How Two Meshing Gears Turn Shaft Speed Into Predictable Fluid Flow

How two meshing gears turn shaft speed into predictable fluid flow

An external gear pump converts shaft rotation into a nearly steady stream of fluid. Two gears rotate inside a close-fitting casing, carry fluid around the outside of the gear set, and release it at the discharge.

The mechanism is straightforward; applying it successfully is not. Nominal displacement establishes theoretical flow, but pressure, speed, viscosity, temperature, inlet conditions, clearances, contamination, materials, and wear determine how much flow reaches the system—and how long the pump lasts.

The practical approach is to calculate a preliminary displacement, evaluate the complete duty point, and confirm the result against performance curves and compatibility data for the exact pump, fluid, and operating conditions.

What an external gear pump is—and what makes it different

An external gear pump is a rotary positive-displacement pump with two externally toothed gears on separate shafts. The prime mover turns the drive gear, which turns the driven or idler gear in the opposite direction. Viking Pump’s external gear pump definition identifies these drive and driven gears and the transfer of liquid between their teeth.

The principal components are:

• A casing or housing
• Drive and driven gears
• Separate shafts
• Shaft-support bearings or bushings
• Inlet and discharge ports
• A shaft-sealing arrangement
• Side plates, wear plates, or other clearance-control components where used

Exact construction varies. A compact hydraulic pump, a chemical-metering pump, and a jacketed polymer pump may use the same pumping principle while having substantially different bearings, seals, materials, clearances, and temperature-control arrangements.

External and internal gear pumps should not be confused:

• An external gear pump uses two gears whose external teeth mesh with one another.
• An internal gear pump uses differently sized elements that mesh internally, commonly with one rotating inside the other.

External pumps often use spur gears. Some products use helical or herringbone gears instead. Gear geometry can affect engagement, axial loading, noise, and other operating characteristics, so the term “external gear pump” does not describe one universally interchangeable design.

The pump is normally described as fixed displacement. That means it encloses a nominal volume during each shaft revolution. It does not mean delivered flow remains perfectly fixed as pressure, viscosity, temperature, or wear changes. Some liquid leaks internally through necessary running clearances, and that slip varies with operating conditions.

Those clearances create the central design trade-off:

• Tighter clearances can reduce backward leakage and improve volumetric performance.
• Those same clearances make abrasive contamination, thermal growth, rubbing, and dimensional changes more consequential.
• Larger or worn clearances provide a greater leakage path, reducing delivered flow—particularly as differential pressure rises.

A successful selection therefore requires more than matching nominal displacement to a desired flow rate.

How an external gear pump moves fluid

The pumping cycle has three stages:

1. Filling: At the inlet, the gear teeth move out of mesh. The opening tooth spaces create expanding volume, allowing liquid to enter and fill the cavities between adjacent teeth.
2. Transfer: Rotation traps the liquid between the tooth spaces and casing. The liquid travels around the outside perimeter of each gear toward the discharge.
3. Delivery: At the outlet, the teeth re-enter mesh. The available cavity volume contracts, displacing the liquid through the discharge port.

The principal flow path does not pass through the center between the gears. Their teeth remain meshed in that region, forming a barrier between the discharge and suction sides. Michael Smith Engineers’ illustrated explanation of external gear pumps describes the same filling, perimeter-transfer, and remeshing sequence.

Radial clearance exists between the gear tips and casing. Lateral or axial clearance exists between the gear faces and adjacent plates or housing surfaces. These gaps provide necessary running clearance, but they also create leakage paths from the high-pressure side toward the inlet.

Reducing the gaps generally limits slip. Reducing them too far can leave inadequate allowance for lubrication, manufacturing variation, deflection, thermal expansion, or contamination. Appropriate clearance is therefore a design choice tied to the intended fluid and operating envelope—not simply a gap that should always be minimized.

Because each revolution encloses a similar nominal volume, output is nearly steady and proportional to speed. It is not necessarily pulse-free. Individual tooth cavities fill and discharge in sequence, and tooth engagement can produce flow and pressure ripple.

The pump primarily creates flow. A pump may be rated to withstand a specified pressure, but it does not independently impose one fixed discharge pressure regardless of the system.

Flow, displacement, pressure, and efficiency

Displacement is the nominal volume moved during one shaft revolution. It is commonly expressed in cubic centimetres per revolution, or cm³/rev. Shaft speed is expressed in revolutions per minute, or rpm.

With metric units, theoretical flow is:

Q_theoretical\;(L/min) = V_d\;(cm³/rev) × n\;(rpm) ÷ 1000

where:

• Q_theoretical is theoretical flow
• V_d is displacement per revolution
• n is shaft speed

This is the standard fixed-displacement relationship: theoretical flow equals displacement multiplied by rotational speed, with the factor of 1,000 converting cubic centimetres to litres (hydraulic gear-pump sizing guidance).

For example, a 20 cm³/rev pump turning at 1,500 rpm has a theoretical flow of:

20 × 1500 ÷ 1000 = 30 L/min

The result is theoretical; it does not account for internal leakage.

Actual delivered flow can be estimated by applying volumetric efficiency:

Q_actual = V_d × n × \eta_v ÷ 1000

where \eta_v is volumetric efficiency expressed as a decimal. Rearranging the equation gives a preliminary displacement estimate:

V_d = Q_required × 1000 ÷ n × \eta_v

These relationships follow the displacement-speed-efficiency sizing method used in rotary gear-pump guidance, although the efficiency value must come from data relevant to the actual pump and duty point (NAPCO sizing guide).

Suppose a system requires 30 L/min at 1,500 rpm and the preliminary analysis assumes a volumetric efficiency of 0.85:

V_d = 30 × 1000 ÷ 1500 × 0.85 ≈ 23.5 cm³/rev

The approximately 23.5 cm³/rev result is a calculated starting point based on the stated 0.85 assumption—not a final pump selection. The chosen model must still deliver the required flow at the actual pressure, speed, viscosity, and temperature.

As differential pressure increases, the driving force for discharge-to-suction leakage generally increases. Delivered flow can consequently fall even though displacement and shaft speed remain unchanged. Wear can enlarge the leakage paths, making the pressure-related decline more pronounced.

Three efficiency terms must be kept separate:

• Volumetric efficiency compares actual delivered flow with theoretical displacement flow.
• Mechanical efficiency reflects torque or power losses associated with friction, viscous drag, bearings, seals, and related mechanical effects.
• Overall efficiency combines volumetric and mechanical performance.

Published percentages are not meaningfully comparable unless they use the same definition and comparable test conditions. A no-load efficiency, volumetric efficiency, and overall efficiency are not interchangeable. Fluid, pressure, speed, temperature, and test method also matter.

For the same reason, there is no useful universal pressure, speed, viscosity, or efficiency range for every external gear pump. General references and vendor guides publish conflicting limits, while specialized process designs can operate in substantially different envelopes. Use the selected model’s ratings and performance curves rather than treating a category-wide figure as a specification.

The clearance trade-off: viscosity, temperature, contamination, and wear

Clearance connects four issues that are too often evaluated separately: leakage, viscosity, temperature, and contamination.

A tighter internal gap can reduce slip. But it also means that a particle, thermal growth, or minor surface damage occupies a larger fraction of the available running clearance. Once surfaces rub or score, wear can enlarge the gap and increase leakage—the opposite of the original objective.

Viscosity creates competing effects. A more viscous fluid may leak less readily through internal gaps, improving volumetric efficiency under some conditions. At the same time, it resists flow through the inlet piping and into the tooth cavities.

The relevant viscosity is not merely the value listed at room temperature. Selection should account for:

• Maximum viscosity during cold startup
• Viscosity during normal operation
• Minimum viscosity at the highest credible temperature
• Changes caused by dilution, blending, or degradation
• Non-Newtonian or shear-thinning behavior where applicable

Treating cP and cSt as interchangeable can lead to a mismatched pump.

Temperature affects both the liquid and the hardware. A temperature change can alter viscosity, while the casing, gears, shafts, plates, bearings, and bushings also expand. Differential expansion can reduce operating clearances.

Contamination attacks the same close-tolerance surfaces that control leakage. Hard particles can score gear tips, gear faces, side plates, casing bores, bushings, or bearings. The resulting clearance growth increases slip and reduces volumetric efficiency.

A suction strainer may intercept damaging debris, but it is not a complete contamination-control strategy. Filtration decisions must balance required cleanliness against the pump’s allowable inlet loss.

Specialized high-viscosity pumps show why category-wide assumptions are unsafe. A peer-reviewed study of a GNP2200 external gear pump for aerospace-composite polymer service used jacketed pump-body components for temperature control and reported performance findings tied to that particular design and test program. Its numerical speed and efficiency findings should not be transferred to unrelated pumps (GNP2200 polymer-pump study).

Fluid behavior can also be more complicated than one viscosity value suggests. A finite-element study modeled Newtonian and shear-thinning flow through a specific external gear-pump geometry, examining interactions among rheology, speed, pressure difference, leakage, and residence time. It also identified inlet-channel vortices that could retain material longer in the modeled pump. These are useful considerations for reactive or degradable materials, but they remain geometry- and model-specific findings (finite-element external gear-pump study).

Where external gear pumps fit—and where caution is warranted

External gear pumps are considered across several broad application groups:

• Hydraulic power: mobile and industrial hydraulic circuits
• Lubrication and fuel transfer: oils, fuels, and circulating-lubrication duties
• Chemical processing: metering, dosing, blending, and transfer
• Coatings and formulated products: paints, inks, resins, and adhesives
• Polymer processing: specialized transfer and metering of viscous polymeric materials

The attraction is straightforward. The mechanism is compact, displacement is fixed by geometry, and theoretical flow is proportional to shaft speed. When the fluid, clearances, inlet conditions, drive control, and system resistance are properly matched, this behavior can support repeatable transfer or metering.

A common hydraulic-oil pump should not, however, be treated as equivalent to a purpose-built process pump. Their approved speeds, pressure cycles, seal systems, bearings, materials, clearances, temperature-control provisions, and allowable viscosity ranges may differ substantially.

Before accepting a pump for a fluid, check:

• Viscosity at startup and normal operating temperature
• Lubricity and film-forming behavior
• Abrasiveness
• Particle size, hardness, and concentration
• Required fluid cleanliness
• Chemical composition and concentration
• Corrosion potential
• Shear sensitivity
• Tendency to foam, entrain air, or release vapor
• Minimum and maximum temperature
• Potential for curing, crystallization, solidification, or degradation

Every wetted component matters, including gears, shafts, side plates, bushings, bearings, seals, elastomers, coatings, and any wetted internal passages.

Abrasive solids are generally a wear concern. A supplier may describe a particular product as particle-capable, but that claim needs defined limits for particle size, hardness, concentration, filtration, and clearance. The pump family as a whole should not be characterized as universally solids-tolerant.

High-viscosity service is possible, particularly with purpose-built pumps, larger inlet passages, appropriate clearances, lower speeds, adequate drive torque, and temperature control. It does not follow that a standard high-speed hydraulic pump is suitable for a polymer, adhesive, or similarly viscous material. Roper Pump’s high-viscosity selection guidance emphasizes viscosity, temperature, chemistry, abrasiveness, shear sensitivity, particles, seals, and materials rather than viscosity alone.

Shear-sensitive or reactive products need a broader assessment. Consider local shear, recirculation through clearances or relief paths, product temperature, residence time, inlet flow behavior, startup procedures, and cleaning requirements. A pump that meets nominal flow may still compromise product quality.

Self-priming, reverse rotation, bidirectional pumping, and operation as a hydraulic motor are design-specific capabilities. Porting, seals, bearing loads, lubrication paths, pressure plates, and relief arrangements may depend on one direction of operation. Require explicit manufacturer approval for the exact model and configuration.

How to select and size an external gear pump

A disciplined selection process moves from system requirements to model verification.

1. Define the required delivered flow. Specify normal, minimum, and maximum flow. For batch transfer, derive flow from the required volume and transfer time. For hydraulic service, calculate the flow needed by the actuators at their required operating speeds.

2. Determine pump differential pressure. At the pump flanges:

\Delta P = P_discharge - P_suction

If specified or measured suction- and discharge-flange pressures are available, use them directly and do not add the same system losses again. If flange pressures must be calculated, establish them from the relevant system components, which may include:

• Outlet back pressure
• Elevation or static head
• Straight-pipe losses
• Fittings, valves, and hoses
• Filters and strainers
• Heat exchangers or process equipment
• Nozzles or metering restrictions
• Available pressure at the pump inlet

Elevation pressure depends on fluid density or specific gravity. A pressure-per-unit-height value derived for water must not be applied unchanged to a fluid with a different density. Include credible clean and dirty filter conditions where relevant.

3. Establish available shaft speed. Determine the actual speed range from the motor, engine, gearbox, or variable-speed drive. Confirm the pump’s approved minimum and maximum speeds rather than relying only on the drive’s nominal rpm.

4. Define fluid properties at every operating state. Record fluid identity, composition, concentration, density, lubricity, vapor behavior, solids, abrasiveness, and tendency to aerate. Obtain viscosity at cold startup, normal operation, and the highest expected temperature.

5. Estimate displacement. Use:

V_d = Q_required × 1000 ÷ n × \eta_v

Choose the preliminary volumetric-efficiency assumption from relevant pump data, not from an unrelated product family. Then identify nearby commercially available displacements.

6. Check ratings and duty limits separately. Ask the manufacturer to verify:

• Continuous differential-pressure rating
• Intermittent or peak-pressure rating
• Permitted peak duration and frequency
• Continuous-duty capability
• Starting conditions
• Approved minimum and maximum speeds
• Direction of rotation
• Permitted external shaft loading, where relevant
• Mounting and coupling requirements

7. Review inlet conditions. Assess inlet pressure, pipe diameter and length, fittings, valves, fluid level, lift, startup viscosity, strainers, and vapor-release behavior. Use the manufacturer’s inlet-pressure or cavitation criteria.

8. Verify materials and seals. Check every wetted material against the exact fluid composition, concentration, temperature, and cleaning chemicals. Also consider environmental exposure around the pump.

9. Confirm the complete drive arrangement. Ask the pump and drive suppliers to verify the required running and starting torque, speed range, coupling, shaft connection, mounting, and credible upset conditions.

10. Validate the duty point on manufacturer curves. Request flow-versus-pressure data at or near the actual speed, viscosity, and temperature. Ask whether the curves show new-pump performance, minimum guaranteed performance, or typical test results. Clarify whether stated efficiency is volumetric, mechanical, or overall.

A fixed oversizing percentage is not a substitute for an efficiency-adjusted calculation and model-specific curves. Oversizing can waste energy, add heat, and accelerate wear, according to NAPCO’s vendor guidance; its numerical examples and product data should be applied only to the relevant pumps.

A supplier data sheet should include:

Category	Information to provide or confirm
Performance	Required flow, differential pressure, preliminary displacement, rpm
Fluid	Identity, composition, density, viscosity, lubricity, solids, abrasiveness
Temperature	Cold-start, normal running, maximum process and ambient temperatures
Inlet	Available inlet pressure, lift or flooded condition, line and strainer details
Duty	Continuous or intermittent operation, starts per cycle, peak duration
Drive	Motor or engine details, speed control, starting conditions
Configuration	Rotation direction, port orientation, mounting position
Construction	Casing, gears, shafts, bearings, plates, coatings
Sealing	Seal type, elastomers, flush or barrier requirements
Protection	Filtration, pressure-relief arrangement, instrumentation
Maintenance	Access, drainage, flushing, spares, inspection requirements

Safety-critical systems, unusual chemicals, severe inlet conditions, hazardous locations, and pressure-relief selection warrant review by the pump manufacturer or a qualified engineer.

External versus internal gear pumps—and other alternatives

External and internal gear pumps both move liquid by trapping and displacing volume, but their mechanisms differ:

• An external gear pump has two externally toothed gears meshing on separate shafts.
• An internal gear pump has differently sized elements meshing internally, generally with one rotating inside the other.

The following is a screening comparison, not a universal ranking.

Consideration	External gear pump	Internal gear pump
Mechanism	Two externally meshing gears	Internally meshing elements of different sizes
Common orientation	Hydraulic power, lubrication, transfer, metering, and specialized process service	Transfer and process duties, including many viscous-fluid applications
Viscosity	Standard hydraulic designs may have limited high-viscosity suitability; specialized external pumps can handle polymers	Often screened for viscous transfer, but suitability remains model-specific
Pressure	Can be suitable for substantial differential pressure, subject to model ratings	Model-dependent; the family name does not establish a universal limit
Ripple and noise	Nearly steady flow, but gear engagement can produce ripple and noise	Often promoted for smoother delivery, though actual behavior depends on design
Shear sensitivity	Requires assessment of shear, recirculation, and residence time	Often considered for sensitive products, but not automatically suitable
Contamination	Close clearances and wetted bearings may be vulnerable to particles	Close clearances may also be vulnerable; limits remain product-specific

Supplier comparisons disagree about universal high-viscosity capability, self-priming performance, pulsation, and particle tolerance. These disagreements are evidence that construction and duty matter more than the family name. A supplier comparison of internal and external gear pumps, for example, favors internal pumps for some viscous duties while characterizing external designs as more pulsating; both points still require verification for the models under consideration.

A specialized external pump may be appropriate for a high-viscosity polymer. In another application, an internal gear pump or a different purpose-built process pump may offer better inlet performance, temperature control, or product handling.

Other positive-displacement technologies should remain on the screening list:

• Vane pumps may warrant evaluation when a candidate gear pump cannot meet the required noise, control, pressure, or fluid conditions.
• Piston pumps may warrant evaluation where pressure, variable control, or efficiency requirements exceed the capability of candidate gear-pump models.

These are screening considerations rather than universal statements that one technology is quieter, more efficient, or more durable.

The practical rule is simple: compare candidate performance curves, compatibility data, allowable inlet conditions, duty ratings, and maintenance needs at the same duty point. Do not select solely by pump family.

Installation, protection, and troubleshooting

A blocked discharge is a critical hazard. Because an external gear pump is a positive-displacement pump, it can continue displacing liquid against a closed valve, plugged filter, frozen line, or other obstruction. Pressure may continue rising until the drive stalls or equipment fails. The system therefore requires a pressure-relief arrangement approved for the pump, fluid, expected flow, and system conditions (external gear-pump safety guidance).

Do not assume another protective device automatically provides the required hydraulic overpressure protection. Confirm the relief arrangement, allowable pressure, and installation requirements with the pump and system documentation. Relief-device selection for a safety-critical system should be reviewed by the manufacturer or a qualified engineer.

Avoid dry running unless the exact pump is explicitly rated for it. In many external gear-pump designs, the pumped liquid lubricates the gears and other internal surfaces, and prolonged dry running is discouraged (gear-pump operating guidance).

Even a pump described as self-priming may have separate limits on initial wetting, suction lift, speed, and permissible dry duration. Follow the model-specific startup instructions.

Before startup:

• Confirm the fluid, composition, viscosity, and temperature.
• Fill, wet, or prime the pump as instructed.
• Verify shaft rotation and port connections.
• Open all valves required for the intended flow path.
• Confirm that the specified pressure-relief path is available.
• Inspect the suction line for closed valves, blocked strainers, air leaks, or collapsed hose.
• Check shaft alignment, coupling installation, mounting, and guarding.
• Confirm drive speed.
• Follow the manufacturer’s startup procedure.

When performance changes, measure conditions before disassembling the pump. Record actual shaft speed, inlet pressure, discharge pressure, delivered flow, fluid temperature, and contamination condition. Compare measured flow at pressure with the manufacturer’s curve at the nearest available viscosity and speed.

Symptom	Plausible causes	Useful checks
Low flow	Higher differential pressure, low viscosity, worn clearances, inadequate speed, inlet starvation, wrong rotation	Measure rpm, inlet and outlet pressure, flow, and temperature; verify rotation and compare with the curve
Noise	Cavitation, aeration, gear or bearing damage, excessive speed, misalignment	Inspect inlet restrictions and air leaks; check inlet pressure, coupling, speed, and fluid condition
Excessive heat	High viscosity, excessive pressure, internal recirculation, restrictive piping, rubbing	Measure temperature and pressure; inspect relief flow, inlet restriction, drive load, and internal condition
Rapid wear	Abrasive contamination, poor lubricity, incompatible fluid, excessive load, thermal clearance loss	Analyze contamination; inspect gears, bearings, plates, seals, and casing surfaces
Leakage	Seal wear, incompatible elastomer, excessive pressure, shaft movement, thermal damage	Verify seal materials, pressure conditions, shaft condition, alignment, and applicable drain arrangements
Unstable flow	Air entrainment, vapor release, fluctuating inlet conditions, speed variation, relief cycling	Trend speed and pressures; inspect liquid level, suction piping, tank return arrangement, and relief operation

Some decrease in flow as pressure rises is expected because internal slip increases. The acceptable amount is model-specific. A larger-than-expected decline can indicate worn clearances, low viscosity, excessive temperature, or operation outside the selected pump’s range.

Inspect gears, side plates, shafts, bearings or bushings, seals, and fluid condition according to the manufacturer’s procedure. Where food, chemical, polymer, or reactive products are involved, follow the approved drain, flush, and cleaning process. Verify that cleaning liquids are compatible with every wetted material. Do not invent a flushing method or interval for a product that may cure, react, crystallize, or create a hazardous mixture.

External gear pump FAQs

Can an external gear pump run in reverse or pump in both directions?

Some can, but the capability is not universal. Rotation can affect which port is the inlet, seal loading, bearing lubrication, pressure-plate operation, and any integral relief arrangement. A pump intended for one direction may perform poorly or be damaged if reversed.

Use reverse rotation or bidirectional pumping only when the manufacturer explicitly approves the exact model, port arrangement, speed, pressure, and duty cycle. Likewise, do not assume a pump can serve as a hydraulic motor merely because pressurized liquid can rotate its gears.

Can an external gear pump handle high-viscosity fluid?

Yes, a purpose-built external gear pump can handle high-viscosity service. Successful operation may require larger inlet passages, slower speed, adequate startup torque, suitable clearances, temperature control, compatible seals, and protection against curing or solidification.

Do not infer suitability from the words “gear pump” alone. A standard hydraulic unit and a jacketed polymer-process pump are substantially different products. Obtain performance data at the actual startup and running viscosities, then check inlet filling, drive requirements, temperature, shear, and residence-time concerns.

Does an external gear pump need a relief valve?

The system needs an approved form of overpressure protection because a positive-displacement pump can continue moving liquid against a blocked or closed discharge.

The relief arrangement must be selected for the pump flow, allowable system pressure, fluid, temperature, and installation. Have safety-critical relief selection reviewed by the manufacturer or a qualified engineer.

Why does external gear-pump flow decrease as pressure rises?

Higher differential pressure generally drives more liquid backward through the pump’s radial and lateral clearances. That internal slip reduces delivered flow even when displacement and shaft speed remain unchanged.

The amount depends on clearance geometry, viscosity, temperature, speed, and wear. Compare measured flow against the manufacturer’s pressure-flow curve under comparable conditions. An excessive decline may indicate enlarged clearances, unsuitable low viscosity, high liquid temperature, or internal damage.

Can an external gear pump handle solids or abrasive particles?

Do not assume that it can. Hard or abrasive particles can damage gear faces, tooth tips, side plates, casing surfaces, bearings, and bushings. Wear enlarges clearances, increasing slip and reducing delivered flow.

A particular model may tolerate defined particles, but that requires manufacturer limits for size, hardness, concentration, and filtration. A suction strainer can intercept some debris, yet an undersized, clogged, or excessively restrictive strainer can starve the inlet. Select contamination control and inlet protection together using the manufacturer’s criteria for the exact fluid and duty.

Final selection rule

An external gear pump makes flow predictable by pairing known displacement with shaft speed. Real output and service life, however, remain governed by differential pressure, viscosity, temperature, inlet conditions, clearances, contamination, materials, and wear.

Calculate a preliminary displacement, evaluate the complete duty point, protect the system against blocked-discharge pressure, and then validate the choice with model-specific curves and compatibility data. Safety-critical sizing, chemical compatibility, severe inlet or cavitation conditions, and pressure-relief selection should receive manufacturer or qualified-engineering review.