Tag Archive for: slewing bearing soft zone

What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. These bearings handle axial loads, radial loads, and tilting moments simultaneously. They are essential in cranes, excavators, wind turbines, and other heavy machinery.

The raceway of a slewing bearing must be hard enough to resist wear and indentation. Manufacturers achieve this through induction hardening, which heats the raceway surface and quenches it rapidly. However, this process creates a section of the raceway that does not receive full hardening. This unhardened section is called the soft zone.

What Is a Slewing Bearing Soft Zone?

A soft zone is the unhardened section of a slewing bearing raceway. It occurs where the induction hardening process begins and ends. When the heating coil starts at one point and travels around the raceway, the start and end points overlap imperfectly. The result is a narrow band that does not reach the required hardness.

Why soft zones form

Induction hardening works by passing a high-frequency current through a coil near the raceway surface. The current induces eddy currents in the steel, heating the surface rapidly. A quench follows immediately to harden the heated zone. When the coil completes a full revolution, the starting point has already cooled. The overlap between the start and end of the hardening pass creates a transition zone that receives less heat or less quench. This transition zone remains softer than the rest of the raceway.

How soft zones are marked

Manufacturers mark the soft zone with an “S” on the bearing ring. The mark appears on the interior or exterior diameter, depending on the bearing design. This mark tells the installer where the soft zone is located so they can position it correctly during mounting.

Why Soft Zones Matter for Slewing Bearing Performance

The soft zone is the weakest point on the raceway. Under load, the unhardened steel deforms more easily than the hardened sections. This deformation creates several problems.

Reduced load capacity at the soft zone

The soft zone has lower hardness and lower yield strength than the hardened raceway. When a rolling element passes over the soft zone under heavy load, the surface can indent permanently. This indentation creates a stress concentration that accelerates fatigue and spalling.

Crack initiation

Research on a 6-ton excavator slewing bearing identified cracks at the soft zone. Metallurgical analysis showed that cracks initiated at the edge of the soft zone where hardness transitioned abruptly. The blockage hole edge and the heat treatment transition zone were identified as crack initiation sites.

Accelerated wear

The soft zone wears faster than the hardened raceway. As the bearing rotates, rolling elements pass over the soft zone repeatedly. The softer surface wears down, increasing clearance and reducing rotational accuracy. Over time, the wear can become severe enough to require bearing replacement.

Seal damage risk

Excessive wear at the soft zone can create a step or ledge on the raceway. This step can damage the seal as it passes over the worn area. Once the seal is damaged, contaminants enter the bearing and accelerate degradation.

Industry Standards for Slewing Bearing Soft Zones

Industry standards recognize the soft zone as an unavoidable feature of induction-hardened slewing bearings. Standards specify limits on soft zone width to ensure acceptable performance.

Chinese standard JB/T 2300-2011

This standard specifies requirements for slewing bearing soft zones. For rings with blockage holes, the soft zone width should not exceed the blockage hole diameter plus 35mm. The blockage hole is a hole drilled into the ring to allow the hardening coil to pass through. The soft zone extends around this hole.

Other standards

Similar requirements exist in other industry standards. The goal is to limit the soft zone width so that it does not significantly reduce the bearing’s load-carrying capacity. A wider soft zone means a larger area of the raceway is not hardened, reducing the bearing’s overall performance.

How to Mitigate Soft Zone Risks in Slewing Bearings

While soft zones cannot be eliminated entirely, their impact can be minimized through proper design, installation, and operation.

Positioning the soft zone outside the load zone

The most important mitigation step is proper positioning. When installing a slewing bearing, the “S” mark should be placed outside the main load area. In most applications, the primary load direction is known. For a crane, the load acts downward on one side of the bearing. For an excavator, the digging force acts in a specific direction. By positioning the soft zone away from this loaded area, the risk of indentation and cracking is significantly reduced.

Optimizing the hardening process

Manufacturers can reduce soft zone width by optimizing the induction hardening process. Using multiple inductors with oscillating movements and preheating can eliminate the seam or reduce its width. Advanced techniques create a more continuous hardened zone with minimal soft zone width.

Material selection

Higher-quality steel with better hardenability can reduce the depth of the soft zone. Steel grades like 42CrMo have good hardenability, meaning the hardness transition is sharper and the soft zone is narrower. Lower-grade steels may have wider transition zones.

Surface treatment after hardening

Some manufacturers apply additional surface treatments after induction hardening to improve soft zone properties. These treatments may include nitriding or other surface modification processes. However, these treatments add cost and may not be suitable for all applications.

Inspection and quality control

Hardness testing verifies that the soft zone meets specification. Manufacturers should test hardness at multiple points along the raceway, including the soft zone, to ensure it meets minimum requirements. If the soft zone is too wide or too soft, the bearing should be rejected.

How to Inspect for Slewing Bearing Soft Zone Problems

Detecting soft zone problems early allows corrective action before catastrophic failure occurs.

Hardness testing

The most direct method is hardness testing. A portable hardness tester can measure the raceway hardness at various points. The soft zone should be tested specifically to verify it meets minimum hardness requirements. If the soft zone hardness is below specification, the bearing may be at risk.

Visual inspection

Visual inspection can reveal signs of soft zone damage. Look for:

• Indentations or depressions on the raceway
• Discoloration indicating overheating or wear
• Cracks, particularly at the edges of the soft zone
• Uneven wear patterns

Vibration monitoring

Vibration monitoring can detect soft zone problems during operation. As rolling elements pass over a damaged soft zone, they generate vibration signatures. Increased vibration amplitude or changes in frequency patterns can indicate soft zone degradation.

Grease analysis

Grease analysis can detect wear particles from the soft zone. Metal particles in the grease indicate active wear. If the particles are concentrated in a specific size range, they may originate from the soft zone.

Practical Recommendations for Slewing Bearing Users

For equipment manufacturers and operators, understanding soft zones helps prevent premature failure.

During installation

Always identify the “S” mark before mounting the bearing. Position the soft zone outside the main load area. If the load direction is not fixed, position the soft zone in the least loaded area. Document the soft zone position for future maintenance reference.

During operation

Avoid overloading the bearing, especially when the load is positioned over the soft zone. Monitor for signs of soft zone damage, including unusual noise, vibration, or grease contamination. If soft zone damage is suspected, reduce load or schedule inspection.

During maintenance

Include soft zone inspection in routine maintenance. Check for indentations, cracks, or uneven wear at the soft zone location. If damage is found, assess whether the bearing can continue in service or needs replacement. In some cases, the bearing can be rotated to move the soft zone to a different position relative to the load.

How LDB Bearing Manages Soft Zones in Manufacturing

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s soft zone management:

• Process control: In-house induction hardening with documented hardness records and controlled soft zone width
• Quality inspection: Hardness testing at multiple points including soft zone verification
• Marking: Clear “S” mark on every bearing to identify soft zone location
• Engineering support: Application engineering to help customers position the soft zone correctly during installation
• Quality certification: ISO 9001-certified manufacturing with documented inspection reports and full material traceability

Serving 73 countries with over 500,000 units in service, LDB delivers the reliability that heavy machinery applications demand. Understanding soft zones—why they form, how they affect performance, and how to mitigate their risks—helps equipment manufacturers and operators achieve maximum bearing life. LDB offers the technical expertise and quality assurance that demanding applications require.

Contact LDB Bearing today to discuss your slewing bearing requirements.

FAQs

1. What is a soft zone in a slewing bearing?
A soft zone is the unhardened section of a slewing bearing raceway. It occurs where the induction hardening process begins and ends, creating a narrow band that does not reach the required hardness.

2. Why does a soft zone form during induction hardening?
When the heating coil completes a full revolution, the starting point has already cooled. The overlap between the start and end of the hardening pass creates a transition zone that receives less heat or less quench.

3. How is the soft zone identified on a slewing bearing?
Manufacturers mark the soft zone with an “S” on the interior or exterior diameter. This mark tells the installer where the soft zone is located.

4. Where should the soft zone be positioned during installation?
The soft zone should be positioned outside the main load area. For most applications, the primary load direction is known, so the “S” mark can be placed away from the loaded area.

5. What are the risks of ignoring soft zone positioning?
Ignoring soft zone positioning can lead to raceway indentation, crack initiation, accelerated wear, and premature bearing failure. The soft zone is the weakest point on the raceway and requires careful handling.