What is a Gear Pump? | How Does Gear Pump Work?

What is a Gear Pump?

A gear pump uses the meshing of gears to pump fluid by positive displacement. They are one of the most common types of pumps for hydraulic fluid power applications. The gear pump was invented by Johannes Kepler around 1600.

Gear pumps are also commonly used in chemical plants to pump highly viscous liquids. There are two main variations: external gear pumps, which use two external spur gears, and internal gear pumps, which use an external and internal spur gear.

Gear pumps are positive displacement (or fixed displacement) pumps, which means they pump a constant amount of fluid with each revolution. Some gear pumps are designed to function as either a motor or a pump.

MORE: What is Pump and Their Types?

What is an External Gear Pump?

An external gear pump consists of two identical, interlocking gears supported by separate shafts. Generally, one gear is driven by a motor and this drives the other gear (the idler). In some cases, both shafts may be driven by motors. The shafts are supported by bearings on each side of the casing.

• As the gears come out of mesh on the inlet side of the pump, they create an expanded volume.  Liquid flows into the cavities and is trapped by the gear teeth as the gears continue to rotate against the pump casing.
• The trapped fluid is moved from the inlet, to the discharge, around the casing.
• As the teeth of the gears become interlocked on the discharge side of the pump, the volume is reduced and the fluid is forced out under pressure.

No fluid is transferred back through the center, between the gears, because they are interlocked. Close tolerances between the gears and the casing allow the pump to develop suction at the inlet and prevent fluid from leaking back from the discharge side (although leakage is more likely with low viscosity liquids).

External gear pump designs can utilize spur, helical, or herringbone gears.

MORE: What is Spur Gear?

What is an Internal gear pump?

An internal gear pump operates on the same principle but the two interlocking gears are of different sizes with one rotating inside the other. The larger gear (the rotor) is an internal gear i.e., it has the teeth projecting on the inside.

Within this is a smaller external gear (the idler – only the rotor is driven) mounted off-center. This is designed to interlock with the rotor such that the gear teeth engage at one point. A pinion and bushing attached to the pump casing hold the idler in position.

A fixed crescent-shaped partition or spacer fills the void created by the off-center mounting position of the idler and acts as a seal between the inlet and outlet ports.

• As the gears come out of mesh on the inlet side of the pump, they create an expanded volume.  Liquid flows into the cavities and is trapped by the gear teeth as the gears continue to rotate against the pump casing and partition.
• The trapped fluid is moved from the inlet, to the discharge, around the casing.
• As the teeth of the gears become interlocked on the discharge side of the pump, the volume is reduced and the fluid is forced out under pressure.

Internal gear pump designs only use spur gears.

How does a gear pump work?

Gear pumps use the actions of rotating cogs or gears to transfer fluids. The rotating element develops a liquid seal with the pump casing and creates suction at the pump inlet. Fluid, drawn into the pump, is enclosed within the cavities of its rotating gears and transferred to the discharge.

Gear pumps work by trapping fluid between the teeth of two or three rotating gears. Often, they are magnetically driven, which means they use less “wetted” materials for greater chemical compatibility. Gear pumps move a cavity that rotates rather than reciprocates.

These pumps move many small cavities per revolution, so they do not pulse nearly as often as diaphragm pumps. The major disadvantage of gear pumps is that increasing the backpressure does decrease the flow rate.

They work best when pumping against stable back pressure. Since gear pumps operate by carrying fluid between the teeth of two or three rotating gears, they are best suited for applications in which fluid shearing or particle contamination from gear wear is not a concern.

These pumps operate well with high system pressure applications and are commonly used for hydraulic fluid power uses, for example in tractors and garbage trucks, and with heavier viscosity fluids, such as oil, that are not compressible.

Gear pumps feature true positive displacement with every revolution delivering a precise volume. Since each pocket of fluid that passes through the chamber is small and so many pockets go through per unit of time, the flow rate is virtually pulseless.

Why choose Gear Pumps?

They offer many advantages, mainly:

• Low Shear. Their design and operating speed are low meaning movement is low shear.
• Self-Priming. They are self-priming up to 6.5M.
• Reversible. Due to their design, they can operate in both directions, ensuring hoses can be emptied and allowing full recovery of any products. However, the relief valve will only operate in one direction.
• Efficient. Models are up to 85% efficient.
• Predictable. Flow is proportionate to speed ensuring repeatable and predictable flow rate.
• Non-Pulsating. The smooth rotary motion at low rpm means pulsations are not experienced as is more common with other positive displacement designs.
• Materials. Designs are in complete metal meaning units can be Atex rated (explosion proof), handle solvents as internals parts are not rubber as in other positive displacement pumps. They can also accommodate high temperatures of up to 350°C.
• Limited dry running. Units can run dry for a limited time providing the gears have been immersed in a lubricating liquid.
• Low NPSH. NPSH requirements are very low due to their slow operation. Internal gear pumps NPSH ranges from 0.5M to 4M based on water, with external designs generally being up to 3M.

Application of Gear Pumps

Gear pumps are commonly used for pumping high viscosity fluids such as oil, paints, resins, or foodstuffs. They are preferred in any application where accurate dosing or high-pressure output is required.

The output of a gear pump is not greatly affected by pressure so they also tend to be preferred in any situation where the supply is irregular.

• Petrochemicals: Pure or filled bitumen, pitch, diesel oil, crude oil, lube oil etc.
• Chemicals: Sodium silicate, acids, plastics, mixed chemicals, isocyanates etc.
• Paint and ink.
• Resins and adhesives.
• Pulp and paper: acid, soap, lye, black liquor, kaolin, lime, latex, sludge etc.
• Food: Chocolate, cacao butter, fillers, sugar, vegetable fats and oils, molasses, animal food etc.

Advantages of Gear Pump

• 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 labor. It is also because of its light weight, the gear pump is more convenient to use, and it is 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 are encountered.
• Low cost. Compared with the conventional pump, the gear pump is smaller in weight and easy to transport, which saves transportation costs to some extent. In addition, the gear pump is cheaper because of its simple structure and lower manufacturing cost. The maintenance procedure is simple in the future and the maintenance cost is low. Therefore, in general, gear pumps are more economical and can effectively save costs.
• High work efficiency. In fact, the fluid loss in the gear pump is small. Although some of the fluid is used to 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 can 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 strainer or a restrictor is placed on the side of the discharge port, 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. This also causes the gear pump to be insensitive to oil contamination and is more suitable for use in petrochemical industries.

Disadvantages of gear pump

• Not easy to repair after wear. Because the gear pump parts are poorly interchangeable, it is not easy to repair after wear. Although the gear pump repair process is simple, if the parts are worn, the entire gear pump is almost impossible to repair.
• Large noise. Because the gear pump has the characteristics of radial force imbalance and large flow artery, it generates very loud noise. If it is in an area where there is a decibel requirement for the surrounding environment, or if it is used in the middle of the night, the gear pump will affect the work or 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 a number of fixed sealed working chambers, so the displacement of the gear pump is not adjustable and can only be used as a dosing pump. This is not possible if you want to increase the displacement of the pump.

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