Working principle of Rotary Gear Pumps

Gear Pump is one type of positive displacement rotary pump. In a gear pump, a fluid moves by enclosing a fixed fluid volume using the interlocking gears and then transferring it as the gear rotates. This pump delivers a pulse-free flow that is directly proportional to the rotational speed of the gears. Mainly, the Gear Pump is suitable for high pressure (where required).

The gear pump is a positive displacement pump and a fixed displacement pump. A positive displacement pump means that this pump provides a constant flow at a fixed speed regardless of the change in pressure and a fixed displacement pump means that it ejects a fixed quantity of fluid per revolution of the shaft. The inventor of the gear pump has been Johannes Kepler around 1600.

In a gear pump, there are generally two gears. One gear is the driver gear or the power gear, and the other is driven or the idle gear. The driver gear connects to some prime movers or any mechanical energy source.

The driver gear is also known as master gear, and the driven gear is also known as slave gear.

The driver gear is rotated using an electric motor, an IC Engine, or manually.

Usually, we divide gear pumps into internal gear pumps and external gear pumps. Do you know the differences between these two types of gear pumps? let's see where the differences are

Internal Gear Pump

The Internal Gear Pump is a new kind of positive displacement gear pump. Internal gear pumps use gears with an external rotor to generate flow and mesh with the internal rotor. When the gear comes out of the mesh on the inlet side, the liquid will suck by the pump. The liquid will discharge through the engagement of gears. Some include a crescent-shaped baffle separating the inlet and outlet volumes between the two gears.

Figure 1 below shows the internal parts of an Internal Gear Pump

Figure 1. Internal Gear Pump

Figure 2 shows internal parts with all details

Figure 2. Internal Gear Pump

External Gear Pumps

The External Gear Pumps use two sets of the same gears with external teeth to create flow. The gears' rotation causes the liquid comes in the inlet port and around the outer of the two rotating gears. As the liquid comes around the periphery, it will discharge to the outlet port. External gear pumps also have a safety valve to protect the pipeline.

Figure 3 below shows the internal parts of an External Gear Pump

Figure 3. External Gear Pump

Figure 4 shows internal parts with all details

 Figure 4. External Gear Pump

A simple comparison between Internal & External gear Pump

External Gear Pump	Internal Gear Pump
Two identical gears take place side by side, rotating with each another. The motor drives one gear only, and then when will turn, it moves the other gear.	Operates a “gear within a gear” principle, whereby one gear (the idler) takes place inside another gear (the rotor).
Suitable for high pressures	Suitable for moderate pressures
Compact in terms of construction and less expensive design.	Bulkier in terms of construction, more expensive design.
For medium/low-temperature fluids.	Can handle high-temperature fluids. They are often supplied with a heating jacket to maintain the fluid temperature.
More significant in terms of outlet sizes and high capacities.	Smaller in terms of outlet sizes and higher capacities.
Bi-directional if spur gears are selected.	Always bi-directional flow.

Design benefits

High pressures - High viscosity fluids can be pumped over long distances using external gear pumps, thanks to their ability to handle high-pressure applications.

Self-priming capabilities - Gear pumps benefit from excelling self-priming capabilities thanks to their rotating gears evacuating any air in the suction line and forcing the liquid into the pump inlet. This gear pump makes it suitable for applications with difficult suction conditions where the pump may be located considerably above the liquid.

Bi-directional - Internal gear pumps have spur gears, which enable flow in either direction. It makes them ideal for filling and emptying containers or applications that require an excess fluid return to prevent it from sitting in the pipework. External gear pumps are usually available with spur gears, making them bi-directional or herringbone gears for viscous fluids, but do not allow reversible flow.

Steady and pulse-less flow - Both types, particularly internal gear pumps, provide a smooth, constant flow as their gears roll in the same direction without interrupting the liquid flow. The tight internal clearances between the gear teeth and the pump casing allow exact fluid volumes to pass for each gear revolution, guaranteeing a reliable, steady fluid output, which requires accurate metering. 

Typical applications of a gear pump

Gear pumps are among the most common pump types for handling clean oil and other self-lubricating, high viscosity liquids such as paints and soaps in relatively high-pressure installations. Thanks to tight internal clearances and positive displacement operation, external gear pumps benefit from a more reliable measure of the fluid that has passed through, making them ideal for batching applications. Internal gear pumps can handle high-temperature fluids such as thermal oil and can be supplied with an external jacket to maintain the temperature level.

Below is the list of the applications where gear pumps usually are used:

• Automotive, lube and fuel oils
• Hydraulic applications
• Paint and ink
• Resins, adhesives and polymers
• Pulp, lime and lye
• Small scale hot oil circulation
• Liquid soap

Whilst suitable for relatively thick oils, they are typically only suited to clean, non-abrasive liquids free of solids due to the meshing gears. If solids make it into the tight clearances between the gears, they can result in immediate wear and a reduced pump lifespan.

Low viscosity fluids should also generally be avoided, particularly with internal gear pumps. They can cause the fluid to leak backwards or “slip” through the tight spaces from the high-pressure discharge side back to the lower pressure suction side, practically can cause reduced flow and efficiency.

Operation guidance

Do not run dry - If a gear pump is run dry, the unlubricated gears will rub together, causing friction. The heat generated by this friction will cause the gears to expand and wear against the pump casing resulting in damage to the internals. It can also result in the tight tolerances lost if the gear teeth effectively become chewed up, resulting in backwards flow and lost efficiency.

Operate at maximum speed - Gear pumps struggle to maintain performance at low speeds and flow rates due to their weak volumetric efficiency. Therefore to get the best from the output, they need to operate as close to their maximum rated speeds.

Inspect bearings - Each gear is supported by a shaft with bearings on either side of the gear; an external gear pump has four bearings, and an internal gear pump has two. These need to be regularly checked for wear as they maintain the balance of the gears.

Check gear clearance -The operation of a gear pump relies on the tight clearances between the teeth and the chamber. A standard test to see if there has been wearing is to try and put a piece of paper between the permissions; if it quickly passes, there is evidence to suspect wear.

Advantages of Rotary Gear Pump

• High pressure
• No overlapping loads for the bearing
• The design is suitable for a wide range of materials.
• Easy to use and maintain - The gear pump is compact and consists of only two gears, the pump body and the front and rear covers. Therefore, compared with other pumps, the gear pump has a small weight, which is convenient for daily transportation and does not require much labour. Also, because of its lightweight, the gear pump is more convenient to use and more convenient when the work content is the same. At the same time, because of its simple structure and fewer components, it is more convenient to repair when problems occur.
• Low cost - Compared with the conventional pump, the gear pump is smaller in weight and easy to transport, saving transportation costs. In addition, the gear pump is cheaper because of its simple structure and lower manufacturing cost. The maintenance procedure will be simple in the future, and the maintenance cost will be low. Therefore, gear pumps are generally more economical and can effectively save costs.
• High work efficiency - The fluid loss in the gear pump is slight. Although some of the fluid will lubricate both sides of the bearing and gear, the pump body can never be fitted without clearance, resulting in a gear pump operating efficiency of 100%. However, the pump can still operate well and achieve an efficiency of 93% to 98%.
• Insensitive to fluid viscosity and density - If the viscosity or density of the fluid changes, the gear pump will not be affected too much. If a filter or a restrictor takes place on the discharge port's side, the gear pump will push the fluid through them. If the filter is dirty or clogged, the gear pump will still maintain a constant flow until it reaches the mechanical limit of the weakest part of the unit. It also can cause the gear pump to be insensitive to oil contamination and is more suitable for petrochemical industries.

Disadvantages of Rotary Gear Pump

• Four bushings in the liquid area
• No solids allowed
• Fixed end clearances
• Large noise- Because the gear pump has the characteristics of radial force imbalance and a large flow artery, it generates a loud noise. If it is in an area where there is a decibel requirement for the surrounding environment, or if it will use in the middle of the night, the gear pump will affect the work or the rest of others, causing inconvenience. The existence of unbalanced radial forces will also affect the service life of the bearings to a certain extent.
• Unadjusted displacement - The inter-tooth groove of the end cap and gear constitutes several fixed sealed working chambers, so the displacement of the gear pump is not adjustable and can only be used as a dosing pump. Practically, it is not possible if you want to increase the pump's displacement.
• It is not easy to repair after wear - Because the gear pump parts are poorly interchangeable, so it is not easy to repair after wear. Although the gear pump repair process is simple, the entire gear pump is almost impossible to repair if the pieces are worn.
• Never drive dry