As a key component of a machine tool , the spindle's performance directly affects parameters such as rotational accuracy, speed, rigidity, temperature rise, and noise, thus impacting the machining quality of the workpiece. To maintain excellent machining capabilities, high-performance bearings must be used.
I. Selection of Spindle Bearings
Bearings used on machine tool spindles should have an ISO P5 or higher precision rating (P5 or P4 are ISO precision grades, typically ranging from low to high: P0, P6, P5, P4, P2). For spindle supports in high-speed, high-precision machine tools such as CNC machine tools and machining centers, ISO P4 or higher precision is required. Spindle bearings include angular contact ball bearings, tapered roller bearings, and cylindrical roller bearings.
1.1 Precision angular contact ball bearings
Precision angular contact ball bearings are widely used. Because the rolling elements of angular contact ball bearings are balls, and because it involves point contact (unlike the line contact of roller bearings), it offers higher speeds, lower heat generation, and higher rotational accuracy. In some ultra-high-speed spindle applications, hybrid bearings using ceramic balls (typically Si3N4 or Al2O3) are also employed. Compared to traditional fully hardened steel balls, the inherent properties of ceramic balls give ceramic ball bearings high rigidity, high speed, high temperature resistance, and long service life, thus meeting the demands of high-end customers for machine tool bearings.
Regarding the contact angle of angular contact ball bearings, 15° and 25° are currently the most popular. Generally, a 15° contact angle offers higher speed performance, while a 25° contact angle provides higher axial load capacity. The choice of preload has a significant impact on the application of precision angular contact ball bearings. For high-load, high-rigidity applications, medium or heavy-duty preload bearings are typically selected. For high-speed, high-precision applications, appropriate preload needs to be carefully chosen during the early selection of the bearing. Preload is generally divided into light, medium, and heavy-duty types, with light preload being the most common. To facilitate customer use, major bearing manufacturers worldwide generally offer pre-ground bearing faces with preload, commonly known as universal mating precision angular contact ball bearings. These bearings eliminate the need for preload adjustment, thus saving installation time.
1.2 Precision Cylindrical Roller Bearings
Double-row precision cylindrical roller bearings are also used in machine tool spindle applications, typically in combination with precision angular contact ball bearings or thrust bearings. These bearings can withstand large radial loads and allow for high speeds. The two rows of rollers in the bearing are arranged in a crossed pattern, resulting in a significantly higher oscillation frequency and a 60%-70% reduction in amplitude compared to single-row bearings. These bearings generally come in two forms: the NN30 and NN30K series bearings have a flanged inner ring and a separable outer ring; the NNU49 and NNU49K series bearings have a flanged outer ring and a separable inner ring. The NN30K and NNU49K series have a tapered bore (1:12 taper) that fits with the tapered journal of the spindle. Axial movement of the inner ring allows it to expand, thus reducing bearing clearance or even preloading the bearing (negative clearance state). Cylindrical bore bearings are typically heat-fitted, using an interference fit to reduce bearing clearance or preload the bearing. For NNU49 series bearings with separable inner rings, the raceway is typically precision machined after the spindle is installed on the inner ring to improve the spindle's rotational accuracy.
1.3 Precision tapered roller bearings
In heavy-duty machine tool applications with specific speed requirements, such as forging grinding, threading machines for oil pipelines, heavy-duty lathes, and milling machines, precision tapered roller bearings are an ideal solution. Because the rollers of tapered roller bearings have a line contact design, they provide high rigidity and load-bearing capacity to the spindle. Furthermore, as a purely rolling bearing design, tapered roller bearings effectively reduce operating torque and heat generation, thus ensuring spindle speed and accuracy. Since the axial preload (clearance) of tapered roller bearings can be adjusted during installation, customers can better optimize bearing clearance adjustment throughout the bearing's service life.
II. Selection of Turntable Bearings
Common rotary tables used in CNC machine tools include indexing tables and CNC rotary tables. When machining certain parts, CNC machine tools require not only linear feed motion along the X, Y, and Z axes, but sometimes also circular motion around these axes, referred to as the A, B, and C axes, respectively. CNC rotary tables can be used to achieve circular feed motion, and also to perform indexing motion. The function of an indexing table is simply to rotate and change the workpiece surface. Used in conjunction with an automatic tool changer, it allows for multiple processes on several surfaces of the workpiece to be completed in a single setup, thus greatly improving work efficiency. The external appearance of a CNC rotary table is not much different from that of an indexing table, but it has a series of structural characteristics. Because a CNC rotary table can achieve feed motion, it shares many structural similarities with the feed drive mechanism of a CNC machine tool. The difference is that the drive mechanism achieves linear feed motion, while the CNC rotary table achieves circular feed motion.
Rotary worktables are widely used in various CNC milling machines, boring machines, vertical lathes, and vertical milling machines. Besides being able to withstand the weight of the workpiece, the rotary worktable also needs to ensure its rotational accuracy under load. As a core component of the rotary table, the bearings must possess not only high load-bearing capacity but also high rotational accuracy, high anti-tipping ability, and high speed capability during operation.
2.1 Thrust ball bearing + cylindrical roller bearing
Thrust ball bearings can withstand a certain axial force, so they are mainly used to support the weight of workpieces; while cylindrical roller bearings are mainly used for radial positioning and to withstand external radial forces (such as cutting forces, milling forces, etc.). This type of design is widely used and relatively inexpensive. Because thrust ball bearings are point contact bearings, their axial load capacity is relatively limited, and they are mainly used in small or medium-sized machine tool rotary tables. Furthermore, lubrication of thrust ball bearings is relatively difficult.
2.2 Hydrostatic bearing + precision cylindrical roller bearing
Hydrostatic bearings are sliding bearings that rely on externally supplied pressurized oil to create a hydrostatic bearing film within the bearing for lubrication. Hydrostatic bearings operate under lubrication from start to stop, resulting in no wear, long service life, and low starting power. Furthermore, these bearings offer advantages such as high rotational accuracy, high oil film stiffness, and the ability to suppress oil film oscillation. Precision cylindrical roller bearings possess excellent radial load capacity, and the use of precision-grade bearings ensures the rotational accuracy of rotary tables. Rotary tables using this design can withstand very high axial forces, with some workpieces weighing over 200 tons and rotary table diameters exceeding 10 meters. However, this design also has some drawbacks. Hydrostatic bearings require a dedicated oil supply system, making maintenance complex and costly.
2.3 Crossed Roller Bearings
Crossed roller bearings are commonly used in turntables. A key feature of crossed roller bearings is that they have two raceways and two rows of cross-arranged rollers. Compared to the traditional thrust bearing + radial centering bearing combination, crossed roller bearings are more compact and smaller, simplifying turntable design and reducing turntable costs. Due to optimized preload, these bearings have high rigidity, ensuring both the rigidity and precision of the turntable. Thanks to the two rows of cross-rollers, the effective span of the bearing can be significantly increased, resulting in high anti-overturning moment. Crossed roller bearings are further divided into two types: cylindrical crossed roller bearings and tapered crossed roller bearings. Generally, cylindrical crossed roller bearings are less expensive than tapered crossed roller bearings and are suitable for turntable applications with relatively low speeds; while tapered crossed roller bearings, employing a pure rolling design of tapered rollers, offer advantages such as high operating precision, high speed capability, and reduced shaft length and manufacturing costs. Crossed 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.
[Editor's Note]
In conclusion, selecting the appropriate size and type of bearing requires comprehensive consideration of various operating conditions, such as operating speed, lubrication, installation type, spindle rigidity, and accuracy requirements. Regarding the bearing itself, only by fully understanding its design characteristics and the resulting advantages and disadvantages can its performance be fully realized. As a core component of machine tool spindles and rotary tables, bearings play a crucial role in the operational performance of machine tools.
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