Gear pump selection guide: What to consider

Gear pumps are a type of positive displacement pump, and can be found in many hydraulic systems.

Typically, applications which use positive displacement pumps are those which require constant flow, high pressure, high velocities or have special fluid requirements – and for this, gear pumps are the most common choice.

Whilst there are two main types of gear pumps, they come in several different forms which means selecting the right pump technology for your application can come with its challenges.

In this blog, we will provide you with a brief overview of gear pumps, and what to consider when selecting one for your application.

Types of gear pumps

The two main types of gear pumps are external and internal. Whilst they have similar pumping principles, there are some clear distinctions between them that will make one type more suitable for a specific application than the other.

External gear pumps

External gear pumps are designed with two identical gears positioned side by side and rotating into each other, with external teeth to generate flow. The motor drives one gear, which in turn rotates the other.

External gear pumps can run pressures beyond 3,000 psi / 200 bar with greater flow control and ensure a reliable, consistent measure of liquid passing through.

They are generally more compact in design, work to higher pressures and offer larger outlet sizes with high capacities. Typically used in fluid power applications, external gear pumps are best suited to medium/low-temperature fluids, as opposed to more abrasive, or highly viscous applications.

Due to their compact and efficient design, external gear pumps are generally less expensive and easier to maintain than internal gear pumps.

Internal gear pumps

Internal gear pumps are designed with one gear (the idler) positioned inside another gear (the rotor) and a stationary crescent fitted between the outer edge of the idler and the inner edge of the rotor.

Teeth are positioned below and above the crescent, and it’s here where the liquid travels through, causing the gears to mesh and liquid to be drawn into the pump and then discharged back out.

By design, internal gear pumps usually have better suction capabilities than external gear pumps, making them more suited to working with high viscosity fluids. They are also more versatile, with the ability to handle more abrasive and high-temperature fluids, alongside thin liquids such as alcohol and fuel.

Internal gear pumps have smaller outlet sizes and higher capacities, with maximum pressures reaching up to 300 psi / 20.7 bar.

Selecting a gear pump for your application

As explained, there are some major differences between internal and external gear pumps, and understanding these differences is crucial in making the right product selection.

When selecting a gear pump, there are a few key things that must be considered:

Media type

Having an understanding of the liquid properties used in the system is crucial when it comes to selecting a pump. The two main properties that should be evaluated are:

• Viscosity – How thick is the liquid? More viscous liquids generate higher pressure and require a greater amount of power to push them through the system. Internal gear pumps perform better with high viscosity fluids, as opposed to external.
• Consistency – What chemicals does the liquid contain? Does it contain any undissolved solids? Most gear pumps generally aren’t designed to work with corrosive chemicals or those with suspended solids due to the wear and damage that may be caused.

Liquid temperature

Whilst internal gear pumps are better designed to handle high temperature fluids, special considerations should be considered for extremely high or low temperatures.

If the fluid you’re working with falls into either category, then your chosen pump may require additional features such as special gaskets, packing materials and added clearances to avoid any mechanical issues.

Pressure

It’s important to define the pressure at which the pump is expected to operate. Gear pumps have a pressure rating which dictates how much pressure they can handle, indicated in bar or psi.

As a general rule of thumb, the rated operating pressure of the pump should be equal to or more than the required pressure of the system.

Flow rate

Gear pumps have a flow rate which dictates the capacity or volume of fluid that they can handle passing through the system over a given timeframe (measured in litres per minute, or gallons per minute).

When it comes to pump selection, the flow rate of both the system and the pump should be matched.

Construction materials

Aside from the properties of the liquid itself, consideration must be given to the materials used to construct the pump itself.

Gear pumps are made using many different materials, and everything from the media to environmental conditions should be taken into account when choosing the most suitable pump for your system to guarantee compatibility.

Some of the most common gear pump materials used are:

• Cast iron – typically comes with high pressure ratings due to its strength, durability and abrasion resistance.
• Stainless steel – protects the pump components from chemical and rust corrosion, with generally a higher pressure rating than plastic.
• Aluminium – has corrosion resistant properties, is easily machinable, and has a high strength to weight ratio.

 

Purchasing a gear pump

Whether you’re designing a hydraulic system from the ground up or need a replacement gear pump, Flowtech can help.

As one of the UK’s leading suppliers of hydraulic components, we can supply an extensive range of gear pumps from best-in-class manufacturers such as Roquet, Marzocchi and Rickmeier.

We can build or modify gear pumps to your exact specifications at our in-house facilities and we aim to honour any urgent needs with our quick turnaround times. To make an enquiry or find out more, get in touch with us.