Friction losses are an important parameter for evaluating the operational behaviour of high-speed rolling bearings. Specifically, in machine tool applications, the bearings are subjected to high radial loads and high speeds, which lead to increased forces in the rolling contact and, as a result, increased bearing friction. In this high-speed application, hybrid spindle bearings, typically made of ceramic balls and steel raceways, show better frictional behaviour compared to full steel-made bearings. Therefore, precise knowledge of the friction characteristics of high-speed hybrid bearings can improve friction models and generalise them to spindle bearings with different types, geometries, and operating conditions. In this article, a new straightforward and cost-efficient method for measuring the frictional torque in spindle bearings is presented. A rigidly arranged 7008 hybrid spindle bearing pair was tested up to rotational speeds of 24,000 rpm and high radial loads of 3 kN. The effects of oil–air and grease lubrication are discussed in characteristic diagrams of the tested bearings. Then, based on the test results, a friction calculation model is presented and validated for the outer race control and minimised power dissipation regarding the influence of radial forces.

The further development of high-precision bearings regarding higher speeds and load-carrying capacity is becoming increasingly challenging. One approach to meet the increased requirements is to use new high-performance raceway steels. Particularly for a highly stressed lubricant film, where seizure may occur, the bearing material properties can make a key contribution in increasing the lifetime. In the following, the test results of hybrid spindle bearings of type 7008 made of nitriding steel are analysed. The bearings were tested in an endurance run for a constant speed parameter of 2.5 million ndm and a maximum contact stress of 2,900 MPa at the inner ring contact. The operational behaviour was validated based on the bearing outer ring temperature and the acceleration level in the test rig. A further sample test in endurance runs with an increased speed parameter of 2.8 million ndm and a maximum contact stress of 3,200 MPa at the inner ring shows that the performance limit of the bearing is even higher than initially assumed. The microscope analyses of the bearing steels after the test show low wear. An analysis of the test conditions in terms of contact stresses, lifetime and the pv value illustrates that the bearings can run far above standard conditions. The experimental results show that the operational limits of hybrid spindle bearings with a raceway made of nitriding steels are significantly higher than expected.

To improve the manufacturing accuracy in the machining process, the stiffness of the machine tool spindle must be increased. One approach is to replace the conventionally used spindle bearings by tapered roller bearings to achieve a higher stiffness and load carrying capacity. However, the speed capability of this bearing type is limited, though can be enhanced by sufficient lubrication, especially in the critically loaded rib-roller-contact. The objective of this paper is to validate the lubricant influence on the speed capability of radially loaded tapered roller bearings. In various speed step runs, the operating behavior of the bearings under oil-air and grease lubrication was evaluated based on the outer ring temperature and the vibration level in the test rig. With oil-air lubrication, a speed parameter up to 900,000 mm/min could be achieved, whereas with grease lubrication, a speed parameter of 540,000 mm/min could be reached. In comparison, the setup with spindle bearings can achieve the highest investigated speed parameter of 900,000 mm/min with both grease and oil-air lubrication. For grease lubricated ball bearings, significantly higher temperature fluctuations are present with variation of the radial load. In the case of oil-air lubrication, the steady-state temperature improved with increasing oil viscosity.

With the increasing electrical energy demands in aviation propulsion systems, the increase in the onboard generators’ power density is inevitable. During the flight, forces coming from the gearbox or gyroscopic forces generated by flight manoeuvres like take-off and landing can act on the generators’ bearings, which can lead to wear and fatigue in the bearings. Utilizing the radial force control concept in the electrical machine can relieve loads from the bearings that not only minimize the bearing losses but also increase bearing life. The objective of the MAGLEV project (Measurement and Analysis of Generator bearing Loads and Efficiency with Validation) is to study, demonstrate, and test a new class of high-speed generators with radial force control. In this paper, design steps of this type of generator and its test rig are presented and the measurement methodology used for radial force control is explained. The concept is developed in an electrical machine and is validated on a test rig by measuring required parameters like shaft displacement, vibrations and bearing temperature. Additionally, the friction moment of each generator’s bearings is measured and validated in a separate test rig under comparable conditions to the bearing loads in the generator. Therefore, a novel approach to determine precisely the bearing friction in a radial load unit, rotatably supported by an additional needle bearing is used, which shows a good agreement with the calculated friction. Furthermore, new calculation methods for the operating behavior of cylindrical roller bearings with clearance are presented, which are utilized in the generator test rig.

Due to increasing demands regarding higher manufacturing accuracy and the machining of high-strength materials, the stiffness and load-carrying capacity of main spindles must be enhanced. One approach is to replace the conventionally used angular contact ball bearings, so called spindle bearings, with tapered roller bearings of the same size, which have a higher stiffness and load carrying capacity due to their larger rolling contact. In case the speed requirement is below a speed parameter $$n\cdot d_{m}=0.9\times 10^{6}$$n · d m = 0.9 × 10 6 mm/min, the poorer speed parameter of tapered roller bearings can be compensated by a sufficiently high lubrication quantity. However, the stiffness increase by using this bearing type in main spindles has not yet been quantified in field tests. Therefore, the aim of this paper is to demonstrate the stiffness advantage of a spindle with an elastically arranged tapered roller bearing compared to a conventional spindle with angular contact ball bearings by machining tests. First, the radial load displacement characteristic of the newly developed tapered roller bearing spindle is tested in advance on an isolated test rig. In machine cutting tests, full slots are milled in an aluminum block. During the tests, the axial and radial displacements of the spindle shaft relative to the housing are measured and evaluated in radial and axial direction. These tests are repeated and compared with the conventional spindle. By varying the process parameters such as cutting depth, feed rate or rotational speed, the increased stiffness of the tapered roller bearing spindle compared to the conventional spindle is illustrated based on the measured displacements. Moreover, frequency response measurements at different speeds underpin the improved damping of the tapered roller bearing spindle. Assuming that the speed requirements are not too high and the tapered roller bearing is sufficiently lubricated, the use of this type of bearing can significantly increase the stiffness and thus the manufacturing precision in the long term for main spindles.

With the increasing electrical energy demands in aviation propulsion systems, the increase in the onboard generators’ power density is inevitable. During the flight, forces coming from the gearbox or gyroscopic forces generated by flight manoeuvres like take-off and landing can act on the generators’ bearings, which can lead to wear and fatigue in the bearings. Utilizing the radial force control concept in the electrical machine can relieve loads from the bearings that not only minimize the bearing losses but also increase bearing life. The objective of the MAGLEV project (Measurement and Analysis of Generator bearing Loads and Efficiency with Validation) is to study, demonstrate, and test a new class of high-speed generators with radial force control. In this paper, design steps of this type of generator and its test rig are presented and the measurement methodology used for radial force control is explained. The concept is developed in an electrical machine and is validated on a test rig by measuring required parameters like shaft displacement, vibrations and bearing temperature. Additionally, the friction moment of each generator’s bearings is measured and validated in a separate test rig under comparable conditions to the bearing loads in the generator. Therefore, a novel approach to determine precisely the bearing friction in a radial load unit, rotatably supported by an additional needle bearing is used, which shows a good agreement with the calculated friction. Furthermore, new calculation methods for the operating behavior of cylindrical roller bearings with clearance are presented, which are utilized in the generator test rig.