Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy (hydrostatic energy i.e. flow, pressure). It generates flow with enough power to overcome pressure induced by the load at the pump outlet. When a hydraulic pump operates, it creates a vacuum at the pump inlet, which forces liquid from the reservoir into the inlet line to the pump and by mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.
A hydraulic pump converts the mechanical energy from the prime mover into hydraulic energy for use by the system. Hydraulic energy is the combination of pressure and flow required by the actuators to perform useful work. It is important to understand that hydraulic energy is both pressure and flow combined, because one without the other cannot achieve work. Pressure would just consist of trapped fluid and flow would have no energy to move fluid alone. A hydraulic pump pushes on fluid, and in this regard, fluid can be considered a solid as it is transmitted throughout the machine and then pushes on actuators to eventually move loads. Motion control professionals will have me point out that oil is compressible, but that’s a discussion for another blog. The point is that a pump could be pushing on sand, ball bearings or any other solid medium capable of taking the shape of its container, and the result would still be the transmission of force.
Transmission of force is really the name of the game with hydraulics, and is the basis for Cosford’s Law, which states that "pressure makes it go, flow is just the rate in which you can create pressure." For fluid to be moving, pressure must absolutely be highest at the pump; always. This flies in the face of the fallacy that pressure is resistance to flow. Pressure will rise as high as it needs to be to overcome downstream resistance, but if it didn’t start at the pump, fluid would move backwards.