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Author Topic: Stepper Torque  (Read 4046 times)

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Offline SimonD

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Stepper Torque
« on: May 22, 2007, 03:37:07 PM »
Hi,

I have a 1.86Nm torque Size 23 stepper directly connected to a 10tpi leadscrew.
I am running it on a "Stepmaster" board, 1/8 microstepping using a 24v supply and have it set to 2Amp output which is what the documentation said to do.
It is working but I am underwhelmed by the available torque. The slightest touch on the loadscrew handwheel makes the stepper stall, even running it at low speeds. I have made sure that the slideway is free to move (i.e. the gib strips are not clamped down)

Have I set something up wrong? or have I seriously underestimated the torque I need?
If I put a spring balance against the bed, the bed can pull about 4 lbs which doesn't feel as though it is enough to me.

What is your experience?

Thanks
Simon

Offline SimonD

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Re: Stepper Torque
« Reply #1 on: May 22, 2007, 04:06:22 PM »
Sorry, forot to say...
The machine is a small bench-top mill.

Offline DAlgie

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Re: Stepper Torque
« Reply #2 on: May 23, 2007, 01:31:52 AM »
Generally it's the more voltage the better, I tune this by the motor temps while running under load, some say you can go to 150F, which I think is correct. 24 volts does sound a bit low as well.

Offline moxy

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Re: Stepper Torque
« Reply #3 on: May 23, 2007, 07:33:26 AM »
Hi, A stepper can operate at up to 25 times the rated voltage but don't go above the rated current (the thing will cook!) If you visit the gecko site you can download a brilliant file about stepper basics, it's well worth it.

Les
Re: Stepper Torque
« Reply #4 on: May 23, 2007, 10:46:07 AM »
Some facts for you. I am still sorting out the bugs on my little mill conversion but I was impressed with the power of the drives. Steppers are Sanyo Denki Nema 23 103H7123-5040 2 Amps 0.83Nm (approx 120 oz.ins). Power supply for the three Routout drivers is 4.5 Amps @ 27 Volts with 9,400 micro Farad capacitors. The toothed belt drive to the 5mm pitch 16 mm ballscrews has 2.5:1 reduction. This thread prompted me to put the bathroom scales under the Z axis and drive it down until it stalled.You can see the result in the picture- 64Kg! I was not expecting as much as that. :)

Ian

Offline SimonD

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Re: Stepper Torque
« Reply #5 on: May 23, 2007, 02:23:56 PM »
Ian,

If my sums are correct...

Assuming that the motors give the the same force in both direction. If the motors can only just lift the head then that is 32kg for the head and 32kg force from the leadscrew. Obviously the ratio is more in favour of the leadscrew otherwise it would not actually be able to lift it due to frictional losses.

I calculated that theoretically your stepper/drive train should be able to lift a 240kg mass in an ideal world. Clearly something is amiss.

Eitherway, I would be more than happy with 32kg from my motor! that would take some force to stop the slide. I am getting nowhere near that!

Simon

Offline SimonD

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Re: Stepper Torque
« Reply #6 on: May 23, 2007, 06:11:24 PM »
I've fixed the problem.  I stripped it all down and started again...

It does help if you read the multimeter correctly! I had set the current way too low!
Oh well you live and learn...

Simon
Re: Stepper Torque
« Reply #7 on: May 24, 2007, 07:04:25 AM »
Simon, glad you found your problem. It's the simple things that bite you!
I did the reverse test on my mill Z and it would lift 39 Kg before it started to miss steps. The two tests are different - down it was a dynamic test with the motor running at jog speed, up it was from a standing start and is carrying the weight of the head and spindle motor. Still, I did not expect such high forces. So I too did the calculations.
Torque = Load x pitch/2pi
The stepper jog speed equates to 160 rpm, from the stepper torque curve this is about 0.5Nm torque, pitch is 5mm(0.005m), 1Newton is 0.102Kg.
Feeding in these values gives 64Kg. Slide friction, and drive efficiencies would make a difference but I can't get to your 240Kg. Did I do it wrong?
If you can do a force test on your setup, it would be interesting to compare and give the forum some practical ballpark figures for designing their systems.

Ian
Re: Stepper Torque
« Reply #8 on: May 24, 2007, 07:23:26 AM »
Should have reread my posting! There is a 2.5:1 speed reduction so figure should have been 2.5 x 64 = 160 Kg.
Can someone explain the difference between theoretical and practical ? I would guess at the fact that the stepper only receives a pulse long enough to reach it's maximum amps, so is operating only on a low duty cycle.

Ian

Offline SimonD

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Re: Stepper Torque
« Reply #9 on: May 24, 2007, 01:11:15 PM »
Ian,

I used...
Practical Efficiency 0.9
Torque 0.83
Ratio 2.5
Pitch 0.005
Gravity 9.81

(0.9 x 0.83 x 2.5 x 2 x PI)/(0.005 * 9.81) = 239.2 Kg

However the efficiency is wrong. it should be...
0.943 Theoretical efficiency * 0.9 Practical Efficiency => 0.85 according to the SKF Ballscrew Manual.

This is for the load acting directly on the leadscrew.

The reason for the losses as you said is the friction in the slideway, not getting the maximum out of the stepper etc etc.

Once I have a more reliable setup I will do a test again. I am using a trapezoidal thread though so will get significantly more losses than a ballscrew. My stepper is "pulsing" at the moment! I think it may be down to the power supply shutting itself down. It does about a full turn of the leadscrew then pauses for a second then does another full turn etc. It is not consistent though, sometimes it runs fine.

Simon