title: Local Temperatures at Electric Contacts in Ball Bearings at Mixed Lubrication
Creators: name: Safdarzadeh, Omid
Publisher: name: UNSPECIFIED
Year: 2025

description: This thesis focuses on the tribological contacts in a laboratory setup with one axial ball bearing, at mixed lubrication, exposed to DC currents above 0.5 A. This work gives the bearing electrical characteristics and the influencing factors in the bearing fluting generation.
The tribological contact between the ball and the raceway of the axial ball bearing type 51208 is described with a focus on several relevant parameters (such as bearing speed, bearing force, and bearing current) for the bearing electric behavior. The surface roughness and the covering oxide layer on the surface of the ball and of the raceway in the Hertz’ian area mainly determine the electrical behavior. White light interferometry (WLI) and X-ray photoelectron spectroscopy (XPS) are used to study the roughness and oxide layer, respectively.
The established knowledge of electric contacts, developed for mechanical switches, is used to describe the measured bearing voltage u_b and bearing current i_b. It turns out that the electric current flows only through tiny bottle-necked areas within the Hertz’ian area, resulting in an electrical constriction resistance and the generation of so-called a-spots (according to R. Holm). The A-fritting and B-fritting of the oxide layer are explained in connection with the measured voltage and current signals. It is revealed that with a non-interrupted bearing current (DC or AC) with an amplitude of typically more than 0.5 A, the bearing voltage remains at values of around 1 V. This behavior resembles antiparallel diodes with a forward voltage of about the fritting voltage.
The temperature-rise ∆T_c, due to the electric current flow at a local metal-metal contact (a-spot), in the Hertz’ian area is analytically and numerically calculated. The finite element simulation results agree with the analytical calculations. Assuming identical electric contacts for all the ball-raceway contacts of the axial ball bearing at mixed lubrication, a bearing voltage of 0.8 V results in local contact temperatures of ca. 1035 °C in less than 1 µs. The findings show that the melting temperature (ca. 1400 °C) and softening temperature (ca. 800 °C) of the bearing steel occur at respectively a bearing voltage of 1.05 V and 0.65 V. At a continuous bearing current, with an amplitude above 0.5 A, bearing voltages in the range 0.65–1.05 V are typically measured, implying the existence of local electric contact points with these locally elevated temperatures. The blackened (burned) lubricant, and the imprints of local melting points in scanning electron microscopy (SEM) of the surface agree with these calculated elevated temperatures. 
The electrical characteristic of the ball-raceway contact is similar to the Ring-Ring metal contact in a second laboratory setup, investigated in this work. With the steel Ring-Ring setup, which is a single rolling contact, the fluting is generated as well as in the bearings. The measured contact voltage in the Ring-Ring setup is almost equal to the ball-raceway contact voltage in the bearing. With rings of carbon steel, aluminum, and copper a contact voltage is observed, similar to the bearing steel. The steel Ring-Ring contact shows that with an increased thickness of the oxide layer, e.g. in a high temperature furnace, the contact voltage increases, and with a decreased thickness of the oxide layer, e.g. using acid, the contact voltage decreases. Unlike steel rings, the contact voltage between two silver rings is almost zero, due to the absence of an oxide layer on the silver rings. 
A study of parameters (such as bearing speed, bearing force, bearing temperature, and bearing current amplitude) on the fluting generation with HF bearing currents at voltage-source inverter operation is generally time-consuming, because the duration of electric current conduction at each inverter switching incident is generally much shorter than of a continuous DC and AC current. In this work, the fluting generation is investigated mainly with DC currents mostly at high current densities above 1 A/mm2. Changing the electrical and mechanical parameters resulted in advancing or postponing the fluting generation. Despite an apparent current density above 1 A/mm2, in some cases no fluting is generated within the selected test duration. An increased bearing current density or increased bearing speed, and a decreased bearing force or decreased bearing temperature, increase the chance of fluting. With DC currents, the fluting almost always appears first on the ring with the negative polarity.
With AC currents (up to 1 MHz) no fluting is observed after up to 168 hours of operation. With DC currents, having the equal amplitude to the AC currents, the fluting is generated in less than 24 hours. In tests with a DC current, the oxide layer thickness on the negative ring may be reduced due to the direction of the electric field. Measurements in long-term tests of up to 168 hours with DC currents, show that the negative ring (except at a very high current density of 58.6 A/mm2) loses more mass than the positive ring. Unlike the negative ring, the positive ring in some cases even gains additional mass. Rectifying the discharge bearing currents resulted in bearing fluting at an early time on the negative rings of the bearings in an inverted-fed induction machine of 90 W.
A test with a deliberately created pit on the raceway was run to cause an additional lateral vibration of the ball on the raceway, in the absence of any bearing current. This test did not result in any fluting. The fluting on a fluted bearing does not relocate or develop in the absence of bearing currents. Hence, the electric current flow is essential in addition to the lateral vibrations for the fluting generation. Tests without lubrication in the Ring-Ring setup result in no fluting. Hence, an elastic medium, like the lubrication film, is essential for the lateral vibrations of the rings. The hardness of the material is essential for the fluting generation, as with copper and silver no fluting is observed. With aluminum, the fluting is observed first on the positive ring. An increased slide-to-rolling ratio (ratio of sliding speed to rolling speed) in the Ring-Ring contact changed the shape of flutings pattern. 
This work achieved a reasonable explanation for the high local temperatures in the Hertz’ian area at an electric current flow in mixed lubrication. A variety of tests are conducted and interpreted to understand the electrical characteristic of the bearing, based on the well-established knowledge of classical electric contacts in mechanical switches. Various test conditions are investigated to enlighten the fluting generation mechanism. The results are explained to the best of the author’s knowledge. Nevertheless, no proof or completeness of the fluting generation mechanism is claimed. Here, still further detailed investigation on the current flow at moving and vibrating electrical contact together with a simultaneous change of the surface properties of the contact partners are necessary.