Using a stepper motor to control a claw in your tabletop claw machine game is a great idea, but you're right to consider the holding strength of the motor when it's turned off. NEMA 17 stepper motors typically don't have great holding torque when unpowered, so you might need some additional mechanisms to prevent the claw from dropping.

Here are a few suggestions:

Planetary Gear Stepper: A 51:1 planetary gear stepper can provide higher holding torque compared to a standard NEMA 17 motor. These gearboxes are designed to increase torque output, making them a suitable choice for applications where you need better holding strength. However, they might be a bit bulkier due to the gearbox.

Worm Gear: Worm gear mechanisms can provide excellent holding strength because they have a self-locking property. Even when unpowered, a worm gear mechanism will prevent the claw from moving. You would need to couple a NEMA 17 stepper with a worm gear set or find a stepper motor with an integrated worm gear mechanism.

Lead Screw: Another option is to use a lead screw mechanism with the stepper motor. Lead screws have self-locking capabilities, similar to worm gears, and they can provide good holding strength. However, they might not offer as high of a reduction ratio as some planetary gearboxes.

Brake Mechanism: You can use a separate electromagnetic brake or a spring-loaded brake mechanism to hold the claw's position when the motor is turned off. This approach is more common in industrial applications but can be adapted for your project.

Counterweight: Depending on the weight of the claw and its payload, you might consider adding a counterweight system. This can help balance the claw and reduce the load on the motor, making it easier to hold position when unpowered.

Regards,
<a href="https://emiratesidofficial.com/">EIDO</a>

Could you put a shelf in the park position and rest the claw on it just before shutdown?

Not a bad idea 👍. Or maybe the spool where it winds has a flat side and a cheap servo flicks out to brake?  I ordered one of these to see if I can modify it to fit a stepper: https://amz.run/7DWs

Something to think about with stepper motors is that their torque falls off as the step rate increases. Modern driver chips help that, but there are still limits. If you use too much reduction gearing, you may run short on torque to lift or carry the claw and prize.

How much the torque will fall off and at what speed is very dependent on the motor and the driver chip, so I can't give you good numbers for your situation. There may be some tuning required.

Thanks for the info on the TMC2226, those sound efficient, I'll have to check them out!  Yes, the goal is to be in a completely powered-off state with the claw positioned at the top of the gantry so that it doesn't unwind if the power is cut.  It'll be coin-operated so most likely, the Arduino will be in a run state but we won't kick on power to the steppers until a coin is inserted.  The kids are developing the code so I'm trying to help them come up with a design that is as simple/safe as possible.  We're using an arcade solenoid-powered 24V claw mechanism for the grabber and it is a bit heavy.  I think a standard arcade claw machine uses DC motors and a worm gear on the claw mechanism but I'm hoping the kids will be able to control steppers/limit switches easier than DC motors

The 51:1 reduction will help with the holding capability when the stepper motor is un-powered. A worm gear drive should also work for this.

Are you thinking of turning off the lift motor while the machine is active or is it just when the whole machine is shutdown? Are you trying to avoid having to home the lift mechanism on powerup?

A some stepper drivers allow reducing the motor current in an idle condition. The Trinamic TMC2226 chips will do this automatically. I expect that there are others that will do this, but that is the only one that I am aware of specifically. Other drivers allow you to set the motor current directly.