Writer: Eric Bearing Limited
Since entering the 21st century, especially with the launch of the national “high-end CNC machine tool and basic manufacturing equipment” special project, it has greatly promoted the development of the machinery and equipment industry, and the development process of “high-speed, precise, complex and intelligent” machine tools has been achieved. It accelerates further. The CNC turntable, which is a functional accessory of CNC machine tools, has also received attention. Several CNC turntables such as high-speed direct-drive CNC turntables, high-precision CNC turntables, large heavy-duty CNC turntables, and oil-immersed CNC turntables are directly included in the national major special projects. Within the development plan.
As an important accessory of CNC machine tools, the CNC turntable is more precisely one of the key functional components of CNC machine tools with more than four axes. It can be used directly as the table of the machine tool, or installed on the table of the machine tool as the fourth axis of the machine tool. , The two-axis turntable installed on the worktable can even upgrade the ordinary three-axis machining center into a pentagonal machining center that can realize arbitrary shape processing in space.
Whether it is used as a CNC machine tool accessory, or as a CNC turntable of a CNC machine tool table, whether it is used on an ordinary machining center, or as a part of a flexible manufacturing unit, the CNC turntable directly affects or even determines the accuracy and overall performance of the machine tool.
The fundamental factor that determines the performance of the turntable lies in the structure and accuracy of its supporting bearing type.
1. First briefly introduce the type of CNC turntable
According to the different installation methods, the CNC turntable can be divided into: vertical and horizontal dual-use CNC turntable and vertical/horizontal CNC turntable; from the shape of the table, it can be divided into rectangular turntable and circular turntable; from the different degrees of freedom, it can be divided into single-axis turntable , Two-axis turntable and multi-axis turntable; there are also linear coordinate turntable and exchange table.
2. The evolution of CNC turntable support
Regardless of the type of turntable, its turntable surface needs axial and radial support. The slewing support of the CNC turntable was originally derived from the slewing support of the mechanical turntable. Before the CNC turntable, the support of the mechanical turntable was basically a sliding bearing.
Taking into account friction, wear and precision maintenance, the first domestic CNC turntable was designed and developed for the Yantai Machine Tool Accessories Research Institute directly under the Ministry of Mechanical and Electrical Engineering in the early 1980s. The radial sliding support of the turntable is changed to a rolling bearing support, and the axial support maintains the cast iron sliding guide of the original mechanical turntable. It was not until the end of the 1980s that the PTFE sheet was applied and verified on machine tools, and the axial support of the CNC turntable was changed from a cast iron sliding guide to a plastic guide, and the sliding friction coefficient was reduced to 1/3 of the original. The effective load of the CNC turntable is increased. NTN ball bearing 6309C3 online , pls click here :
Around 2001, with the continuous application of new technologies and new processes, the machining capabilities of machine tools were improved, and the process methods were more perfect. Large-size NTN end roller bearings were gradually applied to CNC turntables, and the support structure began to change to a full rolling mode.
In smaller specification turntables, there is also a support structure using NTN double row tapered roller bearings, and the load is provided by two tapered roller bearings A and C.
In recent years, there has been a demand for large-scale CNC turntables with a diameter of more than 5m and a load of more than 100 tons. The above support types are no longer suitable for this occasion. The reasonable load-bearing structure is the combination of NTN hydrostatic bearings and NTN precision cylindrical roller bearings.
3. The comparison of several support combinations is as follows
CNC turntables are widely used in various CNC milling machines, CNC boring machines, CNC vertical lathes, machining centers, CNC boring and milling machines and other machine tools. In addition to the requirement that the rotary table can bear the weight of the workpiece well, it is also necessary to ensure its rotation accuracy under load. NTN turntable bearing, as the core component of the turntable, during the operation of the turntable, not only must have a high load-bearing capacity, but also need to have high rotation accuracy, high overturning resistance and high speed capability. In the design of the rotary table, the more widely used bearing types can be roughly divided into the above-mentioned combinations. The specific comparison is as follows:
(1) NTN radial thrust ball bearing + NTN tapered roller bearing + sliding guide
Although NTN thrust ball bearings can withstand a certain amount of axial force, in this combination, the radial thrust ball bearing is not affected by the axial force, and only bears the role of radial force (in view of the bearing installed at this position , It is only affected by the radial force, and the radial roller bearing is also used directly in the occasion of large radial load. The principle is the same, so I will not repeat it), which plays a role in positioning.
The combination of sliding guide rail and tapered roller bearing is used to bear the weight of the workpiece and the applied axial load; the additional overturning moment is borne by the sliding guide rail and tapered roller bearing. Such supported radial thrust ball bearings require preload, and tapered roller bearings and sliding guides also require preload. Due to the sliding friction, the axial preload is prone to produce relatively large idling friction torque, resulting in a large idling force consumption.
(2) NTN radial thrust ball bearing + NTN tapered roller bearing + end roller bearing
This type of combination is similar to the first type of combination in terms of bearing load, but due to the adoption of end roller bearings, the idling friction moment caused by the sliding guide is reduced, and the idling force loss is reduced.
(3) NTN double tapered roller bearing
The structure of NTN double tapered roller bearings has higher requirements on NTN tapered roller bearings, and the axial load has a greater impact on the rotational torque than the end face roller bearings.
(4) NTN composite bearing
NTN composite bearing supports also include NTN crossed cylindrical and NTN tapered roller bearings. These three types of bearings have the characteristics of relatively compact structure and high load capacity. They are currently the preferred support bearings for CNC turntable products.
(5) NTN hydrostatic bearing + NTN precision cylindrical roller bearing
NTN hydrostatic bearing is a sliding bearing that relies on external supply of pressure oil and builds a static pressure bearing oil film in the NTN bearing to achieve liquid lubrication. NTN hydrostatic bearings always work under liquid lubrication from start to stop, with low wear and long life. In addition, this type of support also has the advantages of high rotation accuracy and large carrying capacity; but it requires a special oil supply system and has a complex structure , High maintenance costs. Precision cylindrical roller bearings have good radial load capacity, and the rotation accuracy can also be well guaranteed.
In short, NTN bearing, as the core component of the CNC turntable, plays a decisive role in the performance of the CNC turntable. At present, some bearings with higher precision and larger specifications are still relatively imported. From the perspective of improving the level of CNC turntables, in order to select the appropriate support type, it is necessary to comprehensively consider various working conditions, such as operating speed, lubrication, load, accuracy, etc., and the support type selected on this basis can play the bearing The best performance.