Submitted by bengy5959 t3_xz2esu in askscience
Could you lift a house with a sufficiently large hydraulic cylinder and a small hydraulic pump? If hydraulic force is just pressure times surface area of the cylinder, what happens as you increase the size of the cylinder? Does that put more force back on the pump until it breaks?
In principle, you could.
The force on the pump only depends on the pressure (for the pump of a given size), so that is fine.
Larger piston just means you have to pump large volume of fluid, so you have to be patient.
Of course making a sufficiently large hydraulic cylinder and putting it under your house might turn out to be impractical.
Note that for a piston to sit securely in a cylinder without tipping, it has to be sufficiently long compared to the diameter of the cylinder. If you start making huge diameter pistons, you'll need them to be long, too. The volume of the piston would become huge, and it would be heavy and expensive. And hard to fit where you need it.
That might mean that you'd want many moderate size pistons instead of one giant one to lift a house. You could pipe them all the the same pump, and you wouldn't need any extra pressure because of the many pistons connected, assuming they are plumbed in parallel. You'd just need many pump strokes to move them a significant distance.
Laplace's Law - the greater the radius of a cylinder, the greater the wall stress. More formally, stress = Pressure × radius / thickness as long as thickness is less than 10% of radius.
To generate a very high force, you would need very high pressure and radius, which in turn means you either need very thick walls or very strong material to prevent failure. Neither is impossible but both are expensive.
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This is how Chicago was raised. It was screws instead of hydraulics, but the principal is exactly the same - not one big jack, but many small ones.
I think the limiting factor is kindof as much a practical one as it is a material one. A relatively tiny pump will take an impractical amount of time to do a job, because there will be an operational speed limit for the pump.
In practice, when houses are lifted with hydraulics, it is with many small cylinders because the load map underneath a house is not uniform.
[deleted] t1_irk8o7i wrote
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