Cast Iron Gear Pumps: In-Depth Analysis of Material Characteristics, Application Advantages, and Technical Limitations

I. Classification of Cast Iron Materials and Their Adaptability to Gear Pumps

The performance of cast iron gear pumps is closely related to their diverse material systems. The core classifications and application characteristics are as follows:

1. Gray Cast Iron (HT)

Material Characteristics: Carbon exists in the form of flake graphite, with a compressive strength of 200-300MPa and a tensile strength of 150-250MPa. It has excellent vibration damping (10 times that of steel) but poor toughness.

Pump Body Application: Suitable for low-pressure (≤1.6MPa) and non-corrosive media, such as lubricating oil transfer pumps and food-grade syrup pumps. The typical grade HT200 is used for pump bodies, and HT250 is used for wear-resistant components such as gears.

Applicable Scenarios: Lubricating oil circulation in agricultural machinery and small hydraulic systems, with a cost 30% lower than ductile iron.

2. Ductile Iron (QT)

Material Characteristics: Graphite is distributed in a spherical form, with a tensile strength of 400-800MPa, 30% improved fatigue resistance, corrosion resistance equivalent to gray cast iron, but toughness close to steel.

Pump Body Application: Gear pumps for high-pressure conditions (≤3.5MPa), such as hydraulic pumps for construction machinery and mine emulsion pumps. The grade QT500-7 is used for gears, and QT600-3 is used for pump casings.

Performance Advantages: In particle-containing media, the service life is 2-3 times longer than that of gray cast iron, making it suitable for transporting lubricating oil with trace impurities.

3. Malleable Cast Iron (KT)

Material Characteristics: Graphite is spheroidized through heat treatment, with impact toughness 3-4 times higher than that of gray cast iron and a tensile strength of 300-450MPa, suitable for complex structural castings.

Pump Body Application: Non-critical pressure components such as gearboxes and end covers of small gear pumps, such as fuel pumps and garden irrigation pumps.

Processing Advantages: Malleable cast iron can optimize the tooth surface accuracy of gears through forging, reducing operating noise by 10-15dB.

4. Special Cast Iron

Corrosion-Resistant Cast Iron: Added with chromium, nickel, and other elements (such as high-silicon cast iron), the acid and alkali resistance is increased by 5-8 times, suitable for weakly corrosive media in the chemical industry (such as fatty acid transfer pumps).

Wear-Resistant Cast Iron: The surface of white cast iron is hardened, with a hardness of HRC55-60, used for transporting high-viscosity and abrasive media (such as asphalt pumps), but the processing difficulty is high, and the application is limited.

II. Core Technical Advantages of Cast Iron Gear Pumps

1. Adaptability to High-Viscosity Media Transportation

Operating Conditions: The meshing clearance (0.05-0.1mm) of cast iron gear pumps can stably transport media with a viscosity of 100-1000cSt, such as lubricating oil, syrup, and paint, with a volumetric efficiency of 85-90% (only 60-75% for centrifugal pumps).

Typical Case: In the lubricating oil circulation system, the HT250 gear pump can maintain stable flow at a viscosity of 320cSt, while the centrifugal pump needs to be heated to reduce viscosity for normal operation.

2. Self-Priming Capacity and Sealing Advantages

Self-Priming Height: Relying on the vacuum formed by gear meshing, the self-priming height of cast iron gear pumps can reach 5-8m (only 3-5m for centrifugal pumps), suitable for oil unloading from open-air storage tanks and oil supply for mobile equipment.

Sealing Design: The mating surface of the cast iron pump body and end cover is ground, with a leakage rate ≤5ml/h (the leakage rate of the same specification centrifugal pump is 15-20ml/h), suitable for scenarios where medium leakage is not allowed.

3. Cost and Maintenance Advantages

Manufacturing Economy: The cost of gray cast iron gear pumps is only 1/4-1/3 of that of stainless steel pumps. For example, the selling price of a DN50 cast iron pump is about 800 yuan, while that of a stainless steel pump exceeds 3000 yuan.

Maintenance Convenience: Cast iron gear pumps have a simple structure. Professional tools are not required when replacing gears or bearings, and the maintenance time is 50% less than that of centrifugal pumps, making them suitable for on-site quick repairs.

III. Application Limitations of Cast Iron Gear Pumps

1. Speed and Efficiency Bottlenecks

Speed Limitation: The rated speed of cast iron gear pumps is usually ≤1500rpm (centrifugal pumps can reach 2900rpm). Exceeding this speed is prone to a sharp increase in noise (>85dB) due to increased gear wear.

Efficiency Shortcoming: In low-viscosity media (<20cSt), the hydraulic efficiency of cast iron gear pumps is only 60-70%, 15-20% lower than that of centrifugal pumps, making them unsuitable for low-viscosity scenarios such as clean water.

2. High-Temperature and High-Pressure Performance Limitations

Temperature Upper Limit: The long-term working temperature of the gray cast iron pump body ≤120℃. Exceeding this temperature will cause graphite oxidation and strength decline. Although ductile iron can withstand 200℃, it is prone to stress cracking under high pressure (>3.5MPa).

Medium Limitations: Cast iron gear pumps are not suitable for transporting strongly corrosive media (such as concentrated hydrochloric acid) and rely on coating treatment (such as epoxy resin spraying), which increases costs by 20% and has a coating life of only 1-2 years.

3. Weight and Installation Challenges

Overall Weight: The weight of a DN50 cast iron gear pump is about 18kg (only 8kg for the same specification aluminum alloy pump). Additional fixing brackets are required for installation on mobile equipment, increasing installation costs by 15%.

Precision Requirements: Cast iron gear pumps require an installation coaxiality ≤0.05mm (centrifugal pumps ≤0.1mm). Otherwise, abnormal gear wear will occur due to eccentric loading, making on-site debugging more difficult.

IV. Market Application Scenarios and Selection Suggestions

1. Main Application Fields

Industrial Lubrication Systems: Accounting for 45% of the cast iron gear pump market share, such as machine tool lubricating oil pumps and injection molding machine hydraulic pumps, mostly using HT250 materials.

Agricultural and Construction Machinery: 25% used for tractor hydraulic pumps and harvester fuel pumps, giving priority to QT500-7 to adapt to vibration conditions.

Food and Light Industry: 15% used for syrup and honey transportation, requiring food-grade cast iron (such as passivated HT300) to meet hygiene standards.

2. Core Selection Principles

Medium Viscosity: Select cast iron gear pumps for viscosity >50cSt, and give priority to centrifugal pumps for <50cSt.

Pressure Requirements: Ductile iron pumps can be used for system pressure ≤3.5MPa, and cast steel gear pumps need to be used for >3.5MPa.

Ambient Temperature: High-temperature (>120℃) scenarios require a cooling system or heat-resistant cast iron (such as HTSi5 with 5% silicon).