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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Mon, 2 Aug 2004 14:22:00 GMT
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robots@deeleetef3pSTOPSPAM.com
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1190 times
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On Sat, 31 Jul 2004 17:08:08 GMT, "Mark Bellis"
<mark.bellis@tiscali.co.uk> wrote:
> > 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.)
>
> 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'm not clear on this. It sounds interesting, I just can't picture
how it's done. Where does the tube go? Is it acting like a little
variable volume reservoir?
> > 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.
>
> If TLC did make hydraulic parts, each valve would need to be a reverser with 4
> ports, with the fourth port returning fluid to the reservoir. The pump would
> need to be immersed in the reservoir to fill with fluid. Air bubbles would also
> be a problem.
>
> The system would have to use a non-toxic fluid such as water, since it's meant
> to be a toy. Hydraulic fluid is also highly flammable and brake fluid is the
> most flammable liquid in your car (drip petrol onto the exhaust manifold and it
> smokes, brake fluid flares). Unfortunately, water is more compressable than
> hydraulic fluid, but for a toy this shouldn't matter.
I agree completely here. Hydraulics would be *much* more
complicated, although having a 5-ported 4-way valve with flow controls
would make for some interesting effects, even in pneumatics (For those
who don't know valving, a 5-ported valve has 1 input, 2 exhaust, and
two cylinder ports. Combine it with flow controls and you can control
the extend and retract *speed* of a cylinder, so you can make them go
slower in one direction than the other).
> Is it true that water is used for point to point hydraulics but only oil is used
> for variable movement? This would make sense if water is plentiful but
> compressable. Are there examples of variable water-controlled hydraulic
> systems?
I'm not really up on water hydraulics. I know that it's out there and
enjoying something of a renaissance, but I'm more of an air person.
> I once tried using a non-return valve from the old pneumatic system to pump
> water from a tank to a hose. The valve soon expired! I don't recommend the use
> of water or any other hydraulic medium with Lego parts.
What was the failure mode? From what I've looked at, the parts should
be ok in water, unless it dissolves the lubricant (not sure what the
lubricant is). I'd guess that the seals and other rubber parts are
just buna, which should be fine with water.
> > Here's some info I have determined about some LEGO pneumatic parts.
> > These are the current cylinder and pump designs. All dimensions in
> > inches.
>
> Useful info! I think Kevin should use this to calculate the number of cylinders
> for his scorpion.
It shouldn't be too difficult to work up some spreadsheets to do
calculations, it's just a matter of determining what you want on the
inputs and outputs.
> > 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 think it is. The ones I have aren't.
I looked at mine over the weekend and for some reason I was thinking
the small pump had two ports, but it doesn't. It's only single
acting, so the other direction doesn't do anything usefull.
> > 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.
>
> I run Lego pneumatics off a car tyre air compressor at 25psi (30psi for short
> periods only). I think much more than that would break the seals. Therefore get
> the one with the variable pressure limit if you can.
I didn't test the cylinders. I just hooked the pump up to a small
reservoir and a pressure gage. Chances are that at 50-60 psi you'd be
blowing hoses off the fittings if you just use the soft tubing.
I like the tire compressor idea. I'll have to look for one. I have a
big old tank that I got at an auction that will hold *lots* of lego
air, but I don't have a compressor at home. Yet.
-Jon
Mindstorms, Mindscript, Pneumatics, FLL and more.
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Message has 1 Reply:
 | | Re: Pneumatics Vs. Hydraulics
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| (...) Take two valves, two 2cm tubes with an antenna stuck in the end, a 8cm tube and input and output tubes. Arrange the valves so that they switch together, using "1" joints on their levers, connected by a liftarm (centres 32mm apart for old (...) (20 years ago, 2-Aug-04, to lugnet.robotics)
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Message is in Reply To:
| | 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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