How To Repair Slewing Bearing? [Repair Process And Steps]

How To Repair Slewing Bearing? [Repair Process And Steps]

Slewing bearings are among the most stressed mechanical components in heavy machinery. Their failure not only stops production but can also lead to secondary structural damage. While replacement is the safest option, a properly executed repair can extend service life significantly—if based on correct diagnosis, realistic judgment, and precise process control. This article provides a technically oriented repair framework for engineers and maintenance professionals.

What is a slewing bearing?

A slewing bearing is a large-diameter rolling bearing designed to simultaneously support axial loads, radial loads, and tilting moments. It typically consists of:

• Inner and outer rings (one with integral gear teeth)

• Rolling elements (balls or cylindrical rollers)

• Spacers or cages

• Seals and lubricant channels

Unlike conventional bearings, slewing bearings are usually bolted directly to the equipment structure and operate at very low speeds (0.1–10 rpm) under intermittent rotation.

How does a slewing bearing work?

A slewing bearing transfers loads through contact stress between rolling elements and raceways. The gear teeth (internal or external) engage with a pinion to drive rotation.

Key mechanical behaviors:

Damage Pattern	Most Common Root Cause
Raceway indentation	Overload or shock load
Progressive spalling	Subsurface fatigue (L10 life exceeded)
Tooth root cracks	Reverse torque or pinion misalignment
Seal extrusion	Grease overfill or wrong grease type
Ring deformation	Uneven bolt preload or soft mounting structure

Critical parameter:
The contact angle (typically 45° for four-point contact ball bearings) determines how axial and radial loads are resolved. A deviation of even 0.5° in raceway geometry after repair can cause premature failure.

Applications of a slewing bearing

Slewing bearings are used wherever a heavy structure must rotate under load:

• Cranes (tower, mobile, ship, crawler)

• Excavators and backhoes

• Wind turbines (yaw and pitch bearings)

• Solar tracking systems

• Medical CT scanners

• Radar and antenna rotators

• Rotary kilns and feeders

In wind turbines, a slewing bearing failure can cost $50k–150k in lost production per week, making repair decisions extremely time-sensitive.

What Are the Pre-Repair Inspection Processes?

Before any repair, a quantitative inspection must be performed. Visual checks alone are insufficient.

•  Inspection checklist

1. Axial play measurement – Use dial indicators at 4–8 points around the circumference.
   Acceptable range for repair: <1.5× original spec

2. Radial play measurement – More sensitive to raceway wear.

3. Raceway hardness test – Portable Leeb (HL) or UCI method.
   Minimum acceptable hardness: 350 HB (37 HRC). Below this, repair is not recommended.

4. Tooth wear evaluation – Per ISO 10825 or AGMA 915.
   Allowable local wear depth: ≤10% of tooth height

5. Bolt preload verification – Using hydraulic torque or ultrasonic bolt gauge.

6. Nondestructive testing –

• Magnetic particle (MT) for surface cracks

• Ultrasonic (UT) for subsurface ring cracks

• Typical damage patterns and root causes

Damage Pattern	Most Common Root Cause
Raceway indentation	Overload or shock load
Progressive spalling	Subsurface fatigue (L10 life exceeded)
Tooth root cracks	Reverse torque or pinion misalignment
Seal extrusion	Grease overfill or wrong grease type
Ring deformation	Uneven bolt preload or soft mounting structure

Repair or Replace? – A Critical Decision

This is not a purely technical decision—it must include economic and safety factors.

1. Technical criteria for repair

Repair is technically feasible if:

• No through-cracks in inner or outer rings

• Raceway spalling <20% of circumference

• Gear tooth damage ≤3 adjacent teeth, and not at root

• Remaining ring hardness ≥350 HB

• Axial play <2.0 mm (for bearings up to 2 m diameter)

2. Economic rule of thumb

(Repair cost + remaining machine life × risk factor) < 0.6 × (new bearing cost + replacement downtime cost)

3. Quick decision table

Condition	Repair	Replace
Single localized raceway pit	✅	❌
Spalling >20% of raceway	❌	✅
Cracked ring (any length)	❌	✅
Worn teeth (local, ≤3 teeth)	✅	❌
Worn teeth (continuous arc)	❌	✅
Hardness drop below 350 HB	❌	✅
Seal damage only	✅	❌

Step-by-Step Repair Process of Slewing Bearing

The following 8-step process assumes the bearing is dismantled and repaired in a workshop environment. Field repairs are possible only for minor issues (seals, local tooth re-profiling).

Step 1 – Safe disassembly & marking

• Fully unload the equipment

• Remove all mounting bolts (record torque)

• Mark inner/outer ring orientation for reassembly

• Extract bearing using lifting eyes or dedicated tooling

Step 2 – Thorough cleaning

• Remove old grease with solvent bath or steam cleaning

• Remove rust (light rust: wire brush; heavy rust: chemical rust remover)

• Remove concrete or paint residue from gear teeth

⚠️ Do not use sandblasting on raceways – it damages the precision surface.

Step 3 – Full dimensional inspection

Measurement	Tool	Pass/Fail Criteria
Raceway roundness	CMM or large bore gauge	≤0.10 mm
Axial runout	Dial indicator	≤0.15 mm
Tooth profile error	Gear measuring machine	≤ISO 7–8
Bolt circle concentricity	CMM	≤0.20 mm

Step 4 – Disassembly (if bearing is serviceable)

• Remove seals and spacers

• Extract rolling elements (balls/rollers) and clean them individually

• Inspect each rolling element: discard if pitting >0.3 mm or any flat spot

Step 5 – Raceway & rolling element repair

For minor pitting (depth <0.5 mm):

• Localized grinding with a mounted stone (180–320 grit)

• Polish to Ra ≤0.4 µm

• Re-measure hardness (must remain ≥350 HB)

For deeper damage:

• Raceway re-grinding is possible only on large, thick rings

• After re-grinding, the ring may require re-heat treatment—this is rarely done in repair shops

Rolling elements:

• Always replace rolling elements in a repaired bearing (low cost, high safety gain)

• Use original grade G20 or G40 balls/rollers

Step 6 – Gear tooth repair

Local tooth damage (≤3 teeth, not at root):

1. Preheat tooth area to 150–200°C

2. Build-up weld using low-hydrogen electrode (e.g., E7018)

3. Grind tooth profile to original geometry using CNC tooth grinder

4. Inspect with gear template or CMM

❌ Do not weld on hardened tooth roots – risk of crack propagation.

Extensive tooth wear (>30% of arc):

• Not repairable – replace bearing

Step 7 – Reassembly & lubrication

• Install new seals (always replace seals – they are cheap compared to failure)

• Install new rolling elements and spacers

• Lubricate with exactly the original grease type:

• Standard: lithium EP2 (NLGI 2)

• Low temperature: synthetic PAO-based

• Food-grade: NSF H1

• Grease quantity: typically 80–95% of free internal volume

Step 8 – Post-repair testing

Perform after full reassembly but before installation:

Test	Method	Acceptance
No-load rotation	Rotate manually	Smooth, no binding
Axial play	Dial indicator	Within ±15% of original spec
Radial play	Dial indicator	Within ±0.10 mm of original
Noise test	Stethoscope or sound meter	No grinding or clicking
Load test	50% → 100% rated load	No abnormal deflection

Conclusion

Repairing a slewing bearing is not a shortcut—it is a deliberate engineering decision. When done correctly, repair restores 60–80% of original life at 40–60% of replacement cost, and significantly reduces equipment downtime.

However, repair is not always the right choice. Cracks, widespread spalling, or hardness loss below 350 HB are clear signals to replace. Always combine technical inspection with economic analysis before deciding.

For critical applications (cranes, wind turbines, manlifts), consult the original manufacturer or a specialized bearing repair shop.

LyraDrive: get your slewing bearing 3D drawings for your application

Luoyang Lyra Drive is a professional one‑stop slewing device manufacturer, majored in design and development, customized production, sales and service of slew drives and slewing bearings. LyraDrive provides high‑quality and customized slew bearings, slew drives, and gear rings for truck cranes, excavators, manlifts, and many other heavy-duty applications.

To help you obtain an accurate 3D drawing for your project, please provide the following key parameters: slewing diameter (raceway diameter), gear type (internal/external), module, number of teeth, mounting hole pattern (PCD, hole size, and quantity), axial load, radial load, tilting moment, and operating speed. Our engineering team will generate a ready‑to‑use 3D CAD model (STEP or IGES) specifically for your machine design.

If you are looking for a reliable slewing bearing supplier or need professional technical support—including replacement sizing, load verification, or repair feasibility assessment—feel free to contact LyraDrive. We are here to keep your equipment turning safely and efficiently.

FAQ of Repairing a Slewing Bearing

Q1: How long does a typical slewing bearing repair take?
A: Workshop repair (full disassembly) takes 5–12 working days. Field repair (seals, minor tooth work) takes 1–2 days.

Q2: Can a slewing bearing be repaired without disassembly?
A: Only seals and surface tooth damage. Raceway or rolling element repair requires full disassembly.

Q3: What grease should be used after repair?
A: Always use the same grease brand and base as originally specified. Never mix lithium with polyurea or PAO with mineral oil.

Q4: How much does repair cost compared to a new bearing?
A: Typically 40–60% of a new bearing price, but exclude field labor and crane rental.

Q5: After repair, how long will the bearing last?
A: Under normal working conditions: 60–80% of original design L10 life, provided the raceway hardness and geometry remain intact.