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Technical Trends of Bearings for Machine Tool Spindles

by:JNSN     2021-11-02

The coexistence of high speed and high rigidity

Under the background of the development of machine tools, the tendency of multi-variety and low-volume production, variable varieties and mass production, and total engineering has become even higher. The processing of multiple workpieces on multiple machines is combined into one machine tool, or multiple processes such as rough machining and finishing are combined into a few projects to contribute to improving production performance and saving space. Therefore, for machine tool spindles, there is a need to develop a wide range of machining from low-speed rotating heavy cutting to high-speed rotating light cutting. As a bearing, the coexistence of high speed and high rigidity is required. For high speed, the single dmn value of the bearing achieves 4~5 million for angular contact ball bearings and 2~3 million for cylindrical roller bearings, which meets the requirements of machine tools. However, attaching importance to high speed is disadvantageous to rigidity. For this reason, in the design of the spindle, there are many cases where high speed and high rigidity have to be emphasized at the same time. It can also be seen from this that maintaining high speed and having the necessary rigidity on the heavy cutting side is a subject for future bearing development.

New lubrication method

Lubrication is roughly divided into oil lubrication and grease lubrication. Generally, oil lubrication is used where high-speed machining is important, and grease lubrication is used where low-speed rotation and cost, usability, and reduction of environmental load are important. However, with the advancement of machine tools, it is hoped that there will be a lubrication method that can satisfy both processing occasions at the same time.

As a bearing manufacturer, oil lubrication is improved to maintain high-speed machining while saving energy and improving environmental performance, or the development of technology to improve grease lubrication is underway. In other words, lubrication technology is an element that has a great influence on bearing performance, and there are many issues that need to be developed in the future.

Bearing design

Compared with the original high-speed bearing, the high-speed limit of the R-type bearing is increased to about 1.3 times that of the original bearing, and the temperature rise is reduced by 20 %~30%. R-type bearings with ceramic material used for rolling elements, and C-type bearings have the same tendency compared to the original bearings. The R-type design that emphasizes rigidity according to the rotation speed during use and the C-type that emphasizes load capacity The choice of design becomes possible. According to the effect of improving the high-speed performance of high-performance bearings and reducing temperature rise, it is also possible to use positioning preload with excellent rigidity for high-speed rotating spindles.

Compared with the current high-performance cylindrical roller bearings, it is confirmed that the high-speed performance is improved by about 1.5 times and the temperature rise is reduced by about 15%. The advantages brought by the high speed of cylindrical roller bearings make the spindle structure simple. Generally, the support structure on the rear side of the main shaft needs to be able to absorb the expansion and contraction of the shaft according to temperature changes. Although cylindrical roller bearings are suitable for this function, because the high-speed performance of cylindrical roller bearings has not been very complete so far, the structure of 'angular contact ball bearing sub-housing' is generally used for the back support of the high-speed spindle. However, the new high-performance cylindrical roller bearing developed this time can also be used for high-speed spindles, so the structure around the bearing can be greatly simplified.

Current high-performance cylindrical roller bearings, during high-speed rotation, the cage will be damaged, which makes the high-speed performance have a limit, but as long as this shortcoming is improved, the high-speed performance can be further improved. First of all, as for the material of the cage, compared with the original polyamide resin, it adopts a light weight suitable for high-speed rotation, and a high-strength resin improves the strength. Furthermore, when rotating at a high speed, it becomes a neck-like structure at the contact part of the cage and the roller, which optimizes the contact shape of the roller in the cage pocket, reducing friction and low temperature rise. In addition, the ferrule material is the same as the original product, and the easy-to-handle bearing steel is used, and the high-speed performance is improved through the internal design suitable for high-speed rotation.

High reliability

In order to improve the reliability of machine tools, the reliability of bearings must be improved. For this reason, the life of the bearing is established based on the detected bearing temperature or vibration, so prediction technology is indispensable. Furthermore, if the load on the bearing can be detected, the most suitable preload and lubrication can be self-controlled according to the machining state. As a result, it will be compatible with high-speed performance and high rigidity in a wide machining area. It's possible. Based on this, JTEKT has developed a unique mechanism that applies the most appropriate preload to the bearing according to the speed. This mechanism can have sufficient rigidity from low to high speed. The combination of high-output built-in motor can make high-load processing more efficient. conduct.

The original positioning preload method, the rigidity of the spindle will increase with the increase of the speed, but when this mechanism is used, especially the rigidity of the spindle in the low-speed rotation area when high torque occurs, it will increase dramatically. Preload method suitable for high-load processing. A technological boom is emerging in the mold industry, further accelerating the technological update of my country's mold industry. China's mold industry is facing a 'big wave and scouring period'. Many new-type companies will take the lead in the technological revolution, but more small companies are hanging by a thread.

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