Gear pump

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A pump formed by two gears meshing with each other is called a gear pump, and it belongs to the category of positive displacement pumps. 1 Working principle and structure of gear pumps Gear pumps transport liquids by relying on the changes in volume within the space created by the meshing of gears. As shown in Figure 1(a), two gears of the same shape and size are placed inside the pump casing in mesh with each other; one is the driving gear, which extends out of the pump body and is connected to the shaft of the prime mover, while the other is the driven gear. When the gear pump is in operation, the driving wheel rotates along with the motor, causing the driven wheel to rotate as well. As the meshing teeth on the suction chamber side gradually separate, the volume of the suction chamber increases, creating a low pressure that draws the liquid in the suction tube into the pump. The liquid entering the pump body is divided into two streams, which are pushed by the driving and driven gears respectively into the discharge chamber through the gap between the gears and the pump casing. As the teeth on the side leading out of the chamber continuously mesh, the volume of the discharge chamber decreases, thereby forcing the liquid into the discharge pipe. As the driving gear and the driven gear keep rotating, the pump can continuously draw in and discharge liquid. To prevent damage to the pump when the outlet valve is closed or the pipeline is blocked, a spring-loaded safety valve is installed on the outlet side of the gear pump. When the pressure inside the pump exceeds the specified value, the safety valve opens automatically, allowing the high-pressure liquid to flow back to the suction side. Based on the gear meshing method, they can be divided into external gear pumps and internal gear pumps. As shown in Figure 1(a), an external gear pump uses various types of gears such as straight teeth, helical teeth, and herringbone teeth; among these, the involute tooth profile is the most widely used. The number of gears in an external gear pump ranges from 2 to 5, with two gears being the most common configuration. As shown in Figure 1(b), a internal gear pump consists of two gears with different shapes and different numbers of teeth. One of these gears is an annular gear that can move freely within the pump body. The driving gear is located in the middle, at an eccentric position relative to the pump body; it has one fewer tooth than the annular gear. As the driving gear rotates, it drives the annular gear to rotate as well, and the transfer of liquid is achieved through changes in the space between the two gears. An internal gear pump comprises only these two types of gears. 2 Characteristics of gear pumps: Gear pumps have the ability to self-priming, and their flow rate is independent of the discharge pressure ； Simple and compact structure, uniform flow rate, reliable operation ； Small in size and lightweight. Easy to maintain ； The At/1\" pressure pump is used for transporting liquids with high viscosity, such as lubricating oils and fuel oils. It can be used as a lubricating oil pump, a fuel pump, an oil transfer pump, and a hydraulic pump in hydraulic transmission systems. Its disadvantages include high requirements for manufacturing precision; it is not suitable for pumping liquids with low viscosity such as water and gasoline, nor for liquids containing solid particles; there are fluctuations in flow rate and pressure during operation, and it generates considerable noise. The gear pump uses herringbone gears, which enable smooth operation and eliminate axial thrust. Screw gear pumps also operate quite smoothly; in large gear pumps, herringbone gears or screw gears are commonly used. Spur gears are commonly used in small gear pumps. 3 Maintenance of Gear Systems 3.1 Disassembly, Inspection, and Assembly of Gears 1. Disassembly Sequence The disassembly process of a gear pump can be divided into the following steps, in this order: coupling – rear cover – front cover – packing seal or mechanical seal – gears, gear shafts, bearings. 2. Inspection of Clearance Between Components and Assembly Adjustments After the gear pump is disassembled or its components are removed and cleaned, inspections, measurements, and assembly must be carried out in accordance with the pump’s operation and maintenance instructions. In the absence of specific requirements, gear pumps used for transporting oils at temperatures below 60 degrees can be inspected, measured, and assembled in accordance with the standards specified in SHS01017-2004, \"Maintenance and Repair Procedures for Gear Pumps\". Its maintenance mainly includes the following aspects: (1) Inspection of the housing – The roughness of the two end surfaces of the housing should be R 3.2; the parallelism of the axes of the two holes, as well as the tolerance for their perpendicularity to the ends, must be at least at the specified level ； The cylindricity tolerance for the internal hole in the shell is 0.02–0.03 mm/100 mm. (2) Inspection of gears: The fit between the gear and the shaft is of type H7/m6; the perpendicularity between the two end surfaces of the gear and the centerline of the shaft hole, or between those end surfaces and the centerline of the shaft, is 0.02 mm/100 mm ； The two gears have the same width; the width error of each individual gear shall not exceed 0.05 mm per 100 mm. The parallelism between the axes of the two gears is 0.02 mm per 100 mm ； The top and side clearances of gear meshing can be measured using the lead compression method; the procedure for this can be found in the methods for measuring the top and side clearances of transmission gears. The peak clearance between gear teeth is (0.2–0.3) m, where m is the module ； The side clearance shall comply with the provisions in Table 1. Table 1: Standards for clearance on the gear meshing side, in mm. Center distance: 5051–8081–120; 120–200. Clearance on the meshing side: 0.08, 0.10, 0.13, 0.17. The gear meshing contact must comply with relevant specifications. The inspection method is as follows: First, clean the two driving gears, bearings, pump housing, and other components, then dry the meshing surfaces of the gears with a dry cloth. Apply a thin layer of red lead oil to the meshing surface of the smaller gear. Reassemble the gears and end caps, and slowly rotate the gear pump several times in the direction of operation. After that, remove the pump end caps and take out the gear shafts to check for contact spots. The gear meshing contact area should be uniform, with a contact ratio of not less than 70% along the tooth length and not less than 50% along the tooth height. (3) Adjustment of gaps between gears and the housing, as well as between gears and the pump cover. The gaps between the tooth tips and the housing walls, as well as between the gear end faces and the end covers, should comply with the specifications. If the gap is too large, the leakage of liquid increases ； If the clearance is too small, as the gear rotates, the tooth tips of the gear may experience wear against the pump housing walls, the gear end faces, and the pump cover end faces. Therefore, during maintenance, it is necessary to check the clearances in these two aspects. The radial clearance between the gear and the housing can be checked using a feeler gauge, and the clearance value is. 15-0.25mm, but it must be greater than the radial clearance of the journal in the bearing shell. The axial clearance between the gear end face and the end cover can be checked using the lead wire method. The procedure is as follows: First, remove the end cover and clean all components; once there is no oil or debris on their surfaces, install the gear inside the pump. Place 4 appropriate lead wires symmetrically on both the pump cover end face and the gear end face. Then reinstall the pump cover, tighten the bolts evenly in a symmetrical manner, and finally remove the cover to measure the thickness of each lead wire. If the difference between the wire thickness on the gear’s end face and that on the pump cover’s end face is positive, it indicates that there is a gap between the two end faces ； If the result is negative, it indicates that there is a gap between the two end faces. Based on the measurement results, shims are added or removed to the end face so that the end-face clearance remains within the range of 0.10–0.15 mm. (4) Inspection and assembly of the shaft and bearings: Under normal circumstances, the gear pump shaft journals should be free of defects, with a roughness level of Ra1.6; the tolerance for the cylindricity of these shaft journals is 0.01 mm ； After being in use for a certain period of time, the maximum wear on the shaft journal of a gear pump shall not exceed 0.01D (where D is the diameter of the shaft journal). Gear pump bearings are generally of two types: rolling bearings and sliding bearings, with sliding bearings often taking the form of copper sleeves. In gear pumps that use rolling bearings, the fit between the inner ring of the bearing and the shaft is H7/js6; in the case of needle bearings without an inner ring, it is the fit between the shaft and the needles ； The fit is H7/h6; the fit between the outer ring of the needle bearing and the end cover is K7/h6. For gear pumps that use sliding bearings, the tolerance for the coaxiality between the bearing inner hole and the outer circle is 0.01 mm ； The outer circle of the sliding bearing fits with the end cover at R7/h6 ； The fit clearance between the sliding bearing and the shaft journal (empirical value) shall meet the values specified in Table 2. Table 2 Clearance between shaft journal and sliding bearing. Rotational speed/(r/min): below 1500, 1500–3000, above 3000. Clearance per minute: 1.2/1000D, 1.5/1000D, 2/1000D. The bearings of gear pumps should be replaced when their wear exceeds the specified limits; the method of assembling rolling bearings is the same as that used for centrifugal pump rolling bearings. 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 and threaded at the interface between the bearing and the end cover, and screws should be used to secure it, in order to prevent the copper sleeve from rotating or moving axially, as shown in Figure 2. After the copper sleeve is assembled, it is necessary to check again the fit clearance between the shaft journal and the rivet. If this clearance is too small, the shaft journal should be used as a reference to grind the copper sleeve until the required fit is achieved. On the contrary, if the gap is too large, the copper sleeve must be replaced. (5) Inspection and assembly of the shaft seal: Whether a gear pump uses a mechanical seal or a packing seal for its axial sealing, the assembly method can be based on that of mechanical seals and packing seals in centrifugal pumps. (6) Maintenance of the gear pump relief valve: The gear pump relief valve is located on the outlet side of the pump; its function is to ensure that the pressure at the pump outlet meets the design requirements. When the pressure inside the pump exceeds the specified value, the relief valve opens automatically, allowing the medium from the high-pressure side to flow back to the inlet, thereby maintaining a stable outlet pressure. If the check valve fails, the medium will flow back to the pump inlet through the check valve, resulting in the pump outlet pressure and flow not meeting the required levels; in such a case, maintenance is necessary. The maintenance of the relief valve focuses on ensuring good contact between the valve spool and the valve seat, which can be achieved by grinding the spool and the valve seat. Spring failure can also result in the pump outlet pressure and 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 maximum, a new spring should be replaced. 3.2 Commissioning of Gear Pumps and Fault Handling 1. Commissioning of Gear Pumps 1) Preparatory work before commissioning (1) Check the maintenance records to ensure that the data is correct, and prepare all the necessary record forms for the commissioning process ； (2) No jamming or abnormal noises during shaft rotation ； (3) Check the liquid level; it should meet the pump’s suction height requirements ； (4) The pressure gauge and flow control valve should be flexible and easy to use ； (5) Inject the conveying medium into the pump ； (6) Confirm that the pump outlet valve is open ； (7) Contact an electrician to check the motor’s resistance and then supply power to it ； (8) Use the jog motor to confirm that the rotation direction is correct. 2) Trial operation: (l) Open the outlet valve and the inlet valve to fill the pump with liquid; open the vent valve to remove all air, and then close it ； (2) After the turning gear operates smoothly without any jamming, start the motor ； (3) Check whether the outlet pressure indicator is normal ； (4) Check whether the shaft seal leakage meets the requirements; the standards for seal medium leakage are the same as those for centrifugal pump shaft seal leakage ； (5) Check whether the vibration level of the pump and the bearing temperature are within acceptable ranges; the allowable values for vibration and bearing temperature can be referred to the standards for centrifugal pumps. 3) Precautions: (1) Be sure to confirm that the pump outlet valve is open before starting the pump ； (2) Do not close the outlet valve first when stopping the pump. 4) Acceptance: (1) After 24 hours of continuous operation, all technical specifications meet the design requirements or can satisfy the production needs ； (2) Meet the good condition standard ； (3) The maintenance records are complete and accurate, and the acceptance procedures are carried out in accordance with regulations. 2. Fault handling of gear pumps: The common fault symptoms, causes, and handling methods for gear pumps are shown in Table 3. Table 3 Common Fault Causes and Solutions for Gear Pumps
Serial Number | Fault Symptoms | Fault Causes | Solutions
1 | The pump does not draw in oil | Blocked or leaking suction pipeline; suction height exceeds the allowable vacuum level; motor running in reverse direction; excessive viscosity of the medium | Inspect the suction pipeline; reduce the suction height; change the motor’s rotation direction; heat the medium
2 | Large pressure fluctuations | Leaking in the suction pipeline; overflow valve not adjusted properly or operating pressure too high, causing the overflow valve to open and close frequently | Check the suction pipeline; adjust the overflow valve or reduce the operating pressure
3 | Insufficient flow rate | Insufficient suction height; leaks in the pump body or inlet pipeline; blockage in the inlet pipeline or filter; high viscosity of the medium; excessive radial or side clearance between gears; excessive axial clearance between gears; overly loose spring in the overflow valve or poor contact between the valve disc and seat; insufficient motor speed | Raise the liquid level; replace gaskets; tighten bolts; repair the pipeline; clean the pipeline or filter; reduce the viscosity of the medium; replace the pump casing or gears; adjust the spring; grind the valve disc and seat; repair or replace the motor
4 | Sudden increase in shaft power | Blocked discharge pipeline; severe friction between the gear and the pump interior; excessive viscosity of the medium | Stop the pump; clean the pipeline; inspect or replace relevant parts; heat the medium
5 | Increased vibration | Misalignment between the pump and the motor; misalignment or too small clearance between the gear and the pump; air inside the pump; excessive installation height, resulting in cavitation inside the pump | Adjust alignment; inspect and fix leaks in the suction pipeline; reduce the installation height or lower the speed
6 | Pump overheating | Excessively high temperature of the medium being pumped; excessive or insufficient bearing clearance; too small radial, axial, or side clearance between gears; outlet valve opened too little, causing high pressure | Lower the temperature of the medium; adjust the bearings or replace them; adjust the clearances or replace the gears; open the outlet valve more to reduce pressure
7 | Severe oil leakage from the mechanical seal | Incorrect installation position; sealing gland not pressed evenly; damage to the sealing surfaces of the rotating and stationary rings; damaged sealing rings | Reinstall according to specifications; adjust the sealing gland; grind the sealing surfaces or replace them with new parts; replace the sealing rings