Need a rough calculation: Cylinder to Servomotor:

[ndzied1]

I hope you don't take this the wrong way but I would stick with the air system. It is simpler and seems to do the job.

Just because you "can" do something doesn't mean you "should" do it. I think the servo system would take much longer to implement, be more complex for the end customer to maintain and require a higher level of knowledge of any engineer required to make changes to it in the future.

If you must go the servo route, I'd reccomend finding someone local to really go over all the physics and controls with you. As others have mentioned, it is not as smplle as replacing one manufacturer's cylinder for another or even replacing a servo motor with a different servo motor.

You're basically trying to engineer a new system. This is not a replacement issue.

 

[Peter Nachtwey]

Torque = Force *pitch/(2*pi*eta)

That formula would be used to calculate stall torque from the stall force. Piscis chose to ignore Norm's request for lead or ball screw pitch. I don't know what good the formula will do without knowing the pitch and efficiency. From the required torque we can calculate the current if we knew the ratio of torque to current. This may be guesstimated but I think going through catalogs of motors would give a better value.

Sometimes picking the right components is an iterative process and one can't just give a simple solution in one post or by putting one set of numbers in a form.

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?

What numbers do we use? If you have the information requested above you may be able to fill it in yourself and that is the problem. Real engineers don't need forms to calculate the minimimum requirements. One may need catalogs to find the best motor once you have the minimum requirements. I bet the forms just make it easier to find the motor/drive combination that best meets the requirements.

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.

Yes, someone finally said it.

But the question I have to ask is why?

Because that is what the customer thinks he wants even though he hasn't done any engineering to consider the results.

This is more of a cylinder application so why use a servo motor?

Don't you know it is now the 'cool' think to replace everything pneumatic and hydraulic with motors, even it is is the wrong thing to do? Seriously, motors are best for continuous duty cycle applications where energy can't be stored. Hydraulics and pneumatics work best in applications like this where there isn't much motion but there is a holdling force required like a press, clamp or mold. Hydraulics and pneumatics do not use energy while the clamp is closed, but motors do. One one need to look through catalogs that can withstand the stall current for time the clamps are closed. This can be done easily enough but the resulting motor size will not be as cheap in the up front cost as the pneumatic solution. The motor will require more energy while clamping.

Just because you "can" do something doesn't mean you "should" do it.

Word of wisdom I often use.

I also know these kinds of applications have been done over and over and over and over again. I searched the internet using 'force torque ball screw' and found
http://cr4.globalspec.com/thread/3553/Ball-Screws-Back-Calculating-Torque
as the first hit. See post #3.

A power point from MIT.
http://pergatory.mit.edu/2.007/lectures/2005/Lead_Screws_Gears_Power_Budgets.pdf
Yes, you can see to MIT course material. It is all on the web.
Some general motion info that may be useful.
http://www.danahermotion.com/education/
That is a lot of reading.

 

[TimWilborne]

Another issue that must be addressed in all clamping applications that involve lifting loads is what happens when the Emergency Stop is pressed or the power goes out.

Yes, there may not be any danger to people and the part may not get damaged if dropped but do you really want the part dropping or getting out of the normal clamping position everytime the Emergency Stop is pressed?

Now you could put a brake on the motor to hold it in position when this happens but that is still not a positive clamping force. It is simply holding the motor shaft where it was when the Emergency Stop was pressed. And in the time from the Emergency Stop being pressed to the brake engaging the motor shaft will back off a little bit and you will loose most of your clamping force.

You can select your valving with pneumatics and even hydraulics to store energy to keep positive pressure on your clamping fixture during this. Yes, eventually the stored energy will bleed off if it was a power outage and the air compressor was down but not nearly at the rate of your servo motor

 

[piscis]

I tend to subscribe to the “Keep it simple Stupid” line of thought. If I thought I could do it with cylinders I definitely will, cylinders are much simpler and a great deal cheaper. Problem is that I have 4 different size grips I have to do.

Imagine I have just place my first bundle on the pallet and I’m just about placing the second bundle right next to it.

• With a servo I just open the grippers just enough to clear to bundle and then lift and retract.
• With a cylinder I have to open the grip all the way to the ends and therefore knocking the fist bundle to the floor

The problem with cylinders is that they only have fully extended and fully retracted. Ok, I know there are fancy cylinders able to do 2 or 3 stops along their full extend, but then we are just getting into the realm of servo here.

Thanks all anyway, good try!!!

 

[TimWilborne]

Compound Cylinders fit the application better and are much more reliable than servo.

Your not even close the the realm of servo applications but whatever floats your boat

 

[Peter Nachtwey]

Show a little respect and don't waste our time.

Thanks all anyway, good try!!!

How can we help if you mislead us or don't give all the facts? Below it says a 1.5 inch stroke.

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.

Another one for my s-list.

 

[Tom Jenkins]

This is a problem in fundamental mechanics.

I suggest you go to Machinery's Handbook or your screw supplier's catalog. There are a number of variables that determine the relationship between torque and axial force, including thread type and pitch, pitch diameter, and coefficient of friction. All of these vary widely.

The force of the cylinder is F = P x A, where F = Force, P = pressure, A = cylinder area.

 

[piscis]

I will follow all of your advices and I will redesign the entire gripper system getting rid of the servo and using pneumatics only. I will post back as soon as I'm done with the drawings.

Thanks for all your answers.

 

[TimWilborne]

I'm glad. I think you will be happier and have a happier customer in the long run

 

[Bazoo63]

There's nothing wrong with a servo positioning system, since you would be more flexible (setting for different positions etc). But I know from similar applications that pneumatics is hard to beat in regards of low inertia, high speed, low investment. to size the servo motor you have to do a proper calculation to establish the torque and the speed, which is not difficult once you got the input params such as weight force and friction of the moving parts plus the desired velocity.

 

[piscis]

Done!!!



Thanks all for your great input; I truly feel more comfortable with the all pneumatic design.



Thanks again!!

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