If liquid is not compressible, why do we need special hydraulic fluid? Why not just use water?
In: Engineering
Liquids in general can be compressed, the claim is that water can not be compressed, but even thats not realy true, its just hard and can not be compressed a lot. But water can be more dense and less dense.
Water’s properties aren’t great for hydraulic fluid. For example it’s not very slippery so the cylinders won’t slide as smoothly. Also it’s reactive, the metal hydraulic cylinders will get rusted out if you used water in them, not the case with inert hydraulic oil.
I am not an engineer, but there’s a number of issues with water: Rust, High-ish temperatures (water boils at lower temperatures than oil), and Lubrication.
Hydraulic fluid nowadays tends to be a type of oil with a bunch of additives, but before it was popular (the 1920’s) they *would* use water. ([wiki link](https://en.wikipedia.org/wiki/Hydraulic_fluid#Composition) for further reading)
Water causes rust, does not lubricate components like oil does, *expands* when it freezes (breaking hoses and fittings), produces disruptive steam at high temperatures, and evaporates after it leaks, making it harder to notice.
A couple of things.
Water can cause corrosion – oil doesn’t.
Water can’t be used in environments where temperatures fall below 0C or go over 100C, oil based fluids can do both.
There is also something called lubricity – how well a liquid lubricates machinery. Water has a low lubricity which means it can cause additional wear and tear on machinery compared to oil based fluids.
But you can use water – especially if you need a lot of fluid. In fact, there were once hydraulic networks in several cities that used water. London’s used river water, pressurised it and delivered it across large parts of the city through cast iron pipes. It allowed companies to have powered machinery long before electric motors and an electrical grid became reliable. It was used to drive lifts, cranes, industrial presses, open dock gates and bridges – there were thousands of users. It operated until the 1970s.
1. Water freezes at 0°C. It stops being a liquid, which is very bad for hydraulics. 0°C isn’t exactly an extreme temperature you’ll never encounter.
2. Liquids are compressible, just much less than a gas. Hydraulic fluid does so less.
3. Water is actually pretty corrosive compared to many other liquids.
Pure water is slightly corrosive, the oils they use for hydraulic applications are not. They also double as a lubricant.
… not an answer, but were you listening to Distractible when you thought this question?
Water freezes, so it couldn’t be used in any equipment exposed to freezing climates without destroying the equipment.
Water doesn’t lubricate like oil would.
Water heats up too fast when exposed to those pressures.
Oil won’t react with/cause rust in the metal cylinders.
Adding to the other answers, commonly used hydraulic fluids are actually more compressible than water. The bulk modulus, a measure or resistance to compression, is typically 1.8 GPa for common mineral oils and 2.2 GPa for water. Water’s tendency to freeze, boil, corrode and provide poor lubrication rule it out for modern applications though, as the name gives away, it was the original hydraulic fluid.
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