GAUGING CONSISTENCY
A penetrometer used to measure grease consistency. The standardized cone is released from just above the grease surface, sinking to a depth indicated by the dial. The device lying alongside is for pre-working the lubricant. It screws on top of the sample holder (the upright container under the cone) and is hand-pumped, forcing grease through small holes in the plate.

Oil is the hands-down lubricant of choice, allowing the generation of a fluid film under elastohydrodynamic lubrication. At high speeds, this film separates mating parts, minimizing frictional effects and prolonging component life.

However, due to difficult and expensive arrangements often needed for containment and circulation, liquid lubricant (oil) may be impractical. The next best thing is grease. Grease is essentially an oil-delivery system, used to bind lubricant to working mechanisms.

Sometimes it is unavoidable that different greases will get mixed together. Ideally, we do not want this – there is never an upside. If a machine is working acceptably, pumping in a different form of grease will likely contribute to problems. Nevertheless, greases get changed and mixed, for one reason or another. Perhaps the old grease is unknown or in some ways not wholly effective, and its complete removal uneconomical due to unacceptable downtime and the direct expense of tearing machinery apart.

If we get the mixture wrong, then, what is the danger? The mixture may become soft and run out of the mechanism. At the very least, this bleeding or purging of the grease is unsightly, messy, and can get onto the product.

The results can be a bit more painful, though. As the thinning grease leaves bearing contact areas, and the bearings get increasingly hotter, total lubrication failure sets in, with full metal-to-metal contact and extreme heat. The component eventually, mercifully, seizes up.

Bearing temperatures can exceed 2,000°F, where steel turns to putty. Yet, many industrial machines will continue turning with the incredible amount of horsepower driving the system. Ultimately, the bearing melts enough to give the shaft so much play that other portions of the machine come into contact and force a stall. Or, the horrendous noise alerts the operator to shut it down. What the bearing analysts often see returned are shapeless hunks of melted metal.

This can be avoided by handling greases properly.

Grease makeup

To better grasp incompatibility issues, we should understand the constitution of grease. It consists of three main components, in varying proportions:
• Base oil, 70-95%
• Thickening agent, 4-20%
• Additives, 0-10%

GREASE-THICKENER COMPATIBILITY
This chart covers most of the thickener types available today. The information, available from several major grease manufacturers, is based on standard workedpenetration tests for two-grease mixtures. Here, every thickener type is incompatible with at least one other. Incompatibility usually resulted in significant softening of the mixture, although hardening did occur in several cases.
Select figure to enlarge.

Oil is the primary ingredient, as it should be – it’s what actually lubricates the mating parts. While there are hundreds of bearing greases available, they can be broadly categorized according to their base oil and thickener.

The base oil is either mineral or synthetic. Mineral oils include paraffinic, napthenic, and aromatic varieties. Synthetic oils are sub-categorized as synthetic hydrocarbons, esters, silicones, ethers, and fluorinated.

Each type of oil has its advantages and disadvantages. Mineral oil is inexpensive and therefore the most commonly used. Its primary shortcoming is poor performance outside the “normal” grease operating temperature range of -10 to 250°F. Synthetic oils perform better at extreme temperatures and speed, and cost considerably more.

Thickeners are either soap or nonsoap. Soap thickeners are divided into lithium, sodium, aluminum, barium, and calcium types. Non-soap thickeners are polyurea, carbon black, microgel (clay), and fluorinated.

Soap thickeners are actually the chemical soaps of various metals. Each type has its own unique characteristics, and therefore no broad generalizations are tied to the soap category. However, lithium thickeners are by far the most common, being relatively inexpensive and versatile.

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