dc motor - spindle / shaft drive

[dsc]

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.

[Swarfing]

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?

[Jonathan]

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

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.

[Swarfing]

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

[dsc]

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.

[Swarfing]

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

[Jonathan]

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).

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.

[dsc]

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.

[Jonathan]

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

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.

[Swarfing]

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

[Jonathan]

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

The ones that operate around 900rpm are very large and expensive motors, so he would need a planetry gearbox or something similar to gear it down significantly.

[Swarfing]

Yes but comparison to a Servo still cheaper an more readily available

[dsc]

That was the main problem with DC brushless motors, at least the ones I've found on the net. It's either too low with the torques or too high with the speeds. As I said steppers seems to be ok due to the fact that their torques are reasonable at lower speeds and sizewise they are small, quiet and fit in perfectly in the box I've designed. Even adding a driver and power supply, it seems to be less than £200 which is reasonable for the budget I have. As I said before, I need to squeeze this in a rather tight space, 110mm x 150mm x 2800mm, so this eliminates big, beefy motors. I'd also rather go for an off the shelf kit as I've got enough problems with cuttings parts for the enclosure.

Thank you kindly gents for your suggestions and discussion.

Regards,
dsc.

[JAZZCNC]

Just use a stepper with integrated driver like this SYINTSD233-4 (INTERGRATED STEPPER MOTOR) (http://www.zappautomation.co.uk/en/syints-step-direction-stepper/603-syintsd233-4-intergrated-stepper-motor.html)

Very neat and compact and if you Gear it 5:1 then it should do the job. (Or find larger Nm version and keep 3:1)

Have you thought about the heat a stepper produces.? For short grinds then imagine shouldn't be a problem but still need to consider the heat and possible fans if enclosed and used for lengthy periods.?

[Jonathan]

Just use a stepper with integrated driver like this SYINTSD233-4 (INTERGRATED STEPPER MOTOR) (http://www.zappautomation.co.uk/en/syints-step-direction-stepper/603-syintsd233-4-intergrated-stepper-motor.html)

Very neat and compact and if you Gear it 5:1 then it should do the job. (Or find larger Nm version and keep 3:1)

300rpm is needed, so at 5:1 the motor is spinning at 1500rpm. At that speed the output torque is about 0.46Nm, which with the 5:1 ratio means 0.46*5=2.3Nm. The torque at very low speeds, i.e. when starting is 1.24Nm, so 6.2Nm at the grinder shaft which is below the 9Nm you specified.

Just use this spreadsheet to compare the different motors at Zapp and you'll be able to determine which motor/driver, if any, is suitable. It's substantially cheaper to use a separate motor and driver. The PM752 from Zapp is quite compact and I'm sure there are others.

[dsc]

Thanks again for the replies gents.

Normal usage will be mostly short bursts, as I mentioned before, maybe 10-20s at a time, few times a day. Heavy usage would probably generate loads of heat, so would need additional vents + fans in order to cool it down. Not sure how much heat will be generated, so it's all guesses at the moment.

I don't really need to keep 3:1, I can even use 1:1 if the motor is up to it. Steppers would actually benefit from low ratio, as they provide rather high torques at the lower speed range that interests me.

I've stumbled upon Mark's (blackburn mark) spindle thread and also discussed the details with him. I've looked at this motor:

EMP C6354 KV250 Outrunner brushless motor 2450W for RC airplane match 90A ESC | eBay (http://www.ebay.co.uk/itm/EMP-C6354-KV250-Outrunner-brushless-motor-2450W-for-RC-airplane-match-90A-ESC-/321035257797?pt=UK_ToysGames_RadioControlled_JN&hash=item4abf3147c5)

which can be ran at 500RPM at 2V (which seems oddly low), but I'm not sure of the torques produced at that point. I can also run it at 1000RPM at 4V and then use 3:1 to drop down the speed to around 300RPM.

Regards,
dsc.

[blackburn mark]

with that motor i,m getting 2800rpm at 12v (bit of drag on the angular contact bearings)

I wouldn't get you knickers in a twist about voltages... use a 12v power supply and let the ESC take care of the speeds (just dial it in with the servo tester)
I think between 1500 and 2000rpm would be a better speed with this motor so if you could squeeze a 4:1 you may be getting closer to where all the grunt is

a problem you may have with this route is at startup the toque is pretty low, if the grinder is loaded up it may struggle to start (im not up to speed with coffee grinding procedure, I assume you would run it dry and then knock it off?)

[JAZZCNC]

300rpm is needed, so at 5:1 the motor is spinning at 1500rpm. At that speed the output torque is about 0.46Nm, which with the 5:1 ratio means 0.46*5=2.3Nm. The torque at very low speeds, i.e. when starting is 1.24Nm, so 6.2Nm at the grinder shaft which is below the 9Nm you specified.

Yep correct Boy wonder.!! Didn't really give it much thought and concentrated on the Nm and need for compact design but missed the 300rpm required.!!
Agree that with 300rpm needed that 3:1 and larger motor would be better suited. To keep the compact electrics then the UIM24004 miniture drives go upto 4A so will be ok for a 3Nm motor.!

That said would have thought 6Nm when attached to dedicated grinder should easily crush a Coffe bean.? . . . But hey I drink tea so what would I know.!!

[dsc]

Thanks for the info Mark, the startup torque will be an issue though as the machine will be packed with coffee and that is when the high torque is needed, to get things moving. Once the thing starts spinning, the torque requirements drop to low levels.

Jazz, the 9Nm was measured with a torque wrench, hand cranking manually and changing the settings on the torque wrench (going up from 5Nm, 1Nm at a time, each time it clicked). I was surprised as well, but as I said above, that's static torque, dynamic is much lower.

Regards,
dsc.

[blackburn mark]

I was surprised as well, but as I said above, that's static torque, dynamic is much lower.

owch! that is a pain :(
I cant find any torque info on the DC motor either but im getting the feeling it would struggle in this application without a big ratio

[dsc]

Rubbish:( the only motors I found off the shelf are roughly £300. I thought steppers should be cheaper, but a 12Nm stepper is around the £100 mark:

http://www.zappautomation.co.uk/en/nema-34-stepper-motors/385-sy85sth156-6204b-nema-34-stepper-motor.html

and it needs a rather expensive controller (as it sucks 6Amps), which adds another £100 on top:

http://www.zappautomation.co.uk/en/digital-stepper-drivers/304-am882.html

Of course there's a final slap on the face with the DC supply, which comes to roughly £70:

SPS407 Unregulated switching power supply. (http://www.zappautomation.co.uk/en/power-supplies/442-sps407-unregulated-switching-power-supply.html)

so all together it's close price-wise to the DC motor (and something that Jonathan mentioned before, although at that point I thought I'd be able to get away with the cheaper DM422C controllers).

Jonathan, you've mentioned that if the speed is below the corner speed, the torque should stay at a reasonable level. How does one calculate the corner speed for a given motor? I'm asking because it might be better for me to use a 12Nm stepper 1:1, rather than doubling the speed to 600RPM and then using 2:1 to get the right spindle speed.

Regards,
dsc.

[Ross77]

In case you are still looking have you tried a servodisc/Pancake motor like they use on scooters and electric bikes? Might be a bit big but could be worth a look.