New to hydraulics - question 1 of many

There isn't a standard pump design; you get thousands of different pump designs with different working pressures and displacements. Also note that the pressure quoted is the maximum working pressure. If you connected a pressure gauge to the system it wouldn't necessarily show this pressure. The actual pressure depends on the load on the system.

You do get variable displacement pumps (i.e., you can change the volume of oil discharged per revolution of the shaft), but these are less common than fixed displacement pumps.

Hope this helps,

Chris

Get thee to the library and read up! Or grab inexpensive books off

Audel put out some, like "Pumps, Hydraulics, & Air Compressors", "Pneumatics & Hydraulics", etc. Audel books are all KEEPERS! I have half a dozen of them now covering quite a range of subjects. (They'll be better than gold after "the fall." )

I've found others on Ebay like "Let's Get Into HYDRAULICS, Farm Equipment ELECTRICAL SYSTEMS, Hydraulic Systems DIAGNOSIS", a collection of articles from Implement & Tractor Magazine. It covers both theory and troubleshooting.

Pumps of a given displacement are designed for a certain flow and pressure. Variable-displacement pumps are one exception. Valving can alter flow, too, as can regulation.

I'm fascinated by it, too. From what I've read, flow is pretty much set by the pump volume @ RPM and pressure is adjustable within a given range as determined by the type of regulator.

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.

One off the basic concepts to understand is, pumps only create flow. The pressure is caused by resistance to that flow (ie the load).

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.