Mechanical characteristics of gear pumps


The gear pump consists of two intermeshing hardened ste […]

The gear pump consists of two intermeshing hardened steel precision ground gear assemblies housed in a rugged enclosure with a high-strength front and back cover. A gear assembly includes a drive gear coupled to or mounted to a finely ground polishing drive shaft that extends generally to the exterior of the pump to allow coupling to an external prime mover, such as an electric motor. The other component contains a driven gear, commonly referred to as an idler gear, which is coupled to or mounted on a finely ground polished driven shaft.


The drive gear rotates the driven gear in the opposite direction of its rotation. The movement of the gear causes the liquid retained in the tooth space between the housing bore and the outside of the gear to flow from the inlet side to the outlet side of the pump with little pulsation. The pumped liquid travels around the gear instead of traveling between the gears.


A retaining ring or other arrangement mounted in the groove on the shaft ensures that the gear does not move axially. Most often, another system or component, such as a key, prevents the drive gear from moving radially. A sealing system on the drive shaft (usually a lip shaft seal) prevents external leakage of liquid. If a lip seal is used, the sealing lip in contact with the liquid is typically spring loaded. The exhaust passages in the outer casing and the driven shaft transfer the inlet pressure of the pump to the rotary seal area to apply as low a pressure as possible on the rotary seal to extend the seal life.


An important advantage of gear pumps is their self-priming capability. The rotating gear vents the air in the suction line, creating a partial vacuum that causes atmospheric pressure to press the liquid into the inlet side of the pump. This makes the gear pump an ideal choice when the application requires the pump to be installed above the liquid level. Since the gear pump does not produce the desired degree of vacuum, the total lift (including pipe friction losses) should not exceed half of atmospheric pressure.


However, the tight tolerances and narrow spaces inside the gear pump limit the pumping of the abrasive containing liquid. This is because abrasive particles can cause accelerated wear, which can quickly degrade the performance of the pump. The wear rate depends on the hardness, size and concentration of the particles and the operating speed of the pump. Pumping a thin (relatively low viscosity) liquid can also adversely affect the wear rate due to poor lubrication. Therefore, care must be taken when selecting the material for the gear pump assembly for this type of application. When pumping very thin liquids or liquids containing abrasives, special materials such as carbon graphite can be used to increase lubricity and prevent wear.


Through good design, manufacture and assembly, since the drive contacts form an effective seal at (almost) all positions, there is virtually no leakage through the teeth. The liquid leaks from the pressure to the suction, leaking around the circumference of the tooth and the flat end face of the gear. The end leakage portion is radial and partially on the teeth near the meshing point. Leakage can be controlled by holding the gear approximately at the center of its end gap.


The indenter of the gear pump gradually expands along the leak path, on the one hand overcoming the viscous drag and on the other hand providing kinetic energy in the leakage flow. The viscosity of the liquid in the actual leak path does not have to be the same as the liquid flowing into the pump. Especially due to the heat generated by the power loss, it may change. This power loss can cause a significant increase in the temperature in the high pressure gear pump, where the flow rate and specific heat of the liquid are small relative to the power loss.


A small increase in the end or diameter gap of the gear in its chamber will result in a significant drop in volumetric efficiency. A clean liquid usually does not cause too much wear. However, gear pumps are sensitive to foreign matter in the liquid due to their light alloy casing. Foreign matter that is liable to cause damage includes dirt and scale in the upstream system, as well as metal particles that are worn at high points in the pump itself, especially at the beginning of the pump's service life. Large particles are attracted to the periphery of the gear by centrifugal force and a circumferential groove is engraved on the outer casing, while small particles may embed themselves in components such as bearing cages.


Therefore, it is often meaningful to filter the liquid before it enters the pump. Fine multitage filters generally work well; the so-called 100% filtration system typically consists of three stages: a screen filter for filtering the entire stream, a cloth or felt filter for removing finer particles from the stream, And the final polishing filter. For whatever reason, you can't avoid a certain amount of foreign matter entering


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