Gear Pumps

Gear pumps are used to pump oil, or other thick liquids. The separating gears on the input side of the pump creates a vacuum which pulls in the liquid, which is then carried by the gears to the output side of the pump, where the meshing of the gears pushes the liquid out of the pump. There are several variations on this basic principal.

Exterior Gear Pumps

• Exterior gear pumps are the most common of the gear pumps. They are an excellent choice for pumping thick liquids like oil—they are almost always used for the oil pumps in automobiles. The pump consists of two meshed gears of equal size inside of a housing. Usually, only one of the gears are powered—the other is free to turn. The gears are tightly meshed and there is just the right clearance in the housing around the gears to insure free flow of pumped fluid. Sometimes in industrial uses it is necessary to reverse the flow, and this is one of the great advantages of gear pumps. They are reversible by simply reversing the direction of rotation of the driven gear.

Interior Gear Pumps

• Interior gear pumps consist of one gear inside of another fixed gear and the entire assembly encased in a housing. The arrangement reminds most people of a Wankel engine—the internal combustion that has a rotating triangular cam inside of a larger chamber. Like the Wankle engin, the rotation of the internal gear moves fluid through the system. The remarkable thing about interior gear pumps is the range of viscocities of the fluids it can pump. It can pump very thin solvents, water, oil, chocolate and even concrete. They cannot pump gasses. They can run dry for a while without damage (especially when they are pumping a lubricant) and they are completely reversible.

Centrifugal Pumps

• Centrifugal pumps are the simplest of the gear pumps. They are not classified as gear pumps by some manufacturers and sellers, because they sometimes have impellers instead of gears, they can pump gasses and they are not reversible. Most of the industry, however, does classify them as gear pumps because the method of operation is the same–a rotating “gear” moves a fluid through the pump. A centrifugal pump has one gear centered in the middle of the pump. Fluid moves in the center of the pump near the hub of the rotating gear and exits somewhere near the rim of the spinning gear. Centrifugal force moves the fluid through the pump. These pumps are found in extremely small fluid lines where small electric motors turn tiny plastic gears. They are also found in much larger versions with impellers (paddles) acting as air pumps in HVAC applications.

Hydraulic Pumps

A hydraulic gear pump is a type of rotary pump that creates fluid flow and pressure by the rotation of two gears within the housing. One gear is driven by an external power source and the other is an idler. The hydraulic fluid enters the gearing by suction created at an inlet port; the rotation of the gears pushes/pulls the fluid, creating pressure in the fluid until it arrives at a discharge port. End seals prevent leakage around the gears.


• A hydraulic gear pump basically consists of the drive gear and idler gear, a housing, end covers and a drive shaft. The gears are spur-type (straight line), helical-type (shaped as a helix) or herringbone type (V-shaped). The shape is determined by the fluid to be pumped. The housing has three parts: two end covers and the center section, in which the gears rotate. The end covers contain the end seals and the shaft bearings. The drive shaft is keyed, or splined, to connect to the source of power for the pump. Gear pumps are sturdy; they’re not particularly susceptible to damage and wear, and thus require little maintenance.

Flow Rate

• The gear pump is a fixed-displacement, or positive-displacement, pump. This means that the flow is controlled by the speed of the driveshaft rotation; thus a gear pump requires a variable-speed motor to precisely control the flow. A less efficient way to vary the flow is to use a bypass valve that returns some of the fluid to its reservoir.

Fluid Types

• The hydraulic gear pump is capable of handling fluids of almost any type and viscosity. Engineered and operated correctly, a hydraulic gear pump can handle virtually any liquid petroleum products, foods or other liquids. They can even handle semi-liquid substances.


• The materials used for a hydraulic gear pump depend on what fluid is going to be pumped. The standard-fitted pump has a cast-iron housing, and the other components are determined according to the manufacturers’ choice. An all-iron pump’s housing, and all the other parts that come in contact with the fluid, are fabricated of iron or other ferrous metals. A bronze-fitted pump has a cast-iron housing and other fluid-contacting parts fashioned of bronze. An all-bronze pump is like the all-iron pump except in bronze. Corrosion-resisting pumps are made of materials like stainless steel.