Subject:
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Re: pneumatic cylinder: why not hydraulic ??
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Newsgroups:
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lugnet.technic
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Date:
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Thu, 8 Apr 2004 20:37:00 GMT
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Viewed:
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2734 times
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In lugnet.technic, Kevin L. Clague wrote:
> In lugnet.technic, David Laswell wrote:
> > In lugnet.technic, Harro de Jong wrote:
> > > 7. if you used hydraulics, a cylinder would extend or retract fully with
> > > one stroke of the pump.
> >
> > Not necessarily. Assuming both cylinders are the same size, a full pump
> > cylinder stroke will only result in a full working cylinder stroke if the
> > working cylinder doesn't have significantly more resistance than the pump
> > cylinder. If the working cylinder is trying to lift a heavy load, two or more
> > pumps might be required to generate enough psi to lift the load.
>
> It is my understanding that fliuds cannot be compressed. So a cylinder hooked to
> another cylinder with fluid in between (assuming no leaks), would always provide
> 1 to 1 movement.
>
> I'm not an expert in hydraulics, but I think your claim above only applies to
> pneumatics, because gasses can be compressed.
>
> >
> > > You also couldn't use compressors anymore.
> >
> > Again, not necessarily. Theoretically a fluid with very little viscosity might
> > be pumpable by compressor, assuming you can get away with submerging the
> > compressor. The problem you'd have is that you'd need to keep the motor well
> > removed from the area, and running a drive shaft is the only way I can think of
> > that would prevent the hydrolic fluid from getting sprayed all over.
> >
> > That still doesn't answer the question of why a hydrolic system would be
> > preferable to a pneumatic system.
>
> Hydraulics are preffereable in some cases, because the fluid cannot be
> compressed. This allows more direct control over the articulated pistons. For
> things like steering, hydaulics is preferred over pneumatics because you get 1:1
> movement, no matter how much the wheels don't want to turn.
>
> Kevin
The amount that a fluid can be compressed varies with the fluid, the
temperature, the pressure and other things. Water can be compressed too much -
that's why trucks don't use it for their hydraulic systems. Hydraulic fluid is
special oil that was deliberately developed so that it could not be compressed
very much at all.
Hydraulic fluid could not be used with Lego for three reasons: firstly it's
extremely flammable (brake fluid causes more car engine fires than petrol or
diesel - just drip it onto the hot exhaust manifold and it creates a fireball),
secondly it would be likely to weaken, corrode or dissolve the plastic and
thirdly it would give us (and children!) dermatitis!
Power steering is called Power-Assisted Steering because the fluid must be
compressed by engine power in order to help turn the wheels. I don't think any
fluid is 1:1, but proper hydraulic fluid is as near that as is possible. With
brakes, the brake master cylinder is bigger than the four slave cylinders put
together so that there's a reserve of pressure from the brake pedal.
As has been said earlier, the Lego pneumatic parts are not up to the pressure of
using water. I tried it years ago with the original pneumatic system, creating
a fire engine by using a non-return valve with the input end in a bowl of water,
the middle on a cylinder and the output on a thin tube for the hose to squirt
the water. The non-return valve soon gave up and leaked air thereafter. I
think the water saw off a couple of my early switches too. This may be because
the water forced out the grease that seals the parts. I think hydraulic systems
are self-lubricating.
Mark
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Message is in Reply To:
 | | Re: pneumatic cylinder: why not hydraulic ??
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| (...) It is my understanding that fliuds cannot be compressed. So a cylinder hooked to another cylinder with fluid in between (assuming no leaks), would always provide 1 to 1 movement. I'm not an expert in hydraulics, but I think your claim above (...) (21 years ago, 8-Apr-04, to lugnet.technic)
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