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Re: Mysteries of hydraulicsŠ

To: Triumphs Mailing List <triumphs@autox.team.net>
Subject: Re: Mysteries of hydraulicsŠ
From: "Jim Muller" <jimmuller@pop.mail.rcn.net>
Date: Mon, 1 Oct 2001 22:55:30 -0400
Organization: Southern Rail
Pete Chadwell says:
> And this would support my notion that hydraulics is simply another 
> way to transfer motion.

It would be more accurate to say that hydraulics is a way to transfer 
pressure.

The essense of hydraulics is a simple principle:  The pressure in a fluid is 
the same everywhere.  (This statement neglects the weight of the fluid which 
adds a small variation with height.  We can ignore this because the density 
of most hydraulic fluids is low and the height variations are small when 
compared to the pressures at which most systems operate.)

You can therefore calculate the mechanical advantage of a hydraulic system 
very easily.  The total force that a hydraulic's fluid exerts on a plunger 
is the pressure in the fluid times the plunger's area.  So a large plunger 
will "experience" a greater force than a smaller one.  Similarly, a smaller 
plunger can be "pushed" by an outside force more easily than a larger one.  
Thus you get a mechanical advantage equal to the ratio of their areas.

If the fluid is uncompressible then the total volume in the system must be 
constant also.  (Even hydraulic fluids are compressible, but compared to 
gases just about any liquid is essentially uncompressible.)  If one cylinder 
is larger than another, the smaller one must move a greater distance in 
order to keep the enclosed volume constant.  This gives you another way to 
calculate a system's mechanical advantage.  Conservation of energy says that 
when you move one piston, the same amount of work must be done by the other. 
 So the ratio of the two forces must be the inverse of the ratio of their 
distances.  If one has, say, twice the diameter of the other, then it has 
four times the area.  Hence it will exert four times the force but move only 
one fourth the distance.

There will be a quiz next week.

Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
'70 GT6+ (Nigel)

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