If liquid is not compressible, why do we need special hydraulic fluid? Why not just use water?
In: Engineering
I was offshore in my mate’s boat, a small runabout with hydraulic steering. About 10km out, circling the fishing spot, the steering became weak, and gradually stopped altogether. My mate (an engineer), diagnosed it as a hydraulic leak, and had some spare hydraulic fluid, which he topped up and we headed in. About half way in, lost steering again but had used all the fluid, so he sprayed out a can of WD40 into a cup and used that. Coming back into Sydney heads, lost steering again, had used all the oil, so he used water. It got us home. Later, he rebuilt the steering.
liquid is definitely compressible, it’s just very difficult to compress it for any reasonable length of time. Very difficult being the key words here
Hydraulic fluid is used for its lubrication and anti corrosion properties. But if you take care of some engineering hurdles you can use water as well.
I used to work in a High Pressure Physics lab and we used to say everything is compressible with enough pressure.
Unfortunately water has a few problems with it. Water isn’t very viscous and the sloshing around of it would lead to erosion of the interior of a hydraulic cylinder. Additionally, water is reactive with steel and would oxidize it causing it to rust.
I know a colleague who works EXACTLY on that. The short answer : water is weird man. The long answer : when you compress water, the results in the way it moves are really unpredictable. Sometime you get vertices, sometime you get a pressure gradient etc. What is even worse for this is that it’s not completely dependent on the system’s specs, you can test the same water-based system 10 and obtain 10 different results.
According to him, it’s because water is a small enough molecule to be on the verge of atomic and quantum physics, not sure about that tho.
It can’t be understated how unique water is on a physics/chem level. Its properties are so unlike any other liquid, most of which make it extremely bad for this purpose.
On top of everything else people are listing, the simplest reason is water freezes at 32°F. That’ll break cylinders and hoses and what not. It also boils at 212°F which isn’t that hot in the context of heavy equipment.
It’s similar to how you can use just water for coolant in your engine, but you probably shouldn’t because the results can be disastrous if there’s a cold snap.
Water is compressible, but the compression is negligible at the pressures engineers typically use.
I studied rheology the science of things flowing. You can think of molecule in rest as spaghetti, all tangled up, doesnt really wanna move through small pipes, and when pressure is added the spaghetti gets straighten out and suddenly it moves more and more easily. This is called non-newtonian behavior. Also like bottle of ketchup in a kar that needs a little bang to get it flowing:). Water molecule spaghetti does not straighten out at all so it does not lubricate well at all. It is called newtonian behavior.
Also plain water can make things rust and act all kind of funky ways 🙂
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