Key Takeaways
- Internal gear pumps use an off-center rotor and idler gear, sealed by a crescent piece, to move fluid with almost no pulsation
- They’re built for thick, sticky, or oil-based liquids that would overwhelm a standard centrifugal pump
- Close internal clearances mean less slip, which matters a lot once viscosity climbs
- Common uses include lubricating oils, resins, asphalt binder, adhesives, and fuel oils
- Picking the right pump size and clearance depends on viscosity at actual operating temperature, not room-temperature lab numbers
- Poor pump selection for viscous fluids often shows up later as cavitation, motor strain, or premature wear
Why Thick Liquids Break the Rules
Pump a bucket of water and pretty much anything works. Pump honey, resin, or heavy gear oil, and the rules change fast.
Viscosity, in plain terms, is how much a fluid resists flowing. Water has almost none. Motor oil has more. Asphalt binder or industrial resin can have a lot, especially when it’s cold. Once a fluid gets thick enough, a lot of standard pumping equipment starts to struggle, and that’s exactly the gap internal gear pumps were designed to fill.
This isn’t a niche corner of industrial equipment either. Oils, resins, adhesives, and similar viscous fluids show up constantly in manufacturing, asphalt production, chemical processing, and fuel handling. So understanding how these pumps actually work, not just that they “handle thick stuff,” helps engineers and procurement teams avoid an expensive mismatch.
What Sets an Internal Gear Pump Apart
Gear pumps come in two main flavors: external and internal. Both fall under the positive displacement category, meaning they move a fixed volume of fluid with every rotation instead of relying on velocity like a centrifugal pump does.
External gear pumps use two same-sized gears mounted on separate shafts, meshing outside each other. Internal gear pumps work differently. One gear, the rotor, has teeth cut on the inside and spins around the pump’s center. Sitting inside it, off to one side, is a smaller idler gear with teeth pointing outward.
That offset positioning is the whole trick. A fixed, crescent-shaped piece fills the gap left between the two gears, sealing the space between the inlet and outlet sides of the pump.
How Fluid Actually Moves Through the Pump
Here’s the sequence in practice. As the rotor turns, it drags the idler gear along with it. The teeth separate near the inlet, creating expanding space, and that draws fluid in.
The fluid then gets trapped in pockets between the gear teeth and carried around the outside of the crescent, split into two flow paths on either side of it. As the gears come back together near the outlet, the pockets shrink and force the fluid out under pressure.
No valves. No pulsing surges. Just a smooth, continuous discharge that stays fairly consistent even as speed changes. That last part matters more than people expect when a process depends on steady flow, not surges.
Why the Design Favors Thick Fluids
Thin liquids slip past tight clearances more easily than thick ones do. Internal gear pumps are built with very tight tolerances between the gears and the casing, which limits how much fluid can slip backward from the discharge side toward the suction side.
With water or solvents, that slip is more noticeable. With oils, resins, and similarly viscous fluids, the thickness itself helps seal those clearances, so slip drops and volumetric efficiency climbs. It’s part of why internal gear pumps are often the go-to option once viscosity gets into the hundreds or thousands of centipoise, a range where centrifugal pumps lose efficiency fast.
Where This Actually Shows Up in Industry
Lubricating oil transfer is a classic use case, especially in bulk handling and blending operations where consistent flow matters more than raw speed. Resin transfer in manufacturing and coatings is another, where shear-sensitive fluids need to move without being agitated or degraded.
Asphalt and asphalt-related equipment lean on this pump type constantly, since binder and modified asphalt products can be thick, hot, and unforgiving toward the wrong equipment. Fuel oil handling, adhesive production, and certain food-grade applications round out the list.
What connects all of these? The fluids are viscous, they’re often somewhat shear-sensitive, and the process usually needs steady, low-pulsation flow rather than the high-volume output of a centrifugal system. Positive displacement pumps tend to be the better fit for viscous fluids in general, since they typically run at lower speeds and put less shear stress on the fluid compared to centrifugal designs. If a reader wants to see how viscosity affects pump selection in more technical depth, including friction loss and sizing considerations, it’s worth reading through in full rather than skimming a summary.
Sizing and Selection: Where Most Mistakes Happen
Here’s a mistake that shows up more often than it should: sizing a pump based on a fluid’s viscosity at room temperature, then installing it somewhere the fluid actually runs colder. Viscosity can swing wildly with temperature. A resin or oil that flows reasonably well at 90 degrees can turn sluggish, even semi-solid, at 40.
That single detail changes everything about pump sizing. Clearances, motor horsepower, inlet conditions, even the material of the gears themselves can all shift depending on the real operating temperature range, not a lab-condition number pulled from a spec sheet.
A few other factors worth checking before ordering anything:
- Actual viscosity at minimum and maximum operating temperature, not a single average figure
- Whether the fluid is Newtonian or shear-sensitive, since some thick fluids thin out under agitation
- Inlet conditions and suction lift, since thick fluids create more friction loss on the suction side
- Abrasive content, since even small particulates can accelerate wear on close-tolerance gears
- Required flow consistency, since some processes genuinely need pulsation-free discharge and others don’t
Skip any one of these and the pump might still work. It just won’t work well, and it definitely won’t work for long.
Distributors who supply this kind of rotating equipment tend to see the same pattern over and over: someone orders based on a spec sheet number instead of real operating conditions, and the pump underperforms within months. Suppliers like AMED-US work through those variables with customers before a pump ships, which is a big part of why proper selection upfront saves money later. Distributors that carry established rotary positive displacement lines, including Viking pumps, generally have the technical background to walk through gear sizing, materials, and clearance questions specific to a given fluid rather than a generic catalog answer.
Getting this step right isn’t glamorous. But it’s the difference between a pump that runs for a decade and one that needs rebuilding within a year.
A Quick Word on Maintenance
Internal gear pumps are mechanically simple compared to a lot of alternatives, which helps on the maintenance side. Fewer moving parts generally means fewer failure points.
That said, viscous fluid service isn’t maintenance-free. Seals wear. Bearings need attention. And any fluid carrying even trace abrasives will eventually show wear on the close-tolerance surfaces that make these pumps efficient in the first place.
Routine inspection catches most of this early, long before a small clearance issue turns into a full rebuild.
Frequently Asked Questions
What viscosity range can an internal gear pump handle?
Internal gear pumps typically handle a wide range, from thin solvents up through very thick fluids like asphalt binder or heavy resins, often into the tens of thousands of centipoise depending on the specific pump design and materials.
Are internal gear pumps better than centrifugal pumps for oils and resins?
For viscous fluids, generally yes. Centrifugal pumps lose efficiency quickly as viscosity rises, while positive displacement pumps like internal gear pumps maintain more consistent flow and put less shear stress on the fluid.
Can internal gear pumps handle fluids with solids in them?
Not well, in most cases. The tight clearances that make these pumps efficient with thick liquids also make them vulnerable to abrasive wear or jamming from larger solid particles.
Why does temperature matter so much for viscous fluid pumping?
Viscosity often changes dramatically with temperature. A fluid that flows easily when warm can thicken significantly when it cools, which affects pump sizing, motor load, and suction conditions.
How is an internal gear pump different from an external gear pump?
An internal gear pump uses one gear inside another, with a fixed crescent piece separating them. An external gear pump uses two same-sized gears mounted side by side. Internal designs generally handle higher viscosity fluids more efficiently.
What industries rely most on internal gear pumps for viscous liquids?
Asphalt production, chemical processing, fuel and lubricant handling, resin and adhesive manufacturing, and certain food-grade applications are among the most common users of this pump type.
How often should an internal gear pump be inspected in viscous fluid service?
It depends on the fluid and duty cycle, but routine inspection on a scheduled basis, generally every few months in continuous service, helps catch clearance wear or seal issues before they cause downtime.
