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Working Principle of External Hydraulic Gear Pumps – Full Mechanical Breakdown
2026-06-22 21:09
1. Introduction
- Driving Gear: Connected to power input shaft, transmits rotation torque
- Driven Gear: Meshed with driving gear, rotates reversely
- Brass Side Plates: Compensate axial clearance to reduce internal leakage
- Pump Housing: Forms sealed suction & pressure chambers
- Front/Rear End Covers: Seal assembly, install oil inlet & outlet ports
- Input Shaft & Oil Seal: Prevent external oil leakage
- Bearing Bushings: Support gear shafts to reduce friction wear
Table 1: Core Component Material Standard & Functional Characteristics
| Component Name | Common Material | Core Function | Wear Failure Consequence |
|---|---|---|---|
| Spur Gear Pair | Hardened Cast Iron / Alloy Steel (HRC 55–60) | Meshing to form sealed fluid pockets | Large internal leakage, volumetric efficiency drop below 80% |
| Side Plate | Sintered Brass Alloy | Axial clearance compensation | Severe cross-flow between suction & pressure cavity |
| Pump Housing | Die-cast Aluminum Alloy | Seal chamber, fix mounting flange | Deformation under high pressure, permanent flow loss |
| Shaft Oil Seal | Nitrile Rubber / FKM | Block external oil spill | Oil dripping, system insufficient oil supply & cavitation |
| Bearing Bushing | Bronze Sintered Bearing | Support gear radial load | Shaft vibration, abnormal noise, gear tooth abrasion |
3. Three-Stage Complete Working Cycle (Core Operating Principle)
Stage 1: Oil Suction (Negative Pressure Generation)
When gear teeth separate on the inlet side, the volume of tooth clearance cavities expands rapidly. A partial vacuum (negative pressure) forms inside the pump cavity. Under atmospheric pressure, hydraulic oil from the tank is pushed into the pump through the low-pressure suction port and fully fills every tooth gap.
Key rule: The suction port must connect to low-pressure oil tanks; air intake will trigger cavitation damage, a failure mode fully analyzed in the gear pump operation guide from https://www.firgelliauto.com/blogs/mechanisms/gear-pump.
Stage 2: Fluid Transportation (Isolated Conveyance)
Stage 3: High-Pressure Discharge (Mechanical Energy to Hydraulic Pressure)
On the outlet side, gear teeth re-engage and squeeze the tooth gap volume sharply. Since hydraulic oil is incompressible, confined oil accumulates pressure and is forced out through the pressure port into the hydraulic pipeline to supply actuators.
After completing work in cylinders or motors, low-pressure oil flows back to the tank to restart the cycle.
4. Core Performance Parameter Comparison Table
Table 2: Performance Contrast of Mainstream External Gear Pump Series
| Pump Series | Displacement Range (cm³/rev) | Continuous Working Pressure (Bar) | Max Rotation Speed (RPM) | Volumetric Efficiency (New Unit) | Main Application Scenarios |
|---|---|---|---|---|---|
| Low-flow Mini Series | 1–8 | 140–180 | 4000 | 90–94% | Precision lubrication, small hydraulic power units |
| Medium Standard Series | 10–40 | 210–260 | 3200 | 88–92% | Agricultural machinery, light construction equipment |
| Heavy-duty High-pressure Series | 45–70 | 260–310 | 2500 | 85–90% | Engineering loaders, hydraulic presses |
All flow and pressure benchmarks in this table are extracted from Duplomatic’s official product technical manual available for download at https://duplomaticmotionsolutions.com/docs/2024/11102-ed-224_e53e29295e29.pdf.
Table 3: Gear Pump Volumetric Efficiency – Before & After Overhaul Contrast
| Performance Indicator | Worn Pump (Before Repair) | After Standard Overhaul | Factory New Pump Target Value |
|---|---|---|---|
| Effective Output Flow Ratio | 70%–78% | 90%–98% | 100% |
| Internal Cross Leakage Ratio | 12%–18% | 4%–7% | ≤5% |
| Working Temperature Rise (150Bar Load) | +18~25℃ | +6~10℃ | ≤8℃ |
| Continuous Stable Operation Time | <400h | ≥1200h | ≥2000h |
5. External vs Internal Gear Pump Full Comparison
Table 4: External Gear Pump VS Internal Gear Pump Comprehensive Comparison
| Comparison Item | External Gear Pump | Internal Gear Pump |
|---|---|---|
| Gear Layout | Two identical gears mesh externally | Small inner gear meshes inside large ring gear |
| Max Continuous Pressure | Up to 310 Bar | Max 17 Bar for standard models |
| Operating Noise | 82–90 dB(A) | 70–76 dB(A), low pulsation |
| Manufacturing Cost | Low, mass production friendly | High, high-precision gear processing |
| Viscosity Adaptability | Suitable for 10–1000 cSt hydraulic oil | Optimized for high-viscosity lubricants |
| Common Failure Points | Side plate abrasion, radial gear wear | Crescent separator deformation, idler pin bending |
6. Key Technical Formulas for Reference
- Theoretical Flow Rate: Qₜ = D × n × 10⁻³
- D = single revolution displacement (cm³/rev), n = rotating speed (rpm), unit Qₜ = L/min
- Actual Output Flow: Qₐ = Qₜ × ηᵥ
- ηᵥ = volumetric efficiency (Table 2 reference data)
- Hydraulic Output Power: P = P × Qₐ / 60
- P = working pressure (Bar), unit P = kW
7. Conclusion & Application Guidance
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