Internal Cracks in Swing Bearing: The Invisible Killer and the Path to Intelligent Detection

As the ‘joint’ of heavy equipment such as construction machinery and other rotating equipment, once internal cracks form in the raw materials of swing bearings, they can trigger a series of chain reactions, threatening the safety of the entire machine. These invisible cracks not only accelerate component failure but may also lead to catastrophic accidents. This article will reveal the destructive mechanisms of cracks and introduce cutting-edge detection technologies.

I. The Lethal Impact of Cracks

1. Sudden Changes in Load Distribution and Lifespan Decline

When the outer ring of a swing bearing cracks, the radial load on the inner ring causes a sudden and significant change in the tangential force at the bolt hole positions on both sides of the crack (while the radial force remains largely unchanged), resulting in periodically amplified loads. This alternating stress drastically reduces the fatigue lifespan of the connected castings, and in severe cases, can lead to bolt failure.

2. The Soft Zone Becomes the Crack Hotspot

The process soft zone, with its lower hardness (typically ≤50 HRC), becomes the weakest link. Under alternating rolling loads, the transition zone (where the base material meets the hardened layer) is prone to stress concentration, leading to slip plane separation cracks; the edges of blocked holes may also develop cracks due to plastic deformation.

3. Crack Propagation Direction Determines Risk Level

Cracks propagating from the interior to the exterior pose a far greater hazard to castings than those propagating from the exterior to the interior. Especially during the initial stage of through cracks, the risk is highest, while the impact on safety decreases once the crack surface opens.

II. Breakthroughs in Precise Detection Technologies

1. Acoustic Emission Dynamic Monitoring Technology

Targeting low-speed, heavy-load characteristics, the acoustic emission-based detection system captures elastic waves released during crack propagation to enable early warning. By analysing signal characteristics using wavelet packet energy spectra combined with support vector machine models, the system can not only locate the crack source (with millimetre-level precision) but also assess the severity of the crack.

2. Embedded Closed-Loop Sensing Devices

An innovatively designed recessed embedded sensing layer (e.g., tin foil) forms a self-contained circuit. When a crack causes the sensing layer to fracture, the closed-loop circuit is interrupted, triggering an alarm. This technology seals the detection unit within the recess, offering strong interference resistance and addressing the issue of traditional adhesive sensors prone to detachment.

3. Specialised Inspection of the Soft Belt Zone

Magnetic particle testing (MT) and metallographic re-inspection are conducted around blocked holes, combined with grinding and recessing processes to reduce stress concentration. Industry standards require the soft band width to be ≤ the blocked hole diameter + 35 mm (e.g., for a 27 mm diameter hole, the width must be ≤ 62 mm), and strict verification is required.

III. Key preventive strategies

1. Material Upgrades: Prioritise the use of alloy steels with superior quenching properties, such as 42CrMo and 50Mn, to avoid replacing them with 45# or 50# steel, which can reduce load-bearing capacity.

2. Strengthen incoming inspection of forgings, which is something SWBTEC has consistently emphasised. 

3. Process Optimisation: Strictly control the clearance between the blocked hole and the soft belt zone, and perform grinding and concavity treatment on the soft belt zone to distribute stress.

Swing bearing cracks are like ‘hidden injuries’ within the equipment. Only by combining material gene optimisation, intelligent monitoring, and process innovation can the dilemma be resolved. In the future, the integration of IoT-based acoustic emission online diagnostic systems and soft zone-free quenching technology will redefine the safety boundaries of China slewing ring bearings.