Two Speed Hydraulics with One Pump
                        Ted_E@telus.net

See diagram:  2SPD_HYD.GIF

I would like to properly credit the source for this idea but,
regretably, I have forgotten where I saw it.

I will assume that spring(s) are used to return the cylinder to its
initial position.

To start off, assume V2 and V3 are closed and V1 is open.  If we now 
pump, we extend the ram in the usual fashion.  Fluid flows from the pump
into the right side of the cylinder and forces the ram to extend.  Fluid
flows from the left side of the ram through open valve V1, back to the 
reservoir.  This is the usual arrangement for extending the ram.

Now, if we stop pumping and open valve V3, we permit fluid to flow back
out of the right side into the reservoir and fluid to be sucked into the
left side of the cylinder from the reservoir.

So what?  Nothing out of the ordinary so far.  This is the usual way of 
operating hydraulics such as in a press.

Now, if we wish to extend the ram quickly to contact the work, it would
be nice if we could trade off some the unnecesary force of the device
for speed.  This is often done with a special two speed pump or with two
pumps.  Here is an alternate, much simpler and cheaper way.

Assume we close V1 and V3 and open valve V2 and start to pump.  Fluid 
flows from the reservoir through the pump into the right side of the 
cylinder.  At first sight, it could flow into the left side as well but 
wait a minute:  One of the rules of hydraulics says that, except when 
heavy flows are involved, the pressure is everywhere the same.  So we 
have the same pressure in both sides of the cylinder and, of course, 
pushing on the piston.  But force is pressure times area.  The piston 
area on the right side is pi/4 times D2 squared, call this A2, while the
area on the left side is A2-A1 where A1 is the area blocked off by the 
ram, namely pi/4 times D1 squared.  So the force on the right side is 
larger by a factor of A2/(A2-A1) and will push the piston to the left. 
If the piston moves to left, the fluid in the left side of the cylinder 
has to go somewhere and it can't get back through the pump or into the 
reservoir (V1 is closed) so it has to go into the right side of the
cylinder.  This extra fluid will cause the piston to extend to the left
further than it would if only fluid from the pump were moving it.
Sounds like something for free but it isn't.  We are paying for it with
reduced pressing ability since the net force is down by a factor of
(A2-A1)/A2.  Ok, but the objective is to quickly move the ram to the
work.  Once it's there, we close V2 and open V1 and we are back to
normal operation for full force but slow movement.  TANSTAAFL (There
Ain't No Such Thing As A Free Lunch) after all.

So how much difference does this make?  Suppose, for example, that the
ram is 5.196" in diameter and the piston is 6".  Then, the area of the
piston, A2, is 28.274 and the area of the ram, A1, is 21.205 so the term
A2/(A2-A1) is 3.999 - just a bit shy of 4.  So in fast mode, a twenty 
ton press could move its ram four times as fast as without the extra 
valves but could only exert about five tons.  Switch over the valves and
you are back to twenty tons of slow moving force.

Note that V1 and V2 could be replaced by one three-way valve.  I haven't
had time to modify my own press thus but I definitely plan to.