Stepper Motor Strength

[Cartierusm]

I was wondering how to figure out how much strength a stepper motor has? I have Keling 425 oz. stepper motors, using a 24V power supply, mach 3 is setup to 40 ipm and it's directly connnected to my screws. How do I know how much weight, as in my z axis, can the motor handle? My impression was the 425 oz. was holding power of the motor at a stand still.

I have ordered a 48v 7.3a power supply from keling. Right now I'm not having a problem with my z axis, because everything sounds good and seems to run fine, but that's my perception, my z axis is pretty heavy, I'd say in the 30 pound range...just a guess. But the reason I'm asking is I might be adding a Taig spindle and motor to replace my router for metal cutting and I know that will add quite a bit of weight.

[CarbideBob]

You left out the most important factor. Screw pitch and type (ballscrew?).
With a fine enough pitch screw you could lift a car with this motor (but you won't get 40 IPM).
As far as motor torque I usually use 20% of the rated holding torque for available running power. All depends on the speed and motor torque curves but this gets you in the ballpark.

Bob

[Cartierusm]

Sorry Bob, lol, I am using a 2 start 1/2-10 5 TPI acme lead screw from Mcmaster and a Delrin antibacklash nut from Dumpster, but I am going to be changing to Roton ballscrews, also 5 TPI 5/8", and double nuts next week or so. I already ordered my x axis and I'm going to try that first as I've never used ballscrews before and once that's successful I will order the other 2 axis'. I figure I might only need 1 ball screw on the z as the weight going down will remove the backlash.

As far as your 20% what does that equate to? Do I take the 425oz. and convert that to pounds, 26.5 pounds, and then take 20% of that? That would mean I only get 5 pounds of force, that doesn't seem correct.

[BobF]

Read this thread.
Calculating What kind of power i need ????

[CarbideBob]

Ok, from the above mentioned thread you've probably found the magic formula.
Lbs = (pi X TPI X in-oz) / 8
So using 20% of 425 and a 5 TPI screw we get : Lbs =( 3.14159 X 5 X 85) / 8 = 166.875 Lbs. (with a perfect frictionless screw)

This rotary power to linear motion thrust confuses lots of people so let me give a little different explanation of whats going on here.

First off we start with a 425 oz/in motor and assume 20% power at running speed.
This gives us (425 X 0.20)= 85 oz/in running power. Divide by 16 and we have 5.3125 lb/inch.
What does this mean? It tells us we have enough power to provide 5.3 pounds of force 1 inch from the motor shaft .
So if we were to attach a 1 inch radius (2" dia) pulley to the motor and wrapped a string around it we could lift a 5.3 pound weight.

That's nice you say but I'm not lifting my slide with a string I'm using a 5 TPI screw.

Well you can look at the screw a as ratio increaser or big lever. One turn of the screw moves the load 0.200 inch.
One turn of our motor with the 2" dia pulley lifts our 5.3 pound weight 6.283 inches ( 2 X pi X R ).
So we have a lever that on the long end is moving 6.283 inches with 5.3 pounds of force and the short end moves 0.200 inches.
This is a 31.415 ratio lever ( 6.283 inches in = 0.200 inches out ).
5.3125 pounds X 31.4145 leverage = 166.88 lbs.

A 100% efficent 5 TPI screw is always a 31.415 torque multiplier. Add a 2:1 reducer at the motor and its a 62X multiplier.
2 TPI screws are ( 6.283/0.500 ) 12.566 multipliers.

Now comes the down side. Missing from the the above formula is the screw efficiency.
Acme screws are only about 30% efficient, ballscrews from 80 to 90%.
So using an acme screw we have to multiply our 166.88 lbs by 0.30 giving us about 50 lbs of thust force. ( we're running out of power here )
With a ballscrew we get up to 166.88 lbs multiplied by 0.90 giving 150 lbs of force (no wonder people like ballscrews so much).

You can't use all of this power to just hold the axis up. You need some power to accelerate the system.
How much depends on how fast you want to get up to speed.

As an example it takes about 15 HP to propel a full size auto down the road at 55 MPH.
But as you can imagine it would take forever for a 15 HP car to get to 55 MPH.
So lets say you're getting on the freeway. With a long on ramp (slow accel rate) a 60 HP car gets up to speed in plenty of time to merge into the traffic. If you have a 50 foot on ramp (high accel rate) you're going to need maybe 200 HP to get up to speed in time.

Accel rates have a huge influence on motor power required. You have to calculate the power required to accelerate the screw and the mass of the load along with overcoming the Z gravity loading and friction of the system. You also have to worry about force to overcome screw nut preloads and mounting bearing preloads (drag torque).
These calculations gets messy fast so I use Kollmorgen's Motioneering software to calculate my motor torque requirements.
This allows me to play with pulley ratios and accel/decell rates to optimize system performance.

Bob

[ger21]

An easier way is to go to www.nookind.com Look in their acme catalog, and find a comparable screw. Then look at their specs, which tells you that it takes .896oz of torque to lift 1 lb. Using Bob's example of 85oz, 85/.896 = 95lbs of force. Efficiency is already factored into Nook's numbers. If you have a torque curve for the motor, and know the max rpm you'll use and can find the torque at that rpm, will be a bit more accurate than the 20% rule.

Nook has ballscrews, too, and that catalog shows that your ballscrews need .56oz of torque to lift 1lb. 85/.56 = 151lbs of force. About 60% more force. And pretty close to Bob's 150lbs.

Bob mentioned that acme screws efficiency = ~30%. While this is true with single start screws, the efficiency goes up as more starts are added to the screw. YOUr 2 start screw is actually about 57% efficient, and 1/2-10 5 start is 75% efficient, and can deliver about 80% of the force of a comparable lead ballscrew for usually much less than 80% of the cost.

[Cartierusm]

This is why I love CNCZone...You GUYS Rock!!! Thanks for the help. All though I tried to do my own work as you should around, try first and then ask for help. I read the links Bob gave me and math was never my strong suit and trying to use the Xcell spread sheet confused the hell out of me. Thanks again.

I will be changing to ball screws soon anyway, but it's good to know that I can still use my acme 2 start screw for the z if I want.

P.S. How does the power supply come into effect? I have a 24V 8.3A currently, and I'm getting a 48V 7.3A one soon, I figured even if it's running well extra strength can't hurt, plus I would like to jog faster than 40 IPM.

[CarbideBob]

Oops, missed the 2 start part on the screw (I'm such a dummy).
So yup 166.8 lbs X .57 eff = 95 lbs thrust force.
You really need the motor torque curves to know exactly where you are and how the motor will respond to the higher voltage. My experience has been that you don't gain much but it's been years since I did any serious work with steppers (I'm a servo motor kind of guy).
At 40 IPM with a 5TPI screw the motor is only running a only 200 RPM (666 sps) so you should be well down into the torque curve and more than 20% power should be available. You may be able to jog as fast as 70-80 IPM.

Bob

[Cartierusm]

Not really everytime I adjust the settings in mach3 to above 46 in acceleration (I think it's the acdeleration and not the velocity) the motors stall.

[CarbideBob]

Acceleration eats up power fast.
Back off the accel and you can usually get more top speed but like a low powered car it may take too long to get up to speed. If you are doing lots of short moves lower accels will slow you down as you never reach the max possible speed. One of the many tradeoffs with motion control.
Bob

[Cartierusm]

Bob, you sure know a lot. Any advice on tuning my motors so they are happy when they run? I have my velocity at 40 and my acceleration at 20, but when I jog the acceleration seems the same anywhere from 13 to 20. Mach 3 tells you to use sound and feel for setting the motor parameters, but I wouldn't know where to start to run them at peak. I did notice that jogging sounds different then when running a program, even if they go to the same coordinates. Lately I've notice that when running a program and the motors stop to make a cut they seem, not jerky, but the stopping is a little abrupt. I know this has to do with acceleration, but how to tell when they are in the 'sweet' spot. Do I just change settings and run a familiar program and try to determine a difference?

[CarbideBob]

Bob, you sure know a lot.

No I don't. The older I get the more I realize how little I know.
While I've done a fair amount of motion control work I've never used Mach so I know nothing about how the parameters in Mach are handled.

When I design a machine I size the motors, amps, screws, etc. for specific performance requirements. Once it is built I write a series of programs to "stress" the machine within these limits. I attach accelerometers to each axis and monitor the machines reactions. Accuracy and repeatability are measured with LVDTs, lasers and ballbars. Lots of fancy equipment and out of the range of the normal DIY.

This is really the equivalent of Mach's recommendations to use the sound and feel of the machines response.
Write some programs testing high and low speeds, long smooth moves and lots of short quick moves. Tweak a setting and look (and listen) for changes.
Setup an indicator and make sure the machine returns to the same spot it started from.

I'm sure there are lots of users here that can give you better info on the settings in Mach than I can.

Bob