Main Bitumen Gear Pump Assembly

Function

The pump must handle bitumen viscosities from approximately 1,500 mPa·s at priming temperatures of around 130 °C (266 °F) down to 200–300 mPa·s at full working temperature of 170–185 °C (338–365 °F). Rated flow is typically 400–1,200 L/min depending on unit capacity, sufficient to maintain the spray bar at application pressure while simultaneously keeping the ring main circulating to prevent cold zones. The pump must sustain consistent delivery pressure — typically 3–5 bar (44–73 psi) at the spray bar — across the full range of vehicle road speeds encountered on a surface-dressing contract.

Construction & quality

Housing: precision-bored ductile cast iron. Gear material: case-hardened alloy steel (18CrNiMo7 or equivalent), profile-ground teeth, surface hardness 58–62 HRC case with a core of 35–45 HRC for impact resistance. Shafts: case-hardened alloy steel with ground journals. End plates: close-clearance cast iron with running clearances of 0.03–0.08 mm, selected to minimise internal slip at working temperature. Mechanical shaft seal: carbon or ceramic face, NBR secondary, rated to 220 °C (428 °F). Integral spring-loaded relief valve factory-set at 6 bar (87 psi).

When to replace

• Pump output flow falls progressively below the rated value at constant shaft speed
• Audible whine or knocking from the pump housing — worn gear tooth faces or collapsed bearing
• Bitumen weep at the shaft seal or housing joint face
• Relief valve opens below the factory-set pressure — worn spring or eroded seat
• Pump housing temperature rises noticeably above the surrounding bitumen temperature — internal slip generating excess heat