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The function of the oil pump is to generate a certain pressure in a specific amount of oil, which is then delivered to the friction surfaces of the various moving parts of the diesel engine for lubrication. Currently, the oil pumps used in diesel engines are of two types: gear-type and rotor-type. The 135 series diesel engines, as well as the 6110Z, 6120Q, X6130, and 6140B diesel engines, all use gear-type oil pumps, while the 495Q, 6105Q, and YC6110Q diesel engines utilize rotor-type oil pumps. The following mainly introduces the gear-type oil pump, whose physical shape is shown in Figure 8-2. II. Structure of the gear-type oil pump The gear-type oil pump is mainly composed of components such as the driving gear, driven gear, pump body, oil pump cover, driven shaft, driving shaft thrust bearing, and transmission gears. The drive shaft is mounted on the drive bearing via a half-round key, and the drive bearing is fixed to the pump body and the front cover of the oil pump. The driven gear is installed inside the pump body via a driven shaft. The fit between the pump body and the pump cover is secured by two positioning pins; gaskets are installed between them, and the clearance at the gear end face can be adjusted by changing its thickness. If the gap is too large, it will cause the oil pressure to drop ； Conversely, if the gap is too small, it will increase the frictional resistance when the oil pump rotates, accelerating the wear of the gears. Under normal conditions, the gap between the pump body and the pump cover should be between 0.05 and 0.115 mm. The drive gear is fixed to the front end of the drive shaft via a half-round key and a nut, while a wire retainer is installed at the rear end of the drive shaft to prevent it from coming loose. A thrust bearing is installed between the drive gear and the pump cover. III. Working principle of the gear-type oil pump: When the diesel engine is running, the crankshaft gear drives the transmission gear of the oil pump to rotate through an intermediate gear. The driving gear installed inside the oil pump rotates along with the drive shaft, simultaneously causing the driven gear to rotate in the opposite direction. The teeth of the driving gear and the driven gear mesh with each other, dividing the interior of the pump into two separate sealed spaces. As the gear rotates at high speed, the oil between the gear teeth is carried to the oil compression chamber, causing the amount of oil in that chamber to increase. This rise in pressure pushes the oil out through the outlet. The oil in the oil suction chamber is drawn away, creating a local vacuum; as a result, the oil in the oil pan is continuously drawn in to replenish the oil suction chamber. The oil delivery pressure from the oil pump is generally controlled by a pressure relief valve located on the oil filter. If the oil pressure becomes too high, the pressure relief valve opens, allowing the oil to flow directly into the oil pan, thereby preventing excessive pressure in the oil circuit and avoiding failures in the oil pump. IV. Common faults of gear-type oil pumps Under normal conditions, thanks to good lubrication conditions, the rate of wear on oil pumps is relatively slow. However, after long-term use, varying degrees of wear will occur in the clearance at the gear end faces, as well as on the gear surfaces, between the shaft journals and bearing holes, and on the inner walls of the pump body and casing. As wear increases, various clearances widen; when the degree of wear reaches a certain limit, it leads to a decrease in oil supply volume and a drop in oil supply pressure. V. Methods for maintaining gear-type oil pumps. If it is found during the operation of a diesel engine that the lubrication system does not supply enough oil, the oil pressure is too low, or abnormal noises are heard during operation, the following checks are generally required. 1 Check the oil lines and connections for any signs of oil leakage. If there is no oil leakage, other aspects of the inspection can be postponed. 2 Check the viscosity of the oil. 3 Adjust the oil pressure. If the oil pressure does not increase during the adjustment process, other aspects should be checked. The method for adjusting oil pressure is shown in Figure 8-5. 4. Check whether the oil copper pipes between the rocker arms are broken; the inspection location is shown in Figure 8-6. 5 Check whether the coarse oil inlet filter of the oil suction tray is clogged or not. Only after all inspections have revealed no signs of faults should the oil pump be removed and disassembled for further inspection. VI. Maintenance after disassembling the gear-type oil pump 1. Maintenance for excessive clearance at the gear end faces The clearance between the gear end faces and the pump cover is referred to as the end-face clearance of the gear pump. The gradual increase in this gap is generally caused by wear of the gear axially against the pump cover or pump body. A ruler and feeler gauge can be used for measurements during inspection. This gap is generally 0.05 to 0.14 mm, with a maximum value of 0.15 mm. The size of the end-face clearance has a significant impact on the operating performance of the oil pump. When the end-face clearance of the gear is too large, the gap can be brought back within the specified technical range by thinning the gasket between the pump body and the pump cover. If the end-face clearance cannot be brought back within the specified range by thinning the gasket, fine grinding paste can be applied to the platform, after which the joint surface of the pump body or the flat surface of the pump cover can be hand-ground. When grinding, apply even pressure; otherwise, when the two separated surfaces are reassembled, air or oil leakage may occur due to them not being in the same plane. 2. Repair of excessive gear mesh clearance: The gears inside the oil pump, as a result of prolonged friction against each other, can develop an increased gear mesh clearance. During inspection, a feeler gauge can be used to measure the gear mesh clearance, as shown in Figure 8-7(c). During inspection, three positions separated by 120° each are typically selected for measurement, and the average value of these three measurements is taken as the actual clearance between the gears. This gap is generally 0.05 to 0.25 mm, and the allowable limit shall not exceed 0.28 mm. If the meshing clearance of the gears is too large, the gears are usually used with their sides reversed or new gears are replaced in pairs. 3. Repair of excessive tip clearance: The gap between the inner wall of the oil pump’s pump body and the top of the gears is known as the tip clearance. An increase in this gap is usually caused by an increase in the gap between the drive shaft and the bushing, or between the central hole of the driven gear and the driven shaft; this leads to friction between the inner wall of the pump body and the top of the gears. This gap is generally 0.05 to 0.15 mm, with a maximum gap of 0.45 mm. During inspection, a feeler gauge can be used for measurement. If the measured value exceeds the limit clearance value, the gears or pump body should generally be replaced in pairs. 4. Inspection and maintenance of the drive shaft and bushing: Over time, wear on the drive shaft and bushing leads to an increase in the clearance between them, which affects the transmission accuracy of the oil pump as well as the meshing clearance of the gears. Under normal conditions, the fit clearance between the drive shaft and the bushing is 0.04–0.10 mm, with a maximum value of 0.15 mm. During inspection, a vernier caliper can be used to measure the diameter of the drive shaft and the inner diameter of the bushing respectively; the difference between the two values represents the fit clearance between the drive shaft and the bushing. If the wear amount of the drive shaft and the bushing exceeds the limit value, the fit clearance can generally be restored by replacing the bushing. VII. Assembly and Testing of Gear-Type Oil Pumps: The gear-type oil pump should be assembled in the reverse order of disassembly; after assembly, the driving mechanism should rotate smoothly without any jamming. Then place the oil pump in a container filled with oil; use the thumb to block the oil outlet of the pump, while simultaneously turning the drive shaft of the oil pump by hand. If pressure is felt on the thumb during rotation and oil spills out, it indicates that the oil pump is functioning properly.