Connecting bolts are core load-bearing components for rigid connection between slewing bearings and upper/lower frames. Bolt fracture is a high-risk failure: in mild cases, it causes bearing displacement, slewing jamming and gear tooth damage; in severe cases, it triggers major safety accidents such as upper structure falling and machine overturning. Many maintenance workers simply attribute bolt fracture to poor material quality. In fact, five categories of factors including material defects, design selection, installation process, environmental corrosion and alternating fatigue can induce fracture, with different causes corresponding to distinct fracture surface features. This article systematically analyzes the five core causes and fracture identification methods, with comprehensive preventive measures for full-process control from selection, installation to operation, reducing bolt fracture risk from the source.
1. Five Core Causes of Bolt Fracture and Corresponding Fracture Surface Features
1. Material and Manufacturing Defects
Causes: Impurities in bolt steel, substandard alloy composition; insufficient quenching depth and uneven hardness with internal microcracks; thread rolling process defects causing stress concentration at thread root. These are congenital quality defects, and fractures mostly occur in the early installation and commissioning stage.
Fracture Features: Rough and uneven fracture surface with coarse grains, visible internal inclusions and pores; no obvious plastic deformation, typical brittle fracture; random fracture positions, prone to batch fractures in the same production lot.
2. Insufficient Design Safety Factor
Causes: Low bolt strength grade selection (e.g. replacing Grade 8.8 high-strength bolts with ordinary Grade 4.8), insufficient bolt quantity, overall safety factor below 1.2; actual load exceeds rated bearing limit under heavy impact conditions, leading to overload fracture.
Fracture Features: Obvious necking deformation on bolt shank, cup-cone shaped fracture surface, typical plastic tensile fracture; bright and uniform fracture surface without corrosion traces or fatigue patterns, mostly occurring on the main load-bearing side.
3. Non-Standard Installation Process
Causes: Failure to follow diagonal staged tightening principle, circumferential sequential locking causing uneven flange stress; uncalibrated torque wrenches leading to single bolt torque overload; thread impact damage and forced screwing without lubrication, causing stress concentration fracture at thread root.
Fracture Features: Fracture positions concentrated in thread meshing section, fracture surface develops obliquely along thread root; visible extrusion deformation marks on thread profile with obvious stress concentration features; fractures mostly occur within 100 working hours after installation.
4. Corrosive Environment Erosion
Causes: Electrochemical corrosion and stress corrosion cracking caused by submerged operation, coastal salt spray, mine acid-alkali dust and outdoor rain immersion; corrosion pits become stress concentration points, greatly reducing bearing capacity.
Fracture Features: Fracture surface covered with rust layer and corrosion pits, developing along grain boundaries; no obvious plastic deformation on fracture surface, belonging to brittle corrosion fracture; large-area rust on bolt surface with corroded defective thread profile.
5. Alternating Load Fatigue Fracture (Most Common Cause)
Causes: Slewing bearings bear long-term alternating impact and vibration loads, bolt stress cycles repeatedly, microcracks initiate at stress concentration points of thread root and shank and propagate continuously, eventually leading to instantaneous fracture. Accounts for more than 60% of bolt fracture failures.
Fracture Features: Three typical zones on fracture surface: fatigue source zone (small bright crack initiation point), fatigue propagation zone (shell/beach pattern), instantaneous fracture zone (rough brittle surface); core sign for identifying fatigue fracture.
2. Full-System Prevention and Control Measures
Selection and Procurement Control: Minimum Grade 8.8 high-strength bolts for slewing bearing connection; Grade 10.9 or above is mandatory for heavy mining and crushing conditions; overall safety factor shall not be less than 1.5, ordinary bolts are forbidden to replace high-strength bolts.
Installation Process Standardization: Strictly implement diagonal staged tightening method with calibrated torque wrenches; evenly coat anti-seize grease on threads, forced screwing damaging threads is forbidden; complete first recheck 24 hours after fastening.
Periodic Retightening System: Strictly implement four-level recheck mechanism at 24h, 100h, 500h and quarterly; retighten immediately if torque attenuation exceeds 15% to avoid fatigue fracture from stress relaxation.
Corrosion Environment Protection: Select DACROMET-coated, hot-dip galvanized or 316 stainless steel bolts for submerged, coastal and high-humidity conditions; monthly inspect bolt rust status, apply anti-rust grease and replace severely corroded bolts timely.
Working Condition and Life Management: Avoid long-term overload operation and sudden slewing start/stop to reduce alternating impact load; replace all bolts in complete sets after reaching 2500~3000 working hours fatigue life cycle, extended service is forbidden.
3. Emergency Handling Reminder
Upon finding a single fractured bolt, shut down immediately and fully inspect torque and appearance of all bolts in the same batch. Continued operation after only single bolt replacement is forbidden; simultaneously check bearing displacement and abnormal gear backlash, investigate flange flatness issues, and replace all bolts in sets before resuming operation after eliminating root causes.
Summary
Slewing bearing bolt fracture is not a single quality problem, but a result of combined factors including material, design, installation, corrosion and fatigue. Root causes can be quickly located through fracture surface features. Establishing a full-chain prevention system from selection, installation, recheck, anti-corrosion to life management can effectively reduce more than 80% of bolt fracture failures, ensure long-term stable operation of slewing bearings, and avoid major safety accidents and downtime losses.
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