First, the spindle noise failure analysis In many CNC milling machines, since the spindle's shifting system still uses several transmission shafts, gears and bearings, vibration noise, friction noise and impact noise are inevitably generated during operation. The shifting of the main drive system of CNC milling machine is controlled by computer under the condition that the machine does not stop working, so it is more continuous and more representative than the noise generated by ordinary machine tools. For a CNC milling machine, the noise is larger when it is used for the first time, and the noise source is mainly from the main drive system. As the usage time increases, the noise is getting larger and larger. The sound level meter is measured at the spindle speed of 2000r/min. The noise is 85.2dB. Fault checking and analysis The mechanical system is subjected to any exciting force from the outside, and the system vibrates due to the response to this exciting force. This vibrational energy propagates throughout the system. When it propagates to the surface of the radiation, this energy is converted into a pressure wave that is transmitted through the air, ie, acoustic radiation. Therefore, the three steps of excitation response, system internal transmission and acoustic radiation are the formation process of vibration noise, friction noise and impact noise. When the main drive system of CNC milling machine works, it is precisely because the gears, bearings and other components are excited and responded, and the noise is transmitted and radiated inside the system, and these components are excited due to abnormal conditions. Large so that the noise is increased. (1) Gear noise analysis. The main drive system of the CNC milling machine mainly relies on the gears to complete the shifting and transmission. Therefore, the meshing transmission of the gear is one of the main noise sources. The noise generated by the gears in the main drive system of the machine tool mainly includes: 1 When the gear is engaged, a continuous impact occurs between the tooth and the tooth to cause the gear to generate forced vibration at the meshing frequency and bring impact noise. 2 Because the gear is subjected to the external exciting force, the transient free vibration of the natural frequency of the gear is generated and brings noise. 3 Due to the eccentricity of the gear and the transmission shaft and the assembly of the bearing, the unbalanced inertia force is generated, thereby generating low-frequency vibration consistent with the rotational speed. As the axis rotates, a resonance noise is emitted once per revolution. 4 The self-excited vibration generated by the gear due to the friction between the teeth and the teeth brings frictional noise. If the tooth surface is uneven, it will cause rapid periodic impact noise. (2) Bearing noise analysis. The spindle shifting system of the CNC milling machine has a total of 38 rolling bearings. The assembly of the bearing and the journal and the bearing hole, the pre-tightening force, the concentricity, the lubrication condition, the magnitude of the load acting on the bearing, and the radial clearance all have a great influence on the noise. Moreover, the manufacturing deviation of the bearing itself largely determines the noise of the bearing. The most prone to deformation of a rolling bearing is its inner and outer rings. Under the influence of external factors and self-precision, the inner and outer rings may generate rocking vibration, axial vibration, radial vibration, radial vibration of the bearing ring itself and axial bending vibration. In the same way as the gear, the higher the rotational speed of the bearing, the larger the fundamental frequency of the rotation, and the noise will increase. The accuracy of the rolling inner and outer rings of the bearing, if not high, will become the main factor affecting the bearing noise. There are many dimples on the inner and outer rings, and the higher harmonics of these frequencies are expressed in the spectrum, and the noise caused is higher. Second, the spindle noise troubleshooting Gear noise control Since the generation of gear noise is caused by many factors, some of them are the characteristics of the gear motion of the spindle motion system which are determined by the gear design parameters, and the repair and improvement on the original gear are carried out without changing the original design. To reduce noise. (1) The top of the tooth is trimmed. Due to the influence of the tooth profile error and the pitch, after the gear teeth are elastically deformed, the momentary impact and impact are caused when the gears mesh. Therefore, in order to reduce the meshing impact caused by the unevenness of the tooth tip when the gear is engaged, the tooth tip trimming can be performed. The purpose of the tooth tip trimming is to correct the bending deformation of the teeth and compensate for the gear error, thereby reducing gear noise. The amount of trimming depends on the pitch error and the amount of bending deformation of the gear after carrying, as well as the direction of bending. The trimming is mainly for the pair of gears with the highest meshing frequency of the machine and the different trimming amounts of these gears when the modulus is 3, 4, 5mm. When repairing the edge, we must pay attention to the control of the amount of trimming, and adopt the method of repeated test, so as to avoid damage to the effective working profile without excessive trimming, or the effect of too small trimming can not repair the edge. At the edge, only the top of the tooth or the root of the tooth can be repaired according to the specific conditions of the pair of gears. Only when the top of the tooth or the root of the tooth can not achieve good results, the top and the root of the tooth are repaired together. The radial and axial values ​​can be assigned to one gear or to two gears depending on the situation. (2) Control the tooth profile error. The tooth profile error is caused by a variety of factors. Observing the gears in the faulty milling machine drive system, it is found that the tooth profile error is mainly caused during the machining process, and secondly due to the poor long-term operating conditions. Tooth shape errors are more common when the gears mesh. In general, the larger the tooth profile error, the greater the noise. For the concave tooth shape, the gear teeth are subjected to two impacts in one engagement, the noise is large, and the more concave the tooth shape, the larger the noise. Therefore, the gear teeth are shaped to be appropriately convex, so as to reduce noise. (3) Control the change of the center distance of the meshing gear. The change of the actual center distance of the meshing gear will cause a change in the pressure angle. If the center distance of the meshing gear changes periodically, the pressure angle will also change periodically and the noise will increase periodically. The analysis of the meshing center distance shows that the noise effect is not obvious when the center distance is too large, and the noise is obviously increased when the center distance is small. When controlling the center distance of the meshing gear, the outer diameter of the gear, the deformation of the transmission shaft, The coupling of the drive shaft to the gears and bearings should be controlled to an ideal state. This eliminates noise that occurs due to changes in the mesh center distance as much as possible. (4) Pay attention to the effect of lubricating oil on controlling noise. Lubricating oil also plays a certain damping role while lubricating and cooling, and the noise becomes smaller as the oil quantity and viscosity increase. If a certain oil film thickness can be maintained on the tooth surface, the meshing tooth surface can be prevented from directly contacting, and the vibration energy can be attenuated, thereby reducing noise, so that oil with a large viscosity is advantageous for reducing noise. The main drive system of the fault milling machine uses splash lubrication, while splash lubrication increases the oil's disturbance noise. The actual gear lubrication requires very little oil, and its main purpose is to form a pressure oil film to facilitate lubrication. Experiments have shown that gear lubrication is best for the oil on the biting side. In this way, both the cooling effect and the oil film are formed on the tooth surface before entering the meshing zone. If it is possible to control a small amount of splashed oil to enter the meshing zone, the noise reduction effect is better. Accordingly, the respective oil pipes are rearranged so that the lubricating oil is splashed into each pair of gears in an ideal state to control noise generated due to unfavorable lubrication. 2. Bearing noise control (1) Control the quality of the inner and outer rings. In the main drive system of a fault milling machine, all bearings are rotated by the inner ring and the outer ring is fixed. At this time, if the inner ring is radially deflected, it will cause an imbalance in rotation, and vibration noise will occur. If the outer ring of the bearing, the shape and position tolerance of the matching hole are not good, the radial oscillation will occur, thus destroying the concentricity of the bearing component. If the inner ring and the outer ring end face have a large lateral runout, it will also cause the inner ring of the bearing to be skewed relative to the outer ring. The higher the accuracy of the bearing, the smaller the above-mentioned yaw amount and the smaller the noise. In addition to controlling the geometrical deviation of the inner and outer rings of the bearing, it is also necessary to control the waviness of the inner and outer ring raceways, reduce the surface roughness, and strictly control the surface flaws and scratches of the raceway during the assembly process, otherwise it is impossible to reduce the vibration noise of the bearing. . It has been observed that when the waviness of the raceway is a dense wave or a thinning wave, the contact points of the rolling elements when rolling are obviously different, and the vibration frequencies caused thereby vary greatly. (2) Control the matching precision of the bearing with the hole and the shaft. In the main drive system of the fault milling machine, the bearing and the shaft and the hole should be matched to ensure the necessary radial clearance of the bearing. The optimum value of the radial working clearance is determined by the fit of the inner ring on the shaft and the outer ring in the hole, and the temperature difference between the inner and outer rings in the moving state. Therefore, the choice of the initial clearance in the bearing is important for controlling the noise of the bearing. Excessive radial clearance causes an increase in noise in the low frequency portion, which in turn causes an increase in noise in the high frequency portion. Generally, the gap control is optimal at 0.01 mm. The form of the outer ring in the hole affects the propagation of noise. A tighter fit will increase the sound transmission and increase the noise. An over-tight fit will force the raceway to deform, thereby increasing the shape error of the bearing raceway, reducing the radial clearance and also causing an increase in noise. A loose fit of the outer ring of the bearing also causes a large noise. Only the proper fit of the elastic can make the oil film at the contact between the journal and the hole dampen to the external ring vibration, thereby reducing noise. In addition, the geometrical tolerances and surface roughness of the mating parts should meet the requirements of the selected bearing accuracy class. If the bearing is mounted tightly on a shaft that is not machined accurately, the error in the shaft is transmitted to the inner ring raceway of the bearing and is expressed in a high degree of waviness, and the noise increases.
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