How an external gear pump works
Two gears turn inside a close-fitting body: one is driven by the motor, the other is driven by the first. The pumping cycle has three phases.
- Suction: on the inlet side the teeth come out of mesh, the volume between them grows, a depression forms and the fluid is drawn into the pump.
- Transport: the fluid is trapped between the teeth and the inner wall of the body and carried round the outside of each gear towards the outlet.
- Discharge: on the outlet side the teeth mesh again, the volume between them shrinks and the fluid is pushed out into the delivery line.
Because every revolution moves the same volume, the flow is constant and proportional to speed, practically free of pulsation and independent of the back pressure downstream. The pump is self-priming: it draws fluid even when the suction line is not full.
Why a positive displacement gear pump suits viscous fluids
With thin liquids the small clearances between gears and body let a little fluid slip back from discharge to suction. With viscous products the opposite happens: the fluid itself lubricates the internal surfaces and seals those clearances, so volumetric efficiency rises with viscosity. That is the reason our pumps are chosen for viscous fluids from medium viscosity, indicatively 50–100 cP, upwards, where centrifugal pumps lose head and efficiency.
Typical products are bitumen and its derivatives, lubricating and heavy fuel oils, resins and adhesives, syrups and molasses, molten polymers, high-density paints, glycerine and waxes. The body is cast iron or, for aggressive fluids and food use, AISI 316 stainless steel.
Five series of self-priming gear pumps
- CR — pumps with heating jacket for fluids that solidify at room temperature; ten models, 30.7 to 2,812 l/min at 950 rpm; SEF version for abrasive filled bitumen.
- SB — pumps for bitumen sprayers that flange onto the tank, with two built-in valves for four functions; five models, 57 to 1,093 l/min.
- NAV — high-viscosity industrial pumps with flanged ports DN 40 to DN 125; nine models, 54 to 2,770 l/min, continuous duty.
- ZV — small-flow dosing pumps flanged to the motor, volumetric efficiency close to 100%; ten models, 0.47 to 102 l/min.
- FX — hygienic AISI 316 pumps for food, cosmetics and pharmaceuticals, CIP configuration; six models, 9.2 to 247.1 l/min.
All series share the same principle; they differ in heating, connections, materials and the flow band they cover.
Dosing, heating and special executions
Since flow is proportional to speed, a gear pump driven through an inverter becomes a volumetric dosing unit without any extra metering device. The ZV series was born to dose fuel and lubricant to engines on the test bench: hardened and ground alloy steel gears with tolerances in hundredths of a millimetre and volumetric efficiency close to 100%.
For products that set when they cool, the CR and SB series have a heating jacket fed with thermal oil, steam or superheated water. Where no boiler exists we fit electric cartridges with an electronic controller, or the self-contained hot-spot circuit with electric elements up to 900 W. For strongly abrasive fluids the SEF version moves the support bearings outside the pump body, with external timing gears in an oil bath and four replaceable gland packings, protected by an electronic torque limiter. Stainless steel AISI 316 executions and ATEX documentation complete the range.
Choosing a gear pump manufacturer in Italy
A gear pump is only as good as its selection. Flow, pressure, viscosity at working temperature, suction conditions and the presence of solids decide the model, the speed and the motor; a pump that is right on paper but wrong on one of these points will cavitate, wear or stall. This is why we do not sell from a chart: every enquiry is examined by the people who design and machine the pumps in our Bologna factory, building self-priming external gear pumps since 1955 under ISO 9001:2015.
Tell us the fluid, the viscosity, the temperature, the flow and the pressure. Write on WhatsApp or use the quote form and you will receive a technical proposal with model, drive and any heating or material option, usually within one working day.
Frequently asked questions
What is the difference between an external gear pump and a centrifugal pump?
A centrifugal pump accelerates the liquid with an impeller and loses head as viscosity rises. An external gear pump traps a fixed volume between the teeth and the body, so flow stays constant whatever the viscosity or back pressure.
Are your gear pumps self-priming?
Yes. The depression created as the teeth come out of mesh draws the fluid into the pump even when the suction line is not full, which is why they are used with thick products stored below the pump.
Can a gear pump run in both directions?
Yes. Reversing the motor reverses the flow, so the pump can also be used to empty a line or a tank. The flow stays constant and proportional to speed in both directions.
What viscosity can an external gear pump handle?
From medium viscosity, indicatively 50–100 cP, up to products that are solid at room temperature, provided they are heated. With thicker fluids the volumetric efficiency improves, because the product seals the internal clearances.
Do you build gear pumps in Italy?
Yes. Every series is designed and manufactured in our Bologna factory, where the components have been machined in-house since 1955. Quality is certified ISO 9001:2015 and ATEX documentation is available.