Need a rough calculation: Cylinder to Servomotor:

piscis

Member
Join Date
May 2003
Posts
241
I have an application in which two small cylinders with 2” bore and 1.5” stroke with 80PSI of air is gripping and lifting a part.

I need to replace these cylinders with a single servo motor directly coupled to a twin acme screw.

Question:



Can anyone give me a hint on how to convert the cylinder force to servo motor?



How big of a servo do I need to equal the force these two small cylinders have?



Do you think 200 watts servo motor will do or perhaps 400 watts? This application is not for NASA, so a rough estimate will do.



Thanks
 
You need to give details of time as well - how long does it take for each cylinder to move through it's stroke and how often does it do it ? The force available from each cylinder is approx 1120N, but that's not much use without more details.
Is the actual magnitude of the force applied by the cylinders critical. Getting an accurate controlled force from a motor/screw needs careful consideration - I'm sure some more advice will be forthcoming.
 
Last edited:
This is a crawling application, I have 5 seconds to perform the gripping action and 15 second for repeating it.

The Acme screw is 3/4 inch in diameter and 45 inches long.
 
You need more.

  • What is the lead of the acme screw? (mm/turn).
  • How are you driving the screw? Direct vs. gearbox vs. timing belt? If not direct, you need to know the inertias of each of the elements in the drive train.
  • What power source will you be running on?
  • What is the friction of the screw?
  • When you say crawling application, does that mean your gripper is holding onto something and then the "body" of your device will move forward? In any case, all the forces imparted to the gripper fingers during the cycle need to be considered and calculated back to torques at the servo motor.
  • You mentioned lifting. What do you want to happen when you loose power? Does the motor need a brake?
A picture would be worth a 1000 words here!
 
The Gripper will grab different size boxes. Let me know if you have more questions after seing the photo below.
 
Forget about trying to calculate the size of the motor based on the size, stroke and pressure of the cylinders used in the present design. You need to evaluate the mechanism you're going to be using.

I'm afraid your picture alone isn't as much help as you hoped it would be. You mentioned lifting the parts, but your picture looks like a horizontal machine. Are the four vertical devices the grippers? Is the motor you're trying to size the one connected to the horizontal screw and shown in red or one of the four green things on each of the vertical devices?
 
While selecting the servomotor, you have to consider the stall torque of the selected servomotor. All the connected components of the system should be capable of withstanding this torque, components like shaft key etc.
 
I appears that your 'lifting force' is being generated by air pressure on the rod side of the cylinder. Be sure to subtract the rod area from the piston area before calculating the lifting force with 80 PSI air.
 
I would really recommend you go to this site:
http://www.danahermotion.com/design_tools/sizing_selection.php
And get the Motioneering software. It's free.

It will allow you to model your mechanism and prompts you for all the pertinent information required to size a servo system. Even if you don't want to use the Danaher motors, you will be able to determine the torques and speeds required for your application.

Based on your previous description of using a LH/RH acme screw and looking at the picture above, it looks like you are trying to clamp a box between the two angle plates and pick it up with 4 suction cups. You might have to model only half the screw from a motion standpoint but be sure to include all the forces and inertias in your model.

Right now it sounds like you are shooting in the dark.

<disclaimer> I work for a Daneher distributor.
 
One more thing to know

With your air cylinders doing the clamping, they do not care what size the box is. They just run into the box and stop with whatever force the air regulator is set for. You can even use a pressure switch to tell you when the clamps are in contact with the box.

Depending on the mode of operation, a servo system might care very much that it runs into something. You have to consider this especially if you are tyring to go to specific positions. To really act like an air cylinder you probably want torque.

Then you have to figure out how to limit the speeds and accelertaions (while in torque mode) to get the motion you need.

These are all doable but must be considered in the selection of the components.
 
Thanks you all for your great insight and support. I believe I have complicated the issue myself. To clear things up I have posted all the drawings to my website with better images than those permitted in this forum.

My customer wants to replace an old dedicated palletizing system with a more versatile palletizing robot.

Here is are images:

http://www.telemetrika.com/RoboticsPage8.html


http://www.telemetrika.com/RoboticsPage9.html

The first photo on the link above shows the first prototype I did to show the customers that a single flap style gripper will work much better than a Vacuum Style Gripper (at least for his loosely shrink wrapped, 40 lbs. bundles)

The testing of the prototype came out with excellent results; we were able to lift all of the 3 different bundle sizes. (For those of you with high speed internet, you can see me lifting one of the bundles in the video)

Since I was able to lift this bundle with those small pneumatic cylinders I figure there must be a way for me to pick a Servo Motor able to do the same job.

I know there are many companies selling servo system eager to help you and do the motor sizing for you, I prefer to acquire knowledge first and then call these guys, and this is why I’m requesting your help, to get the knowledge I need.

I hope that the drawings have answered most of your questions, please let me know if there is anything else I’m missing.

Thanks all!!
 
ndzied1

Could you or any other brave souls dare to fill all the fields required using that sizing software called Motioneering and post the results along with the data fields used?
 
I've been watching this thread and really haven't wanted to get in it because there are so many variables.

If you were replacing a servo motor with a cylinder it would be much easier. There are so many variables and things to overlook. Not only must you have enough torque to equate to the cylinder's holding power, but you must look at stall torque, how long the load must be held, the duty cycle of the machine and much much more. It is not just a matter of this cylinder puts out this many lbs of pressure so we need a motor with this much torque.

But the question I have to ask is why? This is more of a cylinder application so why use a servo motor?
 
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