Tribological Improvement of Angular Contact Ball Bearings Operated under Small Pivoting Angles by Surface Texturing

Florian Pape, Gerhard Poll

Institute of Machine Design and Tribology (IMKT), Leibniz Universitaet Hannover, Hannover, 30167, Germany

INTRODUCTION: To reduce the waste of energy for the transportation sector a friction reduction for constant velocity joint shafts provides high potential. To enable a higher ressource efficiency a reduction of friction and wear for such machine elements is of importance. For joint shafts working under small pivoting angles mixed lubrication can be found on the reversal points. Constant velocity joint shafts are mandatory parts for the power train in front-wheel driven vehicles to transmit power from the main driveshaft to the wheels under elongation and shock strut compression. The torque between hub and shaft is carried forward between the joint shafts balls and raceways. Between the balls and the raceway elasto-hydrodynamic contact conditions can be found. For one turn the joint balls travel in a linear motion with two reversal points. On the reversal points the balls’ velocity is zero and the risk of increased friction and wear due to mixed lubrication has to be regarded. A very effective surface modification to reduce friction are micro dimples induced by micro milling, laser machining or milling with a one tooth chamfer^1,2. Ball-end milling operation leads to a characteristic surface topography with a structure similar to micro dimples³. By deep rolling the roughness can be reduced. For the tests, angular contact ball bearings as analogue model to the constant velocity joint shaft were chosen. These bearings were equipped with a textured surface inserting the one tooth chamfer. Angular contact ball bearings can be operated under comparable conditions to constant velocity joint shafts. In order to rotate the bearings under small angles, a test stand was designed and built⁴. To enable the measurement of friction combined with angular resolution a torsional gauge bar and a rotary encoder were installed. Therefore, it is possible to investigate the frictional properties at the reversal points. Additionally the bearing surfaces were investigated by laser scanning microscopy at the reversal points to conclude on the respective surface damage. The results of the structured bearings were compared to bearings from mass production.

PATTERNED ANGULAR CONTACT BALL BEARINGS: The friction reducing efficiency of micro structuring was shown for applications under sliding contact conditions in previous studies. Investigations applying a tribometer proved that with micro structuring the frictional properties can be reduced drastically and the hydrodynamic lubrication is achieved at lower sliding velocities ^2,5. Additionally simulation studies proved that a pattern crosswise to the raceway results in the highest friction reduction². The approach to generate structures within a machining process using a one tooth chamfer after honing offers high flexibility and low effort. Even with a cutting depth lower than 10 microns geometrical errors can be compensated to achieve high geometrical tolerance². Based on these studies angular contact ball bearing surfaces were equipped with different micro structures applying a one tooth chamfer.

TEST SETUP: For the experimental evaluation of the structured surfaces a test rig was set up. The test rig allows to investigate two angular contact ball bearings placed opposite to each other simultaneously. For the motion of the bearings a servomotor is used. The pressure on the bearings is applied by a package of disc-springs. Between the servomotor and bearings a torsional gauge bar is coupled to the transmission shaft to measure the frictional torque of the applied bearings. To examine the pivoting angle of the bearings, a rotary encoder is connected to the main shaft of the bearings. The setup allows to correlate the frictional properties to

the specific pivoting angle of the bearing. The tests were performed with grease.

RESULTS AND DISCUSSION: It could be shown that the surface structuring has a major influence on the achieved frictional properties of the bearings. The micro-texture allowed to reduce the frictional torque by half. The measured frictional torque over the pivoting angle for a standard bearing in comparison to a textured bearing is shown in Fig. 1. The low frictional torque resulted in less wear on the bearing surfaces. Especially in this field, a friction reduction results in significant energy saving. For the textured surface the wear on the surface could be drastically reduced. Subsequently, the findings are transferred to the production of constant velocity joint shaft prototypes.

Figure 1 - Frictional torque over the pivoting angle.

CONCLUSION: The surface structuring with small cavities resulted in a reduction of frictional torque and wear at the reversal points of the bearings. It can be concluded that the cavities provide the tribological contact with lubricant at the reversal points. Thus, structured surfaces enhance the functionality of oscillating bearings. The requirement of higher ressource efficiency could be fulfilled. To save ressources due to lower wear and reduced friction the surface structuring with a one tooth chamfer is a feasible option for bearings oscillating under small pivoting angles. It is expected that in case of constant velocity joint shafts such structures will improve the tribological contact conditions.

REFERENCES: 1. Wakuda, M., Yamauchi, Y., Kanzaki, S., Yasuda, Y., Effect of surface texturing on friction reduction between ceramic and steel materials under lubrication sliding contact, Wear, Vol. 254, 356-363, (2003), 2. Kästner, J., Methode zur spanenden Herstellung reibungsminimierender Mikroschmiertaschen, Thesis, Hanover, (2013). 3. Denkena, B., Böß, V., Nespor, D., Samp, A., Kinematic and stochastic surface topography of machined TiAl6V4-parts by means of ball nose end milling, Proceedia Engineering, Vol. 19, p. 81 – 87, (2011), 4. Pape, F., Neubauer, T., Maiss, O., Denkena, B., Poll, G., Tribological Investigations of Angular Contact Ball Bearings Operated under Small Pivoting Angles for Production Process Analysis, Proceedings, 7th International Conference on Tribology in Manufacturing Processes, Phuket, Thailand, pp 190-199, (2016), 5. Ulmer, H., Dinkelacker, F., Engelke, F., Reithmeier, E., Denkena, B., Göttsching, T., Hübsch, C., Bach, F.-W., Fast, H., Rienäcker, A., Microstructuring of Thermo-Mechanically Highly Stressed Surfaces for Application in Internal Combustion Engines, WTC 2013, 5th World Tribology Congress, Torino, Italy, (2013)