Grease is the hardest lubricant to sample well. It does not flow, it is not uniform inside a bearing, and its condition can change dramatically over a few millimetres. ASTM D7718 is the practice that makes grease sampling repeatable enough to trend — turning a smear on a spatula into a data point you can compare against last quarter's and act on with confidence.
What it covers
D7718 covers obtaining trendable in-service grease samples from a defined range of equipment: motor-operated valves, gearboxes, pillow-block bearings, electric motors, exposed bearings, open gears and failed grease-lubricated components. It recognises that some components do not mix their grease well — large or slow-rotating bearings being the obvious case — and explicitly allows multiple samples from different locations, and the blending of samples where a more homogeneous result is needed.
The practice describes the main sampling tools and when each applies: an active sampling device with a piston and probe that draws grease directly from the lubricated zone, a passive device that captures grease purged during relubrication, a simple tubing method, and a spatula-and-container approach for open and exposed bearings. Around those tools it sets the discipline that actually makes the data usable — cleanliness of the device and access port, capping and crushproof shipping, operator training in the internal layout of the component, and the minimum label information that ties a sample to its equipment, point and run hours. It also carries a clear safety rule: never sample a running component unless you are sampling from a purge path.
Why it matters in practice
The whole value of grease monitoring lies in consistency. A trend only means something if every sample comes from the same place, by the same method, at the same point in the relubrication cycle. D7718 names that principle and gives the procedures to honour it, so that wear metals, contamination and consistency changes reflect the bearing rather than the technician. It also warns against the most damaging field shortcut — pumping more grease to force a purge that will not come — which can wreck the very bearing being monitored.
For a condition-monitoring programme, the first few samples establish a baseline and everything afterward is read against it. Sloppy or inconsistent early sampling poisons that baseline permanently, which is why the practice puts so much weight on technique and documentation.
How we use it
We use D7718 as the backbone of grease condition-monitoring programme design — choosing the sampling method for each component type, defining repeatable sampling points, and setting the timing relative to relubrication. It feeds directly into the analytical suite we run on the grease: wear-particle work, elemental analysis, antioxidant remaining life and consistency. Because the required sample volume depends on which tests are planned, we fix the analytical programme first and let it drive the sampling specification. When advising wind, industrial or marine clients on bearing and gearbox grease, D7718 is the practice the field procedure is built on.