Same NLGI Grade, Different Grease: Plan the Changeover Bearing by Bearing

When a new rate contract brings in a different grease, decide for each bearing whether the old grease can come out, and how, before the first drum reaches the greasing route.

The new grease looks right on paper. Like the old one, it is NLGI 2 with a similar base oil viscosity and passes the plant's purchase specification. Stores issues the first drum, and a fitter on the fan route is about to pump it into bearings still holding the previous product.

Whether that is safe depends on how the two greases' thickeners, base oils and additives behave together in a running bearing. Neither data sheet tells you that.

Take an invented example: a plant moving from a lithium complex grease to a polyurea grease of the same grade. Lithium complex and polyurea are thickeners, the soap or other solid that holds the oil in a grease, much as a sponge holds water. The plant has four kinds of grease point:

• induced-draft (ID) fan pillow blocks (plummer blocks) with split housings and drain plugs;
• low-voltage (LT) motors whose manual names a lithium complex grease;
• a conveyor fed by a progressive centralised system, where one pump feeds many bearings in turn through distributor blocks;
• sealed conveyor idler bearings that are never regreased.

Look past the grade and the specification

An NLGI number describes consistency, nothing more. Schaeffler advises avoiding mixtures where possible. Where a mix cannot be avoided, it sets five conditions: the same base oil, the same thickener type, base oil viscosities no more than one ISO VG class apart, the same NLGI class, and the lubricant maker's agreement. Even then, it says, the mixture may perform worse.

The example pair already fails the thickener condition. A purge pumps new grease into a running bearing to push the old grease out. That mixes the two greases on purpose. For this pair, a purge needs a passed test, because Schaeffler says only suitable tests give a definite answer on mixing. Klüber's changeover procedure also requires both grease makers to confirm that the pair is fully compatible.

A purchase specification does not close the gap either. In its 2019 revision, the Bureau of Indian Standards renamed IS 7623 from lithium base grease to multipurpose industrial grease, because any soap thickener may now be used. Two greases that meet IS 7623 can carry different soaps. The example plant's own specification names no thickener, so polyurea grease meets it while the bearings still hold lithium complex.

The same trap shows up outside grease, in liquid cooling. PG25 names a coolant of roughly one quarter propylene glycol in water, yet Reliability Engine, our sister site, explains that two coolants can share the PG25 label and still use different water quality, additives and material approvals.

ASTM D6185, the standard practice for judging grease compatibility, describes what goes wrong. Most incompatible mixtures soften, sometimes badly. A few harden, and in extreme cases the oil separates from the thickener. A mixture can bleed until grease runs out of an operating bearing. The mixture can start to drip at a lower temperature in a standard dropping point test. The practice notes that equipment makers recommend cleaning out all the old grease before a change.

Use compatibility charts only as a first check

For lithium complex and polyurea, the answer depends on which chart you open.

Lithium complex grease mixed with polyurea grease

One pair, four published answers

Published source	What it says about this pair
SKFBearing catalogue, thickener compatibility table	Incompatible
NSK EuropeTechnical Insight TI/E/0002, guidance credited to Klüber	Partially compatible
MobilGeneric compatibility chart, 2009, shear-stable polyurea	Moderately compatible
Shah, Lotwin and DodosASTM D6185 tests on specific products, 2026	Most compatible of three pairs tested

The reverse also happens. SKF and Mobil rate lithium complex with calcium sulphonate greases as compatible, yet lithium complex with calcium sulphonate complex was the least compatible pairing in the 2026 tests.

Reviewing 17 published charts, Wurzbach and Kowalik of MRG Labs found direct contradictions and few references to the test work behind the ratings. Use a chart to decide which pairs need a closer look. The decision itself needs test data.

Ask for test data on the exact pair

Ask the new supplier, in writing, for an ASTM D6185 report on its grease mixed with the exact product now in your bearings, named by brand and grade. As NSK Europe notes, a supplier should have the data or be willing to run the tests. If it has none, send the supplier a sealed sample of the old grease from stores. Share the report with the old grease's maker too, and ask it to confirm compatibility. Ask early, because the storage stability soak alone takes about three days.

ASTM D6185 compares the mixtures with each grease alone in three primary tests:

• dropping point;
• worked penetration, which shows how far the mixture softens or hardens after 100,000 strokes in a grease worker, a machine that churns the sample;
• storage stability, the change in consistency after a hot soak (DuPont describes 70 hours at 120 °C).

DuPont describes the usual order: a 50:50 mixture first, then 10:90 and 90:10 if it passes. Testing stops at the first failure, and one failed test on any mixture makes the pair incompatible.

A pass lowers the risk without guaranteeing field performance. Mobil points out that these physical tests do not show whether the mixture still protects seals, stops rust, protects yellow metals such as brass and bronze, or controls wear under heavy load. Nor do they show how the two additive packages react over time. The tests also use fresh grease, but the grease in a bearing has aged. If a bearing is exposed to water, runs under heavy load or has brass or bronze parts, ask for corrosion, wear and extreme-pressure tests on the mixture as well.

Suppose the example pair passes at all three ratios and both grease makers confirm it is compatible. A purge is then possible at some points but not all.

Choose the method at each bearing point

Work down the table and use the first row that fits. If a planned stop is close, use row 5 even where row 4 fits, because equipment makers prefer cleaning.

The example motors stay on lithium complex until their maker approves a change at overhaul. ABB's manual shows why: for some high-speed motors it lists two polyurea greases and says they must not be mixed with lithium complex greases.

So stores keeps ordering the old product by name, with purchase approval outside the new rate contract. Another maker's lithium complex is a different grease and needs the same check. The stores list shrinks less than purchasing hoped, but every motor keeps a grease its maker has approved.

The conveyor needs its own plan. A 2023 feature in the magazine of STLE, the Society of Tribologists and Lubrication Engineers, quotes G.R.P. Singh of Tata Steel, Jamshedpur. He saw a caster bearing failure at a steel plant that switched from lithium complex to polyurea without checking compatibility. Such checks, Singh says, are especially important for bearings fed by a centralised system. Besides full cleaning or an interim grease compatible with both, he lists a third practice: run the old grease out, load the new one and run the system in manual mode until new grease comes out of the bearings.

The example plant follows that third practice with one change. With the conveyor stopped and isolated, it disconnects each feed line at its bearing, pumps the line's grease into a container until new grease appears, then reconnects it. The old grease in the lines never reaches the bearings, and the old grease already inside the bearings is pushed out as new grease keeps arriving.

Purge only where the old grease can leave

The ID fan pillow blocks fit row 4, with drain plugs on the side away from the grease inlet. New grease then has to pass through the rollers or balls to reach the drain, so what comes out tells you about the bearing, not just the housing. If the bearing is greased through a lubrication groove and holes in its outer ring, new grease passes through the bearing whichever side the drain is on.

SKF also requires a way out for used grease, through the seals or an escape hole, with no tube on the hole because a tube can restrict the flow.

Opening the split housings at a stop would remove more old grease, but the fan's next planned stop is the annual shutdown, months away. So this plant purges, starting with one bearing and watching it through its first shortened interval.

Plan the access first. Grease a running fan only where the fitting and drain can be reached from outside the guard, under the site's permit and machinery rules, and never open a guard on a running machine. A point whose fitting or drain sits inside the guard waits for extension lines or for cleaning at the next stop.

Housings and purged grease can be hot. Follow the site's glove rule near rotating parts, keep sleeves and rags clear of the shaft, and catch the grease in a tray.

At each visit:

1. Remove the drain plug and check that the hole is clear, as DuPont advises. If clearing it means reaching near moving parts, leave that bearing for the next planned stop.
2. Set the dose: no more than the bearing's replenishment quantity, the grams the bearing or machine maker sets for one normal regreasing. Use a gun whose output has been measured, as our grease gun article shows.
3. Add the new grease slowly in small shots, watching the drain and the temperature. Expect little at the drain on the first visits.
4. If the gun lever stiffens, stop rather than pump harder, because a hand gun can build more pressure than a lip seal can hold.
5. Leave the drain open for the 30 to 60 minutes DuPont suggests, then refit the plug.

Shorten the next intervals

One purge rarely clears all the old grease, so the next few relubrications finish the job. Klüber's procedure gives the pattern. If the normal interval is longer than a week, regrease after one week of running. Then regrease more often than normal for the first two intervals. For the ID fan, the plant agrees the shorter intervals with the bearing maker and grease supplier, and writes into the maintenance plan the date the normal interval returns.

Check that the change worked

Before the first purge, keep sealed, labelled samples of both greases from stores as references, and take one in-service sample from each bearing as a baseline. At each later relubrication, sample the purged grease from the drain the same way each time, following a sampling practice such as ASTM D7718. Ask the laboratory to compare each sample with those references. Drain samples can be small, so ask for a consistency test that needs only about a gram, such as die extrusion under ASTM D7918.

Some softening in service is common. In a 2021 STLE article, Lisa Williams of Spectro Scientific said a drop of more than about 30% in measured consistency should make you ask whether greases have mixed. Treat that as a rule of thumb, not a limit. Infrared and elemental tests can then help confirm which greases have mixed. Colour helps only if the two greases look different, so compare a dab of each on a white card before you start.

Close the fan's changeover only when all four are true:

• the purged grease at the drain matches the new product;
• temperature and vibration have settled into a steady band after greasing;
• the laboratory finds no large drop in consistency against those samples;
• the maintenance plan shows the date the normal interval returns.

Keep the two greases apart

With both greases in use, one fan has lithium complex in its motor and polyurea in its pillow blocks, the kind of split that Wurzbach and Kowalik describe as a common route to accidental mixing. From the first drum:

• label each point with its grease, and give each grease its own labelled gun;
• record in each machine's work history which grease went into which point, as our article on lubrication handover after maintenance describes;
• tell motor repair shops which grease each motor takes, and check what new or rewound motors arrive with;
• for future contracts, check compatibility and changeover needs before any substitute grease reaches stores, as our 90-day pilot guide sets out.

Start before the next drum is issued

If the new grease has already gone into some points, record which ones and ask both suppliers for test data that week. Do not leave those bearings without grease while you wait. Agree with both suppliers which grease those bearings should get at their normal interval. Check their temperature, vibration and leakage more often, and sort them by the table when the data arrives.

Start this week. Write to both grease makers about the exact pair, and ask the new supplier for its test data. No point can be purged until both confirm compatibility. Before the next drum leaves stores, list every point that will receive it and mark one row of the table against each. Then check that enough of the old grease is on order for every point that stays on it. Any point without an answer keeps the old grease until it has one.