Influence of grease rheology on thrust ball bearings friction torque Tiago Cousseau, Beatriz M. Grac - a, Armando V. Campos, Jorge H.O. Seabra Keywords: Greases Grease rheology Rolling bearing Friction torque abstract The friction torque and the operating temperatures in a thrust ball bearing were measured for seven different types of greases, including three biodegradable greases having low toxicity. These friction torque tests were performed using a modified Four-Ball machine. Rheological evaluations of the lubricating greases were made using a rheometer. Bleed oils were extracted from the greases and the dynamic viscosities were measured. In order to compare the performance of the lubricant greases in terms of friction, the grease characteristics were related to experimental results, showing that the interaction between thickener and base oil have strong influences in the bearing friction torque. 1. Introduction The reduction of energy consumption and the increase of the efficiency are, nowadays, an important parameter in the design of machine elements, in particular rolling bearings. Efficiency is also strongly dependant on lubricant formulation and its rheological behaviour. Rolling bearings are mainly lubricated with grease. Thus, grease rheological behaviour is a key feature to understand lubrication inside a rolling bearing [1,2], to predict film thickness between rolling elements and raceways [3,4], as well as evaluate the internal bearing friction torque. In this work the rheological behaviour of different types of greases, including biodegradable low toxicity greases, was eval- uated using cone-plate geometry in sweep and flow tests. The bleed oil was extracted from the greases and its viscosities were measured at different temperatures. Thrust ball bearings were tested with all greases and the corre- sponding internal bearing torque and operating temperature were measured at different speeds. The main objective is to correlate the bearing torque results with the rheological behaviour of the greases. 2. Methods and materials The rolling bearing tests were performed using a modified Four-Ball machine, where the four-ball arrangement was replaced by a rolling bearing assembly, as shown in Fig. 1. The new arrangement was developed to measure the friction torque and the operating temperature in rolling bearings. It is composed by: a shaft adaptor (6), which connects the machine input shaft to the upper race (5); a support (2) and the lower race (3), both clamped to the bearing house (1); a torque cell (11) protected by two plates (9, 13), which connect the bearing house (1) to the lower support of the Four-Ball Machine through the pins (10, 12). The retainer (7) and the cover (8) avoid the lubricant leakage. In operation, the bearing torque is transmitted to the torque cell through the bearing house. This assembly has seven thermocouples positioned in strategic locations in order to measure the lubricant and bearing housing temperatures, so that the lubricant viscosity and the heat evacu- ated through the bearing house can be calculated with reasonable precision. Two of these thermocouples (VI and VII, not shown in Fig. 1) are used to record the temperatures of the air flow surrounding the bearing house and the room temperature. The rolling bearing assembly is submitted to continuous air- forced convection by two fans, having 38 mm in diameter and running at 2000 rpm, evacuating the heat generated during bear- ing operation for all tests. The piezoelectric torque cell KISTLER s 9339 A selected to measure the friction torque ensures high accuracy measurements even when the friction torque generated in the bearing is very small compared to the measurement range available. The output signal is displayed and registered by the virtual instrument running in a computer. In the torque tests a 51107 thrust ball bearing was submitted to an axial load of 7000 N, rotational speeds between 350 and 5500 rpm and air-forced convection. The measurements were