Maintenance method of CNC milling machine spindle noise fault

1. Spindle noise fault analysis

In many CNC milling machines, since the speed change system of the spindle still uses several transmission shafts, gears and bearings, vibration noise, friction noise and impact noise are inevitably generated during operation. The speed change of the main transmission system of the CNC milling machine is completed by computer control without stopping the machine tool, so it is more continuous and more representative than the noise generated by ordinary machine tools.

A certain CNC milling machine has a large noise when it is used for the first time, and the noise source mainly comes from the main transmission system. As the use time increases, the noise becomes louder and louder. The noise is measured at 85.2dB at a spindle speed of 2000r/min with a sound level meter.

Fault inspection and analysis

When the mechanical system is subjected to any external excitation force, the system will vibrate in response to this excitation force. This vibration energy propagates throughout the system. When it propagates to the radiation surface, this energy is converted into pressure waves and then transmitted through the air, that is, sound radiation. Therefore, the three steps of excitation response, internal transmission of the system and sound radiation are the formation process of vibration noise, friction noise and impact noise.

When the main transmission system of the CNC milling machine is in operation, it is precisely because the gears, bearings and other components respond to the excitation, and transmit and radiate noise within the system, and these components also increase the excitation force and noise due to abnormal conditions.

(1) Gear noise analysis. The main transmission system of the CNC milling machine mainly relies on gears to complete speed change and transmission. Therefore, the meshing transmission of gears is one of the main noise sources.

The noise generated by the gears in the main transmission system of the machine tool during operation mainly includes:

① When the gears are meshing, continuous impact occurs between the teeth, causing the gears to produce forced vibration at the meshing frequency and bring impact noise.

② The gears are subjected to external excitation forces, which generates transient free vibration of the gear's natural frequency and brings noise.

③ The eccentricity of the assembly of the gears, transmission shafts and bearings causes rotational unbalanced inertia force, thereby generating low-frequency vibration consistent with the rotation speed. As the shaft rotates, a resonance noise is emitted once per rotation.

④ The friction between the teeth causes the gears to produce self-excited vibration and brings friction noise. If the tooth surface is uneven, it will cause rapid periodic impact noise.

(2) Bearing noise analysis. The spindle speed change system of the CNC milling machine has a total of 38 rolling bearings. The assembly of the bearings with the journal and the support hole, the preload, concentricity, lubrication conditions, the size of the load acting on the bearings, the radial clearance, etc. all have a great influence on the noise. In addition, the manufacturing deviation of the bearing itself determines the bearing noise to a large extent. The inner and outer rings of the rolling bearing are the most prone to deformation. Under the influence of external factors and their own precision, the inner and outer rings may produce swing vibration, axial vibration, radial vibration, radial vibration of the bearing ring itself, and axial bending vibration.

Similar to the gear, the higher the speed of the bearing, the greater the fundamental frequency of the rotation, and the noise will increase accordingly. If the precision of the inner and outer rings of the bearing rolling body is not high, it will become the main factor affecting the bearing noise. There are many depressions on the inner and outer rings of the rolling body or the bearing, and the higher harmonics of these frequencies are shown in the spectrum, causing higher noise.

2. Treatment of spindle noise fault

1. Gear noise control

Gear noise is caused by multiple factors, some of which are determined by gear design parameters. Based on the characteristics of the spindle motion system gear noise of the faulty milling machine, repairs and improvements are made on the original gears without changing the original design to reduce noise.

(1) Tooth tip trimming. Due to the influence of tooth shape error and tooth pitch, after the gear teeth are elastically deformed under load, instantaneous collision and impact occur when the gears are engaged. Therefore, in order to reduce the meshing impact caused by the uneven tooth tip during gear meshing, tooth tip trimming can be performed. The purpose of tooth tip trimming is to correct the bending deformation of the teeth and compensate for the gear error, thereby reducing gear noise. The trimming amount depends on the pitch error and the bending deformation of the gear after load, as well as the bending direction. When trimming, the trimming is mainly aimed at the pairs of gears with the highest meshing frequency of the machine tool and the different trimming amounts of these gears when the module is 3, 4, and 5 mm. When trimming the tooth profile, we must pay attention to the control of the trimming amount and adopt the method of repeated tests to avoid excessive trimming that destroys the effective working tooth profile, or too small trimming that does not play the trimming role. When trimming the tooth profile, we can only trim the tooth top or the tooth root according to the specific conditions of these pairs of gears. Only when the trimming of the tooth top or the tooth root alone cannot achieve good results, the tooth top and the tooth root are trimmed together. The radial and axial values of the trimming amount can be assigned to one gear or to two gears according to the situation.

(2) Control tooth profile error. Tooth profile error is caused by many factors. Observing the gears in the transmission system of the faulty milling machine, it is found that the tooth profile error mainly occurs during the processing, and secondly due to poor long-term operating conditions. Tooth profile error is more common when the gear meshes. In general, the larger the tooth profile error, the greater the noise. For the concave tooth profile, the gear teeth are impacted twice in one meshing, the noise is very large, and the more concave the tooth profile, the greater the noise. Therefore, the gear teeth are trimmed to make them appropriately convex to achieve the purpose of reducing noise.

(3) Control the change in the center distance of the meshing gears. The change of the actual center distance of the meshing gear will cause the change of the pressure angle. If the center distance of the meshing gear changes periodically, the pressure angle will also change periodically, and the noise will also increase periodically. The analysis of the meshing center distance shows that when the center distance is too large, the noise effect is not obvious, while when the center distance is too small, the noise increases significantly. When controlling the center distance of the meshing gear, the outer diameter of the gear, the deformation of the transmission shaft, and the matching of the transmission shaft with the gear and the bearing should all be controlled in an ideal state. In this way, the noise caused by the change of the meshing center distance can be eliminated as much as possible.

(4) Pay attention to the role of lubricating oil in controlling noise. While lubricating and cooling, lubricating oil also plays a certain damping role. The noise decreases with the increase of oil volume and viscosity. If a certain oil film thickness can be maintained on the tooth surface, it can prevent the meshing tooth surface from directly contacting, and the vibration energy can be attenuated, thereby reducing noise. Therefore, using oil with high viscosity is beneficial to reducing noise. The main transmission system of the faulty milling machine adopts splash lubrication, which will increase the disturbance noise of the oil. In fact, the amount of oil required for gear lubrication is very small, and its main purpose is to form a pressure oil film to facilitate lubrication. Experiments have shown that the best way to lubricate gears is to supply oil on the meshing side. In this way, it not only has a cooling effect, but also forms an oil film on the tooth surface before entering the meshing area. If the splashed oil can be controlled to enter the meshing area in small quantities, the noise reduction effect will be better. Based on this, the various oil pipes are rearranged so that the lubricating oil splashes into each pair of gears in an ideal state to control the noise generated by poor lubrication.

2. Bearing noise control

(1) Control the quality of the inner and outer rings. In the main transmission system of the faulty milling machine, all bearings have inner rings that rotate and outer rings that are fixed. At this time, if the inner ring has radial runout, it will cause imbalance during rotation, resulting in vibration noise. If the outer ring of the bearing, the shape of the matching hole and the position tolerance are not good, radial swing will occur, which will destroy the concentricity of the bearing components. If there is a large lateral runout between the end faces of the inner ring and the outer ring, it will also cause the inner ring of the bearing to be skewed relative to the outer ring. The higher the precision of the bearing, the smaller the above-mentioned deflection and the smaller the noise. In addition to controlling the geometric deviation of the inner and outer rings of the bearing, the corrugation of the inner and outer ring raceways should also be controlled to reduce the surface roughness and strictly control the surface bumps and scratches of the raceways during the assembly process, otherwise it is impossible to reduce the vibration noise of the bearing. It has been observed that when the corrugation of the raceway is dense or sparse, the contact points of the rolling elements are obviously different when rolling, and the vibration frequencies caused by this are very different.

(2) Control the matching accuracy of the bearing, the hole and the shaft. In the main transmission system of the faulty milling machine, the matching of the bearing, the shaft and the hole should ensure that the bearing has the necessary radial clearance. The optimal value of the radial working clearance is determined by the matching of the inner ring on the shaft and the outer ring in the hole, as well as the temperature difference generated by the inner and outer rings in the moving state. Therefore, the selection of the initial clearance in the bearing is of great significance to controlling the noise of the bearing. Too large radial clearance will lead to an increase in the noise of the low-frequency part, while a smaller radial clearance will cause an increase in the noise of the high-frequency part. Generally, the clearance is best controlled at 0.01mm. The fit of the outer ring in the hole will affect the propagation of noise. A tighter fit will increase the sound transmission, thereby increasing the noise. An overly tight fit will force the raceway to deform, thereby increasing the shape error of the bearing raceway, reducing the radial clearance, and also leading to an increase in noise. A loose fit of the outer ring of the bearing will also cause a lot of noise. Only with an appropriate fit can the oil film at the contact point between the journal and the hole dampen the vibration of the outer ring, thereby reducing noise. In addition, the form and position tolerances and surface roughness of the fitting parts should meet the requirements of the selected bearing accuracy grade. If the bearing is installed very tightly on an inaccurately machined shaft, the error of the shaft will be transmitted to the inner ring raceway of the bearing and manifested in the form of a higher waviness, and the noise will increase accordingly.