High-performance rolling bearings in machine tools

High-performance rolling bearings in machine tools

Nowadays, the market demand for machine tools supporting efficient production systems is becoming more sophisticated and complicated. Rolling bearings (hereafter referred to as bearings), which are the rotating parts of the machine tool, are a non-negligible component and their performance directly affects the performance of the machine tool.

Development trend of machine tools

The machine tool is currently shifting to several major directions, starting with the increase in the use of 5-axis machining and compounding machines. The 5-axis machine and the compound machine began to appear on the market in the latter half of 1990 and is now widely used in the market. The feature is that in addition to cutting, functions such as grinding, inspection, and workpiece transfer robots are added. In addition, there is a tendency to further increase the number of control axes, a trend that is more complex. The second is that the machine becomes easier to operate. For example, in the machine tool, the function of automatically correcting the mechanical thermal displacement during machining, the function of checking the mutual interference between the grinding tool and the workpiece, the function of monitoring the spindle state during operation (involving temperature, vibration, displacement, etc.), and the intelligence of the equipment are set. Change. These changes will make the machine easier to operate. Then is the development of high-precision, micro-machining machine tools. This improves the performance of the components such as the DD motor and the linear motor, and makes it possible to determine the position accuracy of the sub precision stage. The other is that the machine becomes more space-saving. At present, the high-performance requirements for compact machines for mass production lines are also significant. In order to further suppress the width of the equipment and improve its maintainability, a 5-axis machine with a machine width of 1000 mm has now been introduced. The last point is that the machine tool is being developed to reduce the environmental load. Oil-air-lubricated air flow is reduced and switching from oil-air lubrication to grease-lubrication, etc. There are many machine tools on the market that are aware of environmental issues.

Technical Trends of Bearings for Spindles

1. Coexistence of high speed and high rigidity

Under the background of the development of machine tools to compounding, the tendency of this kind of multi-species, low-volume production, variable variety of batch production, and total engineering becomes higher. Combining multiple workpieces on multiple machines to be combined on a single machine tool, or involving rough machining, finishing machining, etc., can be combined into a few projects to contribute to improving production performance and saving space. Therefore, there is a need for a wide-range machining of a machine tool spindle from a low-speed rotating heavy cutting to a high-speed rotating light cutting. As a bearing, high speed and high rigidity are required to coexist. For high speeds, the single dmn value of the bearing (PCD of the bearing × rotation speed) achieves 400 to 5 million angular contact ball bearings and 200 to 3 million cylindrical roller bearings, which are adapted to the requirements of machine tools. However, if you are concerned about high speed, it will be detrimental to rigidity. For this reason, the design of the spindle must not be accompanied by high speed and high rigidity at the same time. From this, it can also be seen that maintaining high speed and having the necessary rigidity on the heavy-cut side is a problem in the future development of bearings.

2. New lubrication method

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

As a bearing manufacturer, the development of technologies that improve oil lubrication, maintain high-speed processing while saving energy and improving environmental performance, or improving grease lubrication are underway. In other words, lubrication technology is an important factor in the bearing performance. There are many issues that need to be developed in the future.

3. Bearing design

According to the requirements of high-speed, high-rigidity, and reduced environmental load, JTEKT currently offers four types of high-performance angular contact ball bearings (Figure) for R, C, D, and F. There are many R-balls with a large number of balls, which are rigid designs; C-type balls have a large diameter, ceramic balls are used, and design for life is emphasized; D-types are designed with high speed in terms of oil supply methods; F type is Considering the centrifugal force, it is more adaptable to high speed design than the D type. The user can select the most suitable model according to the needs of use. The main dimensions of each bearing are in accordance with ISO standards. The minimum design change of the shaft or housing can replace the previous product, and can also improve the performance of the existing equipment.

Compared with the original high-speed bearing (ACH series), the performance of the high-performance bearing has improved the high-speed limit of the R-type bearing to about 1.3 times that of the original bearing, and the temperature rise has been reduced by 20% to 30%. The rolling elements adopt R-type bearings of ceramic materials. C-type bearings also have the same tendency as the original bearings, making it possible to select rigid R-type designs and C-type designs that emphasize load capacity depending on the rotation speed at the time of use. may. According to the improvement of the high-speed performance of the high-performance bearing and the effect of reducing the temperature rise, it is also possible to use a rigid positioning pre-tensioning for the high-speed rotating spindle.

Under the condition of grease lubrication, the R-bearings of ceramic rolling elements and the temperature rise characteristics of the original bearing under oil-air lubrication conditions are compared. The test results show that high-performance bearings can be grease-lubricated in the field where the original bearings require oil-air lubrication, and there is prevention. The environmental impact caused by the scattering of oil and the cost reduction caused by discarding the lubrication device are effective. With regard to the R model, the seal type with an appropriate amount of grease has also been gradually increased.

These R and C type bearings and the original bearing have the same standard type that can use grease lubrication or oil and gas lubrication. However, they can be used in oil and gas lubrication type D and F. The shape of the bearing developed in consideration of the influence of the wind barrier that occurs when these bearings rotate at high speed.

As a result of comparison between the D-type bearing and the R-type bearing (ball material=ceramic), it has been confirmed that the D-type bearing has about 1.1 times the high-speed performance of the R-type bearing and lowers the temperature by 5% to 10%. Because the oil-air lubrication of the groove on the side of the bearing inner ring side of the D-type bearing is a direct supply structure to the rail portion, it is not easily affected by the wind barrier, and can reduce the air flow rate by 20% compared with the original oil-air lubrication. Moreover, it is also possible to reduce the wind noise generated by the ball. These good features contribute to energy saving or low noise. The F-type bearing is an ultra-high-speed rotating bearing in which oil is supplied to the side of the retainer guide surface from the oil filling nozzle provided on the outer surface of the original bearing. The sliding contact of the cage guide surface during the rapid acceleration or deceleration or ultra-high speed rotation of the cage may not be stable. However, the cage operation can be stabilized by supplying oil gas from the outer ring nozzle.

Compared with current high-performance cylindrical roller bearings, it has been confirmed that the high-speed performance is improved by about 1.5 times and the temperature rise is reduced by about 15%. The advantages of high-speed cylindrical roller bearings enable the simplification of the spindle structure. In general, 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 the temperature change. Cylindrical roller bearings are suitable for this purpose, but because the high-speed performance of cylindrical roller bearings has not been very complete so far, the structure of "angular contact ball bearings + secondary housings" is generally used in the rear support of high-speed spindles. However, since the new high-performance cylindrical roller bearings developed this time can also be used for high-speed spindles, the structure around the bearings can be greatly simplified.

The current high-performance cylindrical roller bearings suffer from damage to the cage during high-speed rotation, which limits the high-speed performance. However, the improvement of this shortcoming can improve the high-speed performance. First of all, as regards the material of the cage, the light weight for high-speed rotation is adopted in comparison with the original polyamide resin, and the high-strength resin increases the strength. In addition, in the high-speed rotation, the neck-like structure has a contact portion between the cage and the roller, so that the roller contact shape in the cage pocket hole is optimized, and friction reduction and low temperature rise are realized. In addition, similar to the original product, the ferrule uses an easy-to-follow bearing steel and improves high-speed performance through an internal design that is suitable for high-speed rotation.

4. High reliability

In order to improve the reliability of the machine tool, the reliability of the bearing must be improved. For this reason, the life of the bearing is established based on the detected bearing temperature or vibration, so the prediction technique is indispensable. In addition, if the load on the bearing can be detected, the most appropriate preloading and lubrication can be controlled based on the machining state. This results in the correspondence of processing fields with high speed and high rigidity and wide width at the same time. It's possible. Based on this, JTEKT has developed a unique mechanism for optimally preloading the bearings according to the rotational speed. This mechanism can provide sufficient rigidity from low to high speeds, and the combination of high output built-in motors enables high-load machining to be performed more efficiently. get on. In the original positioning pretensioning method, the rigidity of the spindle increases as the rotation speed increases. However, in the case of using this mechanism, particularly when high torque occurs, the rigidity of the spindle in the low-speed rotation field increases dramatically, which is a preload suitable for high-load machining the way.

Huatao Bearings included Rolling Bearings, Thrust Bearings, Slewing Bearings, Joint Bearings. They are widely used in Industrial Gearboxes, Electric Motors, Industrial Rollers.