The endurance life of a 3/4" diameter wire rope was tested using nylon and steel sheaves on a simulated lifting device at design factors of 3.5, 5 and 10 employing sheave ratios of 18/1 and 24/1.
The nylon sheaves imparted a distinctively longer endurance life of 1.33 to 4.5 times to the wire rope compared with steel sheaves depending on load level and sheave ratio employed. The performance of wire rope on nylon sheaves was consistent with only a small deviation of data. After-retiring a total of seven (7) ropes from both the nylon and steel sides, the nylon and steel sheaves were still in operational condition.
The substantially longer endurance life of wire rope against nylon sheaves, as compared to steel sheaves, can be attributed to the unique properties of nylon including resiliency and elasticity. It was demonstrated that drastically reduced contact pressure and better rope support with nylon sheaves resulted in delay of initiation of crown breaks as well as tangential breaks.
A linear relationship of remaining strength vs. an introduced fatigue factor was found. This linear relationship can be utilized to evaluate the reliability of wire rope/sheave systems. If the test results from a wire rope/sheave system yield small deviations from the linear relation, it is feasible to predict the remaining strength of cycled ropes for optimal retirement. Test results indicate that a minimum sheave ratio of 24/1 is an important criterion for rope endurance life.
Historically, manufacturers and operators of wire rope/sheave devices have looked for means of increasing rope life by inserting protective materials in rope grooves. In the early 1950's, attempts were made to use wear resistant plastics by strip lining metal grooves and by mounting grooved rings on metal hubs. Nylon was found to be particularly effective. Nylon grooved rings were used on cableways to reduce noise as well as cable wear. l,2
The availability of the nylon monomer casting process in the 1960's led to the development of castto-size nylon sheaves. With this process, the roller bearing is pressed directly into the nylon bore. Early installations on lifting equipment included clam shell buckets, bridge cranes, and tower cranes. 3,4
The growing use of mobile lifting equipment has provided an additional need for cast nylon sheaves to reduce dead weight at the end of the boom and over the axles. The expanded use of offshore lifting equipment has also highlighted the advantages of nylon sheaves for corrosion resistance.
Cast nylon boom point sheaves are now offered by a growing number of equipment manufacturers in both Europe and the U.S.A. They are widely used to improve wire rope life, reduce weight, and eliminate corrosion in a variety of applications including mobile construction cranes, mobile oil-field rigs, and offshore pedestal cranes.
Service experience with mobile lifting equipment has shown that wire rope life is definitely improved with nylon sheaves.
