How is this possible the pressure are the same 2

[moobe]

Hello all,

I am curious to find out. Pressure on a hydraulic cylinder is force/ area. But why in extrusion press where there are two side cylinders and one main cylinder attached together have the same pressure when they are extruded.Shouldnt the side cylinder have more pressure than main cylinder pressure?

https://prnt.sc/tz9xkw

 

[BrianPetersen]

Perhaps the circuit doesn't work the way you think it does ...

The smaller cylinders are for rapid-advance. The big cylinder in the middle is for main tonnage.

Step 1: Rapid-advance. Pressure is applied to the center of the three pipes entering the picture from the bottom, the other two are vented to tank. This applies pressure to the cap end of the two smaller cylinders. The big cylinder is "dragged along for the ride" ... BUT ... See that pilot-operated check valve in the middle of it? Fluid is drawn in from the atmospheric-pressure reservoir at the top left of the photo. The two pipes entering the photo from the left are arranged to hold the PO check open to allow this to happen.

Step 2: Tonnage. The PO check closes and pressure is applied to the left pipe entering the photo from the bottom. This applies pressure to the main tonnage cylinder. (The pressure to the smaller cylinder might continue to be applied, but their tonnage contribution is a small part of the total.)

Step 3: Rapid-retract. The PO check opens (to let fluid otherwise trapped in the main tonnage cylinder to vent back to the reservoir), the center pipe coming in from the bottom is vented to tank, pressure is applied to the right pipe coming in from the bottom. The small cylinders drag the ram towards the left, pushing on the main tonnage cylinder, the fluid in it goes back to the reservoir through the forced-open PO check.

 

[moobe]

Thanks, Brain Peteren for your explanation but please see the figure below. When the press is extruding.It is using the force of the main cylinder and the two side cylinder and since the pressure is force/ area why the pressure on TR 20.1 and TR 20.2 is the same. Shouldn't the side cylinder which has less piston area have more pressure compare to the main cylinder? This is my concern,

https://prnt.sc/tzhf57

 

[LittleInch]

Why do you think the force needs to be the same from the small cylinder as opposed to the big Cylinder??

The big cylinder is doing all the work when it comes to the extrusion process. The side cylinders are just there to move the press and the big cylinder. They might as well be used to add force to the operation, but this looks like a small fraction of the force. The big piston doesn't want to have the smaller ones resisting the force.

I don't think you are quite understanding what the cylinders are doing.

Think of this like weightlifting.

The really big guy lumbers onto the stage.
Two smaller guys lift the bar and the weights onto a stand.
Then the big guy lifts it off the stand with the two smaller guys standing by in case he drops it.
In this case they are the same as helping the big guys lift the weights, but on their own they couldn't lift it.

Now that's not quite the same but I think it works to understand the principle here about what is going on.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

If the cylinders are hydraulically connected at a common point, they will have the same pressure.

Ted

 

[Compositepro]

It appears that the large cylinder is single acting. So the small cylinders are used to retract the large one and follow it in extension.

 

[PNachtwey]

It appears that the large cylinder is single acting. So the small cylinders are used to retract the large one and follow it in extension.

Yes, many presses are designed like this. There is a fill valve that allows the big ram to suck oil , from a non pressurized but usually gravity fed tank, into it while the small cylinders are pushing the the platen to which all cylinders are connect. I have heard different names for the smaller cylinder. Jack cylinders or kicker cylinder are names I have heard. Usually one servo quality valve is. When the ram is extended to the point that pressure starts to increase then the fill valve is shut. The controller must switch to a force or pressure control mode bit controlling the force is relatively easy because the main ram requires a lot of oil to change the pressure much.

This is an example of a press with two 8 inch diameter jack cylinders and a 20 inch diameter main ram. I did this back in 1998.

https://deltamotion.com/peter/Pictures/RMC100/lawton.png

The first 10 seconds the pressure is closing.
The fill valve is shut so now the speed must be reduced.
The motion is not optimal because the platen should have moved closer before slowing down. That would reduce about 3 seconds from the cycle time.
At about 13 seconds the actual pressure ( gray ) starts to increase.
The actual position ( red ) cannot keep up with the target position (cyan).
During this time the control output ( green ) increase a lot because because the main ram was sucking in oil at atmospheric pressure and oil must be added to increase pressure.
When the actual pressure ( gray ) reaches 1500 psi the controller switched to pressure control mode.
The white line is the target pressure that ramps up to the final set point at 2000 psi.
Notice that the actual pressure follows the target pressure almost perfectly. Also notice that control output is proportional to the pressure ramp rate.
After holding pressure for about 2 seconds the target pressure is ramped down in a controlled manner. This keeps any air pockets in the material being pressed from exploding.
Sometimes the pressing and releasing is down a few times to press out any air in the material.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/