External Gear Pumps
November 23, 2020
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.
How External Gear Pumps Work
The cycle of a gear pump can be broken into three distinct actions:
- Unmeshing of the gears expands the volume at the inlet of the pump. The expanded volume creates a vacuum, allowing external pressure to push fluid into the pump.
- As the gears rotate, fluid is trapped between the gear teeth and cavity wall of the casing. This transfers the fluid from the inlet side of the pump to the outlet side. Tight clearances and the speed of rotation minimize fluid from internally leaking backwards.
- The additional fluid delivered to the outlet side, along with the decreasing volume created as the gears interlock discharges the fluid out the outlet.
External gear pump designs utilize one of three gear types:
- Spur: Spur gears are the simplest geometry. They can be made from numerous manufacturing methods including injection molding. Spur gear geometry also lends itself well to grinding or wire EDM, making hardened materials an option. However, the entire length of the spur gear engages simultaneously, leading to louder operation, more sensitivity to cavitation, and reduced life.
- Helical: Helical gears are spur gears but spiral axially along the gear. This reduces noise and vibration because the teeth engage and disengage gradually throughout the rotation, resulting in longer life. However, the helical shape induces an axial force that can cause wear between the gears and housing. This must be addressed through careful design and material choice. Helical gears are slightly more difficult to produce than spur gears; They can still be molded, albeit with reduced accuracy.
- Herringbone: Herringbone gears offer the benefits of helical gears without the resultant axial force. The helix is mirrored about the center plane of the gear, generating a v-shaped “herringbone” pattern. While this is functionally superior to the other gear types, it is considerably more challenging to produce and therefore the most expensive option.
What are the main features and benefits of an external gear pump?
Gear pumps are compact and simple with a limited number of moving parts. Small external gear pumps usually operate at up to 4000 rpm, allowing high flow rates in compact envelopes. They are fully reversible, facilitating complex hydraulic procedures. Unlike centrifugal pumps, gear pumps are self-priming and can dry-lift significant heights.
Since output is directly proportional to speed and is a smooth, pulse-free flow, external gear pumps are commonly used for metering, blending and simple control feedback operations. They can handle both high and low viscosity fluids, but consultation with pump engineers is important to ensure that the pump is operating at its optimal condition. Gear pumps are among the quietest positive displacement pumps and ideal for applications where sound is a concern.
An important subcategory of gear pumps is magnetically coupled gear pumps. These pumps have no dynamic seal, resulting in long life without risk of external leakage.
What are the limitations of an external gear pump?
The gears and their journal bearings are lubricated by the pumped fluid and should not be run dry for prolonged periods. Pumping fluid with abrasives will quickly wear both gear flanks and internal bearings because the pumped fluid is also the lubricating fluid.
The close tolerances between the gears and casing, a tight gear mesh, and limited tooth volumes make pumping fluids with large, suspended solids difficult. If suspended solids are anticipated, a strainer can be installed on the inlet side. However, creating too high of a vacuum on the inlet can lead to cavitation.
A gear pump should not be operated too far from its recommended speed range. The hydrodynamic journal bearings in gear pumps are optimized for specific speed ranges. Running the pump too slow may, surprisingly, lead to accelerated wear.
For high temperature applications, it is important to ensure that the operating temperature range is compatible with the pump specification. Thermal expansion of the casing and gears may change clearances within the pump, altering the performance and potentially accelerating wear.
Given enough power, gear pumps will continue to pump against a back pressure if subjected to a downstream blockage. This can lead to over pressurization and result in system rupture. The internal leakage of a gear pump limits its maximum pressure. Furthermore, pressure relief valves (bypasses) can be integrated into the pump to short circuit the outlet to the inlet at prescribed pressures.
The high speeds, tight clearances, and gear mesh of external gear pumps make them unsuitable for shear-sensitive liquids such as paint and soaps.
Materials of Construction / Configuration Options
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
Why All Gear Pumps Are Not Created Equal
- Life of the pump is a combination of design details, manufacturing precision, and engineer-to-engineer collaboration:
- Bearing size and arrangement are crucial for low wear rates.
- Proper lubrication paths are subtle but important to keep bearings lubricated, cool, and with enough liquid to develop a proper hydrodynamic layer.
- Maintaining small, consistent journal bearing clearances reduces contact but requires high, repeatable machining precision.
- Poor gear profile quality, whether from molding or poor machining practices, will accelerate wear.
- Even the best design and highest quality pumps can perform poorly if improperly used. Access to and engagement with pump engineers are crucial to long life.
- Proper material selection
For OEM applications, pump-to-pump repeatability is a critical yet often overlooked characteristic of pumps. A number of factors influence repeatability in gear pumps including gear tolerances, housing tolerances, bearing tolerances, assembly alignment, and motor consistency. If proper controls are not in place, the pump-to-pump variation can exceed 20% at elevated pressures. Ensuring the pump manufacturer has a robust quality control system, 100% pump testing, and an established reputation are essential.
Seals between the motor shaft and housing are the most life-limiting component in a pump. Careful selection and testing of seals is required using the system designer’s actual fluids. A seal pumping water will perform much different than one pumping an aggressive acid. Furthermore, failure of a seal can result in damage to surrounding equipment and not just the pump. The best solution is to use a magnetic coupling if pressure and speed allow it.