Quick question please...

[dex3361]

Wow, lots of replies - thanks Guys

The reason I posted was getting a headache from thinking about it!

I think the displacement factor wins - I can see now that for the piston to move left 1" would displace 1 cubic inch (actual volume not relevant) but the piston on the right end would try to displace 2 cubic inches therefore hydraulic lock.

Ok although I agree with the above result I want to add a twist - what if the pressure was a constant and supplementry medium was added to allow for displacement increases/decreases?

Lets say it's air and the tank has a compresssor always maintaining pressure and volume.

In this case my first post would be correct.

 

[AKKAMAAN]

Air can be compressed, and do you have a moving force, or is this all in a static state.

So hydr oil cant be compressed??

 

[Kernopelli]

So hydr oil cant be compressed??

Well, that depends on just how picky you want to get on the subject. In general, liquids are "considered" to be incompressible. The atoms are very closely spaced so there is very little room to compress them and make the atoms more dense. The reality though is yes, hyd. fluid can be compressed, however even under VERY extreme pressure the amount of compression is negligible and for almost any scenario it is reasonable to consider that it can't be. This is the basic difference between hydraulics and pneumatics. Pnematics use a gas, hydraulics a fluid/liquid.

 

[J_J]

So hydr oil cant be compressed??

Yes, hydraulic fluid and even water can be compressed, but unlike hyd fluid, water in the oceans can build up pressure by shear volume. The poster set up a static condition. with no pressure. and a solid rod connecting the two cylinders.

How much pressure do you think you would see if you had a gage in the lift cylinders circuit, and were to lift 1000 lbs, and shut off the engine, with valve in neutral?

 

[AKKAMAAN]

Thank you guys for that....
the fact is that liquid compressability play a very important roll in advanced motion control systems.....google "bulk modulus" .......

Hydraulic fluid compresses about 1% in a normal system.......

avreage dept of all oceans are over 12000ft.....and the water compresses so much as 100ft....bye the pressure from gravity.....

This example that started this thread.....100psi will be created by an external force moving cyl to the RIGHT....oil compresses a little and when force are let to balance out, oil decompresses and cyl move back to the left, till we have atmospheric pressure (14psi) in the oil.....how much it moves depends on cylinder dimensions, contained oil volume and oil bulk modulus (compressability).....

 

[J_J]

Apparently I am missing something. Since the fluid and cylinder is in a static state, nothing should happen. If you added pressure to the tank, you will have same pressure on both ends of the piston. If some weight was added to one side of the piston, it might move some, but would then equalize, and stop movement. You now have hydraulic lock. Do the same scenario to your lift cylinder while raised up. Put a 4 x 4 under the bucket, and connect both hose together, and then tell me what is going to happen, when you remove the 4 x 4 . That is assuming that all o-rings are good. and no leakage. You also know that both sides of the piston do not have equal volume, but the pressure will equalize. The weight of the lift arms, plus bucket is very small in terms of pressure. I may be wrong, and I may be right, but I won't know until the dark changes into light.

 

[SPYDERLK]

Apparently I am missing something. Since the fluid and cylinder is in a static state, nothing should happen. If you added pressure to the tank, you will have same pressure on both ends of the piston. If some weight was added to one side of the piston, it might move some, but would then equalize, and stop movement. You now have hydraulic lock.

It is hard to see, but it is only in hyd lock when pushed to the right because pressure builds instantly and continuously as only very slight movement occurs. OTOH, when rightward force is stopped the piston moves leftward only slightly and the pressure goes down to 0 PSIGauge. This is atmospheric pressure - ~ 14.7 PSIAbsolute. If leftward force is applied it will be resisted firmly only while the pressure drops to a full vacuum [0 PSIA]. At this point leftward resistance to motion will no longer increase because pressure can go no lower than 0 PSIA. Zero pressure is an absolute - it can go no lower. The piston will move leftward from this point with no increase in the force pushing it. It will "go soft".
larry