A Kobelco can hold full relief pressure on every function and still be worn out. Pressure proves the pump can reach a number; it does not say how much oil escapes past the pistons and valve plate to get there. Case drain flow says exactly that, takes twenty minutes, and costs one tee and a container. Guangzhou Shenyue Machinery Technology Co., Ltd. asks for a case drain figure on most hydraulic enquiries, and this is why.
What the case drain is telling you
Every piston pump and motor leaks internally by design. A small flow passes the pistons and valve plate and exits through the drain port to tank, carrying heat away. That flow grows as clearances open from wear, and grows sharply once a valve plate is scored. Measured at a known pressure and oil temperature it turns a vague complaint into a number, and the number separates a worn rotating group from a worn valve.
The same logic covers travel and swing motors: high case drain flow means lost torque, felt as a machine that stalls on a grade while relief pressure reads normal.
Setting up a test that means something
Three conditions decide whether the figure is meaningful. The oil must be at operating temperature, because cold oil is thick and under-reports leakage. The drain line must run to a container at atmospheric pressure, because a drain teed into a loaded return reads backpressure instead of flow. And the circuit must be held at a defined pressure, against relief or load, so the reading has a reference.
Warm the machine until the hydraulic tank is hot to the touch, typically twenty to thirty minutes of work. Chock the tracks and hold the circuit at relief for thirty or sixty seconds. A flow meter is ideal, but a graduated container and a stopwatch give a usable number.
Reading the number
There is no single universal pass mark, because rated flow differs by model and pump family. The working rule across piston pumps is a few percent of rated flow at rated pressure and operating temperature, and the model's service data is the authority where it exists. The shape of the result matters more than the absolute figure: leakage rising steeply with pressure points at worn pistons or shoes, while a uniformly high figure points at a scored valve plate or damaged cylinder block.
| Component | Where to measure | What a high figure means |
|---|---|---|
| Main pump | Drain port to open container, at relief | Worn pistons, shoes or valve plate |
| Travel motor | Motor case drain, motor stalled | Worn pistons and block, or scored swash plate |
| Swing motor | Case drain at swing relief | Worn rotating group, slow swing under load |
| Shaft seal area | Visual, after the drain test | Blocked drain or high case pressure, not seal wear |
The two traps that fake a pass
The first is cold oil: a test on a frosty morning can read a third of the true figure, and the machine goes back to work with a pump already on its way out. The second is a blocked breather or crushed drain hose: case pressure rises, the shaft seal weeps, and the seal is blamed when it is the victim.
A third trap wastes money: testing a pump that is being starved. A collapsed suction hose or clogged strainer mimics internal wear, and no case drain reading will show it.
From a high reading to the right part
A high figure rarely means a complete unit. On the travel motors we handle the rotating group is available separately: a swash plate such as C2162122059801, a cylinder block such as C2162221251701 / LC15V00023S040, a ball guide such as C2162420488601 / LC15V00023S042, and a relief cartridge such as C2259321450501 / LC22V00006F1. A pump with a high figure and a healthy regulator may only need the rotating group, while the regulator, such as 29070821-4927 / YN10V01006F2, is a separate decision.
If the figure is high and the oil is dirty, fix the oil first. The travel motor internal parts guide shows how the rotating group goes together and the full pressure slow cycle guide covers the case where pressure is fine and speed is not. The hydraulic components range lists what we hold as assemblies and internals, and a photograph of the drain setup with the figure and oil temperature sent to the contact page is usually enough for Guangzhou Shenyue Machinery Technology Co., Ltd. to say which level to buy at, marked genuine, OEM or aftermarket.
Test at commissioning and at every major service, and keep the numbers: a trend of case drain flow against hours is the cheapest predictive maintenance an owner can run. Seal detail is documented by the seal makers, for example at nok.co.jp, and Kobelco's service data at kobelco.co.jp gives the model-specific limits.
Frequently Asked Questions
What is a normal case drain flow on an excavator hydraulic pump?
A healthy piston pump leaks a few percent of rated flow at rated pressure and operating temperature, and the exact limit varies by model and pump family, so the machine's service data is the authority. The trend matters most: a figure climbing steeply with pressure indicates wear in the rotating group rather than the valve.
Why must the case drain be disconnected for the test?
Because a drain teed into a loaded return line measures backpressure rather than leakage, and it will pass a pump that is already worn out. Run the drain to an open container at atmospheric pressure, keep the hose short and wide, and record oil temperature at the same moment as the flow.
My travel motor shaft seal keeps leaking. Is the seal the problem?
Usually not. A shaft seal fails when case pressure rises, which happens with a crushed or blocked drain line or a blocked tank breather. Fix the drain path and the seal will hold; replacing seals without checking case pressure simply repeats the failure.
Can I repair a worn travel motor instead of replacing the assembly?
In many cases yes. The rotating group is available as separate items including swash plate, cylinder block and ball guide, and Guangzhou Shenyue Machinery Technology Co., Ltd. quotes those internals alongside complete assemblies. The choice depends on the case drain figure, housing bore condition and whether the oil has been contaminated.