Gear pumps are positive displacement rotary units that move a fixed volume of fluid per revolution between meshing gear teeth and the casing, with flow directly proportional to shaft speed, and current commercial lines such as CTP's five-series range span 1.1–55 m³/h at differential pressures up to 1.6 MPa [S3]. The technology is widely specified for hydraulic transmission, lubrication circulation, and transfer of viscous or lubricating liquids in mobile machinery, shipbuilding, power generation, and chemical plants.
Two mechanical arrangements dominate the market, external gear and internal gear, and the choice between them is driven by viscosity range, noise target, pressure class, and whether the fluid contains any solids or shear-sensitive components. This 2026 spec map lines up those classifications against concrete operating envelopes and points to where each variant fits a real plant or machine specification.
External Gear Pumps: Workhorse for Hydraulics and Clean Oil
External gear pumps use two identical externally toothed gears on parallel shafts; fluid is trapped between the teeth and the casing wall and carried from inlet to outlet as the gears rotate, and the design reaches differential pressures up to 70 kg/cm² (about 6.9 MPa) on standard industrial units [S5]. The same three-phase cycle of suction, transport and discharge governs every external pump, with flow pulse frequency tied to gear-tooth count and shaft speed. Spur geometry is the cheapest and most common; helical and herringbone cuts trade a small axial-thrust penalty for noticeably lower noise and vibration, and CTP's 2CY and KCB lines apply an arc-tooth herringbone arrangement to spread tooth load and extend service life [S3][S4].
External pumps cannot run dry, because the gears depend on the process fluid for both lubrication and cooling, and a pressure relief valve is treated as essential rather than optional on any line above a few bar [S5]. Mobile-hydraulic models such as the Komatsu PC56-7 replacement pump 708-3S-00961 and the CAT 777 truck pump 350-9964 are built around this exact topology, where compact envelope and high pressure capability outweigh the inherently higher flow pulsation of an external design [S2]. gear pump selection principles overlap with the fundamentals of positive displacement pumping that apply across rotary categories.
Internal Gear Pumps: Quieter, Smoother, Viscous-Fluid Friendly
Internal gear pumps use a smaller drive gear rotating inside a larger outer ring gear, separated by a crescent-shaped partition, and the cavity between the two gears carries fluid from inlet to outlet in a single, continuous sweep [S5][S8]. Because only one gear is driven, the meshing frequency is roughly half that of an equivalent external pump, which is the main reason internal designs are quieter and produce less flow pulsation. Typical pressure capability is lower than top-end external units but the design tolerates more viscous and shear-sensitive fluids, including food syrups, fuel oils and dosing of viscous lubricants [S8].
Operating limits for current internal units broadly cap differential pressure at around 1.0–1.6 MPa and flow at the lower end of the rotary envelope, so they are rarely the right call for a 350 bar hydraulic circuit but often the right call for a bitumen or polymer line that demands low shear. Where a process fluid changes viscosity by two orders of magnitude across a batch, the internal design's larger tooth-clearance volume also handles viscosity swings that would starve a tight-clearance external pump.
Tooth Geometry: Spur vs Helical vs Herringbone
Spur gears have straight teeth cut parallel to the shaft and are the simplest to manufacture, but they generate the highest noise and vibration because every tooth engages the mating gear abruptly; they are the default choice for moderate-pressure, moderate-viscosity service where cost dominates [S1]. Helical gears cut the teeth at an angle to the shaft, so engagement is progressive, which reduces noise and vibration but introduces an axial thrust load that the bearing arrangement must absorb; they handle higher speeds and slightly higher pressures than equivalent spur units. Herringbone gears pair two opposing helical cuts so the axial thrusts cancel, leaving the low-noise behaviour of a helical pump without the thrust-bearing penalty, which is why CTP's 2CY, KCB, NHG, NHGH and VG series all use an arc-tooth herringbone arrangement [S3].
For the same displacement, a herringbone pump will typically run at lower sound levels and longer bearing life than a spur pump, but at a higher purchase price, and that trade-off tends to decide selection on enclosed factory floors and marine engine rooms. In hydraulic and lubrication circuits where the pump is mounted inside a machine frame and noise is radiated through the structure, helical and herringbone geometries dominate, while in skid-mounted mobile hydraulics spur remains common.
Capacity and Pressure Envelope: Where the 2026 Catalog Lines Land
CTP's published 2026 catalog envelope for general-purpose industrial gear pumps is capacity 1.1–55 m³/h, differential pressure up to 1.6 MPa, speed up to 1,800 rpm, media temperature up to 80 °C, and viscosity 5–1,000 cSt across the five series [S3]. The KCB and 2CY lines share the widest flow band at 1.1–58 m³/h with a 1.45 MPa pressure ceiling, the NHGH line is the dedicated high-pressure option at 1.6 MPa, the NHG is the standard horizontal lubrication pump up to 30 m³/h at 0.6 MPa, and the VG is the vertical-mount high-flow pump up to 55 m³/h at 0.6 MPa. Slurry-rated rotary gear units from lines such as SRS cover much smaller flow bands (0–3, 0–7.5, 0–15, 0–30, 0–60 and 0–120 GPM) and accept NPT threaded, flanged or Triclamp connection ends, indicating where rotary gear technology overlaps with low-flow sanitary or abrasive service [S7].
Where a spec demands very high viscosity, a Unique Pump Systems UA series rotary gear pump can run up to 100,000 cSt, but efficiency drops and the drive must be sized for the higher absorbed power, and a relief valve remains mandatory [S5]. The simplest way to read a catalog sheet is to lock the four envelope parameters first (flow, differential pressure, viscosity, temperature) and only then compare noise, materials and mounting.
Selection Criteria and Trade-Offs at a Glance
External pumps win on pressure, cost and compactness, internal pumps win on noise, pulsation and viscous-fluid handling, and within each family the spur-helical-herringbone choice sets the noise and axial-thrust profile. For bitumen transfer the Australian 2026 selection guidance is to use carbon steel or ductile iron for standard bitumen and stainless steel for polymer-modified bitumen, and to keep the pump body heated whenever it is stationary, because solidified bitumen makes restart impossible without melting the line out [S9]. The same note points to progressive-cavity pumps as the alternative for bitumen emulsions at lower temperatures where heating the pump is impractical, which is the boundary at which a gear pump stops being the right tool.
Material selection follows the fluid: lubricating oils and hydraulic mineral oils run in cast iron or carbon steel, polymer-modified bitumen and corrosive chemicals need stainless, and food or pharmaceutical service demands sanitary finishes and Triclamp connections that match the slurry-range connection list above [S7][S9]. Suction-side NPSH, drive speed, and the need for a relief valve or a pressure-compensated pump body are the next three gates to clear before a model is locked in. The 2026 Jinfujia selection guide repeats the same four gates in this order: flow rate and viscosity, material compatibility, pressure and temperature rating, and only then connection standard and mounting [S10].
Who Should Specify Which Variant
External gear pumps are the right call for clean, lubricating liquids in mobile hydraulics, machine tool circuits, lubrication loops and oil-transfer skids where differential pressure is the priority and the fluid tolerates the small pulsation of a spur or helical mesh. Internal gear pumps are the right call for viscous or shear-sensitive fluids, for low-noise installations such as marine engine rooms and indoor factories, and for hygienic or sanitary lines where crescent-partition geometry and low shear matter more than peak pressure. Helical and herringbone geometries sit between the two, picked where the noise budget rules out spur but the application does not justify an internal pump's lower pressure ceiling. [S5]
Gear pumps are not the right tool for slurries with hard solids above the pump's clearance tolerance, for dry running, for fluids that must not be sheared, or for applications where variable flow is required without a variable-speed drive or a bypass. In those cases a diaphragm, lobe, screw or progressive-cavity pump is normally the better fit, and a 2026 spec map across rotary pump families should always include a cross-check against diaphragm pump topologies and centrifugal pump selection trade-offs before the final line is written. Trackable signals to watch next: a wider publication of 316 stainless internal-gear sanitary units, herringbone pumps at higher than 1.6 MPa differential pressure, and any catalog move that pushes standard viscosity ceiling above 1,000 cSt.
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