How much weight can a stepper motor handle?

[Koni]

Hello All,
I am working on a winter project and was wondering how much weight a reel stepper motor can handle. I am mounting a 16" wheel on a "CDS" reel motor. I know what your thinking- he's building a look alike WOF machine. Well, your right, (my wife thinks I'm crazy too)! I am ready to test it but would like to know the possible result before I do anything to the motor. The CDS reel stops quickly and I don't know if the added wieght will hurt the motor.
What do you think? Thanks- Koni

[PWRSTROKE]

I think it would be better to use the proper part(s) or you could get some shrapnel flying around- .  Thanks B. Could see the internet headlines-"Man stuck in head with slot machine bonus disc"--- .

[PWRSTROKE]

I did not mean any harm in post about this.  Just to clarify.  On a straight up post -try to use the correct motor for top bonus wheel if possible.  I may have a brand new one-will have to chk and see if you are interested.  Thanks B.

[Koni]

Barry, I just saw your post, your right I should wear a face guard when I fire it up (lol). The reason I am not using an actual wof motor is I don't think it will work on a CDS machine. The CDS reels do not have optics, there is a small board on each reel to control the stop. Are you saying the wof motor would work connected to the harness? Thanks- Koni

[Op-Bell]

A reel stepper motor should easily be able to handle that job. BUT... Every stepper motor application is an exercise in matching the software that generates the steps to the inertia of the load. It's a surprisingly complicated mechanical system, far more complex than just fixing a disk to a shaft. With a large wheel, there is going to be new hardware and software involved in this project. I've done it many times, feel free to consult.

[Op-Bell]

A stepper motor only has a certain amount of torque. If it meets more inertia than it has torque, it "slips", ie misses steps, so the trick when starting and stopping is to give it a step, wait for the reel to catch up, then give it another at a shorter or longer time interval, until either it's up to speed or it's stopped. This is called ramping and it's quite critical - just changing a reel strip for a heavier one can throw it off and make the reel miss steps or just sit there buzzing instead of turning.

A key part of the assembly is the little rubber O rings on the pin in the reel hub. The motor steps more or less instantly, but the reel has more inertia and takes a while to move to the new position. So when it's starting, the motor moves one step and compresses the rubber O rings. The rubber rings push the reel, which starts to turn. Its inertia takes it past the new position and compresses the rings the other way, which would slow it down and stop it, but at this point the motor makes another step, overtaking the reel and compressing the O rings again, and the reel keeps turning to catch up. The steps come at shorter and shorter intervals and the reel accelerates to catch up until it reaches full speed. The reverse happens when the reel stops again.

Whenever you start working with a new motor and reel assembly, you have to experiment to work out the ramp tables and write software to make it happen. You're going to need access to some microcontroller skills for that part. On the mechanical side, when you attach the reel or wheel to the motor, you must have those O rings or some other resilient coupling in between, or it puts way too much stress on the hub and may break it, or it may not even turn.

[Koni]

OK, great info however, this is way over my head as far as being able to build myself. What if I make the larger wheel the same weight as the basket that is on the motor now? Whould it work? -Koni

[Op-Bell]

OK, great info however, this is way over my head as far as being able to build myself. What if I make the larger wheel the same weight as the basket that is on the motor now? Would it work?

Unfortunately, no. You have to make the moment of inertia the same. As the center of mass moves further from the axis the moment of inertia increases as the square of the distance, so a wheel twice the diameter of the reel would need to be 1/4 of the weight. Pretty soon even balsa wood and tissue paper is too heavy. I'm afraid if you want to do this, you'll have to have a custom driver. But then you do want this to look like a WoF, right? Slow start and stop, clicky noises? If it spun at full speed and stopped instantly like a reel it would look pretty strange.

If you know the torque of the motor and the mass of the wheel you can calculate the ramps using this equation from Wikipedia -
          
Most engineers (like me) prefer to make the wheel first and work out the tables by trial and error.