1. Overview of Gear Pumps

A gear pump is a positive displacement pump that transfers fluid by using meshing gears. It delivers a

nearly constant flow rate at a given speed, regardless of discharge pressure (within design limits). Gear

pumps are simple, compact, and robust, which makes them ideal for applications where reliability and

predictable performance are critical.

1.1 Basic Principle of Operation

In a typical gear pump, fluid is trapped between gear teeth and the pump housing and then carried from the

suction side to the discharge side. As the gears rotate, they create expanding volume at the inlet (which

draws fluid into the pump) and decreasing volume at the outlet (which forces fluid out).

1.2 Types of Gear Pumps

Gear pumps can be broadly divided into two main types:

• External gear pumps – Two identical external gears mesh together inside a close-fitting housing.
• Internal gear pumps – An external gear meshes with a larger internal gear (or rotor) with the teeth facing inward, usually with a crescent-shaped spacer.

1.3 Typical Gear Pump Applications

• Hydraulic power units and mobile hydraulic systems
• Lubrication systems for bearings, gearboxes and engines
• Chemical and polymer transfer and metering
• Fuel oil transfer and burner feed systems
• Process industry fluid circulation and filtration systems

1.4 Advantages Relevant to Leakages and Pressure Drops

• Simple construction with few moving parts, reducing leak paths.
• Tight internal clearances for good volumetric efficiency and low internal leakage.
• Good performance over a wide viscosity range when correctly selected.
• Self-priming capability under suitable suction conditions.

2. Key Definitions: Leakages and Pressure Drops

Effective gear pump troubleshooting requires clear definitions of leakage and pressure drop. These terms

are often used loosely, but in technical troubleshooting they have specific meanings.

2.1 Internal vs. External Leakage

2.2 Pressure Drop vs. Pressure Loss

In gear pump troubleshooting, technicians commonly observe a “pressure drop”. This can refer to:

• Pressure drop across the pump: Difference between discharge and suction pressures.
• Pressure drop in the system: Losses across piping, valves, filters and other components.
• Unexpected pressure reduction: System can no longer reach the required pressure level.

2.3 Efficiency Terms

3. Common Gear Pump Symptoms: Leakages and Pressure Drops

When a gear pump experiences leakages or pressure problems, it often exhibits multiple symptoms at the same time.

Proper troubleshooting begins by recognizing and documenting all observable issues.

3.1 Symptom Checklist

• Inability to reach required system pressure.
• Pressure fluctuating or pulsating beyond normal limits.
• Noticeable external oil leakage around shaft, cover, or connections.
• Hot pump casing or unusually high oil temperature.
• Excessive noise: whining, grinding, or cavitation sounds.
• Slow actuator movement or reduced machine speed.
• Foaming in reservoir or aerated oil.
• Increased energy consumption for the same output.

3.2 Quick Diagnostic Matrix

4. Root Causes of Gear Pump Leakages

4.1 Internal Leakage Causes

Internal leakage is a major source of gear pump pressure drops and efficiency loss. It occurs wherever there

are clearances between gears and housing, between gear ends and side plates, or across wear plates and

bushings.

4.1.1 Wear of Gear Teeth and Side Plates

• Abrasive contamination erodes tooth flanks and tips.
• Insufficient lubrication leads to scoring and galling.
• Prolonged operation at high pressure causes plastic deformation of components.

As clearances increase due to wear, internal bypass paths enlarge. This increases internal leakage, reduces

delivered flow, and demands higher input torque.

4.1.2 Housing and Bearing Wear

• Worn bearings allow shafts to move radially, opening clearances.
• Misalignment causes uneven contact and localized wear.
• Thermal expansion mismatch can distort the housing.

4.1.3 Fluid Viscosity Too Low

• High temperature or wrong fluid grade decreases viscosity.
• Low viscosity reduces the fluid’s ability to seal clearances.
• Internal leakage increases dramatically at low viscosities.

4.2 External Leakage Causes

4.2.1 Shaft Seal Failures

• Excessive shaft run?out or misalignment.
• Overpressure in the pump casing or seal chamber.
• Incorrect seal material for the fluid or temperature.
• Improper installation or damaged seal faces.

4.2.2 Gasket and O?Ring Problems

• Incorrect gasket material or hardness.
• Age?related hardening, cracking or compression set.
• Over?torqued or under?torqued housing bolts.
• Chemical incompatibility with the fluid.

4.2.3 Cracked or Damaged Housing

• Mechanical impact during handling or installation.
• Thermal shock from rapid temperature changes.
• Fatigue due to repeated overpressure or vibration.

5. Root Causes of Gear Pump Pressure Drops

A pressure drop in a gear pump system can originate within the pump itself or in the connected hydraulic or

process circuit. Accurate pressure troubleshooting requires distinguishing between pump?related and

system?related losses.

5.1 Pump?Related Pressure Drops

5.1.1 Severe Internal Leakage

When internal leakage passes a critical level, the pump cannot build the required pressure at the available

speed and displacement. This appears as a permanent pressure deficit even with no external leakage.

5.1.2 Excessive Clearances from Wear or Incorrect Assembly

If assembly tolerances are not respected during maintenance, side clearances and end clearances may exceed

design values, creating uncontrolled leakage paths.

5.1.3 Cavitation and Aeration

• Cavitation: Local boiling of fluid due to low pressure at pump inlet causing vapor bubbles that collapse in the high?pressure region.
• Aeration: Air entering the system from leaks in the suction line or low reservoir level.

Both phenomena reduce effective flow, cause pressure instability, and damage internal surfaces,

aggravating leakage over time.

5.2 System?Related Pressure Drops

5.2.1 Restrictions in Discharge Line

• Clogged filters or strainers.
• Partially closed isolation or control valves.
• Undersized piping or long lines with high friction.

Excessive system resistance may prevent the pump from achieving its rated flow and pressure, or trigger

relief valve opening at lower pressures.

5.2.2 Relief Valve Malfunction

• Relief valve stuck partially open due to contamination.
• Incorrectly set relief pressure below needed system pressure.
• Worn valve seat or pilot leaks, back?bleeding flow to tank.

5.2.3 Suction Side Problems

• Clogged suction strainer reducing net positive suction head (NPSH).
• Excessive suction lift or small?diameter suction piping.
• Air ingress at fittings, gaskets or flexible hoses.

Suction problems manifest as pressure fluctuations, noise, and gradual performance decline.

6. Step?by?Step Gear Pump Troubleshooting Procedure

The following structured procedure helps technicians locate and correct gear pump leakages and pressure drops

efficiently. It is suitable for hydraulic gear pumps, lubrication gear pumps and general industrial gear pump

installations.

6.1 Preliminary Safety and Preparation

• Isolate power and lock out the drive where required.
• Depressurize the hydraulic or process system safely.
• Clean external surfaces to reveal leakage paths clearly.
• Prepare tools: pressure gauges, flow meter, infrared thermometer, dial indicator, torque wrench.

6.2 Visual Inspection for External Leakages

1. Inspect shaft seal area for oil tracks, spray or residue.
2. Check housing joint faces, cover plates and bolts for wetness or oil staining.
3. Examine port connections and fittings for drips or sweating.
4. Look for cracks, corrosion or mechanical damage on the pump body.

6.3 Pressure Measurements

1. Install calibrated gauges on suction and discharge ports if not already present.
2. Start the pump and allow it to reach normal operating temperature.
3. Record suction pressure, discharge pressure and reservoir temperature at several load conditions.
4. Compare values with design or historical reference data.

6.4 Flow and Efficiency Testing

Flow measurement is critical for quantifying internal leakage. Use a portable hydraulic tester or in?line

flow meter where possible.

1. Determine pump displacement and speed to calculate theoretical flow.
2. Measure actual flow at a known pressure.
3. Calculate volumetric efficiency using the formula in Section 7.
4. If efficiency is significantly below specification, suspect internal leakage.

6.5 Temperature and Noise Monitoring

• Use an infrared thermometer to monitor casing temperature at multiple points.
• Listen for cavitation noise: sharp crackling, rattling or gravel?like sounds.
• Check for abnormal vibration using a simple vibration meter where available.

6.6 Suction Side Evaluation

1. Verify oil level in the reservoir and ensure the suction pipe inlet is submerged.
2. Clean or replace suction strainers and check for collapse or deformation.
3. Inspect suction hose for kinks, collapse under vacuum, or porous material.
4. Perform a vacuum test on the suction line to detect air ingress points.

6.7 System Valve and Relief Valve Checks

1. Confirm all manual valves are in the intended position (fully open or correctly throttled).
2. Check the relief valve setting using a calibrated gauge and adjust if necessary.
3. Inspect filter differential pressure indicators and bypass valves.

6.8 Internal Inspection

If external inspection, pressure tests, and flow tests indicate significant internal leakage, disassemble the

gear pump for internal inspection.

1. Mark the relative position of components before disassembly.
2. Check gear teeth for scoring, pitting, wear and deformation.
3. Measure side clearances and end float against design tolerances.
4. Inspect bushings, bearings and wear plates for uneven wear.
5. Look for cavitation erosion and corrosion on internal surfaces.

7. Key Measurements and Calculations for Troubleshooting

7.1 Volumetric Efficiency Calculation

Volumetric efficiency is a primary indicator of internal leakage in gear pumps. It can be calculated using:

η_v = Q_actual / Q_theoretical

• Q_theoretical = Displacement (cm3/rev) × Speed (rev/min) ÷ 1000 (for L/min)
• Q_actual = Flow measured at pump outlet at the test pressure.

7.2 Example Volumetric Efficiency Table

7.3 Pressure Drop Across Components

To localize system pressure drops, measure pressures at different points:

8. Troubleshooting Tables for Leakages and Pressure Drops

8.1 Internal Leakage Troubleshooting Table

8.2 External Leakage Troubleshooting Table

8.3 Pressure Drop Troubleshooting Table

9. Preventive Measures to Reduce Leakages and Pressure Drops

Preventing gear pump leakages and pressure drops is more cost?effective than corrective maintenance. The

following practices can significantly improve reliability.

9.1 Proper Pump Selection

• Match displacement and pressure rating with system requirements.
• Select appropriate pump type (external or internal gear) for the fluid and duty cycle.
• Ensure compatibility of materials with the fluid (corrosion, swelling, chemical attack).
• Consider viscosity range across full temperature envelope.

9.2 Fluid Selection and Cleanliness

• Use a fluid with viscosity within the recommended range at operating temperature.
• Maintain cleanliness level according to the most sensitive component in the system.
• Install high?quality filters on pressure and return lines where applicable.
• Regularly sample fluid for contamination, viscosity and degradation.

9.3 Suction Line Design

• Use large?diameter suction piping with minimal bends and restrictions.
• Keep suction piping as short as practical.
• Place the pump as close to the reservoir as possible, preferably below fluid level.
• Use appropriate strainers with low pressure drop and easy access for cleaning.

9.4 Operating Practices

• Avoid running the pump dry during startup or after maintenance.
• Warm up the system gradually in cold conditions to avoid thermal shock.
• Avoid frequent cycling between zero and maximum pressure where possible.
• Monitor pressure, temperature and noise regularly to catch early signs of problems.

9.5 Scheduled Maintenance

• Establish inspection intervals based on operating hours and severity of service.
• Check shaft seals, gaskets and connections for early signs of leakage.
• Measure pump performance periodically using flow and pressure tests.
• Plan pump overhaul or replacement before catastrophic failure occurs.

10. Typical Gear Pump Design Specifications (General Reference)

The following table provides typical general ranges for gear pump specifications. Actual ratings depend on

specific pump designs, materials, and manufacturers. Values serve as general guidelines for understanding

limits related to leakages and pressure drops.

11. Best Practices for Gear Pump Troubleshooting

11.1 Establish Baseline Data

• Record initial flow, pressure and temperature data when the system is new.
• Keep documentation of all settings: relief valve, fluid type, filter specifications.

11.2 Use Systematic Diagnostic Methods

• Always start with the simplest checks: fluid level, valves, filters.
• Separate pump issues from system issues using by?pass or test circuits when possible.
• Use temporary test equipment such as portable flow meters and digital pressure gauges.

11.3 Focus on Root Causes, Not Just Symptoms

• If a shaft seal fails repeatedly, investigate misalignment or pressure spikes.
• If internal leakage increases quickly, examine filtration and fluid contamination.
• If cavitation is frequent, redesign the suction line rather than just replacing pumps.

11.4 Documentation and Continuous Improvement

• Document all troubleshooting steps and outcomes.
• Use failure data to refine maintenance schedules and design improvements.
• Train operators to recognize early warning signs of leakages and pressure drops.

12. Frequently Asked Questions about Gear Pump Leakages and Pressure Drops

12.1 How do I know if my gear pump has internal leakage?

Internal leakage is suspected when the pump cannot reach expected pressure even though the drive power is

available and no external leak is visible. Confirm by measuring actual flow and comparing it with theoretical

flow to calculate volumetric efficiency. A significant drop in efficiency, especially at higher pressures,

indicates internal leakage.

12.2 Can low viscosity cause both leakages and pressure drops?

Yes. Low viscosity reduces the fluid’s ability to seal internal clearances, which increases internal leakage

and reduces volumetric efficiency. At the same time, low viscosity can reduce frictional losses in the

system, but the dominant effect in gear pumps is usually reduced capacity to build and maintain pressure.

12.3 What is the difference between cavitation and aeration, and how do they affect pressure?

Cavitation is the formation and collapse of vapor bubbles due to local pressure dropping below fluid vapor

pressure. Aeration is the presence of entrained air bubbles from external sources. Both decrease effective

fluid density and compressibility, disrupt flow continuity, and damage pump surfaces. This leads to pressure

instability, noise, and progressive efficiency loss.

12.4 When should a gear pump be repaired instead of replaced?

Gear pump repair is usually justified when the casing, shafts and major components are structurally sound and

only wear parts such as gears, bushings, seals and plates are affected. When volumetric efficiency drops

below an acceptable threshold and internal wear is extensive, replacement may be more cost?effective.

13. Conclusion

Reliable gear pump operation depends on controlling leakages and minimizing unwanted pressure drops. By

understanding the mechanisms of internal and external leakage, recognizing early warning symptoms, and

following a structured troubleshooting procedure, maintenance teams can extend pump life, improve system

efficiency, and avoid unplanned downtime. Applying best practices in pump selection, fluid management,

suction line design and preventive maintenance will reduce the frequency and severity of gear pump

problems related to leakages and pressure drops across a wide range of industrial applications.