Subject:
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Re: Pneumatics Vs. Hydraulics
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 29 Jul 2004 14:29:15 GMT
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Reply-To:
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robots@DEELEETEf^saynotospam^3p.com
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938 times
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Some of this is not right, some is not very clear. I've been working
in fluid valve design for 14 years. Let me clarify a couple things.
On Wed, 28 Jul 2004 05:28:11 GMT, lego-robotics@crynwr.com (Mr S)
wrote:
> I don't think that pressure shares the same fluid
> dynamics as liquids.
First, some definitions. Pressure is a force, fluid is a type of
medium, and liquid is a type of fluid. To say "pressure shares the
same fluid dynamics as liquids." is like saying that voltage shares
the same properties as copper. I think you meant "I don't think that
gases share the same fluid dynamics as liquids" which is correct, they
don't. Similar, but with differences.
> Certainly on the larger scale,
> fluid works much better at evenly distributing
I think "fluid" here must mean hydraulic fluid, a liquid.
> pressure even though pnuematic systems are capable of
> very strong movements indeed. The viscosity of
> hydraulic fluid is such that it does not compress well
> where air does compress.
Yes. This is one of the fundamental differences between hydraulic
(liquid, whether oil or water) and pneumatic (air or other gases)
systems. Hydraulics are essentially incompressible.
> Even on a smaller scale, the
> air must compress a certain amount before it fluidly
> distributes pressures, and that is the reason that
> LEGO has the air tanks. The more stored pressure that
> you have in the form of compressed air, the more work
> you can do with the LEGO pnuematics.
No. Compression has nothing to do with distribution of pressure. The
pressure is distributed in a fluid system because of the nature of the
fluid to exert pressure evenly in all directions, not because it
compresses.
The air tanks are used as a reservoir to store volumes of compressed
air, because each actuation of a cylinder exhausts some air into the
atmosphere. The air tank in an air system is similar to a water
tower. It stores air and allows you to have a smaller compressor
because the tank evens out the system demand.
> Fine adjustments
> are capable with hydraulic systems and pnuematic
> systems are used generally in open or closed
> situations. If you have a need to have the valve half
> open, hydraulics is your best bet. There is the
> differences in a nutshell.
Fine adjustment and position feedback are difficult in both hydraulic
and pneumatic systems. The compressibility of air further complicates
pneumatic systems.
Having the valve half-open may or may not work in a given application.
A half-open valve is a crude flow-control. But granted, it is easier
to control flow in a hydraulic system with a half-open valve than in a
pneumatic system. Not ideal, but it can work. (Upon re-reading this,
I realize that it really depends on the valve design. Some valves are
designed to use as flow controls, but most aren't. Lego valves are
not.)
> It is not possible to build hydraulic systems on the
> scale of LEGO for the cost of LEGO. Some modelling
> enthusiasts do have small scale hydraulic systems, but
> they are NOT cheap.
This is probably true. Hydraulics tend to be more expensive than
pneumatics, and more complicated. You have more of a mess to deal
with (ever spill/spray hydraulic fluid all over a room? How about
spraying air all over a room? Which is the bigger mess?) with
hydraulics. Your system needs to be tighter to eliminate leaks. On
the other hand, you can operate at far higher pressures with
hydraulics. A typical air system runs at around 100-250 psi.
Hydraulics go upwards of 5,000-10,000 psi.
> BTW, you can make your own air tanks. The air tank in
> the LEGO pnuematic systems with the two port actuators
> is really only there to supply stored pressure. With a
> 12 or 20 ounce plastic soda bottle and some silicon
> sealer, you can make a huge air tank. This makes it
> possible to build a LEGO pnuematic machine which has
> the capacity to do all the work needed before
> recharging the tank, depending on how much work is
> required, of course.
I've been meaning to do some burst tests on this. It is possible to
overpressure a bottle and make it burst, which is an ugly and
dangerous thing. In general, a smaller smooth bottle should be
stronger than a large dimpled one. You can use multiple small bottles
to get the same effect, it's the volume you need, not the shape or
single container.
> Hydraulic systems make use of a principle of physics
> where a cylinder with a 1 inch square plate on one end
> and a 10 inch square plate on the other, takes 10 psi
> on the one inch side, and applies 10 psi to each of
> the 10 square inches on the other end, thus giving a
> power boost by a factor of 10. This is idealized, but
> close to accurate, and the reason that hydraulic fluid
> is such that it does not compress. Any compression of
> the 'power transfer fluid' and the boost ratio is
> decreased accordingly.
I found this not quite clear, but essentially correct. It works for
both pneumatic and hydraulics. If you exert a 10 psi pressure on a 1
square inch cylinder piston, the *force* that the piston rod exerts
will be 10 pounds. You don't exactly get a "boost" (although there
are products that do boost pressure this way), you get a force. But
there is a force multiplier for each cylinder that you can use to
figure out what force you get out based on what pressure you put in.
The force multiplier is equal to the piston area in square inches (or
similar units if you're working in metric). And you do lose some
power because of compressibility, and you lose some because of the
friction of the cylinder seals. Usually you should oversize the
system by about 20% to accomodate this.
The formula is (input pressure) X (piston area) = (output force)
For the extend stroke of a cylinder, the piston area is the area of
the bore of the cylinder. For the return or close stroke, it's the
area of the bore minus the area of the ram.
> Because of the inherent leakage in the LEGO or any
> pnuematic system, it requires not only air under
> pressure, but large volumes of it, replenished often
> or continuously, while a hydraulic system is a closed
> system that requires replenishment only rarely.
Leakage isn't exactly the right term here. Leakage is for
unintentional losses, like around a fitting or seal. What is being
explained is exhaust. A pneumatic system is generally an open system
where the air is compressed, fed to an actuator, and then exhausted to
atmosphere. So by "inherent leakage" I think you were talking about
exhaust. And yes, pneumatic systems use huge volumes of air.
Luckily, air is cheap.
> Okay, see what the Captain can do?
> The answer is that LEGO does NOT make hydraulic parts.
Chances are if you used real hydraulics with your LEGO parts, you'd
break a lot of parts because the forces could be very high.
Here's some info I have determined about some LEGO pneumatic parts.
These are the current cylinder and pump designs. All dimensions in
inches.
Pneumatics Info
Large cylinder
Bore diameter .472
Ram diameter .196
Stroke 1.101
Extend
Area = .175 sq. in.
Force @ 50 psi = 8.75 lbs.
Force @ 10 psi = 1.75 lbs.
Displacement = .193 cu. in.
Retract
Area = .145 sq. in.
Force @ 50 psi = 7.24 lbs.
Force @ 10 psi = 1.45 lbs.
Displacement = .159 cu. in.
Small cylinder
Bore .229
Ram .125
Stroke .595
Extend
Area = .041 sq. in.
Force @ 50 psi = 2.06 lbs.
Force @ 10 psi = .41 lbs.
Displacement = .025 cu. in.
Retract
Area = .029 sq. in.
Force @ 50 psi = 1.45 lbs.
Force @ 10 psi = .29 lbs.
Displacement = .017 cu. in.
Large pump
Stroke =.976
Bore .472
Displacement = .171 cu. in.
Small pump
Stroke =.667
Bore .229
Displacement = .027 cu. in.
It occurs to me now that the small pump is probably double-acting (it
pumps in both directions). I don't have data for it in the other
direction, but it should be trivial to determine.
I don't have the numbers handy, but I think the large pump can
generate somewhere between 50 and 60 psi. I still need to test the
small pump.
-Jon Gilchrist
Mindstorms, Mindscript, Pneumatics, FLL and more.
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Message has 1 Reply:
 | | Re: Pneumatics Vs. Hydraulics
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| (...) One way to change pressure in Lego pneumatics by a small amount is to switch in a short tube (2cm) with an antenna stuck in the end. This lets only a small amount of air out of the cylinder, whilst not exhausting any to the atmosphere. (...) I (...) (20 years ago, 31-Jul-04, to lugnet.robotics)
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Message is in Reply To:
| | Re: Pneumatics Vs. Hydraulics
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| I don't think that pressure shares the same fluid dynamics as liquids. Certainly on the larger scale, fluid works much better at evenly distributing pressure even though pnuematic systems are capable of very strong movements indeed. The viscosity of (...) (20 years ago, 28-Jul-04, to lugnet.robotics)
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