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Monday, 27 May 2013

How Do Greases Differ?

Ever wondered how greases differ – why is wheel bearing grease different to multi purpose?

We all know that using the wrong grease in an application will usually drastically shorten the equipments life, but how do greases differ to cope with the different applications? After all the demands on a wheel bearing are very different to those of a door hinge.

What gives different greases their charachteristics?

Greases are primarily classified by their thickeners, the most common
being metallic soaps. Others include bentonite clay, silica gel, polyurea and inorganic thickeners.

Soap based greases are produced from three main ingredients:
  • One is the fatty material (animal or vegetable), which is usually 4 to 15 percent of the total, called the acid. 
  • The next is the base or alkali, which is the opposite of an acid. Bases used in making greases include calcium, aluminum, sodium, barium and lithium, with 1 to 3 percent normally needed. 
  • The third constituent is the fluid, which can be selected from mineral oils, various types of synthetics, polyglycols or a never ending combination of fluids. 
A more complex structure can be formed by using a salt. This converts the thickener to a soap salt complex, hence the term complex grease. Complex greases offer a higher working temperature than normal soap thickened products - about 38 degrees C higher.

The difference between thickeners used in different greases.

 Inorganic thickeners such as clay and silica consist of spheres and platelets. They thicken fluids with their large surface area. These products produce a very smooth non-melting grease that can be made to perform very well when careful consideration is given to product application. 

Polyurea is a type of non soap thickener that is formed from urea derivatives. It is not a true polymer but rather a different chemical that has a thickening structure similar to soap. Polyurea greases are very stable, have a high dropping point and give outstanding service life.

In terms of use, the lithium 12 hydroxystearate greases are by far the most popular. These are based on 12 hydroxystearic acid, which is a fatty acid that produces the best lithium and lithium-complex greases.

The differences between complex greases.

Complex greases were developed to improve the heat resistance of soap greases, with the most popular being lithium, aluminum, calcium and barium. These products exhibit a higher dropping point (about 260 degrees C), with the exception of barium, which is about 218 degrees C.

The dropping point is the temperature at which the product turns from a semi solid to a liquid. In other words, it is the point at which the lubricant may begin to separate from the thickener.

Complex greases offer a higher working temperature than normal soap types and generally provide outstanding oxidation resistance, although this is not true in all cases. They are also increasing in popularity, with lithium and aluminum being the front-runners.

With modern equipments lubrication demands, synthetic greases are rapidly finding favour for special applications. Habot Synthetic Lubricants produce quality grease to suit most applications, and our technical staff can help you select the right grease for your application - whether for a wheel bearing or a wind turbine.