Why the sample point matters: how wear particles move in oil
Large wear particles settle out quickly and small ones stay suspended, so where and when you sample changes what you see. Includes sample points by machine and typical sampling intervals.
Particles are not spread evenly
Wear particles are a separate phase in the oil, not part of it. Right after a bearing or gear sheds them, they are all present. The largest, which matter most to the analyst, soon drop out in the quiet parts of the system, such as the sump, or are caught by the filter. The smallest stay suspended and pass through all but the finest filters, so they spread evenly through the system.
Large particles are news, small particles are history
When a machine runs steadily, the rate at which particles are generated and the rate at which they are removed settle at a steady level for each size. A change from that level is the signal: a filter bypassing, or a new wear mode beginning. The large particles show the present condition of the wearing surfaces. The very small ones are the residue of what has happened over the whole life of the oil, and their number tends to keep rising until the oil is changed.
How long after an oil change before the results settle
- After an oil change the coarse particle level rises until generation and removal balance again.
- A fine filter and fast oil circulation reach balance sooner. A system with no filter reaches it very slowly.
- Turbulent flow keeps particles suspended longer and delays the balance.
- A small, fast-circulating system, such as an aircraft gas turbine, balances in minutes. A large steam turbine reservoir can take hours or days.
- So avoid sampling straight after an oil change or top-up, and judge the first few samples with that in mind.
Sample points by machine
| Machine | Good sample points | Avoid |
|---|---|---|
| Engine crankcase | Dipstick tube, sample valve in the sump wall, return line before the filter | Sump bottom, cold oil |
| Hydraulic system | Valve on the return line before the filter, or reservoir mid-depth away from the walls | Tank bottom and walls |
| Compressor | Crankcase mid-depth away from the walls, or the return line after the oil separator | Sump floor |
| Gearbox | Sump at mid-depth | Sump floor sludge and side-wall deposits |
| Turbine system | Reservoir mid-depth away from walls and baffles, main bearing return line, after the bypass filter | Corners, baffles, dead legs |
Sampling from pipes and tanks
- Pipe: sample where the oil has just left the wearing part, in turbulent flow so particles stay suspended, and while the oil is hot. Do not sample after it has cooled.
- Large pipe with slow flow: sample near the centre, away from the lower wall where old debris collects.
- Dead-leg valve: flush it fully first.
- Tank: sample from the centre, clear of the bottom and the sides. A fixed stand-off on the sampling tube makes the depth repeatable.
- Label the bottle immediately. Wrong labels cause more confusion than most lab errors.
Typical sampling intervals
| Equipment | Heavy or continuous use | Light or occasional use |
|---|---|---|
| Steam, gas and hydro turbines, large compressors, stationary engines, heavy-duty gearboxes, motors above 2500 hp | About every 500 hours, or monthly | Quarterly |
| Medium-duty gearboxes, motors 200 to 2500 hp, hydraulics | Quarterly | Every six months |
| Low-duty gearboxes | Every six months | Yearly |
| Mobile equipment drivelines | About every 300 hours or 20,000 miles | As the maker advises |
Choosing the interval
Use the maker's interval as a rough guide only. The asset owner knows best. Weigh the safety risk, how critical the machine is, whether there is a spare, the environment, load and speed, the failure history and the cost of a failure. For a new programme, start at monthly and extend as the trends show it is safe. Quick in-house tests make it easier to stretch or shorten the interval based on the trend.