ball.

Thank you for all your input! I'll try to figure out a different design to have it go through the center of mass. It was just going to be a lot simpler to do it this way but it may make it more complicated in the end with the motors. Thanks!