For simple systems with a relatively low level of pressure (about 140 to 180 bar or 14 to 18 MPa) the gearpump is the most used type of pump. The gearpump is a very simple, reliable, relatively cheap and less dirt sensitive hydraulic pump. The pump in the picture is driven in the indicated direction. As the gears rotate and the teeth at the suction side come clear of the meshing point, a vacuum is created and oil flows into the spaces between the theeth. The oil in the chambers is transported to the pressure side of the pump. There the teeth mesh and the oil is forced out the spaces between the theeth into the output port of the pump. The meshing of the teeth prevents the oil flowing back from the pressure to the suction side of the pump. So the oil is transported from the suction side to the pressure side along the housing side of the gear wheels! The pressure at the pressure side is determined by the resistance in the system. The most important resistance is the load on the hydraulic cilinder or hydraulic motor. In order to prevent cavitation, the pressure at the suction side of the pump should not exceed 0.1 to 0.2 bar (10 to 20 kPa) below atmospheric pressure (minimim absolute pressure: 0.8 bar or 80 kPa).