Drill Rig Wire Rope Life Expectancy Calculation

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I am a driller on a Drilling Rig which use the Fork Lift raise and lower concept to lift/lower drill pipes ,casings etc. , we use 1-1/2" 8x26 ( 128.2tons per strand breaking capacity wire ropes x 2 )the ropes are anchored both ends and run over 27" sheaves , the rig has a 100 ton lift capacity and will be on drilling operations 12 to 24 hrs per day lifting minimum 2 ton and max 100 ton intermittantly , SAFETY is the priority - my Question , is there a calculation that can be used to enable the rig operator to say now is time to change the two wire ropes , even though they look to be in good shape .