Antifriction bearings – four-ball tester
The four-ball tester is a testing machine for tribological investigations. The four-ball tester currently used at pmd can be seen on the picture.
Based on the Shell four-ball tester for grease tests according to DIN 51 350, the first test bench was built by modifying at the institute of Machine Elements and Design. By means of this test bench, solid lubricated balls can be examined by a rolling loading. The current testing machine is the fourth one which was developed and built at the institute. The predecessor model is used by a Swiss coater in the development of new layers.
The design modification of the four-ball tester according to DIN 51 350 consists especially in the fact that the three lower balls are not fixed and can rotate freely in the testing cup, driven by the upper ball.
By means of different geometries, rolling and drilling friction can be divided among the three contacts and examined separately. The drive ball can be replaced by a so-called osculation body to represent the convex-concave contact, as it is developed between ball and ring path in the groove ball bearing.
The tests are carried out at a constant temperature, constant Hertzian pressure in the contacts, constant speed and under high vacuum. The buzzing friction moment of the ball set serves as a wear parameter.
If a critical value of the buzzing friction moment is reached within a defined period of time, the test bench turns off automatically. In the four-ball tester, the following experiment results, which are available for evaluation at the end, can be named: the tolerated speed, the continuously recorded measurement data and the specimens (balls, ring and plate) with their signs of wear.