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I need to produce a very slow rotational speed for a disk -- 0.25rpm, or 1 rotation in every 4 minutes. The disk is nominally 12" in diameter. It will be driven by a DC gearmotor, through a worm and gear with a 1:20 drive ratio.

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The slowest gearmotor that I can find (within my budget and space parameters) is the Servo City 12rpm motor here: https://www.servocity.com/motors-actuators/gear-motors/heavy-duty-gear-motors/premium-planetary-gear-motors

But even at that slowest speed, and with the 1:20 worm gear, that still only gets me down to 0.6rpm, more than twice as "fast" as I need.

The options that I can think of would be an additional pair of gears being driven by the worm gear, or a planetary gear reduction unit between the gearmotor and the worm gear.

A few more parameters:

  • All of the components I'm already using for the mechanism are based upon .25" dia. driveshafts (most are from Servo City)
  • I need to go with off-the-shelf components; nothing fabricated (modified is OK)
  • It must be long-wearing and precise (no plastic or cheap alloys)
  • There's limited space between the worm gear and the disk
  • My budget for whatever solution to reduce the speed is =/< $50

I had been considering some of the gearboxes from AndyMark ( https://www.andymark.com/Gearboxes-s/55.htm ) but frankly, I'm put off by the fact that they don't even list the shaft diameters for their gearboxes; most of them seem to be proprietary sizes intended for use only within specific systems.

Any ideas would be appreciated!

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For precise control of speed, if that is required, consider using a motor controller that uses PWM (pulse width modulation) to control the energy available to the motor. With a matched motor & controller very slow rotational speeds are possible.

Using a motor controller in this way, the physical gearing ratio and arrangement is less important.

I would also consider a simple belt & pulley system for your project given the size and mass involved. Very high ratios can be achieved with simple and inexpensive parts.

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  • $\begingroup$ Thank you for the advice, Donald Gibson! The DC gearmotor I already got from Servo City is 165rpm (selected due to a mistake in my calculations). I might try to exchange it for their lowest-speed unit (12rpm), but I first wanted to ask if there's a "sweet spot" for speed-reduction using a PWM chopper. IOW, would the speed, being reduced through a 165rpm gearmotor down to 5rpm, be smoother or stronger than the chopper being used with a 12rpm motor? (I'm thinking of internal-combustion engines as an analogy; some are smoother or stronger at different rpms than others.) $\endgroup$ – RCH Feb 27 '18 at 22:45
  • $\begingroup$ Donald -- I agree that in many applications, a belt & pulley (or chain & cog) would be a good solution to lower the drive ratio, but the reasons I chose to not go with either were these: > the plane of motion is horizontal, which means that chain or belt sag would be an issue > I need precise indexing between the various driven components (of which the disk is just one) > wear would eventually be a concern > Unless a chain is kept under tension (which isn't practical in this application), there tends to be a "snatch" at the start. I need it to be smooth and jerk-free. $\endgroup$ – RCH Feb 27 '18 at 22:52
  • $\begingroup$ The slower gearmotor with the higher ratio gearbox will produce the smoothest motion. Consider that the controller will be able to adjust the motor speed reliably between 35% and 100%. Below about 35% throttle, there may not be enough power (in smaller motors) to overcome cogging, friction and inertia which might result in uneven rotational speeds. The most important aspect of the machine will be rigidity within the drive system if precision speeds are required. $\endgroup$ – Donald Gibson Feb 28 '18 at 17:01
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You can go various ways to solve your case.

  1. You can just use many gears to create a huge rpm reduction.

  2. You can PWM control your motor to go slow, but you'd still need a gearing of multiple gears; it'd be hard to find a motor that can steadily be rotated at 60rpm or lower. You can put a friction load on the motor so you'll have no/less stuttering. See, to drive the motor slow, power should be low(stuttering), or it should be kept at a low speed by a friction load. The latter gives less stuttering behaviour.

  3. It's not clear to me what kind of application you are intending, but you could just drive the motor with pulses. Kind of a ultra low frequency PWM. If you have a controller than powers the motor for a very short time, and then pauses for a longer time. E.g power the motor at 20% power for 0.25sec, stop power for 2.75sec. Repeat. You effectively have created a slow turning disk.

  4. If your application is not power demanding, you can get a quartz clock mechanism and use that.

  5. Recommended option: Use belts and pulleys or sprockets instead of gears. You can easily create large reductions cheaply. You can also create them yourself from wood and rubber bands.

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Cheap price method one: at the scrap-vehicle autoschrott buy the cranckshaft flywheel. Normally it not be over 20 USD, but older cars may be even 10 USD. Buy too the compassing alternator only small sprocket. If find a totally broken alternator, it may cost simbolically, probably 1 USD. Second option - take the backellyte plywood plate, about inch thick, process it completely round and with good glue attach a perimeter with CNC specialized rubber belt. This kind of belt has practically no elasticity (thread inside) and has SINUS teeth profile. It means it gives zero gap between teeth and belt or just superior accuracy. The belt of course must be glued with teeth outside. And over it put the identical belt with teeth inside, what somewhere gets around a sprocket wheel of standard 12mm CNC NEMA17 mass product sprocket. This sprocket costs below 1 USD and belt cost about 1 USD per meter of length. It is sold in cut yourself rolls.

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