Liner bolts are critical components for securing the liners of a ball mill. Once they fracture, the consequences can range from liner loosening and shifting (which accelerates wear on both liners and the shell) to more severe outcomes such as liner detachment, damage to the discharge grate or lifting equipment, and even shell deformation — a major equipment failure. Therefore, it is essential to accurately identify the causes of fracture and to take prompt and appropriate remedial measures.

I. Analysis of Fracture Causes

Liner bolt fracture is rarely caused by a single factor; it is usually the result of multiple combined effects. The main causes include:

1. Stress and Fatigue

• Alternating impact loads: During Kugelmühle operation, grinding media and material exert periodic impact and compression on the liners. The bolts are subjected to long‑term alternating tensile, bending, and shear stresses. Micro‑cracks initiate at stress concentration points and gradually propagate, eventually leading to fatigue fracture — the most common cause.
• Stress concentration: A too‑small transition radius between the bolt head and shank, or sharp roots at the thread, increases the stress concentration factor and makes these locations prone to becoming the source of fatigue cracks.

2. Improper Pre‑Tightening Force

• Excessive pre‑tightening: Overtightening during installation brings the initial tensile stress close to or above the yield strength. When superimposed with impact loads during operation, the stress exceeds the ultimate strength and causes fracture.
• Insufficient pre‑tightening or loosening: If bolts are not tightened to the required torque, or if locking measures fail during operation (e.g., missing lock washers, loosened nuts), gaps appear between the liner and the shell. Relative movement of the liner subjects the bolts to additional bending and shear forces, eventually leading to shear or bending fracture.
• Uneven loading among bolts: Fastening is not carried out in a symmetrical and sequential manner, causing some bolts to become overloaded.

3. Liner and Structural Factors

• Plastic flow or deformation of liners: Liners made of materials such as high‑manganese steel may undergo plastic flow under impact, leading to bulging, buckling, or thickness reduction. This squeezes the bolt head or shank, imposing large additional tensile and bending forces that eventually pull or shear the bolt apart.
• Worn or misaligned countersunk holes: Over long operation, the countersunk holes in the liners may become enlarged or misaligned, resulting in uneven support for the bolt head and creating additional bending stress in the bolt.
• Clogged or seized gaps between liners: Material or broken steel balls become wedged into the gaps between liners. Thermal expansion or material compaction may cause adjacent liners to press against each other, pulling the bolts beyond their limit.

4. Manufacturing and Material Defects

• Substandard material quality: Bolts made from insufficiently strong materials (e.g., using ordinary carbon steel instead of alloy steel) or with improper heat treatment (poor quenching and tempering) result in inadequate strength or toughness.
• Processing defects: Forging cracks, folds, excessive decarburisation layers, burrs on threads, or incomplete thread profiles.
• Aged sealing gaskets: Heat‑resistant rubber or asbestos gaskets between the bolt and the shell may age and collapse, causing bolt loosening or sealing failure. Powder penetration into the bolt hole then accelerates wear.

5. Environmental and Corrosion Factors

• Korrosion: In wet mills or when grinding corrosive materials, cooling water or slurry may seep along the bolt clearance, causing electrochemical corrosion at the thread. Corrosion pits form and act as initiation sites for fatigue cracks, accelerating fracture (stress corrosion cracking).
• High‑temperature effects: In drying mills or when ventilation is poor, high internal temperatures cause bolt materials to creep and lose strength. Additionally, thermal expansion generates extra stresses that increase fracture risk.

II. Behandlungsmethoden

Treatment of liner bolt fracture depends on the fracture location, the condition of the broken part, and the operational status of the mill. Different emergency repair and corrective measures are available.

(A) Routine Preventive Maintenance Measures (Fundamental Solutions)

1. Select high‑quality high‑strength bolts: Use high‑strength alloy steel (e.g., 42CrMo, 40Cr) with a quenched and tempered treatment, and a strength grade not lower than Grade 8.8 oder 10.9. Strictly control thread machining quality.
2. Standardise installation and tightening:
   • Tighten bolts in a symmetrical cross‑pattern in 2–3 steps to ensure uniform loading.
   • Use a Drehmomentschlüssel to tighten to the specified torque; do not over‑tighten by feel.
   • Fit locking washers (e.g., spring washers, Belleville washers) or use double nuts to prevent loosening.
3. Enhance sealing protection: Fit heat‑resistant rubber or asbestos sealing rings between the bolt head and the liner countersunk hole, and between the shell outer wall and the nut, to prevent slurry or dust ingress and thread corrosion.
4. Regular inspection and re‑torquing: After new liner installation or replacement, re‑torque all bolts within 24 hours of operation. During routine inspections, check for nut looseness and abnormal wear of the countersunk hole. Schedule regular shutdown inspections.
5. Select appropriate liner materials: In the high‑impact first compartment, avoid using high‑manganese steel liners that are prone to plastic flow. Instead, use medium‑carbon multi‑alloy steels with better deformation resistance to reduce squeezing forces on the bolts.

(B) Emergency Treatment for Loose or Slightly Bent Bolts

• If routine inspection reveals that a nut has loosened but the bolt is not yet broken, immediately re‑tighten it to the specified torque, check the sealing gasket, and replace it if necessary.
• If the bolt shank is slightly bent but not broken, it should be replaced. Do not straighten and reuse it, as bending will have caused internal damage.

(C) Repair Methods for Broken Bolts

Depending on the condition of the broken end remaining in the shell bolt hole, the following methods can be used to extract it:

Method 1: Welding (most common and reliable)

• Applicable when the broken end protrudes from the outer surface of the shell, or is flush with the hole surface.
• Procedure:
  ① Clean oil and dust from around the fracture.
  ② Place a nut (or washer) slightly smaller than the broken end over it, and weld it firmly to the broken end using electric arc welding.
  ③ While still hot, use a wrench to turn the nut slowly and steadily in the reverse direction to unscrew the broken piece.
  ④ If unsuccessful, repeat with a larger nut.

Method 2: Drilling and tapping (using a screw extractor)

• Applicable when the broken end is completely inside the shell with no exposed end, but the thread hole is still relatively intact.
• Procedure:
  ① Centre‑drill a pilot hole at the centre of the broken end.
  ② Drill through the broken piece with a drill bit smaller than the minor diameter of the original hole.
  ③ Insert a left‑hand thread tap (reverse tap) into the drilled hole. Once it wedges, continue turning in reverse to extract the broken piece.
  ④ Alternatively, use a dedicated “broken bolt extractor” (reverse spiral extractor).

Method 3: Enlarge and re‑tap (last resort)

• Applicable when the broken piece is rusted‑in, welded solid, or the thread is severely damaged, making the above methods unworkable.
• Procedure:
  ① Use a drill bit larger than the original hole diameter to drill out the old hole and broken piece together.
  ② Re‑tap with a larger tap (e.g., M30 to M36) and use a high‑strength bolt of the new size.
  ③ If the shell thickness does not permit enlarging, repair by welding the hole closed, re‑drilling, and re‑tapping to the original size.

Method 4: Remove and replace with a new bolt

• If the broken piece is deeply embedded and the hole is severely deformed, cut a small access window on the outside of the shell (after assessing structural integrity), remove the broken piece from inside, clean the hole, weld the window closed and grind smooth, then install a new bolt.

(D) Quality Requirements After Treatment

• The replacement bolt must be of the same specification as the original (or the new specification after enlargement) and have a strength grade no lower than the original design.
• Before installing the new bolt, thoroughly clean any chips, oil, or cement mortar residue from the hole.
• After replacement, re‑check and re‑torque all bolts on the same liner ring to ensure uniform pre‑tension.
• After restart, intensify inspections of that area during initial operation, and re‑torque again after 24 hours.