External Gear Pumps: A Time-Tested Solution in Fluid Transfer

November 23, 2020

Understanding External Gear Pumps

External gear pumps are a type of rotary positive displacement pump with a rich history dating back to the 16th century when they were powered by water wheels. Today, these pumps remain one of the simplest and most prevalent fluid pump types, utilizing two rotating gears to transfer fluids. Driven by a motor, gear pumps can be customized by size, materials, and drive types depending on the application, with flow rates ranging from 20 ml/min to over 50 L/min and pressures up to 500 bar.

DPP Gears from a Magnetically Coupled Gear Pump 2

External gears from a magnetically coupled Diener precision gear pump

How External Gear Pumps Function?

The operation of an external gear pump revolves around three key actions:

  1. As the gears unmesh at the pump’s inlet, they create a vacuum that draws fluid in.
  2. The rotating gears trap fluid between the teeth and the casing, transferring it from the inlet to the outlet.
  3. As the gears mesh at the outlet, the fluid is discharged under pressure.

Tight clearances and high rotation speeds ensure minimal internal leakage, making external gear pumps efficient and reliable.

DPP Diagram of external gear pump

Diagram of an external gear pump

Gear Types in External Gear Pumps

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.
Spur Helical Herringbone Gear Pumps

The three types of gears in external gear pumps: spur, helical and herringbone

Advantages and Limitations of External Gear Pumps

Main Features and Benefits of External Gear Pumps

External gear pumps are known for their compact design and simplicity, featuring few moving parts. They typically operate at speeds of up to 4000 rpm, allowing for high flow rates in a small footprint. These pumps are fully reversible, making them ideal for complex hydraulic operations. Unlike centrifugal pumps, gear pumps are self-priming and can dry-lift significant heights.

Due to their ability to produce a smooth, pulse-free flow that is directly proportional to speed, external gear pumps are commonly used in metering, blending, and feedback control operations. They can efficiently handle both high- and low-viscosity fluids, but consultation with pump engineers is essential to ensure the pump is operating at optimal conditions. Additionally, external gear pumps are among the quietest types of positive displacement pumps, making them suitable for noise-sensitive applications.

One significant variant is the magnetically coupled gear pump, which eliminates the need for dynamic seals, ensuring a long service life and minimizing the risk of external leakage.

Limitations of External Gear Pumps

The gears and journal bearings in external gear pumps rely on the pumped fluid for lubrication and should not be run dry for extended periods. If the fluid contains abrasives, it can cause rapid wear on both the gear surfaces and internal bearings, as the same fluid lubricates the components.

The tight clearances between gears, the gear mesh, and limited tooth volumes make it challenging for gear pumps to handle fluids with large suspended solids. If such solids are anticipated, a strainer should be installed on the inlet side. However, care must be taken to avoid creating excessive vacuum at the inlet, as this could lead to pump cavitation.

Operating gear pumps outside of their recommended speed range can also lead to issues. Hydrodynamic journal bearings in these pumps are designed for specific speeds, and running too slowly can accelerate wear. Additionally, high-temperature applications require that the pump’s materials be compatible with the operating temperature to prevent thermal expansion from affecting clearances and performance.

In cases of downstream blockages, gear pumps will continue to pump as long as sufficient power is provided, potentially causing over-pressurization and system rupture. Integrated pressure relief valves can help alleviate this by rerouting flow from the outlet back to the inlet when pressures exceed specified limits.

Lastly, the high speeds, precise tolerances, and gear meshing in external gear pumps make them unsuitable for shear-sensitive fluids, such as paints and soaps.

Materials choices and Construction Options

External gear pumps can be constructed from a broad range of materials, from traditional cast iron to modern composites. The proper selection of materials based on the fluid being pumped is essential for achieving optimal performance and durability. For example:

  • Externals: Iron, stainless steel, composites
  • Shafts: Steel, ceramic
  • Gears: Stainless steel, PTFE
  • Bearings: Carbon, bronze
  • Seals: Lip seals, mechanical seals, magnetic couplings

Factors that affect an external gear pump’s performance

Longevity and Precision:

The lifespan of a gear pump hinges on design, material selection, and manufacturing precision. High-quality bearings, proper lubrication paths, and tightly controlled machining tolerances are key to reducing wear and ensuring long service life. Gear profile quality and assembly alignment also play a significant role in pump performance. As an experienced gear pump manufacturer, at Diener Precision Pumps we work closely with our clients to engineer the optimal gear pump solution in line with the requirements of their systems for maximum precision performance and the longest pump service life.

Consistency and Repeatability:

For OEM applications, consistency between pumps is critical. Factors like gear, housing, and bearing tolerances can impact performance. Specialized manufacturers like Diener US, who possess strong quality control systems and robust testing protocols can ensure minimal pump-to-pump variation, maintaining performance within tight tolerances, especially at elevated pressures.

Seal Selection:

Seals are a critical, often life-limiting component of external gear pumps. Choosing the right seal depends on the fluid being pumped, as seals behave differently with water compared to aggressive chemicals. Magnetic couplings can eliminate the need for dynamic seals, offering a longer lifespan in systems where pressure and speed allow. At our California R&D facility, we work tirelessly to design, test and perfect innovative pump components and features that put us and our clients ahead of the competition.

 

With careful material selection, precision manufacturing, and an understanding of the application, external gear pumps can deliver long-lasting, reliable fluid transfer solutions across industries. At Diener we offer four main models of gear pumps which can be fully customized in terms of size, gear type, motor type and other features to best suit the needs of the clients’ systems and their intended applications. For expert advice on the best choice of pump design that best responds to the needs of your fluid management system, don’t hesitate to contact us.

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