Key factor affecting the life of the slewing ring：the quality of the raceway

he slewing ring is a large, circular bearing that enables the smooth rotation of heavy equipment components, such as cranes, excavators, and wind turbines. Slewing ring damage can be classified into two types: raceway damage and tooth breakage. Over 98% of slewing ring failures are due to raceway damage, underlining the importance of raceway quality as a critical factor in determining the service life of a slewing ring.The raceway's quality can significantly impact the performance, durability, and overall efficiency of the slewing bearing. Among them, raceway hardness, hardened layer depth, raceway curvature radius and contact angle are the four most important factors that affect raceway quality.

1. Material Selection:

The choice of material for the raceway plays a vital role in determining its wear resistance, load-bearing capacity, and fatigue life. High-quality bearing steel, properly heat-treated, can help ensure a longer-lasting and high-performing slewing bearing.

2. Raceway Manufacturing Processes:

Precision machining and manufacturing processes contribute to the overall quality of the raceway. Accurate turning, grinding, and hardening of the raceway are essential for smooth and reliable performance.

Raceway hardness: 

The hardness of the slewing ring raceway after quenching plays a significant role in affecting its rated static load. For instance, if we consider the rated static load to be 1 at a hardness of 55HRC, the relationship between the bearing's rated static load and the raceway hardness can be described as follows:

Depth of hardened layer of raceway: 
The appropriate depth of the hardened layer of the raceway is essential to ensure the slewing ring's raceway does not experience spalling. When the slewing bearing is subjected to external loads, the contact between the steel ball and the raceway shifts from point to surface contact, creating an elliptical contact area. This generates both compressive and shear stresses on the raceway, with the maximum shear stress occurring at a depth of 0.47a (the major semi-axis of the contact ellipse) beneath the surface. This is why standards specify the hardened layer depth based on the steel ball diameter, rather than the slewing ring diameter, and provide a minimum guaranteed value.The bearing's rated static load C is proportional to the depth of the hardened layer (H^0.908). If the required hardened layer depth is 4mm, but only reaches 2.5mm, the bearing's static load C will decrease from 1 to 0.65. This reduction significantly increases the risk of slewing bearing damage due to fatigue peeling.

Curvature radius of the raceway：

The raceway curvature radius is defined as the curvature radius of the raceway in the vertical cross-section. The ratio 't', which represents the proportion of the raceway radius to the steel ball radius, also has a considerable impact on the slewing ring's rated static load and fatigue life. When t equals 1.04, both the rated static load and fatigue life attain a value of 1. The interrelationship between the rated static load, fatigue life of the slewing ring, and the value of 't' can be summarized as follows.

It can be seen from the above table that the larger the radius ratio, the lower the rated static load and the shorter the service life.

3. Surface Hardness and Roughness:

A higher surface hardness typically leads to better wear resistance and longer service life. The surface roughness can also determine the performance of the raceway: having a smooth surface reduces friction, which in turn minimizes wear and increases the overall efficiency of the slewing ring.

4. Contact Geometry:

The geometry of the raceway contact surfaces, such as the shape and size of the rolling elements and raceways, impacts the load distribution and stress concentrations. Proper contact geometry can enhance the load-carrying capacity and extend the service life of the slewing ring.The contact angle is defined as the angle formed between the line connecting the point of contact between the steel ball and the raceway, the center of the steel ball, and the radial cross-section (horizontal plane) of the slewing bearing. The rated static load (C) of the slewing ring is linearly proportional to SINα. Typically, the initial contact angle is approximately 45°. However, when a slewing bearing has a clearance, the actual contact angle becomes greater than the original contact angle.Within the standard clearance range, the actual contact angle will usually increase by 2° to 10°, resulting in an actual contact angle of 47° to 55°. This increase in contact angle is generally beneficial for the bearing capacity. However, if the initial contact angle and clearance are initially large, the actual contact angle may exceed 60°. As the raceway wears over time, the clearance will further increase, and the actual contact angle will continue to grow.When the actual contact angle becomes too large, the contact ellipse could surpass the edge of the raceway, and the actual stress on the raceway may exceed the theoretical calculated stress. This condition can cause the edge of the raceway to collapse, ultimately leading to the failure of the slewing bearing.

5. Lubrication:

A well-lubricated raceway can significantly impact the lifespan of a slewing ring by reducing friction, dissipating heat, and preventing corrosion. It also prevents direct metal-to-metal contact, thereby reducing material wear.

Conclusion

Maintaining the quality of the raceway is crucial to increasing the service life of a slewing ring. Proper material selection, manufacturing processes, surface treatment, contact geometry, and lubrication all contribute to the performance, durability, and dependability of the slewing bearing.