On Sun, 31 Jul 2005 11:06:49 +1000, "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

Simple overview:

Pumps deliver a flow rate determined by displacement and RPM.
Pressure is determined by the load and other factors, though the pump
and drive will have a maximum pressure rating.

Hydraulic pumps are "positive displacement" pumps that move a
certain volume of liquid per revolution, nearly regardless of
pressure.

If there is nothing to resist flow then pressure will be very low.
If there is resistance, as perhaps a cylinder with a load, then
pressure will increase until one of the following happens:

* the load moves when hydraulic force exceeds load resistance
* the motor stalls or a clutch or belt slips
* something breaks
* a pressure-actuated relief valve diverts flow around the load

Liquid flow rate is determined by the displacement of the pump and the
speed at which it's driven. Operating pressure is determined by the
load: light load = low pressure. Max pressure is determined by
the ratings of the pump, the hydraulic lines, and whether the drive
(motor, belts, etc) can deliver the torque to produce that pressure.
Pressure is often limited by a bypass valve that will divert flow at
some set pressure, like the 2200 PSI you mention. This valve may be
incorporated in the pump rather than being an external entity.