1. Characteristic frequency analysis of gearbox
The characteristic frequency of the gearbox mainly includes the shaft frequency, the meshing frequency of the gear, the frequency of the inner and outer rings, the rolling elements, and the cage of the bearing. They are combined with the harmonic frequency and the side frequency to become the basis for the fault judgment of the gearbox. The main reasons for gear vibration failure are as follows:
A. Faults caused by gear manufacturing and installation errors. Errors in the gear manufacturing process or assembly process reduce the gear meshing accuracy, resulting in increased gear vibration and noise, and increasing the failure rate of the gear. It is shown as the changes of meshing frequency and the amplitude of each harmonic on the frequency.
B. Faults caused by the inherent movement of the gear itself (or operating environment). During the gear meshing process, the continuous impact between the tooth causes the gear to vibrate and produce noise, which is shown as the meshing frequency of the gear on the frequency.
C. Gear surface damage failure, tooth surface wear. Due to tooth surface peeling, strain and other defects, every time the gear rotates, the rings will collide with each other once, resulting in an obvious impact phenomenon. Each impact is equivalent to an impulse excitation. The impulse response function is the inherent attenuated vibration of the gear, which constitutes a periodic higher frequency shock vibration signal. The cycle period is the rotation period of the shaft, and the attenuated vibration frequency is the natural frequency of the gear. Meshing vibration will occur during gear meshing process if the backlash increases or the gear is worn. The vibration frequency is the gear meshing frequency. For example: when a defect occurs at a certain point, such as pitting, tooth chipping, short-term loading, unloading effects are generated during the gear meshing process, and amplitude adjustment and frequency adjustment signals are generated, which are expressed in the frequency domain as where the meshing frequency is as the center, and the rotation frequency of the shaft as the spacing group of spectral lines, that is, the sideband.
D. The shaft is bent. When the rotating shaft is severely bent, there will usually be obvious shock vibrations at a certain time interval in the time domain, and the number of sidebands is large and dense.
E. The gear is dynamically unbalanced and has an unbalanced mass, or an eccentric gear causes unstable operation of the gear pair during the rotation process. Under the excitation of the unbalanced torque, a vibration dominated by frequency modulation and supplemented by amplitude modulation is generated. Sidebands will be generated on both sides of the meshing frequency and its harmonics, which are influenced by the unbalanced force, and the rotation frequency of the gear shaft and the energy of its harmonics will increase accordingly.
F. The interior of the gearbox is loose, and there will be a sudden random violent sound during the acceleration and deceleration at a low speed. It appears as a sudden and large intermittent rise on the time domain diagram, which has large randomness.
2. Analysis of the reasons for gearbox failure: broken teeth
Part of the broken teeth of the gearbox is caused by manufacturing quality defects.
1. One of the causes of gearbox failure – teeth breakage is manufacturing quality: failure to select suitable steel, poor casting quality of gears, low quenching hardness, insufficient gear hardness, insufficient spheroidization, and fatigue caused by alternating stress during use.
2. Gear meshing accuracy has little effect on broken teeth. Although the meshing accuracy of the gear has a certain degree of influence on the broken tooth, it’s not large. Because the meshing accuracy of the gear is not enough, the contact surface of the gear is insufficient. Then the contact stress will increase, and the load will be uneven. But this is not the main reason for the broken teeth.
Another reason for broken teeth is that the load is too heavy, the tooth root is defective, and the bending strength of the tooth is not enough. (The main factors affecting the bending strength are the load force acting on the gear teeth and the defect degree of the tooth root. The poor meshing accuracy of gears will not change the load force on the gear teeth and the defect degree of the tooth root, but only increase the contact stress of the gear teeth).
In addition to the poor manufacturing quality of the reducer, there are also some problems that may cause gear wear, such as the lack of oil in the reducer and the use of semi-fluid lubricating fluid.
The manufacturing quality factors of abnormal wear include:
① Gear materials do not meet the requirements, causing abnormal wear;
② The gear has defects such as blisters, pores and looseness, and insufficient spheroidization;
③ Insufficient heat treatment hardness or no heat treatment;
④ Gear meshing accuracy and motion accuracy cannot meet the requirements;
⑤ The arc gear is very sensitive to the error of the center distance, especially the positive error of the center distance, which not only reduces the bending strength of the gear teeth but also increases the sliding wear.
3. Common failure analysis of gearbox
(1). What are the reasons for the pitting and peeling of the gear of the gearbox?
a. Material, hardness and defects. The material of the gear does not meet the requirements; the main factor that affects the contact fatigue strength of the gear is that the hardness after heat treatment is low, and the contact fatigue strength of the gear cannot be reached. In addition, defects on the tooth surface or inside are also one of the reasons for the insufficient contact fatigue strength;
b. Gear accuracy is poor. Gear processing and assembly accuracy do not meet the requirements, such as meshing accuracy, poor movement accuracy, etc. In addition, the center distance error of the arc gear is too large;
c. Lubricating oil does not meet the requirements. The lubricant used is of the wrong brand, the viscosity of the oil is low, and the lubrication performance is poor;
d. The oil level is too high. If the oil level is too high, the temperature of the oil rises, which reduces the viscosity of the lubricating oil, destroys the lubricating performance and reduces the working thickness of the oil film.
(2). A brief analysis of the reasons for one of the signs of gearbox failure: the shaft stringing of the gearbox
a. Because of the broken teeth, the input shaft loses its axial restraint and the shaft stringing occurs;
b. The driven gear on the intermediate shaft is not tightly fastened to the shaft. In actual transmission, due to insufficient interference between the driven gear and the intermediate shaft, the driven gear produces axial endplay relative to the intermediate shaft, which in turn causes the input shaft to produce axial endplay. Therefore, insufficient interference is the main reason for the shaft stringing of the gearbox;
c. The steering of the gearbox also has a certain influence on the shaft stringing.
(3). A brief analysis of the reasons for one of gearbox failure symptoms: the high oil temperature of the gearbox
a. The lubricating oil is unqualified or has been used for too long;
b. Too much lubricating oil is not conducive to the heat dissipation of the mechanism in the gearbox;
c. The machine is damaged. Mechanical damage includes severe pitting of gears, broken teeth, damage to bearing cages, inner and outer rings, balls, and gearbox bearing failure or severe shaft deformation;
d. The outside of the gearbox is covered by debris or dust. When things are piled around the gearbox or the surface of the body has not been cleaned for a long time, it is possible that the cover of debris or dust may cause the gear reducer to dissipate incompletely and increase the oil temperature;
e. The cooling device is blocked or fails. The cooling device is placed in a dusty workshop like the gearbox. If the cooling device is blocked or the cooling device is broken due to long-term operation without cleaning the internal pipeline, the oil temperature of the gearbox will increase.