Extended Bearing Life Greases “Tried and True” or New Technology? Gaston Aguilar, Vishal Nandurkar and Anand Karve Afton Chemical Corporation • Richmond, VA Afton Chemical India Pvt. Ltd • Mumbai, India
Abstract
An experimental calcium sulfonate complex (CSC) grease and standard lithium complex grease (LiCx) were produced with the same base oil composition. The base greases were then formulated with two additive systems. The additive systems consisted of a primary antioxidant component and a multifunctional component containing antiwear and extreme pressure additives with secondary antioxidant function. The two base greases were treated with the single components and a combination of the two components. High temperature bearing performance of the six formulated greases was evaluated using ASTM D3527 wheel bearing life (WBL) test. Greases were also evaluated by a protocol designed to study the effect of thermal aging on the oxidative and shear stability of the greases. Specifically, the greases were statically aged at the same temperature (160 °C) and in the same inboard roller bearings used in the WBL test. Bearings were weighed before and after aging to determine weight loss. Pressure Differential Scanning Calorimetry (PDSC) was used to monitor oxidation stability before and after aging, and infrared (IR) spectroscopy was used to measure extent of oxidation. Changes in grease shear stability were measured using oscillatory shear rheology strain sweeps performed at 160 ˚C. The WBL data showed that CSC and LiCx greases had similarly short-lived performance when greases were treated with each of the single components while greases treated with the combined components produced extended lives. Although antioxidant synergism is expected between the two components of this study, the rheological analyses of the fresh and statically aged greases suggest that the additives can alter the thermal and mechanical stability of the greases in ways that positively and negatively impact grease life in high temperature wheel bearings.
are increasingly required to operate at higher loads, speeds and temperatures1. The steady rise in bearing operating temperature has gradually expanded the demand for greases with improved oxidative and thermal stability. To enhance oxidation resistance, grease manufacturers have turned to higher quality base stocks such as Group II and III mineral oils and synthetic fluids like PAO’s and esters. To improve thermal stability, the use of thickeners that impart higher dropping points is also trending upwards2-3. Two grease types that have seen noticeable growth in their production volumes are lithium complex (LiCx) and calcium sulfonate (CS) greases. This increase is demonstrated in Figure 1, which compares production volumes in 2004 versus 20144-5: LiCx grease is an established commodity with a proven service record while CS grease, although not new to the industry, is considered an up and coming technology. Greases based on lithium soap are good all-around performers that have excellent pumpability, shear stability and water resistance. The ability to complex simple lithium soaps and raise dropping points from about 200 °C to >260 °C have made LiCx greases the conventional choice for high temperature applications including automotive wheel bearings . To achieve their high dropping points, the production of LiCx greases involves the use of complexing agents such as boric acid, salicylic acid and di-carboxylic acids. Of these
Introduction
As machinery is built to be more efficient and transmit more power, rolling bearing elements
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