The Running-in Effect of CV Joint Grease Ko Tanimura Kyodo Yushi Co., LTD.
2-2-30 TSUJIDO KANDAI, FUJISAWA-SHI, KANAGAWA 251-8588, Japan
Introduction
Constant velocity joint (CV joint) is a generic term for a joint that transmits rotary movement at the constant speed irrespective of flexion angle. The main application of CV joints includes automotive drive and propeller shafts, where they are usually used in combination of a sliding-type joint such as tripod joints and double offset joints with a fixed-type joint such as ball fixed joints (BJ). Figure 11) shows the typical CV joint assembly. CV joints are generally lubricated with a speciallydesigned lubricating grease. The performance requirements for the CV joint grease have become increasingly severe owing to the market demands for high-performance vehicles of small size and light weight. For instance, the sliding-type joint is required to have reduced vibration. Hatakeyama2) and Yoshihara3) focused on improving vibration performance and developed a low-friction grease with vibration reducing effect by employing some additives in combination. As for the fixed-type joint, a long “flaking life” is of great importance. The typical flaking observed in the fixed-type joint includes subsurface-initiated flaking and surfaceinitiated flaking. For the latter, effective running-in process is thought to be critical for flaking life extension. Taniguchi et al. provided a specific example of surfaceinitiated flaking resulting from insufficient running-in process and reported about a lubricating grease with enhanced flaking resistance developed by employing appropriate additives to improve wear property4). Most fixed-type joint greases are made from molybdenum disulfide (MoS2) 5). MoS2 is a representative solid lubricant well known for its good friction property
and high load capacity. Nevertheless, very little information about the effect of MoS2 on CV joint flaking life can be found in literature. This paper discusses the role of MoS2 in grease lubrication of fixed-type CV joint based on the results of the bench tests using a real CV joint and SRV friction wear tests to evaluate wear property.
Ball fixed joint - Lubrication condition and flaking resistance
Figure 2 shows the typical structure of BJ 5), which consists of an outer and an inner race, six balls, a cage, a boot and the lubricating grease filled inside the joint. Balls are placed between an outer and an inner race and kept by a cage from axial dropping. A boot is used to hold the grease and to keep out dust and water. Balls in rolling-sliding motion between the races travel back and forth in a short stroke. According to Garnham’s calculation of contact pressure for regular passenger cars based on the Hertz theory, the maximum Hertz pressure between the ball and the outer and the inner race are 2.7 GPa and 3.5 GPs, respectively6). The fixed –type CV joint is in boundary lubrication with thin oil films because the rough track surface is exposed to high contact pressures at low speeds, and will supposedly be in mixed lubrication as runningin progresses and roughness projections are worn 7). Therefore, BJ needs to be highly resistant to flaking, wear and heat. It should be noted that one of the major requirements for the lubricating grease filled in BJ is flaking resistance. Figure 3 shows an example of flaking on CV joint track surface.
- 16 VOLUME 79, NUMBER 5
