There are two main reasons for the early failure of slewing bearings in excavators: first, broken teeth; The second is raceway destruction. Among them, broken teeth are the main reason, accounting for more than 90%, and most of them occur within six months after the delivery of the excavator. This not only seriously troubles the product quality reputation of the slewing bearings manufacturer, but also has an adverse impact on the product market of the main engine factory. Therefore, it is the common goal and responsibility of the slewing bearings manufacturer and the main engine factory to solve this problem, and also the fundamental guarantee for the further cooperation and common development of both sides.

What is the root cause of early failure of slewing bearings due to broken teeth? Design problems; Manufacturing problems, material problems; Assembly problem or use problem. The essence of the problem can be easily found through the following phenomena:

(1) In the past 12 years, Maon Shan slewing Bearing Factory has provided more than 20,000 sets of slewing bearings for various main engines. Except for the excavator industry, there is only one record of slewing bearing tooth breaking, which occurred in the late period. Of course, it is an indisputable fact that the working condition of excavator is worse than that of tower crane, automobile crane and other most other industries that use slewing bearing. The speed of slewing is faster, the impact load is larger, and the possibility of tooth breaking is correspondingly greater. Therefore, the rotary bearing modulus of excavator is larger than that of other industries with the same raceway diameter, and it is hard tooth surface (generally choose different hardness segments between 47HRC and 58HRC), which basically meets the requirements of excavator for rotary bearing gear. Although statistical data show that the probability of early tooth breakage of slewing bearings in excavators is greater than that of other main engines, it is only limited to a few two or three kinds of excavators, most of which rarely have early tooth breakage accidents of slewing bearings.

② From the analysis of the data we have, most of the 20 ~ 22 ton excavators at home and abroad use slewing bearing gear module is 10mm (or diameter pitch =2.5), heat treatment and precision grades are basically the same, domestic excavators generally use the standard tooth height and standard pressure Angle. Circumferential allowable force P of slewing bearing gear can be calculated according to the following formula:

P=Kz*m*b/78 (ton) where Kz=(z/150)^(±0.09) external tooth +; Internal tooth take -z- tooth number m- modulus mmb- tooth width mm if set tooth width b=80; Tooth number z=90 ~ 110; And internal meshing, then the allowable circumferential force of the gear is:

p=(90 ~ 110/150)^(-0.09)*10*80/78=10.74 ~ 10.55(ton)

It can be seen that the circumfertial allowable force of the gear can meet the load requirements of the slewing bearing gear of this tonnage excavator, but the early tooth breaking rate of the slewing bearing of some models in this level is as high as 2%, and this phenomenon does not occur in most of the other models.

(3) Through the analysis of a number of early tooth breakage, it is found that most of the tooth breakage occurred in the upper half of the direction along the width of the tooth. More than half of the fracture surface intersected with the upper end face of the tooth, and formed an Angle of 45° ~ 60°. Even if the whole tooth fell off, the crack was caused by top-down expansion. The plastic deformation caused by the extrusion of the gear is also quite obvious, and the upper part is much more serious than the lower part. The groove width of the whole ring varies to different degrees, increasing from the bottom to the top, and from the root to the top.

Can we think that the force causing the early tooth breaking of the slewing bearing of the excavator is not the circumferential rotary driving force, but the radial extrusion pressure exerted by the meshing pinion, and the axis of the pinion and the axis of the slewing bearing gear are not parallel during extrusion? The force produced in mining process occurs in the face of bucket reaction, because of slewing bearing clearance, and the rotary bearing inner and outer ring coupling fluctuation two parts respectively under the action of overturning moment, will occur in rotary bearing axial profile by big arm on the sloping, producing in the opposite direction along the rotary bearing radial and big arm relative displacement, The displacement is equivalent to the radial clearance of slewing bearing. Mesh with the rotary bearing of the small gear installed in arm in the opposite direction, when the backlash over hours, displacement is not done, big gear pinion and pressure, this case should be borne by the rotary bearing raceway load is taken by gear, because of the axis of the pinion is cantilever installation was tilted further under the adverse effects of the extrusion pressure, Causes the extrusion pressure on the large gear to concentrate on the upper part of the tooth width. At the beginning, the plastic deformation of gear is used to compensate the shortage of gear side clearance. With the further running-in of slewing bearing raceway, the radial clearance gradually increases, but the deformation is limited. Through force analysis, it can be seen that the extrusion pressure of the pinion on the big gear is several times or even ten times of the reaction force of the bucket on the ground, and the force acting on the tooth profile will be amplified again. The smaller the pressure Angle, the greater the amplification factor. This twice amplified force is enough to cause tooth breakage in both large and small gears. The conclusion of the above analysis is consistent with the phenomenon of article 3.

Therefore, the author thinks that the fundamental reason of early tooth breakage of slewing bearing is that the backlash with pinion is too small. It is suggested that the backlash should not be less than 1.25 times the radial clearance of slewing bearing. What is worth changing is that the pressure angles of the slewing bearings for the four types of excavators manufactured by our factory for Canada are 25° and 27° respectively, which are also adopted by domestic joint venture factories. It is effective to improve the resistance of gear to radial extrusion.

Of course, such as slewing bearing material defects; After tooth quenching, the residual internal stress is large and the internal crack is found. Due to the failure of slewing bearing raceway, slewing stuck; Illegal operation of excavator can also lead to early failure of slewing bearing gear, but the distribution surface should be wide and discrete.