External Gear Pumps: Operation, Features and Benefits December 2, 2024 What is an External Gear Pump? External gear pumps are a type of rotary positive displacement pump tracing back to the late sixteenth century. They were often driven by water wheels as they can utilize simple rotation of gears to transfer fluids. External gear pumps have advanced to be the simplest and most common type of rotary positive displacement pump. Typically, external gear pumps have two gears on separate shafts with one shaft connected to a motor. The types of drive, size and materials of construction vary widely by industry and application. External gear pumps can be found with flow rates from 20 ml/min to over 50 L/min, with pressures up to 500 bar. Helical gears from an external gear pump by Diener Precision Pumps How External Gear Pumps Work The operation of an external gear pump is quite simple. Two gears rotate within a fixed cavity chamber. The clearances between the rotating gears and the cavity are kept to a minimum so they can pull the fluid into the pump. As the gears turn, the spaces between the gear teeth act as a small chambers that move small volumes of fluid around the edge of the cavity. When these chambers reach the outlet side of the pump, the fluid is expelled as the gear mesh prevents it from escaping back to the inlet side. This process is repeated for each gear tooth, which results in a very smooth delivery of liquid. Fluid flow as the gears mesh in an external gear pump Gear Types Gears in an external gear pump generally fall into three categories: spur, helical and herringbone. Spur: Spur gears have teeth that are parallel to the gear axis and are most commonly used in applications where the liquid has some lubricating properties and/or the pump operates in forward and reverse directions. Helical: The teeth in a helical gear pump are slightly angled (usually 3-10 degrees) relative to the shaft. This helix allows the meshing gear teeth to gradually engage each other during rotation—improving lifetime and reducing noise. The excellent self-priming characteristics of a helical gear are similar to the spur gear. Herringbone: Herringbone gears typically have a “V” shape to the helix angles, thus balancing the axial force. These gears are more costly to produce and are typically used in power-transmission assemblies and/or very high-pressure hydraulic pumps. Gears in an external gear pump can be spur, helical or herringbone Advantages of an External Gear Pump External gear pumps operate at high hydraulic efficiencies and are very compact. They can operate in forward and reverse, are self-priming and thus have high dry-lift capability. Flow and pressure are directly proportional to speed and deliver smooth pulse-free flow. They can pump a wide range of fluid viscosities across a relatively high temperature range. An important subcategory of gear pumps are magnetically coupled gear pumps. These pumps have no dynamic seal, resulting in long life without risk of external leakage. Limitations of an External Gear Pump External gear pumps rely on internal bearings to carry the gears. The fluid lubricates the mating surfaces, so the pumps should not be operated “dry” for prolonged periods. The tight internal clearances that make the pumps so efficient can be damaged if hard solids pass through. These tight clearances also allow the pump to generate high inlet vacuums, which, if not monitored, can result in cavitation damage to the internal surfaces. Careful consideration of the pump’s design capabilities is critical to successful operation. Rotating the pump too slowly may prevent fluid from creating the liquid boundary layers necessary to maintain low friction and enhance life. Failing to provide the necessary clearance for thermal expansion can create problems at high temperatures. An external gear pump will continue to increase downstream pressure if a blockage occurs, so monitoring downstream pressure is highly recommended to prevent damage to the pump and/or system. Gear pumps are not recommended for use with thixotropic fluids. DPP engineers are available to provide guidance on these and other considerations. Materials of Construction As the following list indicates, gear pumps can be constructed in a wide variety of materials. Superior life and optimized cost can be achieved by precisely matching the materials of construction with the liquid. While external gear pumps are commonly found in cast iron, newer materials allow these pumps to handle liquids such as sulfuric acid, sodium hypochlorite, ferric chloride, sodium hydroxide, and hundreds of other corrosive liquids. Externals (head, casing, bracket) – Iron, ductile iron, steel, stainless steel, high alloys, composites, PPS, ETFE Shafts – Steel, stainless steel, high alloys, alumina ceramic, PEEK Gears – Steel, stainless steel, carbides, PTFE, PPS, PEEK Bushings/bearings – Carbon, bronze, silicon carbide, needle bearings, PEEK, Shaft Seal – Packing, lip seal, component mechanical seal, magnetic coupling What Distinguishes High-quality Gear Pumps Long service life Gear pumps are known for their relatively long service life. Proper application is critical though, therefore at Diener Precision Pumps we always provide extensive assistance to our clients to optimally match the design of their custom pump to their application. Key design considerations to take into account include proper bearing sizes, sufficient bearing/shaft lubrication paths, high gear quality and correct gear material selection. From the application perspective, it’s important that the fluid type, cleanliness, pressure, speed and temperature are considered. Pump’s life expectancy is proportional to all of these factors, which is why our design and engineering team works closely with our clients to optimise the pump design and features and ensure minimal wear, long service life and optimum performance. Repeatable Performance DPP’s gear pumps are made to tolerances in the micrometer range, thus assuring reliably repeatable performance from pump to pump. This is particularly important for applications where the pump is expected to deliver flow rates within a tight speed range. It is also important in “on/off” applications in which the pump is cycled up to high pressures for short periods—for example high-pressure dispenses with on/off cycles. Leak-free Most external gear pumps employ a shaft seal to prevent leakage. Seals are a wear component and thus a life-limiting part of the design. Most Diener’s precision gear pumps use a magnetic coupling instead of a seal, ensuring a wider range of chemical compatibility and long life.
