A pump that is formed by two gears engaged with each other is called a gear pump. Gear pump flow formula is: Q = 2qZnηv where Z - the number of teeth; n - number of revolutions, rev / min; ηv - volumetric efficiency, the general gear pump, its value is preferably 0.70 ~ 0.90; q - The pit volume between two teeth, m³. As the gear rotates, the aspirated fluid fills the teeth between the teeth and as the gear moves along the wall of the housing into the pressure space. Here, due to the intermeshing of the two gears, the liquid in the tooth pit is extruded and discharged to the pressure pipe. When the liquid is squeezed, pressure acts on the gear and exerts a radial load on the shaft. After squeezing, the enclosed space gradually increases to form a negative pressure zone, and the external liquid flows into the gear pump inlet under the action of atmospheric pressure. In addition, due to the increased volume of the enclosed space in the negative pressure zone, the air and water vapor in the liquid will be precipitated, and the erosion effect similar to the cavitation phenomenon occurs, so that the gear surface is damaged. Because of this, some gear pump open balance hole or balance tank. In most cases, however, helical gears are used; because the volume of the enclosed space is almost constant when the helical gear is engaged, the volume of one of the segments increases and the other decreases. So the above phenomenon is not serious. Gear pump is characterized by good self-priming performance, and the simple structure, reliable work. As can be seen from the above formula, for a given gear pump (size D, d, b and n are fixed value), the displacement is also determined, is a constant value. Therefore, its characteristic curve is a vertical line (that is, no matter how the external pressure changes, its oil displacement is fixed). And because the gear pump outlet and inlet is isolated, so when the external need to reduce the amount of oil, will cause a sharp rise in the pressure of the outlet pipe, resulting in the export pipeline and casing burst. Therefore, gear pump outlet (or outlet pipe) are provided with safety valve, which rises to a certain degree of pressure on the action, so that part of the oil outlet pipe vent. Characteristic Curves In the high pressure region, the flow is shifted in the smaller direction, mainly due to the leakage of the liquid in the pump from the outlet to the inlet along the tooth gap at high pressures. Gear pump maintenance, the main measurement gap is: 1, the gear end face and the axial clearance of the pump housing, generally take about 0.20 mm. Using pressure lead wire method of measurement. 2, the gear's outer circle and the radial clearance of the pump housing, the general radius of 0.10 to 0.15 mm (but more than the bearing gap), but also by the diameter of 0.003 to 0.005 selected (referring to the total gap). Measure with feeler gauge. 3, the gear meshing at the gap generally take 0.30 mm, such as the gap accelerated wear and tear over the General Assembly, such as too small will cause heat bite. Using pressure lead wire method of measurement. 4, bearing clearance, when the shaft diameter of 20 to 40 mm, take 0.06 to 0.12 mm. Measure with feeler or vernier caliper. It is best that the meshing parts of the gears remain the same before and after the overhaul, so that when the pump is disassembled, the gear can be marked on the gear. When the gear pump is in operation, it is not allowed to close the outlet valve. At the start and stop, the outlet valve should also be kept in the open position, only the inlet valve can be operated at this time.