Mechanical analysis of crane hook deformation failure-Weihua Crane

Mechanical analysis of crane hook deformation failure

2025-07-24 17:35:42

The deformation and failure of crane hooks is a common safety hazard in engineering, which may lead to serious accidents such as breakage and falling off. This paper analyzes the causes of hook deformation based on mechanical mechanism and failure mode , and proposes preventive measures.

1. Hook force analysis

(1) Main load types

Load Type	Mode of action	Typical working conditions
Static load	Constant tension (such as lifting a stationary weight)	Conventional lifting operations
Dynamic load	Impact, swing (such as emergency stop, wind load)	Port loading and unloading, metallurgical casting
Fatigue load	Cyclic stress cycles	High frequency lifting (such as production line)
Eccentric load	Asymmetric load (such as inclined lifting)	Irregular operation

(2) Critical stress area

The stress concentration areas of the hook are usually located at:

Inside the hook mouth (AA section) : area of maximum tensile stress (prone to cracks).
Hook neck bending part (BB section) : bears composite stress (tension + bending + shear).
Threaded connection : stress concentration + thread root fatigue.

2. Mechanical mechanism of deformation failure

(1) Plastic deformation (permanent deformation)

Cause : Local stress exceeds the yield strength of the material (σ > σₛ).
Typical manifestations :
- The hook mouth opens (opening increases by >10%).
- Hook is bent or twisted (eg, hook tip deflected >5°).
Calculation formula :
σ=FA≤σs(F: load, A: dangerous section area)σ=AF≤σs(F: load, A: dangerous section area)
When σₛ is exceeded, plastic deformation occurs .

(2) Elastic instability (buckling deformation)

Cause : Euler buckling occurs when the slender hook neck is under compression.
Critical load formula :
Fcr=π2EI(KL)2(E: elastic modulus, I: moment of inertia, K: length coefficient)Fcr=(KL)2π2EI(E: elastic modulus, I: moment of inertia, K: length coefficient)
- Hooks with a larger length-to-diameter ratio (L/D) are more susceptible to buckling .

(3) Fatigue fracture

Cause : Alternating stress causes cracks to initiate and expand.
SN curve analysis :
- Under high frequency and low load (10⁶ cycles), the fatigue limit is usually 40%~50% of the tensile strength.
Crack growth rate (Paris formula) :
dadN=C(ΔK)m(ΔK: stress intensity factor amplitude)dNda=C(ΔK)m(ΔK: stress intensity factor amplitude)

(4) Creep deformation (high temperature conditions)

Reason : The material continues to flow plastically under long-term high temperature.
Creep rate formula :
ϵ˙=Aσne−Q/RT(Q: activation energy, T: absolute temperature)ϵ˙=Aσne−Q/RT(Q: activation energy, T: absolute temperature)
- For every 50°C increase in temperature, the creep rate increases 10 times .

3. Failure case analysis

Case 1: Plastic deformation of hook

Background : The hook opening of a 50-ton crane hook in a steel plant was expanded from 500mm to 560mm (over 12%).
analyze :
- Material 42CrMo, measured σₛ=650MPa, but actual stress reaches 720MPa (overload 20%).
- The area of section AA is insufficient, resulting in yielding.

Case 2: Hook neck fatigue fracture

Background : The port container hook broke after 5 years of use.
analyze :
- Fractographic scans show conchoidal patterns (a characteristic of fatigue).
- The stress amplitude Δσ=300MPa, which exceeds the fatigue limit (200MPa).

4. Precautions

(1) Design optimization

Increase the size of the dangerous section : reduce the working stress to below 0.6σₛ.
Transition fillet optimization : reduce stress concentration (R ≥ 3 times the plate thickness).
Finite Element Analysis (FEA) : Simulates stress distribution under complex loads.

(2) Material improvement

Improve toughness : Use 34CrNiMo6 (low temperature impact energy ≥ 60J).
Surface strengthening : Nitriding treatment improves wear resistance.

(3) Usage and maintenance

Overloading is strictly prohibited : set a load limiter (such as 90% rated load alarm).
Regular testing :
- Measure the hook size monthly.
- Magnetic particle testing (MT) or ultrasonic testing (UT) is performed annually.
Scrap standards :
- Hook mouth deformation > 10% or crack depth > 1mm.

5. Future Research Directions

Smart Monitoring : Embedded fiber optic sensors monitor strain and cracks in real time.
Additive manufacturing : 3D printed gradient material hook (high hardness on the surface and high toughness in the core).
Digital Twin : Predicting Remaining Lifetime through Simulation.

in conclusion

The deformation and failure of crane hooks are the result of mechanical over-limit, material defects and fatigue accumulation . Safety can be significantly improved through accurate force analysis, optimized design and strict testing . In the future, active early warning of the health status of the hook will be realized by combining intelligent technology.