Field diagnosis
Case drain flow test: the cheapest wear measurement on the machine
Case drain flow shows how worn a pump or motor is without removing it. Here is how to measure it so the number actually means something.
Check these in order
| Check | What it looks like | What it tells you |
|---|---|---|
| Bring the oil to working temperature | Leakage rises with temperature, so measurements taken cold badly understate wear. | Run the machine to normal duty temperature, then measure. |
| Measure at the case drain port | Catch the drain flow in a calibrated container over a measured time, or use a flow meter in the line. | Compare against the specification for your unit, not against a general figure. |
| Compare pump to pump | On a tandem machine a difference between two pumps of the same age is as useful as the absolute number. | Log both sides at the same temperature and load. |
| Load the circuit while measuring | Rotary group wear shows as flow that rises sharply with pressure. | Record the flow at idle and again at full load. |
| Rule out the obvious first | An overfilled case, a restricted drain line or a blocked case drain filter raises flow with no wear at all. | Check the drain line and the case pressure limit. |
| Repeat after repair | The same test after a repair kit confirms the work before the machine leaves the workshop. | Keep before and after numbers in the machine file. |
Units and parts this usually leads to
Pump noisy and oil hot
Correlate the two symptoms.
Datasheets and checklists
Print a test record and keep it with the unit.
Field notes on this complaint
The notes our own team works to on this symptom. Each one is a principle you can check on the machine, a step from the case-drain and cleanliness procedure we follow, or a figure transcribed from our catalogue - not advice copied from a video or a forum. Anything we could not check against a catalogue, a manufacturer document or a working machine is left out.
- Measure it hot. Case-drain flow is only meaningful at working temperature and at a defined engine speed; a cold reading flatters a worn pump. (our own case-drain and cleanliness procedure)
- The figure plus the slipper surface decides. A worn piston and slipper pair is a repair-kit job more often than a whole-unit job - which is why we look at the case-drain number and the part together, never at the pressure gauge alone. (our own case-drain and cleanliness procedure)
- A lip seal goes in with pressure behind the lip - and warmth helps A lip seal is fitted so that system pressure presses the lip against the shaft: pressure from the inside tightens it, pressure from the outside opens it. Warming the seal in oil before fitting makes it pliable enough to seat without being distorted, which matters most on the hard outer seals. A seal fitted dry or the wrong way round is one of the few case-drain problems that comes straight back. (our own case-drain and cleanliness procedure)
- Why the drain line carries a 1-3 bar check valve Parker's instructions for their motors recommend a check valve with a 1-3 bar cracking pressure in the drain line, fitted to keep proper case pressure and lubrication. That single line explains several field symptoms: no case pressure and the bearings and swash plate run dry; too much back pressure and the shaft seal is pushed out; and it is also why a drain line that has been blanked off or kinked turns into a worn unit rather than a leaking one. (manufacturer service or operator manual we hold (title on request))
- Commissioning after a repair: the maker's step-by-step run-in Parker's motor instructions describe the run-in: start at 500 rpm with a 250 bar pressure difference and 10-15 bar at the outlet, run until the drain temperature reaches its maximum and then falls 1-2 C, then step the pressure difference up to 350 bar and repeat, then to 400 bar. If the temperature tends to go above 100 C, reduce pressure immediately, and measure with the probe in the drain oil rather than on the housing. That is what a proper run-in looks like - a repaired unit that is put straight to work never gets it. (manufacturer service or operator manual we hold (title on request))
Common questions
What is a normal case drain flow?
It is unit-specific. Use the figure for your configuration rather than a generic number, and always record it at the same oil temperature and load so the trend is meaningful.
Can I test without removing the pump?
Yes. Case drain flow, relief pressure at the pump outlet and control pressure together separate pump wear from control, suction and relief faults on the machine.
Does high case drain flow always mean a new pump?
Not always. A repair kit restores the rotary group when the housing and shaft are within tolerance; the test result plus a visual check of the valve plate decides it.
Why it happens
The mechanism behind this symptom, from our technical articles: reading the principle first usually shortens the checklist.
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