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 21:57:20 GMT
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Viewed:
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1188 times
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In lugnet.robotics, Jon Gilchrist wrote:
> On Sat, 31 Jul 2004 17:08:08 GMT, Mark Bellis 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?
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 valves, can be 24mm
for new ones).
Connect the 8cm tube between the centre nozzles of both valves.
Connect the 2cm tubes to the left nozzles of both valves.
Connect the input and output tubes to the right nozzles of the valves.
Connect a pump to the input tube
Connect a cylinder to the output tube.
With the valves in the left position, air flows in, through both valves and to
the output. Build up pressure in the cylinder with the pump, ideally have the
pump lift something.
With the valves centred, no air flows. The cylinder retains its pressure.
With the valves in the right position, the air at room pressure in the 2cm tube
of the output valve relases a bit of the cylinder pressure, but not much. The
input valve stopper ensures that there is no leak of the air supply to the
atmosphere when the valves are in the right position.
> > 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.
I think the rubber bits failed to re-seat due to excessive force of the water.
I think I also expired my two oldest switch valves by them being forced apart.
They need the pressure of the plastic parts to keep the seals air-tight.
> > 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
My compressor was £20 ($35?). The one without a pressure limiter was £15. The
output hose gets warm after a while. My compressor came with a football
inflator nozzle. Put a hose onto that, perhaps with an elastic band round it to
give it extra hold under high pressure.
You'll need a 12V 6A power supply for the compressor - I use an ex-computer
switched mode PSU! A small lead-acid or gel battery might do for mobile models,
with a pressure switch to turn the compressor off when pressure is sufficient.
Use flex tubing to reduce the amount of air lost when switching valves, to
maximise efficiency.
Mark
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Message is in Reply To:
 | | Re: Pneumatics Vs. Hydraulics
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| (...) 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? (...) I agree completely here. Hydraulics would be *much* more complicated, (...) (20 years ago, 2-Aug-04, to lugnet.robotics)
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