Research On High Speed Hydraulic Power Chuck For CNC Machine Tools

With the trend of high speed and high precision, modern CNC machines put forward their demand for high speed and high precision function units accordingly. As important function units of high speed turning centers and turning-milling machining centers, the trends of hydraulic power chucks (HPC) also include high speed and high precision. However, compared with high frequency spindles and high speed cutting tools, HPCs are advanced relatively slowly. Lack of high speed and safe HPCs becomes one of the reasons that high speed turning technology has not been widely applied in batch production. Chinese HPC technology falls behind Europe and Japan. All paramount HPCs are from abord. This thesis studies the key technologies of HPCs such as high speed rotary sealing and grip force loss. The objective is to develop a specific theoretical reference to design a new generation of HPC and to improve the rotary speed of HPC.This thesis proposes a numerical model of liquid-solid-air thermal coupled flow in three microscale rotary sealing gaps of a rotary hydraulic cylinder. This numerical model considers the variation of hydraulic oil's dynamic viscosity with temperature, the heat transfer between hydraulic oil and solid walls, and the heat transfer between solid walls and air. Compared with the experimental results, the error of oil temperature solved from this mumerical model is less than 5%. The model can be used to design rotary cylinder. A piecewise model of gripping force loss of power chuck is built, which considers the difference of wedge transmission's radial deformation at low-speed stage and medium-to-high-speed stage, the friction forces of chuck transmission, and the compressibility of hydraulic oil in rotary hydraulic cylinders. The piecewise model explains the principle of gripping force loss more deeply. The experimental validation shows the piecewise model of gripping force loss is more precise than previous models. Two prototypes of high speed power chucks with centrifugal force compensation are developed. For the fist prototype with 250mm outer diameter, the experiments show that the rate of the remaining grip force at 6000 rpm is 82.9%. For the second prototype with 160mm outer diameter, the experiments show that the ratio of the remaining grip force at 7500 rpm is 71.5%. The ratioes of the two prototypes are much higher than the the ratio of 33% according to Chinese occupation standard JB/T 3860. A rotary pulling-pushing force sensor is invented. The sensor integrated strain force measurent technology and non-contact electrical supply and signal tansmission technology. The sensor can be used to measure the pulling or pushing force of the rotary hydraulic cylinder, aiming at testing the performance of the hydraulic lock at high rotational speed.The thesis is outlined as follows:In chapter 1, the evolution process, key technical problems and research state of hydraulic power chucks are introduced. The purpose, significance and main contents are outlined.In chapter 2, the theoretical model of flow and temperature rise in a full circle orifice rotary sealing gap is built. A numerical model of liquid-solid-gas thermal coupled flow in three full circle orifice rotary sealing gaps of a rotary hydraulic cylinder is also established. The results of these models are compared with that of the experiments.In chapter 3, the theoretical models of the static gripping force and gripping force loss of wedge power chucks are established. The transmission efficiency is analyzed. The effects of friction, stiffness and dimension on the gripping force loss are discussed.In chapter 4, the theoretical model of the gripping force loss of wedge power chucks with centrifugal force compensation is established. The optimization method of gripping force loss is proposed. The prototypes of power chucks are developed. Besides, novel control methods of gripping force with pressure control technology are disclosed.In chapter 5, the mechanism of the high speed rotary pulling-pushing force sensor is introduced firstly. Then, the effects of bending deformation, bending vibration and expansion deformation under large centrifugal force on the measuring error are analyzed. Finally, the application of the sensor for hydraulic lock's test and online monitoring is illustrated.In chapter 6, conclusions are summarized and the future research developments and outlooks are proposed in this chapter.