Hydraulics: why not use water?

I see that we have people with signifcant knowledge and experience of hydraulic systems around here, so let me ask a tyro question:

Modern hydraulic systems all seem to use specialised fluids, typically mineral oil based. Why? Is this choice really necessary for low duty-cycle applications?

My very limited knowledge of hydraulics suggests that hydraulic fluids are designed not to degrade or boil at the high temperatures attained during heavy, repetitive use (I know what it feels like to boil brake fluid on a car with inadequately-ventilated brakes descending a mountain pass too quickly).

The reason for going back to square one and asking this very basic question is the failures seen recently in canting keel rams during the Vendée Globe and the Volvo Ocean race. Does anyone here have detailed technical knowledge of the systems these boats use?

It seems to me that the rams should only be used to move the keel to a position where a physical detent can latch it in place, but I suspect that's not how they work. If not, why not?

Back to the basic hydraulic engineering question: if a hydraulic system had a very low duty cycle (like a minute or two during each tack, on a boat which typically takes half an hour to tack) would a suitably-designed system running on seawater be adequate?

 

For operations, yes, water will work, in fact Teleflex Morse make a water based actuator, but it has of course glycol type inhibitors.
Mineral oils are lubricating, and prevent rust. Water is not lubricating for metal contact, the polished metals can corrode very rapidly, corrosion causes pitting, pitting causes leaks on shaft seals and so on down the road.

That is why Oils are used generally instead of water.

 

Hydrolics when they were first invented used water. Oil though, is the obvious choice because of it's lubricating and corrosion prevention properties.

 

If you used seawater, EVEN if you used plastic/rubber sealed, non-corrosive, equipment, you'd still have the problem of foreign matter in the seawater. If you used purified water, that could work though...but you'd still prob. be better with metal & lubricating oil.

 

Thanks. The reason for asking about water was to avoid having to carry spare fluid in long distance races like Vendée Globe. Even if you had to use fresh water from the desalinator, it would be an advantage.

Agreed, oils are required to lubricate metal surfaces. Is anyone here a modern plastics or ceramics expert? (Lets pretend this is a military project, where costs are irrelevant - and assume that if it works the price will come down when it hits the mass market).

I love this forum - you can propose all kinds of engineering projects with a good chance that someone has the knowledge to discuss them seriously.

 

Lead Bronze is "self lubricating" and doesn't corrode..

Messerscmidt engines did run reasonable times without oil..

 

Well, I'm not a plastics/ceramics expert, but I AM somewhat of an expert on military projects...rofl
The US military, at least, buys EVERYTHING from either the lowest-bidder, or the most-connected (with someone in the government) bidder...rofl

Anywise, I think it'd be feasible if you used silicone-impregnated seals on plastic-lined cylinders (maybe the rest of the cylinder would be a good place to use the ceramic)


Good Luck!

 

1. Lubricating qualities to reduce friction (re: heat.)
2. Lubricating qualities to reduce friction (re: wear.)
3. Better high temp heat transfer/temperature stability properties.
4. Higher Viscosity. You'd need very tight tolerances to get water to do the work, and then #1, 2, and 3 rise to the fore very quickly.
5. Anti corrosion properties. Put disimilar metals with water and see what happens.
6. Lower temp operation. Water freezes at common temperatures.
7. Algae growth etc.
8. Cost. Oil is just cheaper. By the time you engineer for water, add chemicals to help with lubricity and corrostion resistance and anti freeze etc, oil is Much cheaper.

But water hydraulic systems do exist for some special apps and might be gaining popularity. Green, y'know.

 

PortTacker-

I think the conversation has moved on a bit...we're now talking about a "special app" and trying to figure out efficient ways to engineer directly FOR the use of water as a hydraulic fluid.

...that's why I was suggesting silicone-impregnated (self-lubricating) O-rings and plastic-lined ceramic cylinders. Also, water's thermal expansion, and freezing properties are reduced significantly when under pressure...though freezing would still be a problem if the system were left un-drained, and un-pressurized in sub-freezing conditions (unless additives were used).

 

Or you could use one of these and avoid the fluid completely:
http://www.precisioneng.com/products/actuators-all-electric/act2100

Rick W

 

Re the Vendee and Volvo boats, from what I've read most of the system failures (as opposed to keel struts snapping) haven't really been the hydraulic parts themselves, but rather the anchor points for the parts, such as the mounts for the rams snapping off, or the 'hinge' for the keel itself. The loads are incredible on a strut-mounted weight held horizontal, and combined with the forces of forging (and Slamming!) through the waves the instantaneous leverage forces must be unimaginable. The designer/engineers clearly don't all have it right yet.

Some of the systems do have some sort of latch to hold the keel in place, but I don't think any swing then latch, I think they are only to hold the keel centered if there's a failure. Lashing it in place doesn't seem to be hot setup... I also wonder if the shock accumulation factor of hydraulics helps cushion the metal bits from fatique breaking, or if once canted it's rigidly in place?

 

You are reinventing the wheel. Hydraulic rams are mass produced and work well with no leaks and are considered reliable enough to use in the marine industry for many applications.

Re designing the rams with sicone or ceramics would be for what advantage? forgetting to take some oil with you. If you are in trouble and need oil to top up, then absalutely any oil, even cooking oil, anything will get you home even diesel fuel and you will have a lot of that

Ide rather be stuck in some far away port looking for hydraulic oil and some common seals than special ceramic seals.

However standardising oils on a boat is a good idea auto trans fluid for the tranny, shaft seal bath, and steering is what I carry

 

PortTacker: True, most of the serious failures have been collapse of mounting points. I am amazed at how short a lever they all use. And you're right, the shock loads when driving to windward in steep seas must be horrendous.

However, I've read of several people replacing rams and then replacing the seal in the failed one so that it's available as a spare. Again, the shock loads seem the likely cause - a bit like using a JCB (backhoe) to cut up rocky ground 24 hours a day for a month.

Rick: good point. Why do they all use hydraulics? Either way, the primary energy source is a diesel engine (although these boats have quite a lot of solar panels, too). How do energy losses compare:

(generator ==> electric actuator)

versus

(pump ==> hydraulic ram)

I principle, you can use a hand pump to drive the hydraulics if you have no power, and I think some of them do - particularly if they use keel canting as a way to right an inverted hull.

 

roherc: you're right about military contracts. Perhaps I should have said Intelligence. I once worked with a guy who had spent some timpe as a prototyping tech for MI5.

 

The US Military has a base somehwere in Alaska where they test every pice of equipment the US Army uses.The General and chief engineer of that base was on the Discovery channel the other day saying "We spend 90% of our time proving that water freezes at 32 deg F"

K9