NOTES OF INTEREST ON A WINDING ROPE SUBJECTED TO SEVERE SHOCK LOADS

by E. A. Johns (Associate Member) - The Certificated Engineer February 1973

Whilst hoisting rock a manually operated Ward Leonard type, direct-coupled electric hoist tripped out on overload at full speed and, according to the driver, pulled up sharply then reversed direction before it finally stopped. At this point, the upcoming skip had travelled one-third of the total length of wind and the hoist was in the slow braking range.

 An examination of the rectangular timbered shaft revealed that the upcoming skip had left the guides 105 metres below the position where it came to rest causing extensive damage to that section of the shaft. A corner of the rim of the skip bucket was pulled up hard against the underside of a wallplate to the extent that the winding rope was taut.

 It was essential to get the fully loaded skip out of the shaft in safety as quickly as possible bearing in mind that the rope and attachments had obviously been subjected to severe strain. An examination was therefore carried out and the following facts were noted during the survey.

 1. Skip. The distortion between bridle and bucket was insufficient to prevent the skip being pulled to surface and tipped.

 2. Attachments. All pins on top and bottom links were bent. Humble Hook centre pin not damaged, copper shear pin intact and alignment between scissor and outer plates correct. The solid type bronze rope thimble was intact but there was a 35 mm gap between the point of the thimble and the invert of the rope splice.

 3. Winding Rope. The rope was relatively new having been in use for two months with a life expectancy of twenty months. A visual examination of the rope in the shaft, from the adjacent compartment and the visible rope coiled on the drum, revealed no external damage whatsoever. A measurement of the rope lay, taken at the surface bank, indicated an increase of 12 mm compared to the length of lay taken at the monthly rope examination three weeks before in approximately the same position.

 It was then decided to bring the damaged skip to the surface and, as the companion skip was clear to be moved downwards, both drums were left clutched in while the hoist was moved slowly until the damaged skip fell clear of the wall plate only after lowering the rope two metres at the drum end.

 The skip was then put back in the guides and slowly hoisted, single drum, to surface when it could be seen that the rope had stretched twenty-six metres. A simple calculation indicated that the percentage increase in the length of rope between the drum and the position of the skip when the hoist stopped was approximately equal to the percentage increase in the length of lay.

 The rope manufacturers, when given the particulars of the rope together with the increase in the length of lay, intimated that rope had been stressed to approximately 85% of its breaking load.

 The rope was discarded and a speciman, cut from the top end of the stretched section was sent to the Mine Equipment Research Unit for tensile test and load elongation diagram.

 The result of this test, together with the original test super-imposed, is shown on the load-elongation diagram below:

 Referring to the diagram the load-elongation curve ABC is from the test of the rope when new and AKL from the test following the mishap.

 It will be seen that the characteristics inherent in the new rope, have changed appreciably and there is evidence that the rope has been stressed beyond the elastic limit to approximately 80% of the breaking load as indicated by the point "K" on the curve "AKL" which registers 855 kN compared to breaking load of 1 085 kN.

In tests of wire rope specimens, the use of the term "resilience" has been extended to mean the energy required to fracture the rope as indicated by the area of the load-elongation diagram.

On this basis, the resilience of the new rope, bounded by the area ABeD is 10 564 kN-cm and for the rope after the discard, bounded by the area AKLM, 8090 kN-cm, a reduction of 24% of the capacity of the rope to absorb shocks and other abnormal stresses.

The slightly increased breaking load of the discarded rope specimen may be attributed to work hardening during the two months it was in use.

It is of interest to note that if this rope was not discarded the problem of matching the distance travelled by both skips during one trip would be aggravated due to the difference in the characteristic of the sections of the rope as seen by the difference in slope of the two curves "AB" and "AK".

The foregoing case history of an actual mishap stresses the importance of taking accurate measurements of the length of lay at the monthly examination of ropes as laid down by The Mines and Works Regulations.