The Case Studies

1.   FAILURE OF A WIRE ROPE ON A CRANE

Third Section of Case Study

Summary and Conclusions

The information gleaned so far from this case study is summarised below: 

1.         Overall, the strength of the rope was reduced by the presence of fatigue cracks - this is evidenced by the observed tensile strength of 232 kN compared with the manufacturers stated breaking load of 332 kN.

2.         The observed breaking load is still very much higher than the stated load being lifted at the time of failure (some 25 - 30 kN).  Thus the rope must still have failed through application of an overload relative to its current strength level.  The failure was not solely due to the presence of fatigue cracks (whose existence is fairly normal in wire ropes and explains the requirement for regular maintenance and high factors of safety).

4.         The cause of this overload is not clear, but bouncing of the load might have allowed the rope to jump from its groove and jam between sheave and boom during winding.  If this state of affairs could exist for a short time undetected, and the rope winding was continued, a very significant overload could be applied to the rope.

5.         The cause of the fatigue cracks needs clarification.  They can initiate as a result of bending stresses induced by too small a sheave diameter.  The recommended diameter is 18x rope diameter which equals 432 mm for the resent rope.  The actual sheave diameter was 520 mm, which should have been sufficient.  As the sheave was older than the rope, however, it is possible that wear of the sheave groove has had an influence.  Fatigue cracks can result from deformed surface regions where ductility thus becomes exhausted, particularly if surface damage from abrasion occurs.  This would be exacerbated by any decrease in sheave groove diameter, which could occur by wear during service, and by poor lubrication practice (which was apparently the case).

The conclusion to be drawn from this investigation is that the presence of fatigue cracks has lowered the breaking load of the rope by some 30%.  However, the breaking load is still 232 kN, very much higher than the stated lifting load of 25 - 30 kN.  The fractographic work has indicated ductile fracture in all wires, demonstrating that the rope metallurgy is up to specification.  The most likely cause of the fracture seems to be rope jamming between sheave and groove, probably due to bounce during lifting.  The cause of the bouncing is unknown.

In insurance terms, poor maintenance is not a prime cause of the failure, which would have been "sudden and unexpected" when it occurred.  Cover should exist for such circumstances.

Recommendations

Any good failure investigation leads to recommendations aimed at avoiding the problem in the future or, at least, reducing its likelihood of occurring.  Recommendations in the present case are:

1.         Ensure adequate lubrication is maintained in the rope.

2.         Re-groove the sheave at regular intervals and, particularly, when the rope is replaced by a new one.

3.         Control lifting to avoid bouncing and install detectors which are activated by rope coming off the sheave.

4.         Monitor condition of rope by surface inspection and tensile testing.  

References - to fatigue of wire ropes

1.         J Llorca and V Sanchez-Galvez (1989) Fatigue and Fracture of Engineering Materials and Structures Vol. 12 No. 1 pp31-45

2.         RE Hobbs and K Ghavami (1982) International Journal of Fatigue April 1982 pp69-72

3.         NF Casey and WK Lee (1989) International Journal of Fatigue Vol. 11 No. 2 pp78-84.

4.          M Alani and M Raoof (1997) Effect of mean axial load on axial fatigue life of spiral strands, International Journal of Fatigue Vol. 19 No. 1 pp1-11

5.          K Coultate (1997) Magnetic attraction of wire rope testing, Materials World Vol. 5 September 1997 pp519-520.

6.          K Schrems and D Maclaren (1997) Failure analysis of a mine hoist rope, Engineering Failure Analysis, Vol.  4 No.  1 pp25-38.

7.          MD Kuruppu, A Tytko and TS Golosinski (2000) Loss of metallic area in winder ropes subject to external wear, Engineering Failure Analysis, Vol.  7 No.  3 pp199-207.

8.         J-I Suh and SP Chang (2000) Experimental study on fatigue behaviour of wire ropes, International Journal of Fatigue Vol. 22 pp339-347.

9.         M Torkar and B Arzensek (2002) Failure of crane wire rope, Engineering Failure Analysis, Vol. 9 No. 2 pp227-233.