Nowadays, the demand for machine tools supporting efficient production systems is growing and becoming more sophisticated. Rolling bearings (hereinafter referred to as bearings) as a rotating part of the machine tool are a non-negligible part, and their performance directly affects the performance of the machine tool.
The development trend of machine tools
Machine tools are currently shifting in several major directions, starting with the increase in the use of 5-axis machines and multi-tasking machines. The 5-axis machine and the multi-tasking machine began to appear on the market around the second half of 1990 and are now widely used in the market. In addition to cutting, they also add functions such as grinding, inspection and workpiece handling robots. In addition, there is now a trend to further increase the number of control axes, a tendency to be more complex. Second, the machine tool becomes easier to operate. For example, in the machine tool, the function of automatically correcting the mechanical thermal displacement in the machining; the function of checking the mutual interference between the grinding tool and the workpiece; the function of monitoring the state of the spindle in operation (involving temperature, vibration, displacement, etc.); and the intelligence of the device Turn. These changes will make the machine easier to operate. Then there is the development of machine tools to high precision and micromachining. This improves the performance of components such as a DD motor or a linear motor, and makes it possible to determine the position accuracy of the sub-precision stage. In addition, the machine tool has become more space-saving. At present, the requirements for high performance of compact machines for high-volume production lines are also significant. In order to further suppress the width of the device and improve its maintainability, a 5-axis processing machine with a device width of 1000 mm is now available. The last point is that machine tools are moving in the direction of reducing environmental load. There are many machine tools designed to recognize environmental problems in the market, such as the reduction of air flow in oil and gas lubrication and the switching from oil and gas lubrication to grease lubrication.
Technical trends of bearings for spindles
1. Coexistence of high speed and high rigidity
In the context of the development of machine tools to composite, the tendency of such multi-variety and small-volume production, variable-variety production and engineering sum has become higher. The processing of multiple workpieces on multiple machines is combined into one machine, or a number of projects such as roughing and finishing are combined into a few projects to contribute to improving production performance and saving space. Therefore, for the machine tool spindle, there is a need for a wide range of machining from low-speed rotary heavy cutting to high-speed rotary light cutting, and as a bearing, high speed and high rigidity are required to coexist. For high speed, the single dmn value of the bearing (PCD × rotation speed of the bearing) realizes 4 to 5 million angular contact ball bearings and 2 to 3 million cylindrical roller bearings, which meets the requirements of the machine tool. However, if you pay attention to high speed, it will be detrimental to rigidity. For this design of the spindle, it is necessary to pay attention to both high speed and high rigidity. It can also be seen from this that maintaining high speed and having the necessary rigidity on the heavy cutting side is a problem in bearing development in the future.
2. New lubrication method
Lubrication is roughly classified into oil lubrication and grease lubrication. Generally, it is recommended to use oil lubrication in the case of high-speed machining, and grease lubrication in the case of low-speed rotation and emphasis on cost, usability, and environmental load reduction. However, with the advancement of machine tools, it is desirable to have a lubrication method that can simultaneously satisfy two processing occasions.
As a bearing manufacturer, the improvement of oil lubrication, the maintenance of high-speed machining while saving energy and improving environmental performance, or the development of technologies for improving grease lubrication are underway. That is to say, the lubrication technology is an important factor affecting the performance of the bearing, and there are still many problems to be carried out in the future.
3. Bearing design
Bearings for machine tool spindles, according to various requirements such as high speed, high rigidity and reduced environmental load, JTEKT currently offers high performance angular contact ball bearing series of R, C, D and F models (figure). There are many R-type ball diameter balls, which are designed to emphasize rigidity; C-type ball diameter is large, ceramic balls are used for life-oriented design; D-type is high-speed design for oil supply method; F type is Considering the centrifugal force, it is more suitable for high-speed design than the D-type. Users can choose 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 past products, and can also improve the performance of existing equipment.
Compared with the original high-speed bearing (ACH series), the high-performance bearing has a high-speed limit of about 1.3 times that of the original bearing, and the temperature rise is reduced by 20% to 30%. The rolling element is made of ceramic material. Bearings and C-type bearings have the same tendency as compared with the original bearings, and it is possible to select a R-type design that emphasizes rigidity according to the rotational speed at the time of use and a C-type design that emphasizes load capacity. According to the high-speed performance improvement of the high-performance bearing and the effect of reducing the temperature rise, it is also possible to use the excellent positioning pre-tightening of the high-speed rotating spindle.
Under the condition of grease lubrication, the results of the comparison between the R-type bearing of the rolling element and the temperature rise characteristic of the original bearing under the oil-lubricating condition are that the high-performance bearing can be confirmed to be grease-lubricated in the field where the original bearing requires oil and gas lubrication, and it is prevented. The environmental load caused by the scattering of oil is reduced, and the cost due to the waste of the lubricating device is reduced. Regarding the R model, the seal type in which an appropriate amount of grease is sealed has also gradually increased the standard.
These R and C bearings and the original bearings also have the standard type of grease lubrication or oil and gas lubrication. However, they can be used in oil and gas lubrication types D and F which are rotated at a higher speed. The shape of the bearing developed in consideration of the influence of the wind barrier generated when these bearings rotate at a high speed.
The comparison between the D-type bearing and the R-type bearing (ball material = ceramic) is that the D-type bearing has a high-speed performance of about 1.1 times that of the R-type bearing, and the temperature is lowered by 5% to 10%. Because of the D-type bearing The groove on the side of the inner ring of the bearing starts to the oil supply lubrication of the rail portion as a direct supply structure, which is not easily affected by the wind barrier, and can reduce the air flow by 20% compared with the original oil and gas lubrication. Moreover, the wind noise generated by the ball is reduced. It has also become possible to make a significant contribution to energy saving or low noise through these good features. The F-type bearing is an ultra-high-speed rotating bearing that is equipped with an oil-filling zone on the side of the original bearing and that supplies oil and gas from the grease nipple provided on the outer ring to the retainer guide surface. The operation of the cage of the sliding contact retainer guide surface during rapid acceleration or deceleration or ultra-high speed rotation may be less stable. However, the cage can be stabilized by supplying oil and gas from the outer ring nozzle.
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 by about 15% is reduced. The advantages of high speed of cylindrical roller bearings make the spindle construction simple. 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 temperature changes. Cylindrical roller bearings are suitable for this purpose, but since the high-speed performance of cylindrical roller bearings has not been perfect so far, the configuration of angular contact ball bearings + sub-housings is generally used for the rear side support of high-speed spindles. However, since the new high-performance cylindrical roller bearing developed this time can also correspond to a high-speed spindle, the structure around the bearing can be greatly simplified.
In the current high-performance cylindrical roller bearings, the cage is broken at high speed, which limits the high-speed performance. However, as long as this disadvantage is improved, the high-speed performance can be improved. First, regarding the material of the cage, a lightweight, high-strength resin suitable for high-speed rotation is used as compared with the original polyamide resin, and the strength is improved. Further, when the screw is rotated at a high speed, the neck-like structure is at the contact portion between the cage and the roller, so that the roller contact shape in the pocket of the holder is optimally optimized, and friction reduction and low temperature rise are achieved. In addition, the ferrule material is the same as the original product, and the bearing steel that is easy to use is used to improve the high-speed performance through an internal design 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 bearing life is established based on the detected bearing temperature or vibration, so the prediction technique is indispensable. Furthermore, if the load on the bearing can be detected, the most suitable pre-tightening and lubrication can be self-controlled according to the processing state, and as a result, the correspondence between the processing fields having high speed performance and high rigidity and wide width becomes It is possible. Based on this, JTEKT has developed a unique mechanism for preloading the bearing according to the rotational speed. This mechanism has sufficient rigidity from low speed to high speed, and the combination of high output built-in motor enables high-load machining to be more efficient. get on. In the original positioning pre-tightening method, the spindle rigidity increases with the increase of the rotational speed. However, in the case of using this mechanism, especially in the case of high torque, the rigidity of the spindle in the low-speed rotation field will increase dramatically, which is a preload for high-load machining. the way.
Concerned about surprises
Label: Rolling bearing performance directly affects the overall performance of the machine
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