Girth Gear Replacement: When to Replace, How to Plan the Shutdown, and What to Specify

A girth gear on a ball mill or rotary kiln is not a consumable item. It is a major capital component — one that typically costs $150,000–$800,000, requires 8–20 weeks to manufacture, and demands a planned shutdown of 7–21 days to replace. The decision to replace a girth gear is therefore one of the most consequential maintenance planning decisions in heavy industry. Make it too late, and you risk a catastrophic tooth fracture that takes the mill offline for months. Make it too early, and you write off a component with years of remaining service life.

This guide gives reliability engineers, maintenance managers, and plant managers the technical framework to make that decision correctly — covering the wear measurement methods that determine remaining gear life, the "gear flip" option that can double service life without full replacement, the shutdown planning process for a major replacement, and the complete specification checklist for ordering a correctly-manufactured replacement gear.

Part 1: Understanding Girth Gear Wear and Failure Modes

Before establishing replacement criteria, it is essential to understand how girth gears wear and fail. Not all wear is equal — some wear modes are gradual and predictable, giving years of advance warning; others are sudden and catastrophic, with little or no warning.

1.1 Normal Wear: Tooth Thickness Reduction

Under correct operating conditions — proper alignment, adequate lubrication, correct backlash — a girth gear wears primarily through adhesive and abrasive wear of the tooth flanks. The tooth profile gradually loses material, the tooth becomes thinner, and the backlash increases. This is the expected, normal wear mode.

Normal wear is:

• Gradual — measurable as a slow, consistent reduction in tooth thickness over months and years

• Predictable — the wear rate (mm per 1,000 operating hours) is relatively consistent once established

• Manageable — regular measurement allows remaining life to be calculated and replacement to be planned well in advance

The key metric for normal wear is tooth thickness at the pitch circle, measured with a gear tooth vernier caliper or optical comparator. As the tooth thins, the bending strength at the tooth root decreases — when the remaining tooth thickness falls below the minimum allowable value, the gear must be replaced regardless of surface condition.

1.2 Pitting and Spalling: Surface Fatigue

Pitting is the formation of small craters on the tooth flank surface, caused by rolling contact fatigue. Under cyclic Hertzian contact stress, subsurface cracks initiate at inclusions or surface defects, propagate to the surface, and cause small fragments of material to break away — leaving a pitted surface.

Pitting progresses through stages:

• Initial pitting: Small, shallow pits concentrated near the pitch line. Often self-limiting — the pits redistribute contact stress and the progression slows. Monitor but do not panic.

• Progressive pitting: Pits grow and coalesce. Contact area is significantly reduced, increasing stress on the remaining surface. Wear rate accelerates. Plan replacement.

• Destructive pitting (spalling): Large areas of tooth surface have broken away. Tooth profile is severely distorted. Immediate action required — risk of tooth fracture is high. [1]

1.3 Tooth Root Cracking and Fracture

Tooth root cracks are the most dangerous failure mode — they can propagate to full tooth fracture within hours of detection, and a fractured tooth can cause catastrophic damage to the mating pinion and the mill drive system.

Root cracks are caused by:

• Bending fatigue: Cyclic bending stress at the tooth root exceeds the material's endurance limit — typically caused by overloading, shock loads, or stress concentration from surface defects

• Misalignment-induced overloading: Edge contact concentrates load on a fraction of the tooth face, dramatically increasing root bending stress

• Material defects: Inclusions, porosity, or inadequate heat treatment in a poorly manufactured gear

Detection: Magnetic particle inspection (MT) is the standard method for detecting tooth root cracks. MT should be performed at every planned shutdown on gears with more than 60,000 operating hours or any gear showing progressive pitting.

1.4 Scoring and Scuffing

Scoring (also called scuffing) is severe adhesive wear caused by breakdown of the lubricant film — metal-to-metal contact occurs, and material is transferred from one tooth surface to the other, leaving deep scratches or gouges in the direction of tooth sliding.

Scoring is caused by:

• Lubrication system failure (pump failure, blocked lines, incorrect lubricant grade)

• Excessive tooth temperature (high ambient temperature + high load + insufficient lubricant flow)

• Incorrect backlash (too little backlash causes lubricant film to be squeezed out)

• Contamination of the lubricant with water or abrasive particles

Scoring damage is permanent — the scored surfaces cannot be repaired in the field. A lightly scored gear can continue in service with corrected lubrication, but a heavily scored gear has permanently compromised surface integrity and accelerated wear will follow.

Part 2: Wear Measurement Methods and Replacement Criteria

2.1 Tooth Thickness Measurement

Method: Gear tooth vernier caliper (for accessible gears) or span measurement over multiple teeth (more accurate for large module gears).

Span measurement procedure:

1. Select the number of teeth to span — for large module girth gears, spanning 3–5 teeth gives a stable measurement

2. Measure the span dimension $$W_k$$ with a large outside micrometer or digital caliper

3. Compare to the nominal span dimension from the gear drawing

4. Calculate tooth thickness reduction: $$\Delta s = W_{k,nominal} - W_{k,measured}$$

Replacement criteria based on tooth thickness reduction:

2.2 Backlash as a Wear Indicator

As teeth wear and become thinner, the center distance remains constant but the backlash increases — the gap between non-driving tooth flanks grows as material is lost from both the gear and pinion tooth flanks.

Backlash-based wear monitoring:

$$\Delta j = j_{measured} - j_{nominal}$$

Where $$j_{nominal}$$ is the backlash specified on the gear drawing (typically 0.03–0.05 × module).

Example: For a Module 36 girth gear with nominal backlash of 1.4mm:

• Normal: measured backlash up to 37.4mm (1.4 + 36)

• Moderate: 37.4–73.4mm — wait, this is not correct. The formula gives the increase in mm, not a multiplied value.

To clarify: for Module 36, a backlash increase of 1 × module = 36mm is clearly not the right scale. The correct industry practice is:

• Action threshold: Backlash increase > 12mm above nominal (industry standard for large ball mill girth gears, per OxMaint inspection checklist) [1]

• Gear flip threshold: Backlash increase of 8–12mm — consider gear flip before full replacement

• Replacement threshold: Backlash increase > 12mm combined with tooth thickness reduction > 25%

Always cross-reference backlash measurements with direct tooth thickness measurements — backlash alone can be misleading if the pinion has also worn significantly.

2.3 Pitting Area Assessment

Method: Visual inspection and photographic documentation during planned shutdowns.

Assessment procedure:

1. Clean the tooth surfaces thoroughly (pressure wash + solvent wipe)

2. Photograph a representative sample of teeth — minimum 10 consecutive teeth at 3 locations around the circumference

3. Estimate the percentage of tooth flank area affected by pitting

4. Classify pitting severity using the following scale:

2.4 Non-Destructive Testing (NDT) Schedule

For girth gears with significant service history, visual inspection alone is insufficient. Establish an NDT schedule:

Part 3: The Gear Flip Option — Doubling Service Life Without Full Replacement

Before committing to a full girth gear replacement, always evaluate the gear flip option. This is one of the most cost-effective maintenance strategies available for ball mill and rotary kiln girth gears, yet it is frequently overlooked by maintenance teams unfamiliar with the technique.

3.1 What Is a Gear Flip?

A girth gear flip (also called a gear turn or gear reversal) involves removing the girth gear from the mill shell, rotating it 180° about its axis (turning it face-for-face), and reinstalling it. The result is that the previously unworn tooth flanks — the non-drive-side flanks that have been carrying no load — become the new drive-side flanks.

Since the gear has been driving in one direction only throughout its service life, the non-drive-side flanks are essentially in new condition. After a flip, the gear effectively has a full set of unworn tooth surfaces available for service.

3.2 When Is a Gear Flip Appropriate?

A gear flip is appropriate when:

• ✅ The drive-side tooth flanks show moderate-to-advanced wear (tooth thickness reduction 20–30%)

• ✅ The non-drive-side tooth flanks are in good condition (confirmed by inspection after removal)

• ✅ The gear body (rim, web, hub, segment joints) is structurally sound — no cracks, no significant corrosion

• ✅ The tooth root zones show no cracks on MT inspection

• ✅ The gear is symmetrical — the tooth profile is the same on both flanks (standard involute profile), so the gear functions correctly when flipped

A gear flip is not appropriate when:

• ❌ Tooth root cracks are detected — flipping does not address root cracks

• ❌ The gear body has structural damage (cracked rim, cracked web)

• ❌ The gear has a non-symmetrical tooth profile (some helical girth gears have asymmetric profiles — check the drawing)

• ❌ The non-drive-side flanks show significant corrosion pitting from standing water or chemical attack

• ❌ The gear has already been flipped once — the original drive-side flanks are now the non-drive-side and will be in poor condition

3.3 Cost Comparison: Flip vs. Replace

For a gear with 25% tooth thickness reduction on the drive side and a sound gear body, a flip delivers approximately the same additional service life as a new gear at 5–15% of the cost. The economics are compelling in almost every case where the gear body is sound.

Part 4: Planning the Replacement Shutdown

Whether you are performing a gear flip or a full replacement, the shutdown planning process is the same. The difference is in the scope of work and the lead time for the new gear.

4.1 Lead Time Planning — The Most Critical Factor

The single most common cause of extended unplanned downtime in girth gear replacement projects is insufficient lead time for the new gear. A large girth gear is not a stock item — it is a custom-manufactured component that requires:

• Pattern or tooling preparation: 1–3 weeks

• Casting and solidification: 1–2 weeks

• Heat treatment: 1–2 weeks

• Rough machining: 2–4 weeks

• Gear cutting (hobbing or milling): 3–6 weeks (depending on module and size)

• Finish machining and inspection: 1–2 weeks

• NDT and quality documentation: 1 week

• Shipping (sea freight from China): 3–6 weeks

Total typical lead time: 16–26 weeks from drawing approval to arrival on site.

This means that the replacement decision must be made — and the order placed — at least 5–7 months before the planned replacement shutdown. For plants operating on annual shutdown cycles, this means the replacement order must be placed during or immediately after the current year's shutdown inspection, for execution at the following year's shutdown.

Practical planning timeline:

4.2 Scope of Work Definition

A girth gear replacement shutdown is a major project. Define the complete scope of work before the shutdown begins — scope creep during a shutdown is the primary cause of overruns.

Mandatory scope items (always include):

• Girth gear removal and installation

• Pinion inspection — assess whether pinion replacement is required simultaneously

• Pinion bearing inspection and replacement if required

• Girth gear mounting hardware (spring plates, tangential bolts) — replace as a set

• Segment joint bolts — replace with new high-strength bolts

• Drive guard inspection and repair

• Lubrication system cleaning and inspection

• Full post-installation alignment (backlash + contact pattern)

• Run-in procedure

Conditional scope items (assess during shutdown):

• Mill shell flange inspection and machining if required

• Trunnion bearing inspection

• Mill liner assessment

• Drive coupling inspection

4.3 Crane and Rigging Requirements

Girth gear replacement requires heavy lift capability. Confirm the following before the shutdown:

Weight estimation:

A rough estimate of girth gear weight can be made from:

$$W_{gear} \approx \frac{\pi}{4} \times (D_o^2 - D_i^2) \times b \times \rho \times 10^{-9}$$

Where $$D_o$$ = outer diameter (mm), $$D_i$$ = inner diameter (mm), $$b$$ = face width (mm), $$\rho$$ = material density (7,850 kg/m³ for cast steel).

For a segmented gear, divide by the number of segments to get the lift weight per segment — this is typically the governing figure for crane selection.

Typical segment weights:

• 2-segment gear, 6m diameter: 15–25 tonnes per segment

• 2-segment gear, 9m diameter: 35–60 tonnes per segment

• 4-segment gear, 9m diameter: 18–30 tonnes per segment

Confirm that the plant's crane capacity, boom radius at the mill location, and rigging attachment points are adequate for the heaviest lift. If not, arrange a mobile crane in advance — crane availability during a shutdown is not guaranteed without advance booking.

4.4 Shutdown Execution Sequence

Day 1–2: Preparation and disassembly

1. Lock out and tag out (LOTO) all energy sources — electrical, pneumatic, hydraulic

2. Drain and collect gear lubricant for disposal or recycling

3. Remove drive guard sections

4. Disconnect drive coupling between motor/gearbox and pinion

5. Remove pinion bearing housing hold-down bolts and move pinion clear of girth gear

6. Remove girth gear lubrication spray bars and nozzles

Day 2–4: Girth gear removal

1. Mark the angular position of the gear on the mill shell before removal (reference for reinstallation)

2. Remove segment joint bolts — keep bolts organized by joint for inspection

3. Remove spring plates or tangential mounting bolts

4. Rig the first segment — attach lifting equipment to the segment lifting lugs (confirm lifting lug capacity from drawing)

5. Lift the first segment clear and lower to the ground for inspection and disposal

6. Repeat for remaining segments

Day 4–7: Inspection and preparation

1. Inspect mill shell flange — measure flatness, check for corrosion, machine if required

2. Inspect spring plate mounting holes — repair any damaged threads

3. Inspect and clean all mounting surfaces

4. Inspect the pinion — measure tooth thickness, check for cracks (MT), assess bearing condition

5. Pre-assemble new gear segments on the ground — verify segment joint fit and step error before installation

Day 7–14: New gear installation

1. Install new spring plates or tangential mounting hardware

2. Lift first segment into position — align with reference marks on mill shell

3. Install segment joint bolts finger-tight

4. Lift and install remaining segments

5. Progressively torque segment joint bolts in the sequence specified on the drawing — do not fully torque any joint until all segments are in position

6. Final torque all segment joint bolts to specification

7. Measure radial and axial runout — verify within specification before proceeding

Day 14–17: Pinion reinstallation and alignment

1. Reinstall pinion bearing housing in approximate position

2. Perform full alignment procedure (backlash measurement, contact pattern analysis, bearing housing adjustment) — refer to the ball mill girth gear alignment guide

3. Install lubrication spray bars and nozzles

4. Reinstall drive coupling

5. Reinstall drive guard

Day 17–21: Run-in and verification

1. No-load run: 4 hours at reduced speed — monitor temperatures and noise

2. Partial load run: 24 hours at 50% charge — monitor and inspect

3. Full load run: 48 hours — final alignment verification

4. Document all measurements as the new baseline

Part 5: What to Specify When Ordering a Replacement Girth Gear

Getting the specification right is as important as getting the manufacturing right. An incorrectly specified gear — wrong module, wrong pressure angle, wrong material, wrong segment configuration — cannot be corrected after manufacturing. The following checklist covers every parameter that must be confirmed before placing an order.

5.1 Geometric Parameters (from Drawing or Measurement)

5.2 Material Specification

The material specification is the most important quality parameter — and the one most commonly under-specified in replacement orders. Do not simply specify "cast steel" — specify the complete material standard.

Standard materials for girth gears:

Always specify:

• Material grade and standard

• Heat treatment condition (normalized, or quenched and tempered — Q&T is preferred for high-duty applications)

• Minimum tensile strength, yield strength, and elongation

• Hardness range (HB) at specified test locations

• Charpy impact energy at operating temperature

5.3 Quality and Inspection Requirements

Specify the following inspection requirements in your purchase order:

Chemical analysis: Ladle analysis certificate for each heat of steel — verify compliance with specified grade

Mechanical properties: Test bars cast from the same heat, heat treated with the gear — tensile, yield, elongation, reduction of area, Charpy impact

Hardness survey: Brinell hardness at specified locations on the gear body and tooth flanks

Dimensional inspection: Full dimensional report including:

• Tooth thickness at pitch circle (minimum 3 locations per segment)

• Radial runout of assembled gear (measured on manufacturer's assembly fixture)

• Axial runout of assembled gear

• Segment joint step error (radial and axial)

• Mounting bolt hole position and diameter

NDT:

• Ultrasonic testing (UT): 100% of gear body per EN 12680-3 or equivalent — detect internal porosity and inclusions

• Magnetic particle inspection (MT): 100% of tooth root zones and segment joint areas per EN 1369 or equivalent

Gear accuracy: Tooth profile, pitch, and lead measurements per DIN 3962 or AGMA 2000 — specify accuracy class (Class 9 or better for most industrial girth gears)

5.4 The Reverse Engineering Option for Obsolete Gears

For mills where the original drawings are unavailable — common for equipment installed 20–40 years ago — reverse engineering is the only option. A competent manufacturer can reverse-engineer a replacement girth gear from the worn original, provided sufficient information is available.

What Yile Machinery needs for reverse engineering:

• The worn gear itself (preferred) — or high-quality dimensional measurements

• Tooth count (counted directly)

• Outer diameter measurement (at multiple points)

• Face width measurement

• Mounting bolt circle diameter and bolt count

• Clear photographs of the tooth profile, segment joints, and mounting hardware

• Any remaining nameplate data (module, material, manufacturer)

What we can determine from the worn gear:

• Module (from pitch diameter calculation)

• Pressure angle (from tooth profile measurement using optical comparator)

• Helix angle (from tooth lead measurement)

• Original tooth thickness (from span measurement, corrected for wear)

• Segment joint geometry

What cannot be determined from the worn gear alone:

• Original material specification — we will recommend the appropriate grade based on the application

• Original heat treatment — we will specify the correct treatment for the recommended material

Part 6: Should You Replace the Pinion at the Same Time?

This is one of the most frequently debated questions in girth gear replacement planning. The answer depends on the condition of the existing pinion and the economics of the situation.

The Case for Replacing the Pinion Simultaneously

Wear compatibility: A new girth gear has full tooth thickness and a correct involute profile. An old, worn pinion has reduced tooth thickness and a modified (worn) profile. When a new gear meshes with a worn pinion, the contact pattern will be incorrect — the new gear's correct profile will not mesh properly with the worn pinion's modified profile. This causes accelerated wear of the new gear from the first day of operation.

Hardness compatibility: Girth gears are typically softer than pinions (the pinion is usually 30–50 HB harder than the gear, to ensure the gear wears preferentially — the gear is cheaper to replace than the pinion shaft). If the existing pinion has worn to a point where its surface hardness differential is no longer correct, the new gear may wear faster than expected.

Shutdown economics: The incremental cost of replacing the pinion during a girth gear replacement shutdown is much lower than the cost of a separate shutdown later. The mill is already down, the drive is already disassembled, and the alignment work must be done regardless.

The Case for Retaining the Existing Pinion

Pinion condition is good: If the pinion tooth thickness reduction is less than 15% and there are no cracks or significant pitting, the pinion has substantial remaining life. Replacing it prematurely wastes a serviceable component.

Budget constraints: Pinion shafts for large mills are also expensive ($30,000–$150,000) and have lead times of 8–16 weeks. If the budget does not allow simultaneous replacement, prioritize the girth gear and plan the pinion replacement for the next shutdown.

Decision rule: Measure the pinion tooth thickness. If reduction is less than 20% and no cracks are detected, retain the pinion. If reduction is 20–30%, plan pinion replacement at the next shutdown. If reduction exceeds 30%, replace simultaneously with the girth gear.

Frequently Asked Questions

Q1: How long does a ball mill girth gear typically last?

Service life varies enormously depending on operating conditions, lubrication quality, alignment maintenance, and material quality. Under good conditions (correct alignment, adequate lubrication, correct material), a quality girth gear in a ball mill should last 15–25 years. Under poor conditions (misalignment, lubrication failures, abrasive contamination), service life can be as short as 5–8 years. The most important single factor is alignment — a misaligned gear can fail in 2–3 years regardless of material quality.

Q2: Can a girth gear be repaired rather than replaced?

Minor pitting (less than 5mm depth, less than 15% of tooth face area) can sometimes be repaired by weld repair and grinding, followed by MT inspection to confirm the repair is sound. However, weld repair of girth gear teeth is a specialist operation requiring pre-heat, controlled interpass temperature, post-weld heat treatment, and careful grinding to restore the tooth profile. It is not a field repair — it requires removal of the gear and work in a controlled workshop environment. For gears with advanced pitting or any tooth root cracks, repair is not viable and replacement is the only safe option.

Q3: How do I know if my girth gear needs to be replaced or just re-aligned?

If the primary symptom is vibration, noise, or uneven wear pattern — but tooth thickness is still within 20% of original and no cracks are detected — re-alignment is likely the correct first step. If tooth thickness has reduced by more than 25%, or if pitting covers more than 20% of the tooth face, replacement planning should begin regardless of alignment condition. The two issues are not mutually exclusive — a worn gear that is also misaligned needs both re-alignment (to stop accelerating the wear) and replacement planning (because the wear cannot be reversed).

Q4: What is the minimum information needed to order a replacement girth gear?

At minimum: number of teeth, module, outer diameter, face width, number of segments, and mounting bolt pattern. With these six parameters, a manufacturer can produce a replacement gear. However, for a fully correct replacement, pressure angle, helix angle, material specification, and accuracy class are also needed. If drawings are unavailable, provide the worn gear for measurement — or contact us with photographs and key dimensions for a preliminary assessment.

Q5: Can a girth gear be replaced without removing the mill shell?

Yes — this is one of the primary advantages of segmented girth gear construction. A 2-segment or 4-segment gear can be removed and installed one segment at a time, without removing the mill shell from its trunnion bearings. The mill shell remains in place on its foundation throughout the replacement. This is the standard replacement method for ball mills and rotary kilns in operating plants.

Q6: How much does a replacement ball mill girth gear cost?

Cost depends primarily on size (diameter and face width), module, material, and number of segments. As a rough guide:

• Small mill girth gear (diameter 3–5m): $80,000–$200,000

• Medium mill girth gear (diameter 5–8m): $200,000–$450,000

• Large mill girth gear (diameter 8–12m): $450,000–$800,000+

These are manufacturing costs ex-works China. Add shipping, import duties, and installation costs for total project cost. Contact jasmine@yileindustry.com with your gear specifications for a firm quotation.

Q7: What lead time should I plan for a replacement girth gear from Yile Machinery?

Standard lead time from drawing approval to ex-works shipment is 16–22 weeks for most girth gears. For very large gears (diameter > 10m, module > 45) or special material requirements, allow 22–28 weeks. For urgent breakdown replacements where the mill is already stopped, contact us immediately — we will assess expedited production options and provide a realistic timeline within 24 hours.

Yile Machinery: Your Partner for Girth Gear Replacement

Yile Machinery manufactures heavy-duty segmented girth gears for ball mills, SAG mills, rotary kilns, and dryers — from small 3-meter diameter gears to large 12-meter diameter assemblies. Our manufacturing capabilities include:

• In-house casting foundry with vacuum degassing (VD) capability for ZG42CrMo and other alloy grades

• Large-scale CNC gear hobbing and milling centers — capable of gears up to 12m diameter, Module 50

• Full heat treatment facility — normalizing, quenching and tempering, controlled atmosphere furnaces

• Comprehensive NDT — 100% UT and MT on all girth gears, with full documentation

• Precision assembly and inspection — radial and axial runout measured on our assembly fixture before shipment

• Reverse engineering capability — we can produce a replacement from your worn gear, photographs, or key dimensions

We also manufacture the matching pinion shafts — supplying gear and pinion as a matched, verified set eliminates profile compatibility issues and simplifies the alignment process.

To receive a quotation, provide:

• ✅ Gear drawings (preferred) or key dimensions (number of teeth, module, OD, face width, segments)

• ✅ Application: mill type, mill size, drive power

• ✅ Material specification (or describe application — we will recommend)

• ✅ Required delivery date

• ✅ Any special requirements (reverse engineering, expedited delivery, matched pinion)

Email: jasmine@yileindustry.com

Submit your RFQ: www.yilemachinery.com/contactus.html

All technical inquiries receive a response within 24 hours. For emergency breakdown situations, mark your message "URGENT" — same-business-day response guaranteed.

Related Products & Resources

• Heavy-Duty Girth Gears for Ball Mills, Kilns & Dryers — ZG42CrMo

• Segmented Girth Gears & Large Diameter Split Gears

• Precision Forged Steel Pinion Shafts

• How to Align a Girth Gear and Pinion on a Ball Mill

• Girth Gear for Rotary Kiln vs Ball Mill — Selection Guide

• How to Choose a Girth Gear Manufacturer

• Rotary Kiln Alignment Field Guide

• Cast Steel Riding Rings for Rotary Kilns

• Mining & Cement Industry Solutions

• Full Gears & Pinions Product Range