Bridon - Teyseer Group

Technical InformationBend FatigueService life curve for various D:d ratiosBend fatigue testing of ropes usually consists of cycling a lengthof rope over a sheave while the rope is under a constant tension.As part of its ongoing development program, Bridon has testedliterally thousands of ropes in this manner over the years on itsown in-house design bend testing equipment.Through this work, Bridon has been able to compare the effectsof rope construction, tensile strength, lay direction, sheave size,groove profile and tensile loading on bend fatigue performanceunder ideal operating conditions. At the same time it has beenpossible to compare rope life to discard criteria (e.g., as laiddown in ISO 4309) with that to complete failure of the rope, i.e.,to the point where the rope has been unable to sustain the loadany longer. As part of the exercise, it has also been possible toestablish the residual breaking strength of the rope at discard levelof deterioration.Relative Rope Service Life10080604020005 10 15 20 25 30 35 40 45 50 55 60 65D:d ratioEffects of D:d Ratio and loading on fatigue lifeTypical example Dyform 6Number of bends to rope failure10% MBL20% MBL5% MBL30 29 28 27 26 25 24 23 22 21 20 19 18 17 16Sheave D:d ratioWhat needs to be recognized, however, is that very few ropesoperate under these controlled operating conditions, making it verydifficult to use this base information when attempting to predictrope life under other conditions. Other influencing factors, such asdynamic loading, differential loads in the cycle, fleet angle, reevingarrangement, type of spooling on the drum, change in ropedirection, sheave alignment, sheave size and groove profile, canhave an equally dramatic effect on rope performance.However, the benefit of such testing can be particularly helpful tothe rope manufacturer when developing new or improvingexisting products.If designers or operators of equipment are seeking optimum ropeperformance or regard bending fatigue life as a key factor in theoperation of equipment, such information can be provided byBridon for guidance purposes.Oversize ToleranceWire ropes are manufactured slightly larger than the nominaldiameter. The maximum allowable oversize tolerances providedby industry standards are shown in the following table:Nominal Rope DiameterBending Ratios D:dUnderTypical minimum bending ratios (sheave or drum dia : ropedia) provided by some industry standards are as follows:DrumToleranceSheaveMobile crane Load hoist 18 18Boom hoist 15 15Load block 16Tower crane Hoist 18 18Load block 16Rotary drilling Drill Line 20 20Offshore pedestal crane Hoist 18 18Surface mining Hoist 24 24Drag 22 22OverUp to 1/8" -0 +8%Over 1/8” to 3/16" -0 +7%Over 3/16” to 5/16" -0 +6%Over 5/16" -0 +5%42 BRIDON North American Catalog