dc motor - spindle / shaft drive

Gents,

again this isn't my area of expertise, so please excuse if what I'm about to ask seems silly.

I'm looking for a DC motor which can do around 3Nm at around 800-900RPM. I'm guessing this means it will need a gearbox of some sort, although it has to fit into an area of 110mm x 150mm x 2800mm, so something tube-like is more than welcome. This will run a shaft / spindle via a toothed belt, at a ratio of 1:3, so in the end the shaft will 'see' 9Nm at around 300RPM.

Any help with this is greatly appreciated. I was thinking a stepper can handle this, but I can see that it's probably above what steppers are made for. I've found a few motors which roughly fit the above, although their power rating was rather low, sitting at around 100W. Looking at speed vs torque vs power charts, the motor will supply max power at 0.5 top speed and 0.5 top torque. Should I then look for a motor which is twice the requirements I need or simply ignore that and have a motor which slows down when the torques get high?

Regards,
dsc.

DSC

I made a small spindle using a 100w dc motor which was fine for circuit boards. As a suggestion have a look at servo motors instead, tend to have a better torque curve and a lot more controllable? A stepper with a planetary gearbox could work? just rob a drill for its gubbins.

At the speeds you are quoting are you wanting to cut steel?

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Originally Posted by 2e0poz

At the speeds you are quoting are you wanting to cut steel?

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No, I think those are the cutting speeds for coffee!

I think using a drill motor is a good idea since they're very cheap.

300 is a reasonable speed for steel to a machinists, i also suggested the use of a planetary gearbox from a drill not the motor, thats only good for toffee

Hmm wouldn't mind using a drill motor, although they tend to get a bit noisy, plus are not easily available off the shelf (apart from replacement parts).

Steppers seem like an interesting idea that I keep coming back to, one can find some with rather high holding torques, which should mean rather high torques at low speeds as well (I'm guessing a loss of around 10-20% of torque at 200-300RPM). The problem with those is you need a controller and a power supply and I'm not so sure I can fit everything inside the box. I'm also tempted to go down this route as I'm planning to use a small stepper on the lead screw, so using steppers for both would solve all problems.

Oh yeah, forgot, this is indeed for grinding coffee, Jonathan is right, although it might sound like a joke.

Regards,
dsc.

There are quite a few Independent controllers on the net which will connect with a standard stepper. Completely standalone

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Originally Posted by dsc

Steppers seem like an interesting idea that I keep coming back to, one can find some with rather high holding torques, which should mean rather high torques at low speeds as well (I'm guessing a loss of around 10-20% of torque at 200-300RPM).

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If operating below the 'corner speed', which at 200-300rpm almost certainly is the case, you'll get about 2/3rds the rated holding torque as rough guide. However with a 3:1 ratio it's going to be less - try the motor calculation spreadsheet irving posted a long time ago and you'll see what I mean.

It looks like you're after 9Nm at 300rpm? Power=torque*angular velocity, so P=9*300/60*2pi=283W
So if you definitely need 9Nm at 300rpm, your motors needs to output about 300W which is not really realistic for a stepper motor. That's realistic for a drill motor with planetry gearbox, but as 2e0poz has hinted, the brushed motors in those, although cheap, are not likely to last long. However if the duty cycle is low that may still translate to several years use.

Hmmm all good stuff Jonathan, thanks for the info. Shame about the steppers not being up to the task, as it's dead easy to change their speeds and generally driver them via off-the-shelf drivers / components. Does it matter that the 9Nm torque requirement is only momentary ie. it's static torque needed to get the grinder moving sometimes. Once past that point the thing can be driven with anything as low as 1Nm (although it's prone to stalling from time to time). There's an existing grinder which uses a DC motor which only has 0.3Nm and is driven via a 30mm dia pulley : 90mm dia pulley (so effectively 1:3), which I thought would triple the torques and divide the speed by 3. That stalls from time to time but manages to do the job even with those low specs.

Duty cycles are low, say on for 20-30s a couple of times a day maybe. Although that would change of course in a busy environment.

Regards,
dsc.

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Originally Posted by dsc

Does it matter that the 9Nm torque requirement is only momentary ie. it's static torque needed to get the grinder moving sometimes. Once past that point the thing can be driven with anything as low as 1Nm

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Yes, that helps a lot since you just need a motor with high starting torque, so again the motor with planetry gearbox is probably a good bet. The average power whilst switched on is now 1/9th, so about 32W which is more realistic for a stepper motor. The problem with stepper motors is in addition to the motor you'll need a driver which is probably quite expensive.

Why not have a go at DC brushless motor spindle. There are some good threads on here for those?