Ask A Scientist© Engineering Archive

 >    name      Larry
 >    status       other
 >    age          40s

 >    Question -   If you can manually put ten pounds of pressure into a
 > grease gun and get 6,ooo lbs out the other end, why can't that be
 > converted into rotory power and thusly converted into usable energy?  It
 > seems there is more energy created and made available than is being used
 > to start the process, i.e., 10 pounds in to 6,000lbs out. Am I
 > understanding this correctly?  Hydraulics seem to be the most efficient
 > use of energy other than nuclear.  Do you concur with this?  Your answers
 > are very important to me.  I look forward to hearing your response.
 >
 >Kind regards,
 >Dr. Larry D. Blanton


Doctor, even you know that energy cannot be created or destroyed ;-).  It is
always conserved by changing forms.  As you probably know, you can't get
something without sacrificing something else.  Hydraulics and pneumatics are
a transfer of power from one location (the hydraulic pump) to another
location (linear or rotary actuator).  You put energy into it and get energy
out of it, but there are always losses of energy due to friction, heat loss,
etc.  So, you can never get out more energy than you put in.  In addition,
the hydraulics that you mention (10 lbs into 6,000 lbs) is a case of
mechanical advantage much like a lever.  The principle of hydraulics is
based on Pascal's law which states that a confined liquid that has a
pressure place on it will act with equal force on equal areas at right
angles to the area.  The key words here are equal area.  If I change the
area I change the forces produced.  Take for instance if I place that 10 lb
force on the end of a grease gun that has an area of 1 in^2, I will generate
a pressure of 10 lbs/in^2 (or psi).  If the end of the grease gun has an
area of 1 in^2 as well, the grease will come out at 10 lbs.  But, if I
increase the area at the end of the gun to say 10 in^2, the grease will come
out at 100 lbs.  Great, but what did I sacrifice?  Well, let's have a look
at the definition of work.  Work is defined as a force over distance.  If I
think about moving that grease through the gun, I look at the work created
by moving the 10 lbs say 10 inches in the chamber.  That is 10 cubic inches
of grease (1 in^2 * 10 in) moved in the gun with 100 inch-lbs of work done.
Conservation of energy says that I still transfer this 100 inch-lbs of work
(ignoring friction for now) and conservation of mass says I transfer this
100 cubic inches of grease.  Well, 100 cubic inches of mass moved in a 10
in^2 area at the other end means that I only move 1 inch in distance, and 1
inch distance times 100 lbs equals my 100 inch-lbs of work.  I have
sacrificed distance to increase my ability to take 10 lbs of force to move
100 lbs of force.  This is the same principle of a simple lever.  Now, due
to losses of friction and heat, I will not exactly get 100 inch-lbs of work
out at the other end, so the distance I moved that 100 lbs is now even less.
Hydraulics and pneumatics are great sources for transferring energy to
different locations.  At your local gas station for instance they might have
three of four hydraulic lifts for raising cars, but they will need only one
pump to work the four lifts instead of needing 4 pumps for 4 lifts.  That
saves the owner money, which he passes the savings on to you, right?  The
key word is transfer of energy.  He still has to pay the electric bill to
run the electric motor that is attached to the hydraulic pump.  This is
where the energy comes into the hydraulic system: through the electric motor
driving the pump.  Now, where does that electricity come from?  From a power
GENERATING station that uses fossil fuels, nuclear energy, or solar power,
etc.  Nuclear energy is a way to generate power by harnessing the energy
stored in the atom.  Nuclear power and hydraulics are two separate entities.
Hope this explained it well enough.
Dr. C. Murphy

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