When one thinks of transmission lubricants, oil is of course the first one that comes to mind. However, grease also plays a vital role in driveline lubrication. This importance was highlighted in the NASCAR Cup series at the start of the 2013 season, when a regulation change increased the loads at the rear axle. The regulation in question allowed for an increase in rear camber from the previously specified 1.8° to 3°. Although increasing the camber in an independent front suspension is relatively straightforward, developing a robust set-up with the larger rear camber in a solid axle proved a challenge.
Teams struggled with broken axles, flanges and other rear suspension parts, and with prematurely worn-out bearings and seals leaking under the increased stresses. As Eric Warren, director of competition at Richard Childress Racing, explains, “It was a huge learning experience. The reason that the rule had been 2º for years was to prevent failures. NASCAR figured that as people were running housings with toe-in (a practice outlawed for 2013), you should be able to run with a degree more camber.
“Everybody was in a mad dash to get the rear axle grease and the materials for the half-shafts and drive plates to survive under the new loadings. The major issue was heat generation, and that was breaking down the greases we were using at the time. So we had to move to greases that were rated at much higher temperatures.”
So how are different types of grease formulated, and what sorts of additives are used to tailor their performance to particular applications?
Much as with oils, greases are formulated around base oil stocks and additive packages (see RET-Monitor November 2012 and December 2013). However, where grease differs is in the addition of thickeners, which give it its consistency, with the concentration and type of thickener dictating how ‘thick’ the final grease is. It should be noted that a grease can have a very thick consistency but in fact be based around a low-viscosity base oil.
Numerous types of grease thickeners are currently in use, each with its own advantages and disadvantages, depending on the particular application. The most common thickener types are simple lithium soaps, lithium complex and polyurea.
Simple lithium soaps are often used in low-cost general-purpose greases, and perform relatively well in most performance categories at moderate temperatures. Lithium complex greases provide improved performance, particularly at higher operating temperatures. A common upper operating temperature limit for a simple lithium grease might be 250 F (120 C), while that for a lithium complex grease might be 350 F (176 C). Polyurea is growing in popularity, and like lithium it has good high-temperature performance as well as high oxidation stability and bleed resistance.
The type of thickener in a grease is key to its performance, so in an application such as NASCAR Cup, where high temperature stability is key, only a lithium complex or polyurea grease should be considered. Fortunately for the motorsport customer, there are a lot of high-performance greases on the market, many of them originating from the aerospace and power generation industries, that provide the performance needed in even the most demanding environments.
Written by Lawrence Butcher