[Univ of Cambridge] [Dept of Engineering]

Abrasion and Friction in Parallel-Lay Rope Terminations



Ropes made from parallel aramid yarns have been used for many years due to their light weight, good electrical properties and utilisation of the material's stiffness. The standard termination for these parallel-lay ropes is a "spike-and-barrel'' developed by the rope manufacturer. The terminations perform very well under static loading but fail due to abrasion of the rope near the nose of the spike when exposed to cyclic stresses.
Spike-and-barrel terminations for parallel-lay ropes are much more complex than they first appear. The functioning of the termination is heavily influenced by geometry, the material properties of both the termination and fibre, and the frictional properties. Any alteration to one of these factors alters the behaviour of the whole termination. Over their 25 year history, Parafil terminations have evolved into a sophisticated design which works well most of the time. To improve on their performance this study has looked inside the terminations and exposed hitherto unknown mechanisms and processes of slip and wear.
No straightforward analysis is possible because the simplifying assumptions mask the subtleties of the design. The use of a modified Howell's equation for friction, tex2html_wrap_inline6173 , is proposed here as a valid means of modelling the friction between polymeric materials. This re-expression in terms of stress, enables its use in a finite element analysis. Experiments on Kevlar 49 yarns over aluminium capstans were performed to determine the stick and slip coefficients of friction. Experiments on pads of 1000 yarns were also performed to determine the non-linear transverse stress-strain properties of Kevlar 49. Two sets of transverse moduli can be derived, one for first-loading, and a much stiffer set for unloading-reloading. This data is integrated into the analysis via subroutines written by the engineer.

Using the Kevlar 49-on-aluminium abrasion tests performed here, a general formula has been devised for lifetime, in terms of contact pressure, amplitude of slip and thickness to be abraded. This formula is coupled with the calculated severities in 6 and 60 tonne terminations, to predict lifetimes for ropes under various cyclic regimes. These predictions are very close to those reported in the literature.

The predictions from the finite element model compare favourably with strain-gauge and displacement readings measured here on an actual 60 tonne spike-and-barrel termination; so this work, which focussed on understanding behaviour, can be extended to optimise the design in terms of materials and geometry for bigger and longer lasting ropes.

The design procedures developed in this work, may be used as a model for the development of similar procedures for terminations for other very high strength materials.

Keywords: parallel-lay ropes, Parafil, Kevlar, friction, abrasion, aramid, non-linear, finite element, Abaqus, Howell.

The complete thesis is available on-line.

[Cambridge University | CUED | Division D | Structures Group ]

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