dc motor - spindle / shaft drive

[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)

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)

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

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