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Truck Topics

Grease – that sticky stuff

By Lee Kurtzmann
Posted Sep 4th 2002 9:00AM


Just the word grease presents a negative image. It brings to mind a black, gooey mess that sticks to everything it touches, is difficult to wash off your hands and stains your clothes.

Something to be avoided, right? Well, as we all know, the machines we co-exist with on a daily basis need grease to live. Grease (along with oil), keep the bearings turning freely and quietly and keeps the gears meshing and not grinding.

Unlike oil, grease does not cool, flush and clean the parts. But grease remains in place as it lubes the metal surfaces that would otherwise grind together. It also forms a barrier to keep contaminants out of the joints and housings.


Q: When is grease used instead of oil?

A: Grease is used in places where the oil is mechanically confined, like crankcases and gearboxes, non-thickened motor or gear oil; if oil cannot be held in place, use grease.


Q: What turns oil into grease? A: Soap thickener mixed with liquid oil and chemical additives. The thickener holds the oil in place so it can do its lubricating job.

Thickeners of various kinds are found in automotive greases, but the most common thickener in truck grease is lithium or a lithium complex.

Q: How important is tackiness?

A: Tackiness comes from additives, which make grease cohesive (it sticks to itself) and adhesive (it sticks to metal surfaces). Grease must have enough of these two characteristics to do its job, but too much impedes the function of the mechanical parts.

Grease remains in place because it's tacky. One test of a grease's quality is to put a dab between thumb and finger and pull it apart; if it forms a nice, long strand, it's thought to be good stuff. Or, it might mean it's too tacky.

Sticky joints caused by overly tacky grease can present their own set of problems. These joints require more engine energy and can burn more fuel to overcome the resistance and in the case of a fleet situation, it can add up.

Q: What about the hardness of grease?

A: Just as oil's thickness is measured by viscosity, grease has it's own measurement.

The National Lubricating Grease Institute has a standard test which involves dropping a weight into a sample of grease; the deeper the penetration, the softer the grease.

The softer the grease, the lower the NGLI Consistency Number. The numbers can range from a semi-fluid "triple-ought (.000) to a block grease's NGLI rating of 6.


Q: What about other additives?

A: Chemicals such as Lithium, graphite and others are often used For their lubricating and other properties.

"Lithium grease" has a nice, buttery texture and resists water washout.

Solid lubricants like graphite and molybedenum disulfide are sometimes added to reduce friction. Other additives enhance the oil's properties and guard against corrosion.

Q: What makes a grease "synthetic"?

A: Synthetic oil, with esters and various synthetic hydrocarbons being the most often used. These can have benefits but shouldn't be used in existing joints unless it's compatible with what's already in there. That's also true with any grease that's formulated differently in any way from what is currently being used.

Greases that use the same types of oils and thickeners will usually be compatible. A simple test of mixing the old with the new will usually tell the tale. Incompatibility will be demonstrated if they thin or thicken.

Incompatibility can be a serious issue, so if another product appears superior but is not compatible with the grease currently being used, it would be best to flush out the joints by pumping in enough of the new product until it appears out of the joint.

Problem Areas

Oil sometimes comes to the surface, making the grease soft and runny. "Bleeding," as it's sometimes called, will happen when the grease is in storage, and a little of it is thought to indicate good lubrication characteristics, especially for roller bearings. Too much means the oil may leave the bearing or joint and lubrication will suffer.

As you'd expect, bitter-cold winter temperatures may dictate use of thinner greases. These are also used in automatic lubing systems, which pump grease through pipes and manifolds.


Q: Which grease should you use?

A: Your safest bet is to follow the recommendations of the company that made the bearing or device being greased.

Usually the truck builder will pass on these recommendations and major components builders also maintain toll free numbers to answer these “greasy” questions.


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