What’s life like inside a rolling element bearing? Let’s say you are a dollop of grease and you’ve just been pushed by a grease gun into the dark recesses of a bearing cavity. You are now in a combat zone. What are your orders? Maybe you are on a suicide mission. Your bearing has been screaming for reinforcements and you are it – the new recruit – all gooey and slimy.

Inside you see grease casualties all around. There’s the stench of oil oxidation on one side and the stiff lifeless remains of a soap-based thickener on the other. In an instant, the floor and walls begin to vibrate, then you hear a low rumbling sound. You are pushed deeper into the bearing cavity and suddenly you can’t move – the heat and the pressure are excruciating. Then more pressure … crack … silence.

What just happened? How could regreasing a bearing have contributed to sudden-death failure? Doesn’t fresh grease prolong bearing life and restore reliability? Not so fast. The problem here is new grease entering an old, infrequently lubricated (and overly lubricated) bearing. As the new grease enters, it must make room, and in doing so, it pushes around the crusty remnants of grease past.

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Many who claim to be purists tout that they prefer oil over grease. But then again, there are those die-hard grease advocates who have their own orthodoxy. Indeed, both schools of thought are supported by strong, compelling arguments. It seems for every point in support there is an equally strong counterpoint. As it has been for decades, the grease versus oil debate does not die or even fade away. Let’s hear what they are saying today, including several classic arguments along with a couple of new ones.

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One of the main disadvantages of using grease over oil is the annoying tendency of grease to cake and dry out. We all know that oil has a natural tendency to drain out of the grease thickener over time. We see conspicuous evidence of this when oil puddles in unleveled grease pockets in pails of new grease. We’ve also seen the dribble of oil from stored grease cartridges and prepacked bearings on storeroom shelves.

More serious is the hardened byproduct of oil drainage that builds inside the cavities of operating bearings and gears over time. Often this takes years to develop, while in other cases the condition can reach catastrophic levels in just a few weeks. The failure can be self-propagating. As grease becomes dry, frictional forces escalate, causing concentrated heat within the bearing.

The elevated heat continues to force more rapid and complete drying of the grease. In time, the grease can reach a state of consistency ranging from hard putty to sandstone, depending on the thickener type and degradation conditions.

Frequently the most serious offending conditions leading to grease dry-out are aided by other causes working in concert. Let’s begin by reviewing the primary causes of grease dry-out. Once the causes are determined, we have the information necessary to prescribe steps to grease your bearings properly.

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There are three opportunities to inspect the state of in-service grease. One is by disassembly (such as by removing the bearing cap), the second is by sampling the grease using a probe (ASTM D7718), and the third is by examining the purge discharge.

The purge discharge is the grease that’s extruded from exhaust ports, seals and other openings during relubrication or machine operation.

Not all grease-lubricated machines have a purge stream, but many do. Machines (mostly bearings) that purge grease provide a valuable opportunity for inspection. The opportunity is significant because of the frequency and simplicity of the inspection.

Machines that purge are generally “total loss” systems, meaning the grease is not recovered for reuse but instead is discharged to a catch-pan, trap, grease thief, exterior surface or straight to the floor (Figure 1).

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