The Gear Pump Manufacturers states that the gea […]
The Gear Pump Manufacturers states that the gear pump consists of two intermeshing hardened steel precision grinding gear assemblies housed in a rugged enclosure with high-strength front and rear covers. A gear assembly contains drive gears - connected or mounted on a precision ground and polished drive shaft that typically extends outside the pump to allow connection to an external prime mover, such as an electric motor. The other component contains a driven gear, commonly referred to as an idler - connected or mounted on a precision grinding and polishing driven shaft.
The drive gear rotates the driven gear in a direction opposite to its direction of rotation. The movement of the gear causes the liquid collected in the gear 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, not between them.
A retaining ring or other arrangement mounted in the groove provided on the shaft ensures that the gear does not move axially. Most often, another system or component, such as a key, does not move the drive gear radially. The 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 housing and the driven shaft transfer the pump inlet pressure to the rotary seal region to apply the lowest possible pressure at the rotary seal to extend the seal life.
An important advantage of gear pumps is their self-priming capability. The rotating gear evacuates the air in the suction line, creating a partial vacuum that allows atmospheric pressure to force 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 above the liquid level. Since the gear pump does not produce a perfect vacuum, the total lift (including the pipe friction loss) should not exceed half of the atmospheric pressure.
However, tight tolerances and tight spaces within 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, as well as the operating speed of the pump. Pumping thin (relatively low viscosity) liquids can also adversely affect wear rates because of their poor lubricity. 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 abrasive-containing liquids, special materials such as carbon graphite can be used to increase lubricity and wear resistance.
With good design, manufacturing and assembly, there is almost no leakage through the teeth because the drive contacts form an effective seal at (almost) all points. The liquid leaks from the pressure around the circumference around the gear to the flat end face of the gear. The end leakage portion is radial, partially on the teeth near the point of engagement. Keeping the gears roughly at the center of their end gaps can control leakage.
The indenter of the gear pump itself is consumed along the leak path, partially overcoming the viscous drag and partially providing kinetic energy in the leakage flow. The viscosity of the liquid in the actual leak path is not necessarily the same as the viscosity of the liquid flowing into the pump; it may change the amount of heat generated by the loss of electricity. This power loss can result in a significant increase in temperature in the high pressure gear pump where both the flow rate of the liquid and the specific heat are small relative to the power loss.
A small increase in the end or radial clearance of the gears in its chamber will result in a significant drop in volumetric efficiency. Cleaning the liquid usually results in little wear. However, gear pumps are sensitive to foreign matter in the liquid due to their light alloy casing. Foreign matter that is prone to damage includes dirt and scale from the upstream system and metal particles that wear from the high point of the pump itself, especially at an early stage of its life. Large particles tend to be around the circumference of the gear under centrifugal force and engrave circumferential grooves in the casing, while smaller particles can embed themselves in components such as cages for bearings.
It is often meaningful to filter the liquid before it enters the pump. Fine multi-stage filters generally work well; the so-called 100% filtration system typically consists of three stages: a wire mesh filter for obtaining the entire flow, a cloth or felt filter for removing finer particles from the flow, And the final polishing filter. If for any reason it is unavoidable to have a certain amount of foreign matter in the liquid, do not choose a light alloy casing and ensure that the cylindrical parts of the gears and casing are as hard as possible. However, the flat end of the cannula should face a softer material to minimize the risk of seizures. In this case, consider using a lower speed to minimize centrifugation of the foreign particles.