motor stalling

[planescott]

Hello everyone, I am having problems with my step motor on my x-axis stalling. Sometimes it goes, sometimes it don't go. But if it does start moving it sounds fairly ok. It seems to have the problems at the lower rpm range.

My gantry weighs around 85lbs I am guessing. And I have 640oz/in motor, 5tpi ballscrew.

So my question is, how much more torque do I need. If I went to 1200oz/in is that going to be enough? What can I do to figure out how much power I need.

Here are some pics of my machine to see what I am working with.

[CJL5585]

You will only need enough torque on the X Axis to move the gantry. From your photos of the rails that you are using, you can probably move the gantry very easily with the leadscrew not attached. (My guess is 3 to 7 pounds of pulling or pushing force manually) I made a calculation of the total motor torque and ballscrew combination and this moving force is 16.66 pounds max.

I am thinking that your problem is not with the torque of the stepper, but that the stepper is experiencing what is called "resonance" at some speed.

Does this happen when you are jogging the machine or when you are running a test program?

What are you using for software? Mach, TurboCNC, Other?

Your problem might be created by the Max Frequency, Acceleration, and Start frequencies.

What is your driver supply voltage? This could also be a factor.

Give us some more information on what you are using, and we might be able to get you going.

Jerry

[CJL5585]

Originally Posted by planescott So my question is, how much more torque do I need. If I went to 1200oz/in is that going to be enough? What can I do to figure out how much power I need.

THE FOLLOWING IS BASED ON A NEAR PERFECT BALLSCREW WITH NO RESISTANCE WHEN MOUNTED IN PLACE AND TURNED BY HAND.

The method to determine how much torque you need is calculated as follows:

Disconnect the ballscrew from the gantry.

Get a weight scale - The type held in the hand at the top with the item to be weighed attached to the bottom. Scale range 0 to 20 lbs.

Move the gantry to one end of the machine.

Connect the scale to the gantry and slowly pull until gantry begins to move. Note the weight reading on the scale.

Take this reading and multiply by 1.5. ( Adds an additional 50% torque factor for fast feedrates)

Divide this number by Turns per inch of ballscrew.

Multiply this number by 12.

Then multiply this number by 16.

This last number will be the oz/in rating of the motor you will need to move the gantry.

-------------------------------------------------
Example: Scale reads 2 lbs.
Multiply by 1.5 = 3 lbs.
Divide by 5 = 0.6 (5 is the # of TPI of ballscrew used)
Multiply times 12 = 7.2
Multiply times 16 = 115.2 oz/in stepper
-------------------------------------------------

This may not be the PERFECT ENGINEERING FORMULA for some folk, but it will give you a very close number for the oz/in rating you will need.

Jerry

[planescott]

Hi Jerry, If I jog the x-axis it usually won't start moving. It just sits there and makes a horrible sound. But really when it does start moving it doesn't sound near as good as the y&z axis.

I am using mach3, gecko 202's, 50vdc and approx. 17 amps. I believe it just does not have enough torque. I tried the acceleration and velocity at all different ranges, nothing worked. It just don't want to move it.

All my ballscrews are lined up very straight, there is no binding any where.

I guess I will try putting a scale on it and see what I come up with. This sounds like it should be a good starting point, Thanks Jerry

Scott

[CJL5585]

Turn off all power to the drives and motors.

Check the motor wire connections on the X axis. Sometimes if all the strands of wire are not under the terminal, it can act like that. Also, sometimes the connector will be tightened down on the wire insulation and not making good contact. Do you have a capacitor across the voltage terminals 1 and 2? (positive on 2 -- negative on 1)

Do you have the same type motor on the Y axis or Z axis?

Jerry

Another option is that there might be something wrong with the stepper.

[ger21]

A 640oz stepper should have no problems at all moving an 85lb gantry. Something's not working right.

[planescott]

What would the capacitor do? But no I do not have one on there.

I have 640oz/in on y axis. And 270 on z axis. They both work very well.
They sound very smooth and soft kinda sound. They work very well.

[2muchstuff]

How wide is the gantry, is it racking putting it in a bind. If every thing is lined up, you should be able to turn the screw by hand.

[planescott]

I can twist the gantry about 1/16" but it takes pretty much force to do that. And I can turn the coupler by hand on the x-axis, it maybe just a tiny bit harder to turn the x than the y by hand.

[CJL5585]

Originally Posted by planescott What would the capacitor do? But no I do not have one on there. I have 640oz/in on y axis. And 270 on z axis. They both work very well. They sound very smooth and soft kinda sound. They work very well.

Gecko recommends a filter capacitor across terminals 1 and 2 if the drive is located farther than ?? (I think it is 18) inches from the power supply. The filter capacitor should be at least 470 uF. On a large stepper a 1000 uF capacitor would be better. With a 50 Volt supply, you should get capacitors with at least a 75 volt rating.

Without a filter capacitor across the voltage input on the Gecko drive you can get noise and possibly voltage spikes induced into the drive which causes crazy things to happen.

I have put input filter capacitors on every drive I have ever built, even if the power supply is 6" from the drive.

Jerry

[planescott]

Thanks Jerry I will put some on. I was told I didn't really need them.

What capacitor would I put on the 640oz/in? What motor is considered large.

Scott

[CJL5585]

[QUOTE=planescott]Thanks Jerry I will put some on. I was told I didn't really need them.

What capacitor would I put on the 640oz/in? What motor is considered large.

Scott[/QUOTE

Get some Electrolytic capacitors rated at 470 uF. (They have a + and - terminal and are marked). Make sure the capacitors have a rating of at least 75 volts. You can use capacitors rated higher than this voltage.

If you cannot lay your hands on the above value, Any of the below will work.

680 uF at 75 or 100 VDC
1000 uF at 75, 100 V DC

The higher the voltage -- and the value -- The larger the capacitor. So try to stay with the 75 or 100 Volt rating.

In the hobbiest context, I would say 1200 oz/in would be large steppers.

Jerry