# Servo Sizing



## HARRY304E (Sep 15, 2010)

mutabi said:


> Hey guys, I need to replace a pneumatic cylinder with a rotary servo. The cylinder is a 4 inch diameter cylinder with 90PSI air. So this is approx 1100 lbs of force.
> 
> I am getting confused however, trying to figure out what torque a servo would need to be able to produce, to provide 1100lbs of force in a linear movement.
> 
> I believe pulley size affects things but I am not quite sure how. Can anyone provide some insight for me? Thanks.


You might get an answer over on Contractors Talk

I think you are right about the pulley size but i do not know for sure.


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## John (Jan 22, 2007)

What are you going to use for the mechanical drive....as in ball screw. rack and pinion, cable and pully, etc.? What's the application that you want to replace and why?


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## mutabi (Jun 2, 2009)

The servo will run a cable and pulley system which will be attached to a metal piece that runs along a shaft. The servo will move that metal piece linearly. The maximum force needed to move that metal piece will be whatever force the pneumatic cylinder can put out. 
This is a complete machine redesign so pulley diameters and any other factors are changeable if needed


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## joethemechanic (Sep 21, 2011)

1,100 pounds of force acting on how long of an arm?

The product of force X Arm length will give you a torque value expressed as a "moment"

You got a picture?


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## mutabi (Jun 2, 2009)

Pardon my poor drawing skills. I understand that torque is a rotational force and that is what is causing me some confusion. Trying to convert the linear motion into a rotational force. I am getting confused.


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## McClary’s Electrical (Feb 21, 2009)

mutabi said:


> Pardon my poor drawing skills. I understand that torque is a rotational force and that is what is causing me some confusion. Trying to convert the linear motion into a rotational force. I am getting confused.


 
You peumatic cylinder is coupled to a shaft that it is rotating. The length of that arm is a factor in the force being delivered to the shaft. 100 lbs on 12" of arm rotating, is 100 ft lbs. 100on 6" arm is 50 ft lbs, 100 lbs on 24" arm is 200 ft lbs. Use the numbers to determine how much work your cylinder is doing. I'd be willing to bet this servo will be 5' long.


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## McClary’s Electrical (Feb 21, 2009)

mutabi said:


> Pardon my poor drawing skills. I understand that torque is a rotational force and that is what is causing me some confusion. Trying to convert the linear motion into a rotational force. I am getting confused.


 

Is this a paper splicer?


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## mutabi (Jun 2, 2009)

Right now the cylinder moves the piece about 12 inches at a time in a linear direction. It is not actually shown in the drawing. For all intents and purposes you could forget about the fact that there is a cylinder altogether and just assume that we want to move that piece using a maximum force of 1100 lbs. 

Could we use a gearbox to get some torque multiplication?


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## mutabi (Jun 2, 2009)

> Is this a paper splicer?


No, its a machine used to machine 30 foot lengths of tubing. The piece the servo will be controlling will move the tubing as it is being machined.


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## McClary’s Electrical (Feb 21, 2009)

mutabi said:


> No, its a machine used to machine 30 foot lengths of tubing. The piece the servo will be controlling will move the tubing as it is being machined.


 

That's gonna be the wrong application for a servo motor. I would recommend a standard motor and gearbox. How fast does it move?


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## mutabi (Jun 2, 2009)

What makes you say that it is th wrong application for a servo? The reason we were looking at a servo was because we need precise control/feedback.

I don't have the exact numbers for the speed and this is gonna sound really unprofessional but it currently moves anywhere from a slow walk speed to a sprinti speed. I can't believe I just wrote that lol. 

Regardless of whether or not a servo will work, I still need to figure out how to translate the linear force into a torque number :-(


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## joethemechanic (Sep 21, 2011)

That will depend on the mesurements of the pulley indicated by the red arrow.

What we need to know is the radius. If you can see the blue dots connected by the line. One blue dot represents the centerline of the shaft. The other dot represents the center of the wire rope.


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## mutabi (Jun 2, 2009)

Let's say I make the pulley radius 2 inches.


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## joethemechanic (Sep 21, 2011)

Better pic


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## joethemechanic (Sep 21, 2011)

1,100 pounds X 2" =2,200 inch pounds of torque

Or divide by 12 to get foot pounds

183.33 foot pounds


You may want to add a bit for design factor


And your cable will move 12.56 inches for every revolution of your shaft. That is provided your cable is a single layer thick on your pully/drum 

2" + 2" X 3.14 = 12.56

3.14 being pi


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## joethemechanic (Sep 21, 2011)

You probably want to have your pulley/drum machined with lagging and have both your cable ends terminate at that point if you need a lot of accuracy.

See the lagging groves in these winch drums?












They keep everything in place and nice. Just remember that they slightly decrease your radius


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## joethemechanic (Sep 21, 2011)

OOOPs

hang on

Got to edit the post above


Fixed it, answer was right, work was wrong, story of my life lol


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## John (Jan 22, 2007)

If you want the high precise control/feedback I would go with a rack and pinon system using a servo motor

Go here for ideas

A cable drive system will become loose after a while and you will not have the accuracy that you would have with a rack and pinon system. The servo motor encoder will not be a good position reference because of cable looseness and some type of encoder to reference position will be required.

Pictures of the existing setup would help. :whistling2: We all love pictures.:thumbsup:


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## John (Jan 22, 2007)

Here is a rack and pinion drive with a motor and the length can be as long as you want.

View attachment 11075



Here is a example of a ball screw movement which would be extremely accurate. 

View attachment 11076


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## nolabama (Oct 3, 2007)

I will send you some links later for where to get those ball screw linear actuators. IMHO a cylinder puts out more force than engineers say they do, having said that they seem to grossly undersize the actuator. Those ball screws are problamtic. I would recomend designing this without pulleys if at all possible. Encoder problems as stated, maitanence issues. Their is an air powered linear actuator that uses internal cables, but i dont think it moves very slow at all. And it needs to be as long as it moves similar to a hydraulic cylinder. A servo cylinder is gonna cost a fortune. The only app I have found foe them is areospace and aircraft, really pricy stuff.


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## nolabama (Oct 3, 2007)

we use these and IMHO they are junk
http://www.duffnorton.com/

re reading all the posts and i was a little confused - thinking you needed a servocylinder not a servo motor -sorry - 

so your gonna do the rotational to linear conversion yourself lol

good luck with your machine


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## wildleg (Apr 12, 2009)

what's the application. sounds like you are trying to reinvent the wheel


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## mutabi (Jun 2, 2009)

Thanks guys for all of your input. joethemechanic answered my question as to how to calculate the torque necessary to move the piece. It turned out to be super simple but for some reason I was thinking it was more complicated. 

I wish I could post pics of the actual machine but I can't. Trade Secrets etc etc. 

As for reinventing the wheel....well possibly. We are looking to innovate. To create the only machine of its kind in the world. Its a challenge but I am learning a lot. What we are trying, may and may not work. BUT if it does work, there is a lot to be gained from it. 

Thanks also to the other suggestions with the rack and pinion and ball/screw type mechanisms. They are definitely something I will be looking into, to see if there is any way they can be incorporated into our machine design. 

As always +1 to the members of electriciantalk.com!


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## John Valdes (May 17, 2007)

Why not ask someone who reps servo manufacturers? They do design work. If they smell business I bet they come out and help you. For free. Tell them you have some projects coming up and need some help. 
Baldor builds servos and has tremendous helpful people on board. As a matter of fact, there home office is in Fort Smith Arkansas. Go to their web site and find a "authorized service center" Give them a call and see if someone is interested in coming out for a look. You might even get a free lunch out of it along with some help.


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## nolabama (Oct 3, 2007)

mutabi said:


> As always +1 to the members of electriciantalk.com!



are you a google plusser ?


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## joethemechanic (Sep 21, 2011)

nolabama said:


> are you a google plusser ?



WTF is that? "Google plusser"???? Not sure I would want to be known as a "google plusser"


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## nolabama (Oct 3, 2007)

its that or circle jerk - whichever you prefer - its googles version of face book 

the +1 is the google version of the "like" hand thing since he +1ed us i figure he is a geek on the google like me:thumbsup:


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## joethemechanic (Sep 21, 2011)

Well, maybe better to be a Google Plusser that a Foamer :no:


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## mutabi (Jun 2, 2009)

Haha Yes I am a Google Plusser. I have no idea what a foamer is??


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