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Bearing solutions for

Bearing solutions for machine tools manufacturers Authors: Dr. Michael Pausch, Head of Product Development Rotary - Industrial Automation at Schaeffler Technologies AG & Co. KG Dr. Martin Voll, Head of Application Engineering Production Machinery at Schaeffler Technologies AG & Co. KG With Schaeffler’s new series of high-speed spindle bearings, machine tool manufacturers can now optimize the designs and increase the load carrying capacity of their motor spindles and thus offer their customers higher productivity and competitiveness. As compared to conventional spindles, the motor spindles have certain advantages in terms of speed, dynamics and vibration behavior. Apart from these advantages, they also have a very compact design. But this also causes certain drawbacks: Due to the integration of the motor into the spindle, the electrical losses in the form of heat must also be dissipated in a suitable manner to ensure correct function and a long spindle operating life. New designs for a new series of spindle bearings 3 fundamental designs have established themselves on the spindle bearing market, and these can be categorized according to the size of the rolling elements: Large-ball series with a high load carrying capacity, small-ball series for very high speeds, and medium-size ball series, which try to combine the positive characteristics of large-ball and small-ball bearings. With its new M-series spindle bearings, Schaeffler has managed to optimally combine the kinematic suitability of small-ball series for high speeds along with the robustness of large-ball bearings. In this context, the word “optimally” means matching the ball size, raceway contour and contact angle in combination with the radial internal clearance so that the bearing pre-load increases as little as possible in all operating conditions. The diagrams below clearly shows the minimal increase in bearing pre-load of the M-series compared to the HC and HCRS series due to radial expansion of the inner ring. Requirements for modern motor spindle bearings MACHINE ELEMENTS Rotor temperatures of up to 150 °C are quite common due to the high power density. This leads to a substantial temperature gradient and a varied thermal expansion between the shaft and the housing, which must be compensated axially and radially by the bearing support. In addition, inner rings of the bearing expand due to the centrifugal forces. For manufacturers, while developing the spindle bearings the goal is to always minimize the change in bearing pre-load across a wide range of operating parameters. The increase in pre-load is only desirable within certain limits, because parameters such as friction and heat generation in the rolling contact also dramatically increase with a higher pre-load, which reduces the maximum permissible limiting speed. Furthermore, spindle bearings are made to be insensitive to contamination – particularly particle inclusions and contamination with lubricant – they are also made to be tolerant to higher friction and severe lack of lubrication. Motor spindle bearings are therefore subject to the highest requirements. About Schaeffler AG Schaeffler Group is a leading integrated auto motive and industrial supplier. The company based in Herzogenaurach, Germany, generated revenues of approximately 13.3 billion Euros in 2016. With around 86,600 employees, Schaeffler is one of the world’s largest family owned companies. The group has approximately 170 operating locations in more than 50 countries with a worldwide network of manufacturing plants, research and development centers, and sales subsidiaries. Schaeffler makes key contributions towards ‘mobility for tomorrow’ with precision components and systems in engines, transmission and chassis applications as well as rolling and plain bearing solutions for industrial applications. WORLD OF INDUSTRIESMOTION, DRIVE & AUTOMATION 4/2017

series series 01 Bearing pre-load as a function of the radial expansion of the inner ring due to interference between the shaft and bearing inner ring, and expansion of the inner ring due to high speeds and temperatures Due to the optimized X-life internal construction, the M-series spindle bearing is significantly more resistant to stresses occurring due to varying operating conditions and at the same time makes it possible to operate the spindle at extremely high speeds. It is available in 3 different versions: M, HCM and VCM. These versions are based on the dimensions series 70 and 719 and have a nominal contact angle of 17 and 25 degrees. The 3 versions differ from each other as follows: n “M” version: The balls and rings of the “M” version are manufactured from the proven 100Cr6 rolling bearing steel. This version provides a high-performance, cost-effective bearing solution for motor spindles. n “HCM” version: The HCM version is equipped with ceramic balls and rings made from 100Cr6 rolling bearing steel. Due to its highperformance capability and suitability for extremely high speeds, this version allows the performance of motor spindles to be increased even further. n “VCM” version: The VCM version also features balls made from ceramic material, but the bearing rings are made from the newly-developed ‘Vacrodur’ high-performance material. This material is mechanically and thermally stable even at temperatures over 400 °C. Thus spindle bearings made from ‘Vacrodur’ remain dimensionally and geometrically stable and retain a high load carrying capacity even under high thermal loads. Vacrodur also makes a convincing case when the lubricant is contaminated with foreign particles. This steel is characterized by a very high mechanical load carrying capacity, particularly in the case of abnormal lubrication conditions such as insufficient lubrication or contamination. In tests under controlled contamination conditions, the operating life up to bearing failure was increased almost by 25 times compared to the existing benchmark. Under good lubrication conditions, the dynamic load carrying capacity of ‘Vacrodur’ is around 240 % higher than that of standard 100Cr6 rolling bearing steel. This represents a 13-fold increase in the bearing’s nominal rating life. The VCM version specifically offers high levels of freedom when designing motor spindles compared to conventional spindle bearings. The significantly higher load carrying capacity of VCM bearings opens up opportunities for design engineers, for example, to replace a set of matched bearing supports with a single bearing. The advantages are obvious: The spindle can be shortened by one or two bearing widths, the significantly reduced mounting outlay reduces costs, and the amount of heat transferred to the spindle via the bearing support is reduced. In some cases, VCM bearings make it possible to dispense with a very complex series 02 Overview of the performance and suitability for high speeds of the 3 versions of spindle bearings M, HCM and VCM 03 Change in the relative preload due to the influence of the fit, kinematics and temperature spring-loaded arrangement in the spindle in favor of a costeffective, rigidly adjusted bearing support. The M-series X-life high-speed spindle bearings can be designed in open versions as well as in sealed and greased versions. They are also available as a “direct lube” version with lubrication holes and seal rings in the outer ring. They are matched for universal use, which means they can be assembled in any configuration without additional outlay. Added value for machine tool manufacturers With the M, HCM, and VCM versions of the new high-speed spindle bearings series, spindle manufacturers can achieve a significant increase in the mechanical and thermal load carrying capacity and thus the performance ranges of their spindles. The advantages of the ‘X-life’ internal construction are demonstrated particularly under undesirable operating conditions, which are usually associated with significantly increased temperatures in the rolling contact. This is where the new X-life high-speed spindle bearings offer a significant factor of safety reserve and are thus effective in preventing premature failures of the spindle. Longer machine running times, reduced maintenance costs and lower unit costs ensure the OEMs a competitive advantage. Photographs: Schaeffler AG WORLD OF INDUSTRIESMOTION, DRIVE & AUTOMATION 4/2017