# Hydraulic practical question

If an inert gas is used to force piston with a Pressure say P (Assume the volume of gas to be sufficiently large). I would like to know the following: (consider piston to be horizontal)

1) If say one puts a small volume of a hydraulic fluid in front of the piston. Keeping the same pressure of gas, will he be able to achieve more force off the piston.

2) Go on increasing the volume of fluid and decrease the volume of gas, but keep the pressure of gas constant. What will happen to the force in this case ?

I feel it is the compressibility which should make a difference. But how should I quantify it?

I understand the derivation of hydraulic, but th concept seems impoosible to imagine. How can the same pressure be transferred to the entire huge area generating a huge force differential?

I would like to thank Physics Forums. Cheers!!

256bits
Gold Member
Pressure P whether it comes from a gas or a liquid will have the same effect.
As long as P is constant, the ratio of the volume of gas/ volume of liquid has no bearing upon the force the piston can produce.

Thank you for the reply 256bits. I was relating it to the need of an incompressible fluid in hydraulics. If this incompressible fluid is replaced by a compressible one, wouldn't it make a diffrence on the force. So I still am not completely convinced with the

Nugatory
Mentor
I was relating it to the need of an incompressible fluid in hydraulics. If this incompressible fluid is replaced by a compressible one, wouldn't it make a diffrence on the force.

You are correct, the incompressibility makes an enormous difference.

(For example, if the hydraulic fluid in the brake lines of your car boils from incompressible liquid to compressible gas, your brake pedal goes to the floor without any braking action at all - that's why brake fluid is selected for a high boiling point as well as low compressibility, and why race cars have arrangements for cooling the brakes).

256bits
Gold Member
force = pressure/ area of the piston for a fluid. If as you say the pressure is constant, the force the piston can provide is also constant, and not dependant upon the type of fluid.

Having said that,
If you are worried about the springy-ness of a gas filled cylinder, then as the piston bobbles back and forth the pressure in the clinder is not constant, and neither is the force on the piston.

In a lot of cases the following is where you would use either, though not a full guide:
Pnematics when you need a quick action of the piston and you need the piston to go from start to finsih quickly.You are not too worried about the pressure just as long as the acceleration of the piston matches what you need.
Hydraulics when you want constant speed of the piston. The pressure of the fluid will reflect the force against the piston if the pump is a positive displacement ( it puts out a Q irregardless of the pressure ). Hydraulics have good holding power - close the valve from the pump and the cylinder stays put.

256bits
Gold Member
You are correct, the incompressibility makes an enormous difference.

(For example, if the hydraulic fluid in the brake lines of your car boils from incompressible liquid to compressible gas, your brake pedal goes to the floor without any braking action at all - that's why brake fluid is selected for a high boiling point as well as low compressibility, and why race cars have arrangements for cooling the brakes).

If air is in the brake line you will not get the same pressure to the brake pads. If a car's braking system was set up so that by pumping the brakes one could always increase the pressure in the line rather than increase-release with each stoike the same pressure could finally be supplied to the brake pads.

Train brakes use an air supply, and they seem to work fine on air with a constant pressure from the air brake tank.

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