Picking the right rope28 July 2011
MRE Matron Rope and Wire Rope Engineering, an Israeli firm, has developed a test to show how a specific crane might be fatiguing its rope with the aim of improving rope life.
The firm’s fatigue testing machine gives crane owners the ability to improve rope life for ropes used on heavy-duty cranes such as those in shipyards, but also on ropes used in large asset applications such as aerospace and deep mining shafts, where the cost of rope failure is high. The testing machines at MRE Matron’s laboratory compare life lengths of different ropes more quickly than generic testing machines, says its founder Dr. Ronen Ashkenazi, wire rope specialist. It does this by using testing machines to simulate the loading cycle of the application, and giving an approximation of rope life in only two and a half months, rather than the year typically required.
The tests replicate the exact loading cycle of the crane using nearly identical crane interfaces. They expose rope to similar environmental conditions, such as extreme temperatures of -55 degrees, and maintenance including greasing, under laboratory conditions.
Ashkenazi, says, “We have to consider the interface of the rope and the machine: the sheaves, the drum, the grooves on the drum and the alignment of the sheaves. You can specify friction forces, rotation forces as well as controlling parameters such as the braking of the drums and the control of the operator. All of these will affect the rope life.”
In the analysis stage, MRE Matron assists with rope selection by analyzing the test results and determining which of different available ropes is the most economical and safe for the specific application.
MRE Matron sometimes works with engineers at rope manufacturer’s firms to modify structures in order to improve rope's performance. Crane modifications are also considered. As Ashkenazi says, “The service life is not just the rope, it’s also the interfaces with the machine.”
Based on the tests for the chosen rope type, MRE Matron will determine the discard criteria, predict the rope life, and recommend maintenance practices suited for the usage.
Ashkenazi presented a case study that shows how important it is to test ropes for heavy duty and critical applications. Seemingly identical ropes made by different companies may be better or worse for a given application.
“With regard to a specific application, fatigue testing compared three different ropes under the same loading cycle,” he said. “The results are that three ropes of the same construction performed extremely differently. We opened one rope and found that the core was completely broken, and this is invisible, while the outer surface showed no damage. Another rope performed very well with respect to its core.”
Preventing rope failure at all costs is important for obvious reasons on satellites, but it is important as well on shipyard cranes and in other situations where neither the hire firm nor the contractor can afford waiting times.
Ashkenazi says, “On heavy duty cranes for ships the fine is €100,000 just to occupy the space. The crane owner pays the shipyard for the waiting time. We are helping them to choose the right rope.”