How to choose the type of bearings used for spindle bearings and turntable bearings in machine tools
The machine tool industry, as an important component of the mechanical equipment industry, plays a pivotal role in the development of the entire national economy. According to statistics, in general machine manufacturing, the processing workload undertaken by machine tools accounts for 40% of the total manufacturing workload. Therefore, the development level of the machine tool industry is usually a symbol of a country is industrial development level. With the increasing development of modern manufacturing industry, customers have higher and higher requirements for industrial mother machines - machine tools. At present, the continuous emergence of new materials, new processes, and new equipment has brought many challenges to the processing capabilities of machine tools. In order to meet the processing needs of the above-mentioned structural components, modern machine tools are developing towards the direction of "high speed, precision, composite, and intelligence".
With the deepening of China is industrial structure upgrading, the machine tool industry also shows a trend of industrial structure upgrading, that is, the proportion of CNC machine tools is gradually increasing. In the field of CNC, the proportion of high-end CNC machine tools (such as heavy-duty machine tools, intelligent machine tools, etc.) is also constantly increasing. Heavy machine tool products mainly serve major industrial pillar industries such as national energy (thermal power, hydropower, nuclear power, wind power), shipbuilding, engineering machinery, metallurgy, aerospace, military, transportation (railway, automobile), as well as national key engineering projects. National key engineering projects provide a broad market for heavy-duty machine tools.
This article mainly introduces how to choose the type of bearings used for spindle bearings and turntable bearings in machine tools.
1、 Main shaft bearings
As a key component of machine tools, the performance of the spindle directly affects the rotation accuracy, speed, rigidity, temperature rise, noise and other parameters of the machine tool, which in turn affects the machining quality of the workpiece, such as the dimensional accuracy and surface roughness of the parts. Therefore, in order to maintain excellent machining capabilities, high-performance bearings must be used. The accuracy of bearings used on machine tool spindles should be ISO P5 or above (P5 or P4 is the ISO accuracy level, usually P0 from low to high), P6, P5, P4, P2)， For the spindle support of high-speed and high-precision machine tools such as CNC machines and machining centers, ISO P4 or higher precision should be selected; Main shaft bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.
Precision angular contact ball bearing
Among the above-mentioned bearings, precision angular contact ball bearings (see Figure 2) are the most widely used. We all know that the rolling element of angular contact ball bearings is a ball; Because it is a point contact (different from the linear contact of roller bearings), it can provide higher rotational speed, smaller heat generation, and higher rotational accuracy. In some high-speed spindle applications, hybrid bearings with ceramic balls (usually Si3N4 or Al2O3) are also used. Compared with traditional fully quenched steel balls, the characteristics of ceramic ball materials endow ceramic ball bearings with high stiffness, high speed, high temperature resistance, and long service life, thus meeting the needs of high-end customers for machine tool bearing products.
As for the contact angle of angular contact ball bearings, the currently popular one is 15? And 25? Contact angle; Usually 15? The contact angle has relatively high rotational speed performance, while 25? The contact angle has a high axial load-bearing capacity. Due to the significant impact of preload selection on the application of precision angular contact ball bearings, for example, in high load and high rigidity situations, medium or heavy-duty bearings are generally selected for preload; For some high-speed and high-precision applications, we need to pay attention to selecting the appropriate preload in the early selection of bearings. Generally, light preload is more common. Preloading is generally divided into three types: light, medium, and heavy; For the convenience of customers, several major bearing manufacturers in the world generally provide bearings with pre ground bearing end faces and preloaded bearings, which are commonly referred to as universal paired precision angular contact ball bearings. This type of bearing eliminates the need for customer preload adjustment, thereby saving installation time.
Precision tapered roller bearings
In some heavy-duty machine tool applications with certain speed requirements, such as rough grinding of forgings, threading machines for oil pipelines, heavy-duty lathes and milling machines, choosing precision tapered roller bearings is an ideal solution. Due to the linear contact design of the rollers in tapered roller bearings, they can provide high rigidity and load-bearing capacity for the spindle; In addition, tapered roller bearings are a pure rolling bearing design that can effectively reduce bearing operating torque and heat generation, thereby ensuring the speed and accuracy of the spindle. Because tapered roller bearings can adjust the axial preload (clearance) during installation, this allows customers to better optimize the adjustment of bearing clearance throughout the life cycle of the bearing.
In addition, in some high-speed applications where the inner ring edge speed is greater than 30 meters per second, certain specially designed tapered roller bearings can also meet the requirements, such as TSMA bearings or Hydra Rib hydraulic floating edge bearings (see Figure 3). The TSMA bearing has multiple axial lubricating oil holes on the edge, which can be lubricated with circulating oil or oil mist. Centrifugal force can distribute oil to the contact area between the roller and the edge, thereby more fully lubricating the bearing and increasing its speed. The Hydra Rib hydraulic floating edge bearing is designed specifically for optimizing the preload of the spindle system. The bearing has a floating edge that contacts the large end of the roller, unlike common bearings that have fixed inner ring edges; And the floating gear edge is positioned by the "pressure" system, which ensures that the bearing can perform at its best under various working conditions, and has been successfully used in domestic customers.
For the precision selection of tapered roller bearings, ISO P5 grade bearings are mainly used in traditional lathes and milling machines; For applications such as machining centers and grinders, precision bearings with ISO P4 or higher are usually chosen. There are two common layouts of tapered roller bearings in spindle design. The first method is to use a tapered roller bearing at both the front and rear ends, and adopt a face-to-face installation method (see Figure 4); This type of design has a compact structure, high rigidity, and is easy to install and adjust.
The second method is to use two tapered roller bearings installed face to face as the front end of the main shaft, while the rear end uses a double outer ring and two single inner ring bearings (TDO) as the floating end; Due to its floating ability, this type of design can effectively withstand the axial thermal expansion of the spindle and has high stiffness, ensuring the accuracy of the machine tool.
Precision cylindrical roller bearings
In the application of machine tool spindles, double row precision cylindrical roller bearings are also used, usually combined with precision angular contact ball bearings or thrust bearings. This type of bearing can withstand large radial loads and allow for higher rotational speeds. The two rows of rollers in the bearing are arranged in a cross pattern, and the fluctuation frequency during rotation is significantly higher than that of a single row bearing, reducing the amplitude by 60%~70%. This type of bearing usually has two forms: NN30 and NN30K series bearings with inner rings with blocking edges and outer rings that can be separated; The outer rings of the NNU49 and NNU49K series bearings are equipped with blocking edges, and the inner rings can be separated. Among them, the inner rings of the NN30K and NNU49K series are tapered holes (with a taper of 1:12), which are matched with the tapered journal of the main shaft. Axial movement of the inner rings can cause the inner rings to expand, so that the bearing clearance can be reduced or even preloaded (negative clearance state). Cylindrical bore bearings are usually hot installed, using interference fit to reduce bearing clearance, or pre tightening the bearing. For NNU49 series bearings with separable inner rings, the raceway is usually precision machined after the spindle is installed on the inner ring to improve the spindle rotation accuracy.
2、 Rotary table bearings
The commonly used rotary worktables in CNC machine tools include indexing worktables and CNC rotary worktables. When CNC machine tools are used to process certain parts, in addition to the linear feed motion around the X, Y, and Z coordinate axes, they sometimes also require circular motion around the X, Y, and Z coordinate axes, which we call the A, B, and C axes, respectively
The CNC rotary worktable can be used to achieve circular feed motion. The CNC rotary worktable (abbreviated as CNC turntable) can not only achieve circular feed motion, but also complete indexing motion. The function of the indexing worktable is only to rotate and change the surface of the workpiece, and when used in conjunction with an automatic tool changing device, it can complete multiple processes on several surfaces of the workpiece in one installation, thus greatly improving work efficiency. The appearance of the CNC turntable is not much different from that of the indexing worktable, but it has a series of structural characteristics. Due to its ability to achieve feed motion, the CNC turntable shares many similarities in structure with the feed drive mechanism of CNC machine tools. The difference is that the driving mechanism implements linear feed motion, while the CNC turntable implements circular feed motion.
Rotary worktable is widely used on various CNC milling machines, boring machines, vertical lathes, vertical milling machines, and other machine tools. In addition to requiring the rotary worktable to withstand the weight of the workpiece well, it is also necessary to ensure its rotational accuracy under load. As the core component of the turntable, the turntable bearing not only needs to have high load-bearing capacity during the operation of the turntable, but also needs to have high rotation accuracy, high anti overturning ability, and high speed capability. In the design of rotary workbenches, the commonly used types of bearings can be roughly divided into the following categories:
Thrust ball bearing and cylindrical roller bearing
Thrust ball bearings can withstand a certain axial force, so this bearing is mainly used to bear the weight of workpieces; Cylindrical roller bearings are mainly used for radial positioning and bearing external radial forces (such as cutting forces, milling forces, etc.). This type of design is widely used and the cost is relatively low. Due to the fact that the thrust ball is a point contact bearing, its axial bearing capacity is relatively limited and it is mainly used in small or medium-sized machine tool rotary worktables. In addition, lubrication of the thrust ball is also quite difficult.
Static pressure bearing, precision cylindrical roller bearing
Static pressure bearing is a sliding bearing that relies on external supply of pressure oil to establish a static pressure bearing oil film inside the bearing for liquid lubrication. Liquid hydrostatic bearings work under liquid lubrication from start to stop, so there is no wear, long service life, and low starting power; In addition, this type of bearing also has the advantages of high rotational accuracy, high oil film stiffness, and the ability to suppress oil film oscillation. Precision cylindrical roller bearings have excellent radial bearing capacity, and due to the use of precision grade bearings, the rotational accuracy of the rotary worktable can also be well guaranteed. The rotary worktable designed with this type can withstand high axial forces, with some workpieces weighing over 200 tons and a turntable diameter exceeding 10 meters. However, this type of design also has some shortcomings, as static pressure bearings must come with a dedicated oil supply system to supply pressure oil, which is more complex to maintain and costly.
The application of cross roller bearings (see Figure 7) on turntables is also quite common. The characteristic of a crossed roller bearing is that there are two raceways and two rows of crossed rollers in the bearing. Compared with the traditional radial centering bearing combination of thrust bearings (see Figure 8), the cross roller bearing has a compact structure, small size, and simplifies the worktable design, thereby reducing the cost of the turntable.
In addition, due to the use of optimized preload force, this type of bearing has high stiffness, thus ensuring the stiffness and accuracy of the turntable. Thanks to the design of two rows of intersecting rollers, the effective span of the bearing can be significantly increased, so this type of bearing has a high anti overturning moment. In cross roller bearings, there are two types: the first is cylindrical cross roller bearings, and the second is tapered cross roller bearings. Generally, cylindrical cross roller bearings are cheaper than tapered cross roller bearings and are suitable for turntable applications with relatively low rotational speeds; And the tapered cross roller bearing adopts the pure rolling design of tapered rollers, so this type of bearing has
High operational accuracy
High rotational speed capability
Reducing shaft length and processing costs, limited changes in geometric dimensions due to thermal expansion
Nylon divider, low moment of inertia, low starting torque, easy to control angular division
Optimize pre tension, high stiffness, and small bounce
Wire contact, high rigidity, and high precision of guide roller operation
Permeable carbon steel provides excellent impact resistance and surface wear resistance
Simple but well lubricated
During bearing installation, customers only need to pre tighten the cross roller bearings to the recommended values, without the need for a complex installation and adjustment process like hydrostatic bearings. The installation of cross roller bearings is simple and easy to adjust the original installation form or maintenance method. Cross roller bearings are suitable for various types of vertical or horizontal boring machines, as well as applications such as vertical mills, vertical lathes, and large gear milling machines.
In summary, bearings, as the core components of machine tool spindles and turntables, play a crucial role in the operational performance of machine tools. In order to select bearings of appropriate size and type, we need to comprehensively consider various working conditions, such as operating speed, lubrication, installation type, spindle stiffness, accuracy, and other requirements. As for the bearings themselves, only by fully understanding their design characteristics and the advantages and disadvantages they bring can we maximize their performance.
