With the rapid development of numerical control technology, “compound, high-speed, intelligent, precision, environmental protection” has become the main trend of the development of machine tool industry technology.
Among them, high-speed machining can effectively improve the machining efficiency of the machine tool and shorten the machining cycle of the workpiece, which requires the machine tool spindle and its related components to adapt to the requirements of high-speed machining.
The assembly quality of precision bearings directly affects their working conditions and service life. Many failures of CNC machining centers are caused by improper assembly of bearings, so enough attention should be paid to the assembly technology of precision bearings.
The key points of high-speed precision bearing assembly are summarized as follows.
Removal and cleaning of precision bearings
When the precision bearing is taken out of the package, the operator’s hands should be kept clean and dry, because the sweat on the hands will cause rust, and gloves can be worn when necessary.
Ensure good lubrication effect
The precision bearing taken out shall be greased and oiled immediately. After taking out the greased precision bearing, it shall be installed without pollution and shall not be greased or oiled. During grease filling, the clearance between cage and bearing outer ring must be greased. It is better to grease the precision bearing to 30% of the cavity, and grease the high-speed precision bearing to 20% of the cavity.
The package shall be sealed
Precision bearings can only be removed from their original packaging for cleaning before assembly. Several sets of precision bearings sealed with volatile corrosion inhibitor shall be sealed immediately after being taken out of the package, because the protective gas of VIC paper can only be kept in the sealed package.
The greased precision bearing cannot be cleaned before assembly, while the non greased precision bearing must be cleaned before assembly. After cleaning, it should be dried and immediately coated with antirust oil or grease to avoid corrosion.
Precautions for precision bearing installation
The precision bearing assembly site shall be kept as clean as possible; Avoid the contamination of precision bearings or the entry of foreign matters, which have a great negative impact on the operation and service life of bearings; Check the geometric accuracy, dimensional accuracy and cleanliness of the bearing seat hole and the mating surface on the shaft; When installing, apply a little oil or grease on the mating surface of the ring, so that the precision bearing is easier to install to the design position.
When designing the bearing seat hole and shaft, there should be a 100-150 installation guide chamfer; Do not over cool the bearing, because condensation may lead to corrosion of the bearing and its mating surface.
The assembly of bearing inner race and main shaft shall adopt the directional assembly method or angle selection method, that is, manually control the direction of radial runout error of each assembly, so that the errors cancel each other rather than accumulate; After the bearing is pressed into the bearing, it shall rotate flexibly without blocking.
After installation, check whether the precision bearing system operates normally.
Use installation tools reasonably
1) When the interference is large, the bearing can be installed by heating box, oil bath heating or inductor heating. The heating temperature range is 80 ℃~100 ℃, and the maximum temperature cannot exceed 120 ℃. At the same time, the bearing shall be tightened with nuts or other appropriate methods to prevent the gap between the collar and the shaft shoulder due to the contraction of the width direction of the bearing after cooling.
(2) When installing the bearing, equal pressure must be applied on the circumference of the end face of the ring to press the ring in. It is not allowed to directly hit the end face of the bearing with a hammer or other tools to avoid damage. When the interference is small, press the end face of the bearing ring with a sleeve at room temperature, tap the center of the assembly sleeve with a hammer, and press the ring evenly through the sleeve. The assembly force must not be transmitted through the rolling element to avoid direct hammering on the bearing ring. When pressing in, ensure that the outer ring end face and the housing shoulder end face, and the inner ring end face and the shaft shoulder end face are tightly pressed without clearance.
(3) When mechanical press or hydraulic press is used, assembly sleeve must be used for press mounting, and it must be kept in a horizontal state. If there is inclination, bearing channel will be damaged due to force, resulting in abnormal bearing noise. The assembly sleeve shall fully contact the entire circumference of the end face of the bearing inner race or outer race.
Disassembly of precision bearing
The designer should consider the problem of disassembly at the installation position of precision bearings. Generally, precision bearings are installed with interference fit. During design, grooves for disassembly should be set on the shaft or bearing seat hole to facilitate the disassembly of bearings.
If the disassembled precision bearing needs to be reused, the disassembly shall be handled with more care. Pay attention to the following points when disassembling precision bearings: avoid directly hitting bearing rings; The disassembly force shall not be transmitted through the bearing roller. When disassembling the precision bearing from the shaft, the disassembly force shall be transmitted through the bearing inner race. When disassembling the precision bearing from the bearing seat hole, the disassembly force shall be transmitted through the bearing outer sleeve; Once the precision bearing is disassembled, it should be cleaned, dried and then coated with antirust oil or greased to avoid bearing corrosion.
If we pay attention to the above points in the assembly process of precision bearings, it will have an ideal effect on the control of the spindle temperature rise, the high-precision high-speed trouble free operation of the spindle, and the control of the transmission accuracy.