Every gear pump leaks internally by design, and that fact is often misunderstood. The clearances that let the pump turn are the same clearances that let a little oil slip back from outlet to inlet. The real question is never whether leakage exists, but how much there is, how fast it is growing, and whether it has reached the point where the pump no longer does its job. Framing the issue that way turns a worrying symptom into a measurable one.

Leakage as a Design Feature

A pump with zero clearance could not turn. Because clearance is unavoidable, internal leakage is built in from the first day, and a new pump's volumetric efficiency is already below one hundred percent. Leakage becomes a fault only when wear widens those clearances until the leakage exceeds what the application can tolerate.

The Leakage Paths and Their Wear

Each path wears in its own way, and knowing them helps locate the problem:

• Gear tips and flanks. Meshing clearances that open as the gears wear.
• Side faces against the end plates. Often the largest path, sensitive to wear and pressure.
• Outer ring against the housing. Grows with wear and heat.
• Shaft seals and bearings. Wear that lets oil out and air in.

How Wear Accumulates

Leakage grows because conditions accelerate wear over time:

1. Contaminated oil carrying abrasive particles through the clearances.
2. Oil that is too thin at working temperature, raising leakage directly.
3. Air and cavitation eroding metal surfaces.
4. Long service without attention to oil condition.
5. Overheating, which thins oil and weakens the lubricating film.

The important insight is that these causes compound. Contaminated oil wears the surfaces, the worn surfaces leak, the leakage adds heat, and the heat degrades the oil further. This is why a pump's condition tends to worsen faster late in life than early.

Measuring Leakage in Practice

Internal leakage is judged through volumetric efficiency. Measure flow at low pressure and again at rated pressure, with oil at working temperature, and compare the fall against the pump curve. A small, predictable fall is normal. A fall well beyond the specification is excessive leakage and, almost always, internal wear.

The Replacement Threshold

Replace the pump when leakage has reached the point where the machine can no longer meet its duty, and when the cause is wear that cannot be corrected. The deciding evidence is a measured volumetric efficiency below what the application needs, with correct oil, correct temperature and a sound suction line. When those conditions are met and flow is still poor, the clearances have opened and the pump is finished.

One rule safeguards the new pump: correct the conditions that wore out the old one. Oil cleanliness, oil grade, suction condition and operating temperature should be checked before a replacement is fitted, or the new pump will follow the same path at the same speed.