While reading through this month’s issue of Lubes n Greases (be nice, I really do have a life), I was surprised to learn that Alaska Airlines Flight 261 crashed three years ago because of a lack of grease.
Last December, the National Transportation Safety Board closed its investigation into the crash and concluded that the jackscrew and nut assembly, which raises and lowers an MD-83’s horizontal stabilizer, had not been properly lubricated. Threads from the nut had been stripped off and were found wrapped around the jackscrew. With the nut free to ride up and down the jackscrew, there was no control of the stabilizer, which pounded the nut into its stop until it broke.
The rest is a tragic bit of history, but it highlights the importance of a centuries-old substance. Roman charioteers, I’m told, used animal fat, which they called crassus, to lubricate their wooden wheels and axles. Thousands of years later, grease of one type or another still is widely used to reduce friction and wear between sliding and rolling surfaces. In some instances – for example, where leaking wheel oil seals have been a problem – grease has even enjoyed newfound popularity.
Why do we still use such old technology? For one thing, grease is basically a sponge that holds oil and gradually releases it. That’s why it’s useful where frequent relubrication is inconvenient, as in a clutch pilot bearing.
Grease also is useful where the lubricant is needed to perform part of the sealing function, to help keep lubricant in and contaminants out, like between a spring shackle-pin and its bushing.
The limitations of grease aren’t in its ability to lubricate, but in the fact that it does stay put. Of course, it can’t be expected to perform the cooling and cleansing functions normally associated with oils.
So, what’s in the greases we use today? Modern greases have three main components:
- Thickeners, often called soaps. These make up the sponge part and can be derived from calcium, lithium, sodium or other elements or compounds. Final properties, such as appearance, stability, pumpability and heat and water resistance, depend largely on the type of thickener used;
- Oils, which can be petroleum-based, synthetic or a blend. These perform the primary lubricating function;
- Additives, which provide additional properties, such as corrosion-resistance, adhesiveness and extreme-pressure (EP) capability. Sulfur, phosphorus, zinc, molybdenum disulfide (moly) and graphite are common additives.
Based on its ingredients, the finished product will display a number of properties, the most distinguishing of which is consistency. Per standards set forth by the National Lubricating Grease Institute (NLGI), a grease’s consistency can range from 000 (the softest or thinnest) to 6. Commonly used automotive greases have ratings of 1 to 3, with 2 being the most common. There also are ratings that characterize low- and high-temperature performance and all the other properties baked into a grease.
So, if you had the luxury of stocking several different kinds of grease, there would be preferences for different vehicle areas and applications. For example, because wheel bearings endure wide variations in speed and temperature, greases with good mechanical and high-temperature stability are desired. Normally, a smooth, lithium-based product works well.
Rubbing or sliding surfaces, such as metal bushings and ball joints, require superior anti-wear and EP capability. Lithium and aluminum-based greases, with an anti-wear agent like moly or graphite are well suited.
If you need to choose one grease that will perform well in most applications, make sure it conforms to all minimum specs published by your vehicles’ manufacturers.
Then, check with your lube supplier to make sure the grease is compatible with what’s already on your vehicles. Some greases don’t mix well with others, which can result in thinning, runoff and inadequate lubrication. And inadequate lubrication is something you don’t want to mess with.