How to Align a Girth Gear and Pinion on a Ball Mill: Step-by-Step Technical Guide

A ball mill girth gear and pinion that are out of alignment do not simply wear faster — they destroy each other. Edge-loaded tooth contact concentrates the full transmitted force onto a fraction of the available tooth face, multiplying the contact stress by a factor of three to five compared to correctly aligned gears. The result is accelerated pitting, spalling, and ultimately tooth fracture — a failure mode that can write off a girth gear worth $200,000–$800,000 and shut down a concentrator or cement plant for four to eight weeks.

Yet girth gear misalignment is one of the most common preventable failures in heavy industry. The root cause is almost never a manufacturing defect in the gear itself. It is almost always the result of incorrect initial installation, inadequate post-installation verification, or alignment drift that was not detected and corrected during routine maintenance.

This guide provides the complete technical procedure for aligning a ball mill girth gear and pinion — from pre-alignment inspection through backlash measurement, tooth contact pattern analysis, runout correction, and final verification. It is written for maintenance engineers and reliability professionals who need actionable, field-proven procedures rather than general principles.

Why Girth Gear and Pinion Alignment Is Uniquely Challenging

Aligning a ball mill girth gear and pinion presents challenges that do not exist in conventional gearbox alignment:

Scale. A large ball mill girth gear may be 8–12 meters in diameter, weigh 30–80 tonnes, and have a module of 30–50. At this scale, even a 1mm positional error at the pinion bearing housing produces a tooth contact shift that would be catastrophic in a smaller gear set. 

Thermal and structural flexibility. The mill shell is not a rigid body. It deflects under the weight of the charge, expands thermally during operation, and can develop shell ovality over time. All of these effects change the position of the girth gear relative to the pinion after the mill starts running — meaning that a perfect cold alignment does not guarantee a correct hot alignment.

Segmented gear construction. Most large ball mill girth gears are manufactured in two or four segments, bolted together on the mill shell. The segment joints introduce the possibility of step errors (radial and axial discontinuities at the joint faces) that must be measured and corrected before alignment can be meaningful.

Dual-pinion drives. Many large mills use two pinions driving a single girth gear, one on each side. In this configuration, load sharing between the two pinions is critically dependent on alignment — a misaligned pinion will carry disproportionate load, accelerating its wear while the other pinion is underloaded.

Understanding these challenges is essential for interpreting measurement results correctly and for setting realistic alignment targets.

Essential Tools and Equipment

Before beginning any girth gear alignment work, confirm that the following instruments and equipment are available and calibrated:

Measurement instruments:

• Dial test indicators (DTI) with magnetic base stands — minimum resolution 0.01mm, range 0–10mm

• Feeler gauge set — range 0.05–3.00mm, calibrated

• Outside micrometer or vernier caliper — for tooth thickness measurement

• Laser alignment system or total station (for large mills where dial indicator reach is insufficient)

• Engineer's blue (marking compound) and brush — for tooth contact pattern assessment

• Infrared thermometer — for bearing temperature monitoring during run-in

Equipment:

• Hydraulic jacking equipment — for pinion bearing housing adjustment

• Precision shim stock — stainless steel, range 0.05–5.00mm

• Torque wrenches — for girth gear segment bolts and pinion bearing hold-down bolts

• Slow-speed drive (barring gear) — essential for rotating the mill under controlled conditions during alignment

Documentation:

• Mill general arrangement drawing showing nominal center distance, backlash specification, and pinion bearing housing adjustment range

• Girth gear manufacturing drawing showing tooth profile, module, pressure angle, and face width

• Previous alignment records (if available) — for trend comparison

Phase 1: Pre-Alignment Inspection

Never begin alignment work on a girth gear and pinion without first completing a thorough pre-alignment inspection. Attempting to align components that have underlying defects will produce incorrect results and may cause further damage.

1.1 Girth Gear Segment Joint Inspection

For segmented girth gears, inspect all segment joint faces:

Bolt torque verification: Check that all segment joint bolts are torqued to the specified value. Under-torqued joints allow the segments to shift relative to each other under load, making stable alignment impossible. Torque values are specified on the gear drawing — typical values for large girth gear joints are 800–2,000 Nm depending on bolt size.

Step error at joint faces: Using a dial indicator mounted on a fixed reference (not on the mill shell), measure the radial and axial step at each segment joint as the mill is slowly rotated through the joint. A step error greater than 0.3mm at the pitch circle indicates that the joint faces are not correctly aligned — this must be corrected before proceeding. [1]

Joint face gap: Inspect the segment joint faces visually and with feeler gauges for gaps. Any gap greater than 0.1mm indicates that the joint is not fully seated — recheck bolt torque and joint face condition.

1.2 Girth Gear Mounting Inspection

Spring plate or tangential bolt condition: Most girth gears are mounted to the mill shell via spring plates or tangential bolts that allow the gear to float slightly relative to the shell (accommodating differential thermal expansion). Inspect all spring plates for cracks, deformation, or looseness. Damaged spring plates cause the gear to shift position during operation, making stable alignment impossible.

Shell flange condition: Inspect the mill shell flange (the mounting surface for the girth gear) for corrosion, deformation, or debris. The flange must be clean and flat — any high spots will cause the gear to run with axial wobble (face runout) that cannot be corrected by pinion adjustment alone.

1.3 Tooth Surface Condition Assessment

Before measuring alignment, inspect the tooth surfaces of both girth gear and pinion for:

• Pitting and spalling: Note the location and distribution — is pitting concentrated at the tooth tips, roots, or one end of the face? The pattern reveals the nature of the misalignment.

• Scoring and scuffing: Indicates lubrication failure or excessive sliding velocity from misalignment.

• Plastic deformation (ridging): Indicates overloading — the tooth material has yielded under contact stress.

• Tooth fracture: Any fractured teeth must be documented and assessed for root cause before alignment proceeds.

Interpreting wear patterns before alignment: Wear concentrated at one end of the tooth face (edge loading) confirms axial misalignment. Wear concentrated at tooth tips indicates excessive backlash or incorrect tooth profile. Wear concentrated at tooth roots indicates insufficient backlash or profile error. These patterns guide where to focus alignment corrections.

1.4 Pinion Bearing Condition Check

Check pinion bearing temperatures (should be at normal operating temperature, not elevated) and listen for abnormal noise. Inspect bearing housing hold-down bolts for looseness. A pinion running on a failing bearing cannot be correctly aligned — the bearing must be replaced first.

Phase 2: Measuring Girth Gear Runout

Girth gear runout — the deviation of the gear from true circular rotation about the mill axis — is the foundation measurement for all subsequent alignment work. All other alignment parameters are meaningless if runout is not first quantified and, where possible, corrected. 

2.1 Radial Runout Measurement

Setup: Mount a dial indicator on a rigid, fixed support (not on the mill shell or any component that rotates with the mill). Position the indicator tip to contact the gear tooth tips (outside diameter) or, preferably, the gear pitch cylinder if a reference surface is available.

Procedure:

1. Rotate the mill slowly using the barring gear — one complete revolution at minimum

2. Record the dial indicator reading at every 10–15° of rotation (24–36 readings per revolution)

3. Mark the angular position of the maximum and minimum readings on the gear

4. Calculate total radial runout = maximum reading − minimum reading

Acceptance criteria:

• Excellent: ≤ 0.5mm TIR (Total Indicator Reading)

• Acceptable: 0.5–1.5mm TIR

• Caution: 1.5–3.0mm TIR — investigate cause; correct if possible

• Unacceptable: > 3.0mm TIR — must be corrected before proceeding with pinion alignment

Causes of excessive radial runout:

• Segment joint step errors (most common)

• Incorrect mounting of gear to shell flange

• Shell ovality causing the gear mounting diameter to be non-circular

• Gear manufacturing error (rare in quality-manufactured gears)

2.2 Axial Runout (Face Runout) Measurement

Setup: Reposition the dial indicator to contact the gear face — the side surface of the gear, as close to the pitch cylinder as possible.

Procedure: Same rotation procedure as radial runout — record readings every 10–15° through one complete revolution.

Acceptance criteria:

• Excellent: ≤ 0.5mm TIR

• Acceptable: 0.5–1.0mm TIR

• Caution: 1.0–2.0mm TIR

• Unacceptable: > 2.0mm TIR — causes the gear to wobble axially, driving the pinion in and out of correct mesh with each revolution

Causes of excessive axial runout:

• Shell flange not perpendicular to mill axis

• Debris or high spots on shell flange mounting surface

• Segment joint step errors in the axial direction

• Damaged or missing spring plates causing uneven gear seating

2.3 Runout Correction

If runout exceeds acceptable limits, the correction approach depends on the cause:

• Segment joint step errors: Adjust segment joint shims (if the design permits) or machine the joint faces. This requires specialist equipment and should be performed by the gear manufacturer or a qualified service provider.

• Shell flange issues: Machine the flange face to restore flatness and perpendicularity. This is a major intervention requiring mill shutdown and specialist machining equipment.

• Spring plate issues: Replace damaged spring plates and recheck.

Important: If runout cannot be corrected to within acceptable limits, the subsequent alignment measurements must account for the runout variation. The pinion must be positioned to give correct alignment at the average gear position, and the backlash specification must be widened to accommodate the runout variation.

Phase 3: Backlash Measurement and Adjustment

Backlash — the clearance between the non-driving tooth flanks of the meshing gear pair — is the most frequently measured and most commonly misunderstood alignment parameter in girth gear drives.

3.1 What Is the Correct Backlash?

Backlash serves three essential functions:

1. Prevents tooth interference — allows for thermal expansion of the gear and pinion without the teeth locking together

2. Provides space for the lubricant film — the lubricant that prevents metal-to-metal contact on the tooth flanks needs space to form

3. Accommodates manufacturing tolerances — small errors in tooth spacing and profile are absorbed by backlash

Calculating target backlash:

For large module open girth gear drives, the target backlash is typically specified on the gear drawing. As a general reference, the following formula is widely used in industry:

$$j_{min} = 0.03 \times m_n$$

$$j_{max} = 0.05 \times m_n$$

Where $$m_n$$ is the normal module in millimeters.

Example: For a Module 40 girth gear:

• Minimum backlash: $$0.03 \times 40 = 1.2\text{ mm}$$

• Maximum backlash: $$0.05 \times 40 = 2.0\text{ mm}$$

Always verify against the specific gear drawing — some manufacturers specify different backlash ranges based on their tooth profile design.

3.2 Measuring Backlash with Feeler Gauges

Procedure:

1. Rotate the mill to position a tooth mesh point at the most accessible location (typically the side of the mill, at the 3 o'clock or 9 o'clock position)

2. With the mill stationary and the drive locked out, insert feeler gauges between the non-driving flanks of a meshing tooth pair

3. Select the thickest feeler gauge combination that slides through the gap with light resistance — this is the backlash at that point

4. Record the measurement and the angular position of the girth gear

5. Rotate the mill to bring the next measurement point into position — measure at minimum 4 positions equally spaced around the gear circumference (0°, 90°, 180°, 270°)

6. For a segmented gear, also measure immediately before and after each segment joint

Interpreting backlash variation:

• Backlash variation around the circumference = radial runout of the girth gear

• If maximum backlash − minimum backlash ≈ 2 × radial runout: this is expected and correct

• If variation exceeds 2 × measured runout: investigate for segment joint errors or pinion bearing looseness

3.3 Adjusting Backlash

Backlash is adjusted by moving the pinion bearing housing radially toward or away from the girth gear center:

• Too much backlash (center distance too large): Move pinion bearing housing toward the girth gear. Remove shims from under the bearing housing base, or adjust the radial positioning screws if provided.

• Too little backlash (center distance too small): Move pinion bearing housing away from the girth gear. Add shims under the bearing housing base.

Adjustment increment guidance:

• 1mm of radial pinion movement changes backlash by approximately $$2 \times \sin(\alpha)$$ where $$\alpha$$ is the pressure angle

• For a 20° pressure angle gear: 1mm radial movement ≈ 0.68mm backlash change

• For a 25° pressure angle gear: 1mm radial movement ≈ 0.85mm backlash change

Make adjustments in small increments (0.5–1.0mm maximum per adjustment) and re-measure after each adjustment.

Phase 4: Tooth Contact Pattern Analysis

Backlash measurement confirms the center distance is correct, but it tells you nothing about whether the gear axes are parallel or whether the contact is distributed correctly across the tooth face. Tooth contact pattern analysis is the definitive test of girth gear and pinion alignment.

4.1 Applying Marking Compound

Procedure:

1. Clean the tooth surfaces of both girth gear and pinion thoroughly — remove all lubricant, grease, and debris from at least 10 consecutive teeth on each component

2. Apply a thin, uniform coat of engineer's blue (Prussian blue marking compound) to the pinion teeth only — 6–10 consecutive teeth

3. Apply the compound with a brush or roller to achieve a uniform film approximately 0.05–0.10mm thick — too thick gives a misleading pattern; too thin gives insufficient transfer

4. Rotate the mill slowly through the marked teeth using the barring gear — one pass through the mesh is sufficient

5. Examine the transfer pattern on the girth gear teeth — the blue transferred from the pinion shows the actual contact zone

4.2 Interpreting the Contact Pattern

The contact pattern tells you everything about the alignment condition. Learn to read it correctly:

✅ Correct alignment — ideal contact pattern:

• Contact covers 70–80% of the tooth face width

• Contact is centered on the tooth face (not shifted to either end)

• Contact extends from approximately 30% tooth height to 70% tooth height (centered on the pitch line)

• Pattern is uniform — no isolated high spots or gaps within the contact zone

❌ Pattern shifted to one end of the tooth face (edge loading):

• Contact concentrated at the drive-end or non-drive-end of the tooth

• Indicates axial misalignment — the pinion axis is not parallel to the girth gear axis in the axial plane

• Correction: Adjust the axial position of one pinion bearing housing (move one end of the pinion shaft axially) to bring the axes into parallel alignment

❌ Pattern concentrated at tooth tips:

• Contact on the addendum (tip) of the driving gear teeth

• Indicates excessive center distance (too much backlash) or profile error

• Correction: If backlash is within specification, the profile may be worn — assess tooth thickness. If backlash is excessive, reduce center distance.

❌ Pattern concentrated at tooth roots:

• Contact on the dedendum (root) of the driving gear teeth

• Indicates insufficient center distance (too little backlash) or profile error

• Correction: Increase center distance to achieve correct backlash. Check for interference.

❌ Diagonal contact pattern:

• Contact band runs diagonally across the tooth face

• Indicates combined radial and axial misalignment — the pinion axis is skewed relative to the girth gear axis in both planes simultaneously

• Correction: Requires simultaneous adjustment of both radial position and axial parallelism — the most complex alignment condition

❌ Intermittent or spotted contact:

• Contact appears as isolated spots rather than a continuous band

• Indicates surface irregularities — high spots on tooth flanks from manufacturing error, previous damage, or uneven wear

• Correction: If the gear is new, contact the manufacturer. If the gear is worn, the high spots may need to be dressed by a qualified gear specialist.

4.3 Quantifying Contact Pattern Coverage

After interpreting the pattern qualitatively, quantify the contact coverage:

$$\text{Face contact ratio} = \frac{\text{Contact width (mm)}}{\text{Total face width (mm)}} \times 100%$$

Minimum acceptable face contact ratio:

• New installation: ≥ 70%

• Run-in period (first 500 hours): ≥ 50% (contact will improve as surfaces bed in)

• Established operation: ≥ 60% (some wear of high spots is normal and acceptable)

If face contact ratio is below 50% on a new installation, do not proceed to full-load operation — the gear is not correctly aligned and damage will occur rapidly.

Phase 5: Pinion Bearing Housing Adjustment Procedure

With the measurement data from Phases 2–4, you now have a complete picture of the alignment condition. This phase covers the physical adjustment procedure for the pinion bearing housing.

5.1 Understanding the Adjustment Degrees of Freedom

A pinion bearing housing typically has four adjustment degrees of freedom:

For dual-pinion mills, each pinion has its own bearing housing with the same four degrees of freedom — plus the additional requirement that both pinions share load equally.

5.2 Adjustment Sequence

Always follow this sequence — adjusting in the wrong order creates interactions that make convergence difficult:

Step 1: Correct axial runout of the girth gear first

If axial runout exceeds 1.0mm TIR, address the root cause (spring plates, flange condition) before adjusting the pinion. A pinion correctly aligned to a wobbling gear will be incorrectly aligned once the wobble is corrected.

Step 2: Set approximate radial position (backlash)

Adjust the pinion radial position to achieve backlash in the middle of the specified range. This is a coarse adjustment — you will refine it after setting the contact pattern.

Step 3: Set axial parallelism (contact pattern end shift)

If the contact pattern is shifted to one end of the tooth face, adjust the axial position of the appropriate pinion bearing housing end:

• Pattern shifted to drive end: move the drive-end bearing housing axially away from the gear (or move the non-drive end toward the gear)

• Pattern shifted to non-drive end: opposite adjustment

• Adjustment increment: 0.5–1.0mm per step; re-apply marking compound and re-check after each adjustment

Step 4: Refine radial position

After correcting axial parallelism, re-measure backlash — the axial adjustment may have slightly changed the effective center distance. Refine the radial position to bring backlash back to the target value.

Step 5: Verify contact pattern

Apply fresh marking compound and re-check the contact pattern. The pattern should now show centered contact covering ≥ 70% of the face width. If not, identify which misalignment mode remains and repeat the appropriate adjustment.

Step 6: Check and tighten all fasteners

After achieving correct alignment, torque all pinion bearing housing hold-down bolts to specification. Re-check backlash after tightening — bolt tightening can shift the housing slightly.

5.3 Shimming Procedure for Bearing Housing Adjustment

Radial adjustment (shims under bearing housing base):

1. Calculate required shim change from backlash measurement

2. Loosen (do not remove) bearing housing hold-down bolts

3. Use hydraulic jacks to lift the bearing housing slightly — sufficient to remove/add shims

4. Remove or add shim stock to achieve the calculated position change

5. Lower the housing onto the shims and snug the hold-down bolts

6. Re-measure backlash before final torquing

Shim selection guidance:

• Use stainless steel shim stock — do not use soft metals (copper, aluminum) that will creep under load

• Use the minimum number of shims — a stack of many thin shims is less stable than fewer thick shims

• Ensure shims cover at least 80% of the bearing housing base area — do not use small shims that concentrate load

Phase 6: Run-In Verification and Ongoing Monitoring

Correct static alignment does not guarantee correct dynamic alignment. The mill must be run in under controlled conditions to verify that the alignment is maintained under operating load and temperature.

6.1 Initial Run-In Procedure

Stage 1: No-load run (empty mill), 2–4 hours

• Start the mill at reduced speed (50% of normal operating speed if variable speed drive is available)

• Monitor pinion bearing temperatures every 15 minutes — should stabilize below 65°C

• Listen for abnormal noise — clicking, grinding, or periodic impact sounds indicate tooth interference or contact problems

• After 1 hour, stop and re-check tooth contact pattern — the run-in contact should show improvement over the static pattern

Stage 2: Partial load run, 8–24 hours

• Charge the mill to 30–50% of normal ball charge

• Run at normal operating speed

• Monitor bearing temperatures continuously

• After 8 hours, stop and inspect tooth surfaces — look for evidence of correct contact (polished contact band) and absence of distress (scoring, pitting)

Stage 3: Full load run, 48–72 hours

• Charge to normal operating level

• Monitor bearing temperatures and vibration levels

• After 48 hours, stop and perform a full alignment re-check — measure backlash at 4 positions and re-apply marking compound for contact pattern verification

• Document all measurements as the baseline for future maintenance reference 

6.2 Alignment Monitoring Schedule

Girth gear alignment is not a one-time activity. Establish a regular monitoring schedule:

6.3 Key Indicators of Developing Misalignment

Train your operators and maintenance team to recognize these early warning signs:

• Increasing vibration at the mill drive end — particularly at the gear mesh frequency (shaft RPM × number of pinion teeth)

• Rising pinion bearing temperature — particularly if one bearing runs hotter than the other on the same pinion shaft

• Unusual noise — a periodic "clunk" or "thud" at each revolution of the girth gear indicates a local problem (segment joint step, damaged tooth, or severe runout)

• Lubricant condition deterioration — increased metal particle content in the gear lubricant indicates accelerated tooth wear

• Visible wear pattern shift — if the polished contact band on the teeth moves toward one end of the face, axial misalignment is developing

Common Mistakes in Girth Gear Alignment — and How to Avoid Them

Mistake 1: Aligning cold without accounting for thermal growth

The mill shell, girth gear, and pinion all expand thermally when the mill reaches operating temperature. For a large ball mill, the thermal expansion of the mill shell diameter can shift the girth gear center position by 1–3mm relative to the cold position. If the pinion is aligned to the cold gear position, it will be misaligned in operation.

Solution: Either perform alignment at operating temperature (hot alignment, as described in this guide), or calculate the expected thermal growth and pre-offset the cold alignment accordingly. The thermal offset should be calculated from the mill shell material (typically carbon steel, coefficient of thermal expansion ≈ 12 × 10⁻⁶ /°C) and the expected temperature rise.

Mistake 2: Measuring backlash at only one position

Measuring backlash at a single point and declaring the alignment complete is the most common alignment error. Backlash varies around the circumference due to gear runout — a single measurement may happen to fall at the maximum or minimum point, giving a completely misleading picture of the average center distance.

Solution: Always measure at minimum 4 positions, 90° apart. Calculate the average backlash and the variation. The average should be within the specified range; the variation should be consistent with the measured runout.

Mistake 3: Ignoring segment joint condition before alignment

Attempting to align a girth gear that has loose segment joint bolts or step errors at the joint faces is futile — the gear position changes every time the joint passes through the mesh, making stable alignment impossible.

Solution: Always inspect and correct segment joint condition as the first step of any alignment campaign, before any other measurements are taken.

Mistake 4: Over-tightening bearing housing bolts before final verification

Tightening the pinion bearing housing hold-down bolts to full torque before verifying the final contact pattern is a common mistake that wastes time. The act of tightening the bolts can shift the housing position by 0.2–0.5mm, changing the backlash and potentially the contact pattern.

Solution: Snug the bolts (hand-tight plus one quarter turn) for all intermediate measurements. Only torque to final specification after the contact pattern and backlash are both confirmed correct. Then re-check backlash one final time after torquing.

When to Replace vs. Re-Align: Decision Framework

Not every girth gear alignment problem can be solved by adjusting the pinion position. Use this decision framework to determine the correct course of action:

Yile Machinery: Precision-Manufactured Girth Gears Built for Correct Alignment

A girth gear can only be correctly aligned if it was correctly manufactured. Dimensional errors in the gear — runout, tooth spacing error, profile error — create alignment problems that no amount of pinion adjustment can fully correct.

Yile Machinery manufactures heavy-duty segmented girth gears for ball mills, SAG mills, and rotary kilns to the following quality standards that directly support correct field alignment:

• Radial runout of finished gear: ≤ 0.5mm TIR (measured on our precision vertical lathe before shipment)

• Axial runout of finished gear: ≤ 0.5mm TIR

• Segment joint step error: ≤ 0.1mm (controlled by precision machining of joint faces as a matched set)

• Tooth spacing error: Per DIN 3962 accuracy class 9 or better

• Material: ZG42CrMo alloy cast steel, vacuum degassed (VD), with full chemical and mechanical property certification

• NDT: 100% ultrasonic testing (UT) + magnetic particle inspection (MT) on all tooth root zones and segment joint areas

Every girth gear ships with a complete dimensional inspection report — including runout measurements, tooth spacing data, and segment joint step measurements — so your alignment team knows exactly what to expect before the gear arrives on site.

For segmented girth gears requiring field installation without mill disassembly, we manufacture matched segment sets with precision-machined joint faces and supply full installation instructions.

We also manufacture the matching pinion shafts for ball mill and kiln drives — supplying gear and pinion as a matched, verified set eliminates the most common source of alignment difficulty: geometric incompatibility between gear and pinion from different manufacturers.

Frequently Asked Questions

Q1: What is the correct backlash for a ball mill girth gear?

Correct backlash depends on the gear module. The general industry guideline is 0.03–0.05 × module (normal). For example, a Module 36 girth gear should have 1.08–1.80mm backlash. Always verify against the specific gear drawing — some manufacturers specify different values. Measure at 4 positions around the circumference and use the average; variation around the circumference reflects gear runout, which is normal and expected.

Q2: How often should girth gear backlash be measured?

At minimum, measure backlash quarterly and after any maintenance event that could affect alignment (foundation work, bearing replacement, mill shell repair). If the mill shows increasing vibration or noise, measure immediately. Backlash increases as teeth wear — a progressive increase over time is normal; a sudden large change indicates a problem.

Q3: Our contact pattern shows good coverage in the center but poor at both ends. What does this mean?

This "hourglass" or "center-only" contact pattern indicates that the pinion shaft is deflecting under load, causing the teeth to contact only at the midpoint of the face. This is a structural problem — the pinion shaft is undersized for the applied load, or the bearing span is too wide. Alignment adjustments cannot correct this. Contact the equipment manufacturer or a gear specialist for assessment.

Q4: We have a dual-pinion mill and one pinion is running much hotter than the other. What is the cause?

Unequal bearing temperatures in a dual-pinion drive almost always indicate unequal load sharing — one pinion is carrying more than 50% of the total drive torque. This is caused by a difference in the center distance (backlash) between the two pinions. Measure backlash on both pinions — the one running hotter will typically have less backlash (closer to the gear). Adjust the hotter pinion slightly outward (increase its backlash by 0.3–0.5mm) and monitor temperatures.

Q5: How long does a full girth gear alignment take?

A complete alignment campaign — including pre-inspection, runout measurement, backlash measurement, contact pattern analysis, adjustments, and run-in verification — typically takes 3–5 days for a single-pinion mill and 5–8 days for a dual-pinion mill. This assumes no major corrective work (segment joint correction, foundation repair) is required. Plan accordingly when scheduling planned maintenance shutdowns.

Q6: Can we perform girth gear alignment ourselves, or do we need a specialist?

The measurement procedures described in this guide can be performed by a competent maintenance team with the correct instruments. However, interpreting complex contact patterns, diagnosing root causes of excessive runout, and managing dual-pinion load sharing require experience. For initial installation alignment or after a gear replacement, we recommend engaging a specialist for at least the measurement and interpretation phases, with your team performing the physical adjustments under guidance.

Q7: What information do I need to provide to get a quote for a replacement ball mill girth gear?

Provide: mill make and model, girth gear outer diameter, number of teeth, module, face width, number of segments, material grade (if known), and whether you need a matched pinion. If drawings are available, please include them. If not, we can work from key dimensions. Contact sales@yilemachinery.com — we respond to all technical inquiries within 24 hours.

Get Expert Support for Your Ball Mill Girth Gear

Whether you need a replacement girth gear manufactured to your drawings, a matched gear-and-pinion set, or technical support for a difficult alignment problem, Yile Machinery's engineering team is ready to help.

Email: jasmine@yileindustry.com

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

All technical inquiries answered within 24 hours. For urgent breakdown situations, mark your message "URGENT" for same-business-day response.

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