How to measure case drain flow

How to confirm it

1. Find the case drain line and confirm which unit it serves

   The case drain is the smallest-diameter hose leaving the pump or motor housing, and it returns to the tank or to the return line rather than to any circuit. On a tandem pump, each section has its own. Confirm from the service documentation rather than by tracing hoses by eye, because on some machines the drain lines of two units are routed together.

2. Warm the machine to operating temperature

   Cold oil is thick and slips less, so a worn unit reads better than it is. Work the machine until hydraulic oil is at normal operating temperature, and note the temperature. A case drain figure without a temperature attached to it is not a measurement, it is an anecdote.

3. Release hydraulic pressure and make the machine safe

   Shut down, release residual pressure from the circuit, and place a container of known volume under the open drain line. Expect oil to flow as soon as the engine restarts, so the container needs to be positioned and steady before the engine turns over.

4. Run at rated speed with no functions operated

   Start the engine and hold rated speed with all controls in neutral. Idle speed produces a lower figure than rated speed, and the manufacturer's limit will normally be stated at rated speed. Do not operate any function during the test, because the reading you want is the leakage at standby.

5. Time sixty seconds and measure the volume

   Sixty seconds is long enough to average out fluctuation and short enough to be practical. Measure the volume, then repeat the test a second time. Two readings that agree are a measurement; two readings that differ by more than a few percent mean the oil temperature moved between them and the test should be repeated.

6. Convert to a percentage of pump output

   Manufacturers state the acceptable case drain as a share of the pump's rated output at the same speed. Work out the nominal output of the pump from its displacement and speed, divide the measured drain by it, and compare the result with the specification. That conversion is what makes the number meaningful; a bare figure of so many liters per minute tells you nothing on its own.

7. Repeat on every section if the machine has a tandem pump

   Tandem and triple pumps share a common failure pattern in which one section carries the others. Testing each section separately is the only way to know which one to repair.

What to do about it

Within specification and the complaint is real: case drain is not the problem. Move on to flow and pressure testing, because a unit can leak acceptably and still deliver less than the machine needs.

Slightly above specification and the machine is sluggish: the unit is worn but may still have useful life. Plan the replacement, watch the temperature, and check that the hydraulic oil is the correct viscosity and is not diluted or degraded. Oil that has sheared down in viscosity raises case drain on a serviceable unit.

Well above specification with heat and loss of function: the rotating group is worn and the unit should be replaced or exchanged. This is the point at which a reseal is usually wasted money, because a new seal in a worn pump leaks through the same clearances as the old one did.

High case drain on both sections of a tandem pump at once: treat it as an oil or contamination problem first. Two sections rarely wear identically unless the oil has been carrying abrasive material, and in that case the whole system should be flushed, not just the pump replaced.

Guangzhou Shenyue Machinery Technology Co., Ltd. supplies pumps, motors and the case drain hoses and fittings that protect them, from Guangzhou since 2007, quoting each line as genuine, OEM-quality or aftermarket so a replacement decision can be made against the measured figure rather than against a guess.

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