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Originally Posted by CJO
Ummm, isn't air a fluid? I mean, to all intents and purposes doesn't fluid dynamics deal with "liquids" and "gasses?"
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Yes it is a "fluid", but please don't confuse the behavior of "liquid" vs "gas" forms.
It's mostly a question of semantics. You'll mostly find the term "Hydraulics" is traditionally used to refer to "liquid only" power transfer mediums, and "Pneumatics" to refer to "gaseous only" power transfer mediums. As long as you're not compressing the "working fluid" enough to phase change the medium, this distinction is fine.
The major difference is "compressibility". Gasses are compressible, whereas fluids are not. IOW, pneumatic systems provide its own "spring function" between the energy source and the load, while hydraulic systems act more like a hard mechanical linkage. Otherwise, they have similar characteristics. Both will give you controllable Mechanical Advantage, very flexible energy transfer without complex mechanical linkages (hoses are simpler than gears and shafts!), and (unlike electric motors) "infinite stallability without harm".
"Fluid Power" is the superset term, that covers BOTH pneumatics AND hydraulics. It is often used by university departments when teaching the subject. You should also definitely use it when referring to any power transfer medium that may undergo a phase change between liquid and gas depending on condition. (But honestly, most designs normally try hard to AVOID those situations... You either want compressibility, or you don't!)
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The confusion comes in because the COMMON usage of the term "fluid" in English IMPLIES a liquid. In scientific terms though, the "working fluid" can be any phase of material.
Oh yes, before someone chimes in with a joke, solid working fluids ARE possible, as long as they are particle systems. Things like sand, or a bunch of tiny ball bearings CAN be used as a "working fluid" (though I'm not sure it would be a good IDEA for use in OUR situation!) . Particle systems are a rather "weird" working fluid, but you CAN transfer force through them!

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- Keith