​ Gear pump

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A gear pump is a pump that is formed by two gears meshing with each other; it belongs to the category of positive displacement pumps. 1 Structure and working principle of gear pumps: The working principle of a gear pump involves a pair of meshing gears located within the pump’s casing. Due to the very small gap between the gear end face and the housing end cover, as well as the very small gap between the gear tooth tips and the inner surface of the housing, it can be considered that the gear pump housing is divided into two sealed chambers on the left and right sides. As the gears rotate, the gears on one side gradually lose engagement, exposing the spaces between their teeth. Therefore, the volume of the sealing chamber on this side gradually increases, creating a local vacuum. The oil in the tank enters this chamber through the pump’s suction port under the effect of atmospheric pressure; hence, this chamber is called the suction chamber. As the gear rotates, the oil in each tooth gap is carried from one side to the other. In one of the sealed chambers, the gear teeth gradually enter engagement, causing the volume of that sealed chamber to decrease gradually. The chamber in which the oil between the teeth is forced out through the oil pressure port is referred to as the oil pressure chamber. As the gear pump keeps rotating, its suction and discharge ports continuously draw in oil and push it out, thereby enabling the delivery of oil to the hydraulic system. In a gear pump, the suction area and the discharge area are separated by the intermeshing gear teeth and the pump body. 2 Repair of gear pumps: (1) Disassembly, inspection, and assembly of gear pumps 1) Disassembly sequence The disassembly process of a gear pump can be divided into the following steps, in this order: coupling – rear end cover – front end cover, packing seal or mechanical seal – gears – gear shafts – bearings. 2) Inspection of fit tolerances between components and adjustment during assembly ① Inspection of the housing The two end surfaces of the housing should be flat without any protrusions; the parallelism of the axes of the holes and their perpendicularity to the ends must meet the IT6 standard. The cylindricity tolerance of the internal holes of the housing is 0.02–0.03 mm/100 mm. ② Inspection of the gears The fit between the gears and the shafts is H7/m6. The perpendicularity of the end surfaces of the gears to the axis of the shaft holes, as well as the perpendicularity of those end surfaces to the centerline of the shaft, should be 0.02 mm/100 mm. The widths of the two gears should be equal; the width error for any single gear should not exceed 0.05 mm/100 mm. The parallelism of the axes of the two gears should also be 0.02 mm/100 mm. The top and side clearances between the gears can be measured using the lead compression method. The top clearance should be 0.2–0.3 m (where m is the module value), while the side clearance should conform to the values specified in the table below: Center distance ≤ 50: 1–80; 81–120; 120–200. Side clearance: 0.08, 0.10, 0.13, 0.17. Method for checking gear meshing: Clean the gears, bearings, pump housing, and other components thoroughly, then apply a layer of red lead oil on the mating surfaces of the small gears. Reassemble the gears and end covers, and rotate the pump slowly several times. Remove the end covers to inspect the contact points between the gears; these contacts should be even, with the contact area accounting for no less than 70% of the tooth length and no less than 50% of the tooth height. ③ Adjust the clearance between the gear and the housing, as well as between the gear and the pump cover. The radial clearance between the gear and the housing is measured using a feeler gauge; the desired clearance value is 0.15–0.25 mm. However, it is also necessary to consider the clearance between the shaft diameter and the shaft bearings. The axial clearance between the gear end face and the end cover is measured using the lead compression method. Two wires are placed on each of the pump cover end face and the gear end face; the pump cover is then reinstalled, and the bolts are tightened symmetrically and evenly. The compression plates are removed to take out the wires for measurement. The difference between the thickness of the wires on the pump cover and those on the gear end face is calculated – a positive value indicates the presence of clearance, while a negative value suggests no clearance. Based on these measurements, the thickness of the end cover shims is adjusted so that the clearance remains within the range of 0.1–0.15 mm. ④ The shaft and bearings were inspected; the mating surfaces were free of scratches and burrs. Gear pump bearings generally use either rolling bearings or sliding bearings. For rolling bearings, the fit between the inner ring and the shaft is of the H7/Js6 standard, while the fit between the outer ring and the end cover is of the K7/h6 standard. For sliding bearings, the fit between the outer ring and the end cover is of the R7/h6 standard; the fit between the sliding bearing and the shaft diameter should comply with the values in the table below: Rotation speed (r/min): Below 1500, 1500–3000, Above 3000. Clearance (mm): 1.2/1000d, 1.5/1000d, 2/1000d. Note: d represents the diameter of the shaft in mm. The bearings of gear pumps should be replaced when their wear exceeds the specified limits. The assembly method for rolling bearing sets is the same as that for centrifugal pumps. For gear pumps that use copper sleeves as bearings, when replacing the copper sleeves, it is first necessary to check the fit between the copper sleeves and the end caps. Once the requirements are met, lubricant should be applied to the outer surface of the copper sleeve, which is then pressed into the pump end cover using a press. Finally, holes should be drilled at the interface between the bearing and the end cover, tapped, and screws used to secure it, in order to prevent the copper sleeve from rotating or moving axially. After the assembly is completed, the fit clearance between the journal and the copper sleeve must be checked again. If the fit clearance is too small, the journal should be used as a reference to lap the copper sleeve until the required specifications are met. On the contrary, if the gap is too large, the shaft sleeve must be replaced. (5) Inspection and assembly of the axial seal: Whether a gear pump uses a mechanical seal or a packing seal for its axial seal, the assembly method can be based on that used for the mechanical seals and packing seals in centrifugal pumps. (6) Maintenance of the gear pump relief valve: The maintenance of the relief valve focuses on ensuring good contact between the valve spool and the valve seat, and this can be achieved by grinding the valve spool and the valve seat. Spring failure can also result in the outlet press flow not meeting the requirements; insufficient spring elasticity can be adjusted using an adjustment nut. If the problem persists even after adjusting the nut to the fullest extent, a new spring should be replaced.