I need to slip the coupling on a small stepper - ideas?

[julianf]

This is for an audio project, so i may as well describe it in those terms, although the job will be much smaller...

On one of these record players -

the motor is directly coupled to the platter. There's normally a felt style mat between the platter and the record, so, when the DJ wants to adjust the record, the platter and record slip, relative to one another.

Hence the felt thing being called a "slip mat"


All well and good, but i want to do this kind of thing, but on the vertical. The motor will be way smaller, probably one of these -

https://www.rapidonline.com/rvfm-my355-2-phase-bipolar-stepper-motor-12v-37-0507


(cant link to the image)


Or similar.

The stepper will be controlled by an arduino, but i need the end user to be able to stop the rotation without stooping the motor. Of course, if this were horizontal, i could just use a felt mat, but its not, so i cant.

Is there a sort of coupling that i can use that slips when the torque rises? But that would slip at really quite minimal torque, as the stepper will be small.


The alternative is to to it in software, with a momentary button, but that's kind of not as nice.

 

[thinfourth]

Rubber hose?

 

[sako243]

What about effectively creating a clutch plate?

If you had a straight spindle attached the stepper motor you could put a spring over it and then a circular plate that has the centre drilled out so it slides up and down the shaft. The spring will effectively be trying to push the disc off the shaft but you can then line it up with the platter so that in normal circumstances the friction is enough to rotate the platter. Someone stalling the platter then allows slip to occur.

You could tune the friction required by adjusting the spring and / or materials on the clutch surface. To become even more advanced if the stepper motor end of the shaft was threaded you could put a couple of nuts on there so that you can adjust the length of the spring to tune it more easily.

I suspect you would need a slightly rough surface on the two clutch faces otherwise once the platter has stalled but the motor is spinning then you'll need some friction to restart the platter.

Did any of that make sense?

Do you need to use a stepper or can you use a normal DC (or brushless) motor? If you can ditch the stepper then you might be able to play some tricks by monitoring the current being drawn by the motor and disconnecting it when you detect a stall situation. Reconnecting it might be a bit trickier.

 

[rtbcomp]

Slack belt

 

[waddycall]

Magnet or pair of magnets?

 

[tommullin]

Limit the current to the stepper?

 

[Hood]

Would the stepper not just stall when you increase the pressure then start going again when you relieve it?

 

[tommullin]

Yep. Isn’t that what a slip mat would do or did I miss the point? It wouldn’t goto it’s intended position. It’d just keep going

 

[Anguz]

Stop the motor using the capacitive sense library... touch the sensor attached to the shaft and it would stop

 

[Hood]

Yep. Isn’t that what a slip mat would do or did I miss the point? It wouldn’t goto it’s intended position. It’d just keep going

I wasn't disagreeing with you, just thinking you wouldn't even need to limit the current if it is a small motor anyway. Like you however I may be reading things wrong.

 

[tommullin]

Yeah I’m not sure on the intended purpose. And he did say without stopping the motor, so may not be an option. But some of those steppers are surprisingly torquey (depending on the current supplied).

 

[hotponyshoes]

Magnetic drive.
Round disk (x2) on the output for the motor
Magnets on each disk so they lock together.
Use a few magnets then reduce the force if required by packing washers between the disks.

Will generate a small flywheel effect on the motor side but that might actually be an advantage for your application?
A record/platter/etc is quite a decent mass to spin up quickly and it does need to be quick or it sounds horrible.

 

[optima21]

slightly off topic, but Ive got a worm wheel to use as as fine feed on the quill of my milling machine

so I'll be machining the boss off the work wheel and clamp it on both faces so that it can be driven or free to rotate.

so I'll be using something like this arrangement (not to scale) but without the wavy washer. the pressure plate cant rotate so needs to either have a square hole in it or slide along a pin in the mounting boss. the nut and wavy washer (or spring) if you need alot of slip can be used to change the torque required to make it slip.

its a modified version of the ball type differentials that are used in radio controlled cars. basically if the output drives of the differential cant rotate, the gear will the balls in it will slip depending on the tension set by the screw that runs though the middle.

Im guessing you might be wanting something simpler though

 

[Screwdriver]

Build a clutch unit from a driven felt pad and a steel washer pickup. The clutch "pack" could be sandwiched in a small adjustable housing like a 15mm compression fixing. An alternative "clutch" or friction drive would be to use a close fitting spindle in a bore. You could probably get away with plastic on plastic (eg Delrin) if the torque requirement is low enough.