Introduction
External and internal gear pumps work on the same basic principle, and from the outside they can look much alike. In service, however, they behave very differently. Put an external gear pump on cold bitumen and it struggles to fill. Put an internal gear pump on a high-pressure hydraulic circuit and it runs out of headroom.
Choosing between them comes down to a few clear factors: fluid viscosity, pressure, suction conditions and how gently the product must be handled. This guide explains how each design works, sets out the key differences side by side, walks through a step-by-step selection method and lists the typical applications for each.
Gear Pump Basics
How Both Designs Move Fluid
Both are rotary positive displacement pumps. Fluid enters where gear teeth separate, travels in the pockets between the teeth and the casing, and is pushed out where the teeth come back together. Each revolution moves a fixed volume, so flow stays steady even as discharge pressure changes.
The difference lies in how the gears are arranged.
What Is an External Gear Pump?
Design and Components
An external gear pump has two gears of the same size, side by side, each on its own shaft. One gear is driven by the motor and turns the other. The gears sit inside a close-fitting housing, with end plates on both sides. Typically, each shaft is supported by bearings on both sides of the gear, giving four bearings in total.
Gear Types
- Spur gears – straight teeth; simple and economical
- Helical gears – angled teeth; quieter and smoother
- Herringbone gears – double helical; smooth running with balanced axial thrust
Strengths and Limitations
Strengths
- High pressure capability
- Compact and relatively low cost
- Runs well at motor speed
- Bearings support the gears on both sides, so there is no overhung load
Limitations
- Weaker suction performance with very thick fluids
- Higher shear on the product at mesh
- Tight clearances wear quickly with abrasives
What Is an Internal Gear Pump?
Design and Components
An internal gear pump uses a large outer gear, called the rotor, with teeth on its inner surface. A smaller idler gear sits inside it, mounted off-centre. As the rotor turns, it drives the idler. A crescent-shaped spacer fills the gap between them and separates suction from discharge.
Crescent vs. Gerotor Designs
- Crescent type – uses the fixed crescent seal; common in industrial transfer pumps
- Gerotor type – no crescent; the inner rotor has one tooth fewer than the outer rotor, and the tooth profiles seal against each other
Strengths and Limitations
Strengths
- Excellent suction and filling of viscous fluids
- Low shear and smooth, low-pulsation flow
- Runs well at low speeds
- Most designs can pump in both directions by reversing rotation
Limitations
- Lower maximum pressure than external designs
- Idler is often supported on one side only, so bearing loads are overhung
- Generally less efficient with very thin fluids
External vs. Internal Gear Pumps: Key Differences
Comparison Table
| Factor | External gear pump | Internal gear pump |
| Gear arrangement | Two equal gears side by side | Small gear inside a ring gear |
| Pressure capability | Higher | Moderate |
| Viscosity handling | Low to medium | Medium to very high |
| Suction capability | Good | Better |
| Typical speed | High, often direct motor speed | Low to moderate |
| Shear on product | Higher | Lower |
| Bearings | Typically four, both sides of each gear | Typically fewer, idler often overhung |
| Reversible flow | Possible on some designs | Common |
| Initial cost | Lower | Higher |
Pressure Capability
External gear pumps handle higher pressures because both gears are supported on both sides and the casing can be made very stiff. This makes them the standard for hydraulic power and high-pressure lubrication.
Viscosity Handling
Internal gear pumps handle thick fluids far better. Their large, slow-moving pockets fill completely even with heavy liquids such as asphalt or molasses. Viscosity also helps here: thicker fluids seal the clearances, so slip falls and volumetric efficiency rises.
Suction Performance
Internal gear pumps usually have better suction capability. The inlet area is larger and fluid speed at the inlet is lower, so the pump is less prone to cavitation on difficult suction lines.
Speed
External gear pumps typically run at full motor speed. Internal gear pumps are often run slower, through a gearbox or variable frequency drive, especially on viscous duties.
Shear Sensitivity
Products such as emulsions, polymers, chocolate, cosmetics and some food products can be damaged by high shear. The gentler action of internal gear pumps protects product quality.
Bearings and Maintenance
External designs place bearings on both sides of each gear, so there is no overhung load and wear is predictable. Internal designs often have the idler pin and bushing as the main wear point. Many internal gear pumps are designed so the head can be removed for inspection without disconnecting pipework.
Reversibility
Most internal gear pumps can pump in either direction simply by reversing the motor. This is useful for loading and unloading tanks with one pump.
Cost
External gear pumps are generally cheaper to buy and simpler to maintain. Internal gear pumps cost more but can be the more economical choice for viscous duties because they last longer and handle the product better.
How to Choose the Right Model (Step-by-Step)
1. Fluid and Viscosity
Start here, because viscosity drives most of the decision.
- Thin to medium fluids (fuel oil, diesel, hydraulic oil, lube oil): external gear pumps work well
- Thick fluids (bitumen, resins, adhesives, syrups, heavy fuel oil): internal gear pumps are usually the better fit
Also note the viscosity at the lowest operating temperature. A fluid that is easy to pump at 60 °C may be very thick on a cold start.
2. Pressure and Flow
If you need high discharge pressure, such as in hydraulic power units, an external gear pump is the natural choice. For transfer and circulation at moderate pressure, both may fit, so other factors decide.
3. Suction Conditions
Long suction lines, suction lift or low tank levels favour internal gear pumps. If suction conditions are poor and the fluid is thick, avoid high-speed external designs.
4. Product Sensitivity
If shear can damage the product, or if solids such as sugar crystals or fillers must stay intact, choose an internal gear pump running at low speed.
5. Material Compatibility
Match casing, gear and seal materials to the fluid:
- Cast iron or ductile iron for oils and non-corrosive liquids
- Carbon steel for higher temperatures and pressures
- Stainless steel such as CF8M for chemicals and food products
- Duplex or nickel alloys for highly corrosive service
6. Maintenance Access
Consider how the pump will be serviced on site. Back pull-out or removable-head designs reduce downtime on critical lines.
Typical Applications by Pump Type
External gear pumps
- Hydraulic power units and mobile machines
- Lubrication systems
- Fuel injection and fuel transfer
- Chemical additive dosing and metering
- Low-viscosity solvents and oils
Internal gear pumps
- Asphalt and bitumen
- Heavy fuel oil and crude
- Adhesives, resins and polymers
- Paints and inks
- Chocolate, syrups, molasses and vegetable oils
- Tank loading and unloading with reversible flow
A useful pattern for buyers: where the pump sits close to a machine and the job is pressure, external designs dominate. Where the pump sits beside a tank and the job is moving a thick product gently, internal designs dominate.
Materials and Casting Considerations
In both designs, the casing and heads are usually castings. They need:
- Accurate, concentric bores to hold tight gear clearances
- Sound metal, free of porosity, near pressure-retaining walls
- A material that resists the fluid’s corrosion and temperature
For stainless and alloy pumps, investment casting is often preferred. It produces housings and rotor cases close to final shape, so less hard-to-machine alloy is cut away, and surface finish on internal passages is smoother.
FAQs
Which is better, an external or internal gear pump?
Neither is better in all cases. External gear pumps suit high pressure and thin to medium fluids. Internal gear pumps suit thick, shear-sensitive fluids and difficult suction conditions.
Can an external gear pump handle viscous fluids?
Yes, up to a point, if it is run slower and sized larger. For very high viscosities, internal gear pumps fill better and handle the product more gently.
Are internal gear pumps reversible?
Most are. Reversing motor rotation reverses the flow direction, which is useful for transfer between tanks. Check that any built-in relief valve also works in both directions.
Which gear pump is better for food applications?
Internal gear pumps are often preferred for viscous food products because of their low shear. Stainless steel construction and suitable seal materials are needed for hygiene and cleaning.
Why do internal gear pumps often run slower?
Viscous fluids need time to fill the gear pockets. Running slower prevents cavitation, reduces wear and protects shear-sensitive products.
Conclusion
Choose an external gear pump when pressure and compact size matter most. Choose an internal gear pump when viscosity, suction conditions or product care matter most. In both cases, sound and accurate castings decide how long the pump holds its performance.
List the fluid, viscosity range, pressure and suction conditions for your next application, and match them against the comparison table above.
About Rainbow Technocast
Rainbow Technocast supplies precision investment castings to gear pump manufacturers from Shapar, Rajkot. We cast housings, rotor cases, heads, brackets and covers for both external and internal gear pumps, in carbon steel, stainless steel, duplex and nickel alloys.
Our three foundries and two machining shops cover the full route from tooling to machined, tested part. Every melt is verified by spectrometer, and castings are checked by DPI, MPI, radiography, mechanical testing and hydro-pressure testing as required. With more than 20 years in investment casting and approvals including ISO 9001, PED and NORSOK M-650, we build castings that hold tight clearances in demanding service.
Our promise: accurate gear pump castings that keep your pumps sealing and performing.
Send your gear pump casting drawing through our contact page for a technical review and quote.