Re: What is a hydraulic press and how does it work?

Dear Michael,

A hydraulic press is any sort of device that uses hydraulic pressure to 
squish something.  (I know squish isn't a very technical term, but it gets 
the idea across).  This can be as simple as a hand jack set in a frame, to 
a large machine that compresses old cars into small boxes.  I will give a 
short explanation of hydraulics and then give some examples.

The main principle that governs how hydraulic jacks and presses works is 
Pascal's principle.  In equation form it is F0/A0 = F1/A1 where F0 is the 
force applied to the fluid over area A0 in a cylinder which is transferred 
to the second cylinder with area A1 resulting in the force F1.  (Physics 
for Scientists and Engineers, 2nd Ed. By Douglas Giancoli)  This derives 
from the fact that the hydraulic fluid is mostly incompressible, so the 
pressure at one cylinder is equal to the pressure at the other.  A better 
representation is given at the following website: http://www.lerc.nasa.gov/Other_Groups/K-12/WindTunnel/Activities/Pascals_pr
inciple.html

This sort of hydraulic system has many uses as you likely will know, but 
your focus is on presses.  It is hard to define a good explanation since 
there are so many different uses for a hydraulic press.  As I said before, 
they can be as simple as a hydraulic jack on a frame.  Usually with two big 
springs that return the jack to its shortest stroke after use.  These are 
frequently used to make press fits for things like dowels and bearings and 
such things.  (A press fit means that the hole you are shoving the part 
into is smaller in diameter that the part itself.  This requires a good bit 
of force, hence the need for the press).  Sometimes they are also used for 
breaking stuck items apart, such as getting gears off of a shaft.  That is 
the most simple use for a hydraulic press.  But there certainly is more.

MTS corporation is a manufacturer of hydraulic testing equipment.  They 
make a version of a hydraulic press that has a cylinder on the bottom and 
at least one force gauge.  This is mostly used to do vibration testing on 
materials. By pushing and pulling many times until a material or joint 
fails, you can determine how long something can operate under such 
conditions until it breaks. An example of one of these machines is at the 
following site: http://www.mts.com/menusystem.asp?DataSource=0&NodeID=1011

Now I'm sure that the MTS engineers wouldn't want to call their product a 
press, but at its essence that's what it is.  If you want to see an even 
more complex one, GEMCOR manufactures hydraulic presses to form rivets.  A 
stationary one is shown at this webpage: http://www.gemcor.com/G300.html
and a huge one on tracks is shown here (this is one of the Boeing Aircraft 
wings being made) http://www.gemcor.com/G300.html 

There are others that are used to bend, punch, and even form metals.  I'm 
sure there are some that I haven't even thought of (grape press maybe? )  
There are many possibilities.  I hope that this somewhat answers your 
questions and I hope the sites I provided helped.
BK