Hydraulics: why not use water?

[chabrenas]

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?

[Landlubber]

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.

[Splint]

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

[robherc]

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.

[chabrenas]

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.

[TeddyDiver]

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

Messerscmidt engines did run reasonable times without oil..

[robherc]

(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.
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!

[PortTacker]

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.

[robherc]

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).

[Guest625101138]

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?

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

Rick W

[PortTacker]

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?


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?

[Frosty]

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

[chabrenas]

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.

[chabrenas]

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.

[Kay9]

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

[chabrenas]

Frosty: I agree with you from the point of view of normal sailing folks. I was just using the bleeding edge of non-stop round-the-world racing as a trigger to investigate whether we got where we are by continuously honing an established technology, or whether there was long-term potential for a different approach.

Many changes come about because the enabling technology matures to a level where it is reasonably well understood. For instance, my own lifetime covers a long period where GRP couldn't compete with wood for serious boatbuilding, particularly in racing dinghies.

[El Sea]

I don't think you can compress H2O....

[mydauphin]

Things are they way they are for a reason.
Physics, chemistry, metallurgy have something to do with it.
Hydraulics is the multiplication of forces by increasing and decreasing pressure upward of 1000 psi to 4000psi.
Oil is more stable than water, does not expand or compress as easily, also it has a wider temperature range. It is also a barrier against corrosion as compare to water or even glycol. I have small diesel Deutz that is oil cooled and it is virtually bullet proof.
Also a properly setup hydraulic system has very little oil usage, a quart of oil might last years.
Now you want something to change the world, a biodegradable oil at a reasonable price, that dissolves into water not leaving a slick.
Biodiesel works good but eats some of the seals and retains water...

later

[Splint]

I don't think you can compress H2O....

I was going to answer your question and say that all gases can be compressed and all liquids cannot be compressed as this is what I have been led to beleive. I though I might just do a quick search before making that claim and found this interesting article on a Physics web site.

Question

Can you compress a liquid (water)?

Answer

The answer is yes, You can compress water, or almost any material. However, it requires a great deal of pressure to accomplish a little compression. For that reason, liquids and solids are sometimes referred to as being incompressible.

To understand what happens, remember that all matter is composed of a collection of atoms. Even though matter seems to be very solid, in actuality, the atoms are relative far apart, and matter is mostly empty space. However, due to the forces between the molecules, they strongly resist being pressed closer together, but they can be. You probably have experienced compressing something as hard as steel. Have you ever bounced a steel ball bearing off a sidewalk? When you do that, the 'bounce' is due to compressing the steel ball, just a tiny little spot that comes into contact with the sidewalk. It compresses and then springs back, causing the bounce.

The water at the bottom of the ocean is compressed by the weight of the water above it all the way to the surface, and is more dense than the water at the surface.

A consequence of compressing a fluid is that the viscosity, that is the resistance of the fluid to flow, also increases as the density increases. This is because the atoms are forced closer together, and thus cannot slip by each other as easily as they can when the fluid is at atmospheric pressure.

That should clarify the situation.
Cheers
Splint

[robherc]

Yes, but for most practical applications you're not going to compress water very far at all beyond it's state at 40*F/7*C (highest naturally-occurring density at +1atm). The magnetic structure of water keeps it from being compressible to solid form, except maybe under EXTREME pressures (millions of PSI).
I THINK that at some point you could compress oil into a solid at room temperature, but it would probably take unreasonable amounts of pressure, too.

[chabrenas]

It hadn't occurred to me that viscosity would go up under pressure (I'm sometimes a bit slow...). I guess that is one reason why the fluid gets so hot.

Do hydraulic fluid developers try to minimise viscosity increase in order to reduce heat generation?

[Poida]

There seems to be a lot of people that think that it is uncommon to use water for hydraulics. It may be more common than you think.

I have worked on water hydraulics, the last time about three years ago.

Powerboats work on water hydraulics. It's the compression of the water on the prop blade that propells the boat, hence hydraulic.

There are reasons for using water hydraulics but I can't think of any application on a boat where it would be an advantage over oil.

And in a pleasure craft I can't see any advantage in hydraulics over electric.

Incidently the heat from hydraulics comes from friction.

There is no good or bad in any system, you use whatever is suitable for that application.

[Frosty]

Thats a point ever felt the bounce of a hammer on an anvil?

[ancient kayaker]

When I was working in a high pressure project I noted that water compressed about 30% at 50,000 psi. In case anyone wanted to know! Cylinders, pumps etc for water-based hydraulics are available but more expensive, but they don't use pure water, it's a glycol mix usually. I was in a research program that was looking at use of pure water for a situation where contamination could not be allowed but I don't remember it going anywhere, lots of technical problems.

[Fanie]

ever felt the bounce of a hammer on an anvil?
Most haven't :D It makes you envy the strength of iron though.

[Fanie]

I noted that water compressed about 30% at 50,000 psi
Good grief, that much ? I know the air in the water allows some compression but I didn't know it would compress 30%.

[chabrenas]

Kayaker:
I noted that water compressed about 30% at 50,000 psi

I'd have guessed around 17%, if Wikipedia is right in quoting 5.1×10 to power (-5) per bar at O°C. Or have I messed up my arithmetic...

Fanie: 50,000 psi (3,333 bar) is a hell of a pressure. Don't stick your finger over a hole in the pipe.

[ancient kayaker]

I think your math is correct: I was quoting something I was told more than 20 years ago from memory. I amazed myself by finding an ancient document on the system, the pressure was actually 55,000 psi and the compression would therefore have been about 20%. That's still quite a bit of compression though.

It was a waterjet cutting machine and yes, it would easily have cut off a fingertip. Rumour has it you didn't notice for a few seconds but I never met anyone who could speak from experience!

Are we off the topic yet?

[Poida]

Hi Terry

You hit the magic word "expense"
The subject of hydraulics in boats has been bantered around by many that obviously do not know the expense of setting up a hydraulic system.

Last week I got an invoice for making up and connecting 8 hoses about 1 metre long. $2,150.00. That's just hoses. Put in a pump and valves? It is really only an option when absolutely nothing else will do the same job.

[Frosty]

Ide guess those hoses where 6 inch diam ,--about 100 quid each?

It all depends on pressure . if your using a steering pump on a boat well,--its just compression fitting stuff.

There is serious hydraulics and there isnt, same as electricity.

[ancient kayaker]

I'm not a hydraulics guy but worked on the electronics of several installations: if the parts being conected are not moving with respect to each other it's a lot cheaper to use the solid tubing with the compression fittings as Frosty suggested. Also, don't overspecify or use a non-standard pressure, use the closest standard.