231 cubic inches per gallon

B.B. wrote:
In article ,
"Murphy" wrote:


I am learning about hydraulics presently and am fascinated by their
simplicity and potential. There are a number of issues that are still
unclear to me and I would be appreciative if someone could answer these
questions if I post them here from time to time...

The first being with regards to the hydraulic pump. It appears that 2200 PSI
is a standard line pressure used in hydraulics, do all pumps produce this
pressure with the variation between pumps being the flowrate or are all
pumps basically the same with the flow rate and pressure being determined by
the power source for the pump ?

For example does a pump produce either high pressure at a low flow rate or
low pressure at a high flow rate with the flow rate and pressure determined
by the power source ?

I understand that

Thanks

Murphy


Pumps will be sized by displacement. Usually cubic inches of oil
pumped per revolution of the pump. The flow rate will be determined by
how fast you spin the pump. Revolutions per minute times displacement
is cubic inches per minute. Divided by 281 (IIRC) gets you gallons per
minute oil flow. In the case of variable displacement pumps it will
list the maximum possible displacement.
Pumps have a maximum operating pressure, typically 3,000 PSI for
hydraulic equipment. A relief valve somewhere downstream from the pump
will be set lower than that most of the time. That's where that 2,200
PSI number comes from.
Your actual pressure when working depends on how much is required by
the load on the system. If you actually need 2,000 PSI to move it, the
pump will deliver 2,000 PSI as long as it is turning. OTOH, if the load
only needs 50 the pump will only deliver 50.
The relief valve sets an upper limit that system pressure can reach.
That keeps you from killing the engine, blowing off lines, or damaging
the pump. If your load needs more pressure than your relief allows, you
can't move it.
In systems with variable displacement pumps there is often a small
servo attached to the pump that will cause displacement to go down as
pressure goes up. That way the horsepower used remains constant and
heavier loads will just move more slowly without bogging down the
engine. Those systems can have a much higher maximum pressure and
usually don't use a relief valve. The pump displacement will just be
turned to zero before anything breaks.