eli5: If water is non-compressible, then why not use it in hydraulic machines instead of hydraulic oil?
In: 1817
I’m not even close to be an expert but I think it’s cause the boiling temperature. Most machines that require use of hydraulics work at higher temps. Water boils at 100°c and becomes gas that is pretty compressible.
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Water changes density quite a bit with temperature which would affect the performance of the system. Water is also highly polarized and thus may cause corrosion and rust to form in the system. Using an oil helps both with keeping a more stable density profile over various temperatures and is less reactive than water.
Water has a few major issues as a working fluid:
1: it’s corrosive to metals and will dissolve seals and sealants.
2: it gets weird below 0C.
3: it gets weird above 100C.
We’ve engineered various hydraulic fluids to avoid these problems. You wouldn’t want to have your hydraulic cylinders all blow apart the first time you get a light frost overnight.
Water is corrosive and non-lubricating. So if you get water into your hydraulic oil then you get a lot of rust and grinding which reduces the lifetime of the equipment. It is possible to make water based hydraulics, this is essentially what the mains water supply is. However this requires other materials like teflon, PVC and stainless steel. These are more expensive and wear out faster. Especially for the pressures of a typical high power hydraulic system. And some designs such as gear pumps are not feasable with these materials without lubricant.
Hydraulic fluid is more than non-compressible. It’s also non-corrosive, a sealant, lubricant, and coolant for the machinery it’s inside. Water does not have all of the properties required for modern hydraulics.
Early hydraulic machines created thousands of years ago did indeed use water as a hydraulic fluid, but our needs evolved with our technology.
Oil is a lubricant itself, water machinery tends to have short lived seals in moving parts because water makes them seize by washing away the lubricants.
Water freezes, that’s something you really don’t want in a system.
Water is corrosive to certain metals.
Water boil sooner than oil, making problems to hot machinery.
Water boiling point is very low if you reduce the pressure below atmospheric one, this leads to vapors in some parts of the system, and a lot of cavitation in the pumps, cavitation is predominant in low life expectancy for valves and pumps, it’s the biggest factor in the deterioration of those. This doesn’t mean you can’t avoid cavitation, it’s that preventing cavitation requires more complex machinery.
Water-based hydraulic systems do exist but they’re quite unusual.
There are several reasons.
The first is water is corrosive, thus a hydraulic system using water would require expensive stainless steels to avoid rusting and pitting, leading to leaks. Not to mention the fluid becoming extremely dirty from fine rust particles.
Using various kinds of oils allows use of cheaper normal, non-stainless steels.
However the shank of hydraulic pistons is often plated with a layer of chromium. This both resists corrosion on outdoor equipment, but also resists abrasion from ambient dust and dirt. Chromium is one of the hardest elements on the periodic table.
The second reason is lubrication. Oils have better lubrication properties. Thus sliding parts like rod and piston seals and wear bands last longer using oil. This is one reaon why hydraulic systems are very reliable.
The third reason is freezing. Water unlike most liquids expands with extreme forcep when it freezes. This could cause fluid lines to crack, seals to burst, or fittings to separate in subzero weather. In contrast, hydraulic oils contract and form a slushy gel when they freeze, which at worst causes the system to stop moving since fluid can’t flow into various mechanisms. It may also cause small low pressure vapor bubbles to form in confined spaces. This isn’t really an issue.
Other people pointed out the primary issues. But I just wanted to correct some thing. Water boils/freezes at 0c/100c at normal sea level pressure. As pressure increases the boiling point goes up and freezing point goes down (though it gets weird in very high pressures). So if the system stays pressurized the water will stay liquid. Until a leak forms and the water starts vaporizing, and if it drops enough you have a bomb.
Because water is corrosive, promotes the growth of algae, has a narrow operating temperature range, will seep through the narrowest gaps and has no significant lubricating properties.
Hydraulic oil is much better at all of these things. The only advantages of water is that it’s cheap, readily available and any spillages won’t have any environmental impact.
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