Discussion on the causes of damage to turntable bearings and improvement measures

1, turntable bearing damage phenomenon in the truck crane, excavator and other types of construction machinery, turntable bearing is an important component to transfer the axial load, radial load and tilting torque between the turntable and chassis. In light load conditions, it can work normally and rotate freely. But in heavy load, especially in the maximum lifting weight and the most substantial, heavy rotation difficulties, or even simply can not rotate and thus jammed. At this time usually take to reduce the amplitude, adjust the legs or move the position of the chassis and other methods to make the body tilt, in order to help achieve the rotary movement of heavy objects, to complete the scheduled lifting and other operations. So in the maintenance work is often found in the turntable bearing raceway has been seriously damaged, the inner seat circle on both sides and the front of the work area under the raceway generated along the raceway direction of the ring-shaped crack, at the seat circle on the raceway of the largest area of force indentation, and in the entire indentation area produced radial cracking, its damage state as shown in Figure 1.

1.inner seat ring; 2.outer seat ring; 3.roller column Figure 1 Raceway damage state diagram

2, turntable bearing damage reasons to explore
(1) the influence of the safety factor turntable bearing often in the low-speed large load operating conditions, its bearing capacity is generally available static capacity, rated static capacity recorded as C0a. the so-called static capacity refers to the raceway permanent deformation δ reached 3d0/10000 when the bearing capacity of the turntable bearing, d0 for rolling body diameter. The combination of external load is generally used equivalent load Cd to express. The ratio of static capacity and equivalent load is called the safety factor, recorded as fs, which is the main basis for the design and selection of turntable bearings. When using the method of checking the maximum contact stress between the roller and the raceway to design the turntable bearing, take the permissible line contact stress [σk line] = 2.0 ~ 2.5 × 102kN/cm. At present, most manufacturers are based on the size of the external load on the turntable bearing selection calculation. According to the available information, the current small tonnage crane turntable bearings compared to large tonnage crane turntable bearings contact stress to be small, the actual safety factor is higher. The larger the crane tonnage, the larger the diameter of its turntable bearing, the lower the manufacturing accuracy, its safety factor is actually lower, which is the fundamental reason why large tonnage crane turntable bearings are more prone to damage than small tonnage crane turntable bearings. At present, it is generally agreed that the turntable bearings of cranes above 40t, the line contact stress should not exceed 2.0 × 102kN/cm, the safety factor should not be less than 1.10.
(2) the influence of the turntable structure stiffness turntable bearing is an important part of the transfer of various loads between the turntable and chassis, its own stiffness is not large, mainly dependent on the chassis and turntable supporting it structural stiffness. Theoretically, the ideal structure of the turntable is a large rigid cylinder so that the load on the turntable can be evenly distributed, but this is not possible due to the height limitation of the machine. Finite element analysis of the turntable shows that the deformation of the base plate connecting the turntable to the turntable bearing is large, more so under large deflection conditions, resulting in the load being concentrated on a small number of rollers, thus increasing the pressure on individual rollers; in particular, the deformation of the turntable structure changes the contact between the rollers and the raceway, significantly reducing the contact length and leading to a significant increase in contact stresses. The widely used methods of calculating contact stress and static capacity are based on the premise that the bearings are uniformly stressed and that the effective contact length of the rollers is 80% of the roller length. Obviously, this premise is not in line with the actual situation. This is also another reason why turntable bearings are easy to damage.
(3) heat treatment state of the influence of the turntable bearing itself processing quality by manufacturing accuracy, axial clearance and heat treatment state of the influence of the great, here easy to ignore the factors is the impact of heat treatment state. Obviously, to make the raceway surface does not produce cracks and indentation, it requires the raceway surface in addition to have sufficient hardness, but also must have sufficient hardening layer depth and heart hardness. According to foreign information, the depth of the hardened layer of the raceway should be thickened with the increase of the rolling body, the deepest can be more than 6mm above, the hardness of the heart should also be higher, so that the raceway will have a higher resistance to pressure collapse. So the turntable bearing raceway surface hardening layer depth is not enough, the heart hardness is low, is also one of the reasons for its damage.

3、Improve countermeasures
(1) Through finite element analysis, appropriately increase the thickness of the plate connected to the turntable and turntable bearing parts, in order to improve the structural rigidity of the turntable.
(2) When designing the large diameter turntable bearing, the safety factor should be increased appropriately; the number of rollers should be increased appropriately to improve the contact condition between the rollers and the raceway.
(3) Improve the manufacturing accuracy of the turntable bearing, focusing on the heat treatment process. Can reduce the medium frequency quenching speed, and strive to obtain a larger surface hardness and hardening depth, to prevent quenching cracks on the surface of the raceway.

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