Technical Foundation
What Is a Hydraulic Gear Pump?
A hydraulic gear pump generates flow by trapping fluid in the spaces between meshing gear teeth and carrying it from the pump's inlet to outlet port as the gears rotate, delivering a fixed volume of fluid per revolution (displacement) regardless of outlet pressure, up to the pump's rated maximum. External gear pumps use two identical gears meshing outside one another, are the simplest and most economical gear pump design, and are the standard choice for general hydraulic power units where variable flow control isn't needed. Internal gear and gerotor pumps use one gear rotating inside another with a different tooth count, producing smoother flow with lower pulsation and noise than external gear designs, at a higher cost.
Because a gear pump is fixed-displacement, actual flow output scales directly with shaft speed — the pump doesn't compensate for downstream pressure changes the way a pressure-compensated piston pump can, so system flow control for a gear-pump circuit is typically achieved with flow control valves or by varying pump drive speed rather than at the pump itself. Pressure rating and displacement should be selected together against the circuit's required flow (GPM) at the prime mover's actual rotational speed, and against the maximum system pressure the pump will see, including any pressure spikes from cylinder deceleration or relief valve cracking pressure.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Displacement | Cubic in./rev or cc/rev | Sets flow rate (GPM) at a given shaft speed — flow = displacement x RPM |
| Maximum Pressure Rating | Continuous and intermittent (peak) pressure, psi | Must exceed max system pressure including relief valve setting and pressure spikes |
| Gear Type | External gear, internal gear, gerotor | Internal/gerotor gives smoother, quieter flow; external gear is simpler and lower cost |
| Shaft/Mounting Configuration | SAE A/B/C flange, keyed or splined shaft | Must match the prime mover's mounting pattern and coupling |
| Rated Speed Range | Minimum and maximum RPM per manufacturer spec | Running below minimum RPM can starve pump lubrication; above max RPM accelerates wear |
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Applications
Where Hydraulic Gear Pumps Get Used
Hydraulic gear pumps are specified wherever a simple, economical, fixed-displacement pump meets the circuit's flow and pressure requirements without needing variable displacement control.
Industrial Hydraulic Power Units
External gear pumps as the standard fixed-displacement pump for general-purpose hydraulic power packs.
Mobile Hydraulic Equipment
Gear pumps driving cylinder and motor circuits on agricultural and construction equipment for their simplicity and durability.
Hydraulic Lift & Press Systems
Fixed-displacement gear pumps supplying flow for lift table and press cylinder circuits.
Lubrication System Pumping
Internal gear and gerotor pumps used for smooth, low-pulsation lubricant circulation in machine tool and engine lubrication.
PTO-Driven Hydraulic Systems
Gear pumps matched to variable PTO shaft speed on tractors and mobile equipment where flow varies with engine RPM.
FAQ
Hydraulic Gear Pumps Questions, Answered
How do I calculate the flow rate of a gear pump?
Flow rate equals the pump's displacement (in cubic inches or cc per revolution) multiplied by shaft speed in RPM, then converted to gallons per minute — because a gear pump is fixed displacement, this relationship is linear and flow only changes if shaft speed changes, unlike a variable-displacement pump which can maintain flow or pressure independent of speed within its control range.
What's the difference between external and internal gear pumps?
External gear pumps use two identical gears meshing outside each other and are the simplest, most economical, and most common design for general hydraulic circuits. Internal gear and gerotor pumps use one gear rotating inside a larger internally-toothed gear, which produces smoother, lower-pulsation flow with less noise — a meaningful advantage in noise-sensitive or precision applications, at a higher unit cost than external gear pumps.
Can a gear pump be used in a variable flow or pressure-compensated circuit?
Not directly — a gear pump delivers fixed displacement per revolution regardless of downstream pressure, so it can't inherently vary flow to maintain a set pressure the way a pressure-compensated piston pump can. Flow or pressure control in a gear pump circuit is achieved externally, with flow control valves, relief valves, or by varying the pump's drive speed.
What causes a gear pump to lose flow or pressure over time?
The most common cause is internal wear — as the gears, side plates, and housing wear from years of operation, internal clearances increase and more fluid slips back from the outlet to the inlet side internally (a phenomenon called slippage), reducing volumetric efficiency. This shows up as reduced actual flow output at a given speed even though the pump still turns and builds some pressure; a repair kit with new gears and wear plates can often restore performance.
What pressure rating do I need for my hydraulic circuit?
Select a pump rated above your circuit's maximum system pressure, including the relief valve's cracking pressure and any transient pressure spikes from cylinder deceleration or valve shifting, not just the nominal working pressure of the circuit. Running a pump consistently near or above its maximum continuous pressure rating significantly shortens its service life.
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