Anchor bolt fracture in a reducer (gearbox) is a common equipment failure in the cement, mining, and other industries. In severe cases, it can cause displacement of the reducer, damage to gears, or even scrapping of the entire unit. Below is a systematic analysis of the causes and the corresponding treatment methods.

I. Cause Analysis

(1) Design and Selection Issues

1. Insufficient bolt diameter or insufficient number of bolts: The design did not fully consider the torque, vibration, and impact loads during reducer operation, resulting in individual bolts being subjected to excessive stress beyond their load‑bearing capacity.
2. Improper bolt material selection: Bolts of insufficient strength grade (e.g., 8.8 or 10.9 grade) were not selected, and instead lower‑strength bolts (e.g., 4.8 or 5.6 grade) were used, which are prone to fatigue fracture under alternating loads.
3. Inadequate locking design: Only ordinary spring washers were used without effective locking methods such as double nuts, locking washers, stop washers, or anaerobic adhesives, allowing bolts to gradually loosen under vibration and eventually fracture.

(2) Installation and Foundation Issues

1. Uneven foundation surface: Poor contact between the reducer base mounting surface and the foundation surface, with local voids or high spots. After tightening the bolts, the base deforms, subjecting the bolts to additional bending stress, making them prone to fracture at the thread root or the head‑shank transition.
2. Improper shim placement: Insufficient shims, improper positioning, or failure to level the equipment results in unstable installation and vibration during operation, aggravating bolt fatigue damage.
3. Deviation in anchor bolt holes: Misalignment between the reserved holes and the equipment base holes forces the bolts into an inclined or strained state during installation, creating additional bending moments.
4. Poor quality secondary grouting: Low grout strength, lack of compaction, or excessive shrinkage reduces foundation load‑bearing capacity, causing bolts to bear uneven tensile forces during operation.

(3) Operation and Load Issues

1. Excessive reducer vibration: Abnormal vibration caused by poor gear meshing, bearing damage, coupling misalignment, or dynamic imbalance is one of the most important causes of anchor bolt loosening and fatigue fracture.
2. Excessive impact loads: Frequent mill starts and stops, uneven feed, or “saturated grinding” conditions cause the reducer to withstand severe impacts, with instantaneous peak loads far exceeding the rated strength of the bolts.
3. Uneven output torque: Uneven distribution of grinding media or material bias inside the mill shell subjects the reducer to periodic alternating torque, causing fatigue fracture of the bolts under alternating stress.

(4) Maintenance and Inspection Issues

1. Failure to retighten periodically: After equipment commissioning, bolts often loosen due to vibration and foundation settlement. If not retightened as required, they gradually loosen and eventually fracture.
2. Failure to detect loosening in time: Routine inspections neglect the condition of anchor bolts. When some bolts loosen, the load on adjacent bolts increases sharply, leading to a chain reaction of multiple bolt fractures.
3. Non‑standard replacement: Only the broken bolt is replaced without also replacing other bolts in the same group that may have already fatigued or have hidden defects. The uneven stress distribution causes the newly replaced bolt to break again soon after.

(5) Corrosion and Manufacturing Defects

1. Stress concentration at thread root: Insufficient fillet radius at the thread root or the presence of machining marks creates stress concentration points, which easily become sources of fatigue cracks under alternating loads.
2. Heat treatment defects: Improper quenching and tempering processes result in either excessive hardness (brittleness) or insufficient strength, or the presence of a decarburized layer, reducing fatigue resistance.
3. Corrosion: In humid environments or those with corrosive gases, the threaded portion of the bolt may rust or suffer stress corrosion cracking, accelerating fracture.

II. Treatment Methods

(1) Emergency Response Methods

1. Stop for inspection: Once anchor bolt fracture is found, stop the mill immediately (if an emergency stop is not feasible and short‑time operation is possible, run at reduced load until a planned stop). Thoroughly inspect the remaining bolts and the reducer condition.
2. Extract the broken bolt:
   • If the broken end protrudes above the foundation surface, use a pipe wrench or a special bolt extractor to remove it.
   • If the broken end is flush with or below the foundation surface, use the reverse tap extraction method: first drill a centering hole with a center drill, then drill to the appropriate size, and finally use a left‑hand thread tap to remove the remaining bolt shank.
   • If the broken bolt is severely rusted and cannot be removed, drill and tap a new hole nearby and install a new bolt.
3. Clean and repair: Remove oil, rust, and residual grout from the foundation surface and the bottom face of the reducer base. Check the base flatness and, if necessary, correct it with an angle grinder.
4. Replace with new bolts:
   • Selection principle: The material and strength grade of the new bolts shall not be lower than the original design requirements; 8.8 or 10.9 grade high‑strength bolts are generally recommended.
   • Torque control: Use a torque wrench for tightening, applying the specified preload torque in stages (usually 3–4 steps) and in a symmetrical sequence to avoid excessive or uneven preload from one‑step tightening.
   • Locking measures: Use double nuts, nylon insert lock nuts, Spiralock locking threads, or anaerobic adhesives (e.g., Loctite 243) to enhance locking.

(2) Fundamental Corrective Methods

1. Re‑align and center:
   • Check and adjust the coupling alignment between the motor and reducer, and between the reducer and the mill. Generally, the radial deviation should be ≤0.05 mm, and the axial deviation ≤0.03 mm per 100 mm.
   • Also check and correct the concentricity between the hollow shaft and the reducer output shaft.
2. Address foundation issues:
   • If the foundation is severely uneven, lift the reducer, re‑arrange the shims (no more than 3 shims per group, and they must be tack‑welded), and level using a spirit level to ensure the base meets the flatness requirement.
   • For secondary grouting, use non‑shrink high‑strength grout (such as epoxy grout or cement‑based high‑strength grout). The grout layer thickness should generally be 40–80 mm, and bolts should only be tightened after the grout has cured to the design strength.
3. Eliminate vibration sources:
   • Inspect gear meshing inside the reducer, adjusting the backlash and contact pattern (contact area should be ≥60% along the tooth length and ≥45% along the tooth height).
   • Inspect bearing wear and replace damaged bearings if necessary.
   • Check whether the mill shell is balanced and whether the grinding media distribution is uniform.
4. Add dowel pins (fitted bolts):
   • Install fitted bolts (dowel pins) at diagonal positions on the reducer base to carry horizontal shear forces, so that the anchor bolts bear only axial tension, significantly reducing the risk of bolt fracture.
   • This measure is usually implemented together with foundation modification and provides the most reliable results.
5. Reinforce foundation rigidity:
   • For concrete foundations with insufficient rigidity, plant additional reinforcing bars around the foundation and increase the grout layer thickness, or add a steel base transition plate to distribute loads and improve overall rigidity.

(3) Preventive Maintenance Measures

1. Establish a regular retightening schedule:
   • For newly installed or overhauled reducers, retighten the anchor bolts after 24 hours, 72 hours, and one week of operation.
   • During normal operation, inspect and retighten at least once a month, and keep records.
2. Regular vibration monitoring:
   • Use a simple vibration meter or online vibration monitoring system to regularly measure the reducer vibration velocity (generally required to be ≤4.5 mm/s). If a significant increase is found, investigate the cause immediately.
   • Use an infrared thermometer to check bearing temperatures as supplementary information for condition assessment.
3. Recording and early warning:
   • Establish an equipment log to record each retightening torque value, bolt replacement history, vibration data, etc. Trend analysis can help identify potential problems early.

III. Summary

Anchor bolt fracture is rarely an isolated problem; its root causes are often closely related to foundation condition, alignment accuracy, and vibration levels. Therefore, treatment should not simply “treat the symptom” by replacing bolts; instead, a systematic approach should be taken, covering installation quality, operating conditions, and foundation rigidity, to solve the problem fundamentally.