The high-speed electric spindle, as the core component of high-speed machine tools, is also the main heat source of such machine tools. In high-speed machine tools, the stiffness and accuracy of each part of the electric spindle unit are high, the workload is small, and the machining error caused by cutting force of the electric spindle is also relatively small. However, the heating of the motor and the friction heating of the bearings in the electric spindle are inevitable. If not handled properly, the resulting thermal deformation will seriously reduce the machining accuracy of the machine tool. Therefore, in high-speed machine tools, the thermal characteristics of the electric spindle become the main factor affecting machining accuracy, directly limiting the increase in electric spindle speed.

The heat source of high-speed electric spindles, and the thermal deformation of high-speed electric spindles is mainly caused by the heating of the motor and spindle bearings. In the machining process of a machine tool, the output power of the motor is the sum of the power consumed during idling and the power consumed during cutting. In high-speed machining, the heat converted from the idle power consumption of the machine tool becomes the main heat source of the high-speed machining machine tool. During high-speed operation, the spindle bearings experience complex friction phenomena, which intensify the heating intensity and directly affect the thermal deformation of the electric spindle system.

At the same time, due to the heat transfer from the motor to the bearings, the temperature rise of the bearings is high, accelerating the wear of the bearings and causing loss of accuracy. In severe cases, metal bonding burns may even occur, leading to bearing failure. The thermal strength of the spindle is closely related to the structure of the spindle system, the type and configuration of the bearings, the preload force, the lubricant, and the transmission method. Experiments have shown that the temperature rise of angular contact bearings with steel balls and ceramic balls of the same size and specifications is similar at different speeds, but with further increase in spindle speed, the temperature rise of the bearings increases sharply. As the preload force of the bearing increases, the heat generation of the bearing will also rapidly increase. In addition, in the oil air lubrication system, oil and gas are mixed together to cool the bearings, with air cooling accounting for a large proportion.

The mechanism of spindle thermal deformation is that the machine tool spindle is under the action of internal and external heat sources during operation, and these heat sources are generally non constant. Due to different processing conditions and varying degrees of variation, the materials, shapes, and structures of each part of the spindle are different, and their thermal inertia is also different. In addition, factors such as thermal resistance at the interface between connectors and different heat transfer conditions on the spindle surface create a complex and variable temperature field on the spindle. Under such a temperature field, the material of the spindle component generates thermal stress and thermal displacement, which is different from the physical properties of the material, the shape of the part, and the state of the support connection, making the thermal deformation problem of the spindle more complex and bringing great difficulties to the research of spindle thermal deformation. During the machining process, the heat sources that affect the machining accuracy of the machine tool can be divided into two categories: internal heat sources and external heat sources.

The temperature rise of the spindle system usually refers to the difference between the typical area temperature and the ambient temperature in the absence of external loads and external heat sources. In engineering, the outer ring of the spindle front bearing is often used as a typical area for measuring system temperature rise. The higher the system temperature rise, the greater the thermal deformation of the parts, the greater the possibility of accuracy loss, and the worse the thermal characteristics of the system. The key factor affecting the working accuracy of the spindle system is not temperature rise, but the distribution of the temperature field, that is, the symmetry and temperature gradient of the temperature field relative to the spindle axis. During the temperature rise process, the spindle itself will extend axially, and the center position of the front and rear supports of the spindle will change radially. Due to the fact that the diameter and load of the front support are usually larger than those of the rear support, the heat generation of the front support is also greater than that of the rear support. Therefore, the temperature of the front support and the front box wall is also higher than that of the rear support and the rear box wall. The working end of the main shaft will be radially displaced due to thermal deformation, resulting in the phenomenon of head up.

The heat dissipation of high-speed electric spindle is affected by internal and external heat sources, resulting in different temperatures in various parts of the high-speed electric spindle, and heat is always transferred from high temperature to low temperature. There are three basic heat transfer methods for electric spindles. The majority of the heat generated by the stator is carried away by the cooling water or oil through convection, while a small portion is transferred to the air around the stator through convection and radiation. A portion of the heat generated by the rotor is directly transferred to the main shaft and bearings through thermal conduction, while another portion is transferred to the stator through convection and radiation.

The motor stator oil water heat exchange cooling system is commonly used in high-speed electric spindles. The oil pump continuously outputs a large flow of cooling oil, which exchanges heat with the motor stator through the spiral groove of the motor stator cooling sleeve, and then exchanges heat with water through the output circuit. After cooling, the oil flows back to the oil pool to achieve circulating cooling. The oil air lubrication system of the main shaft bearing uses compressed air at a certain pressure and a small amount of lubricating oil output in a certain length of pipeline to mix. By the flow of compressed air inside the pipeline, lubricating oil is continuously driven to flow along the inner wall of the pipeline, delivering the oil and gas mixture to the nozzle installed near the bearing, and then spraying it towards the contact point between the inner ring of the bearing and the rolling element through the nozzle, achieving lubrication and cooling.

The heat transfer between the electric spindle and the surrounding air occurs when the surface of the high-speed spindle motor is relatively hot during operation, resulting in free convection heat transfer under large temperature differences, as well as radiation heat transfer. In order to reduce the impact of heating on the performance of the spindle, especially on the performance of the spindle bearings, a cooling ring was installed between the rotor and bearings of the spindle motor during the structural design process. This can effectively reduce the impact of motor heating on the spindle bearings and extend their service life.