Need clarification on this regenerative hydraulic circuit

[xlumax]

Hello everyone,
the circuit in the picture have port A and B closed (no fluidi can pass through them) and the actuators are pulled by an external force.
The pressure at A will increase until the relief valve opens up (number 55).
Can the cylinder extend?
My opinion is that it can't move if port B is not connected to tank for example.

 

[LittleInch]

Yes it can as the fluid now goes round in a circuit once the force is sufficient to overcome the relief valve pressure.

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

 

[zeusfaber]

Yes it can - as the pistons move, the additional volume that's created on the bore side of each piston is greater than that collapsed on the annulus side so there's somewhere for the fluid to go. You'll draw a bubble in the bore, so it won't be pretty.

A.

 

[xlumax]

Hi tanks for your reply.
What do you mean by it will draw a bubble at bore side?
Thanks

 

[LittleInch]

That's true. I'd missed that.

What Zeus means is that the volume per unit length of the cylinder is less as it moves left on the left than the same unit length on the right hand side. So you will pull a vacuum on the right hand side.

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

 

[xlumax]

Ok so the cylinder will still be able to move all the way to the left even though I'm having a vacuum at right side?

 

[LittleInch]

Yes, but it will get harder as you now have a pressure difference across the cylinder face giving you a force in the other direction at the pressure your relief valve is set at plus 1 bar.

As soon as the force is released the cylinder will try to go backwards to close up the void, pulling liquid through valve 54 if A is opened or pulling liquid in through port B if that is opened.

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

 

[Stick.]

You'll need to decide if you're ok pulling a vacuum on the fluid or not. Note that this problem will go away if you're able to use a double rod cylinder. In that case, the volume change in each end of the cylinder will be equal, so no vacuum will be created.

 

[xlumax]

I'm still not convinced that the cylinder can extend all the way. The oil needed for cap end to be filled while moving is greater than the oil coming from rod end so will generate vacuum and it will allow only small movement of the cylinders.

 

[3DDave]

It depends on how hard one pulls, but the resistance to extension is the outside pressure applied to the rod area, usually 1 atmosphere, minus the vapor pressure of the hydraulic fluid, a usually small amount.

One is removing a volume from the system equal to the volume of the rod that is being pulled from the cylinder.

Since the external pressure is constant and the internal vapor pressure is constant, after reaching initial cavitation there isn't any increase in force required to move the piston the full distance.

 

[Compositepro]

In this situation, the maximum resisting force that the cylinder can create when pulling to extend the rod is 15 psi time the rod area. Any force applied that is greater than this can extend the rod through its full stroke.

 

[xlumax]

I've tested both configuration, with and without connection to tank using automation studio and cylinder does not move without connection to tank.
Results in the pictures.
Could it be wrong?

Thanks

 

[3DDave]

You have not specified a limit on the force applied to the rod. If enough load is applied to open the relief valve the rod will move and the cap end will cavitate. I miss where in that simulation you have applied a load to the rod.

 

[LittleInch]

Maybe the simulation won't allow you to pull a vacuum? It really isn't a very good idea so the analysis might just stop at that point (-0.1 bar) and say can't be done?

You need to go to prob -1.0 bar before oil starts to pull a vacuum.

Maybe fix the program by allowing air in at 0 bar and see if it likes that instead?

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

 

[xlumax]

It's not visible in the model but the cylinder is inclined downward of 45° and a mass of 5000kg is connected at rod side.

 

[3DDave]

Put 1 million Newtons tension load on that rod.

 

[xlumax]

Whith 10^6 N cylinder started moving and stopped half way, than started to move forward and backward acting as a spring.
The load limit for my case would be less than 10^5 N.

 

[3DDave]

I doubt it was acting as a spring and more that the simulation failed. Set the relief valve to open at 0 kPA and see what happens.