Page 10 - Wire Rope News & Sling Technology - December 2019
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continued from page 8
         on each side of a beam.
           When two horses are hitched to a
         singletree,  they “balance” their  horse
         power for pulling a wagon. If one horse
         moves ahead, both dived the load
         equally on the drawbar, regardless of
         their position. The same is true when
         two reeving systems are connected
         to each side of a beam to equalize the
         force to raise the load.
           We see that an equalizer beam (Fig.
         3), requires two separate ropes of the
         same length,  they stretch  unequally
         over time, and their length must be ad-
         justable to maintain an equal pulling
         force on the beam’s connection points.
           The pivot range of the equalizer
         beam is limited. When the limit is
         reached as pictured in Fig. 3, the right
         side is contacting the load block, a se-
         rious  imbalance  of  force now occurs
         between the two reeving systems, the
         left side has all the load. The great-
         er rope stretch on the right side has
         caused it to contact the block and re-
         laxes the tension. The maintenance
         team must center the beam and equal-
         ize tension on both sides.
           With the swaged rope stops in-
         stalled on both ropes, to shorten the
         rope is only accomplished at the right
         side drum anchor by shortening the
         rope, difficult. If this situation had oc-
         curred on a crane with turnbuckles on   Fig. 3
         each end, it would easily be adjusted.
         However,  turnbuckles  also  have  their   creasing the force on individual wires   sheaves diameter were increased to be
         adjustment limits, inspections  have   through  unequal wire  loading and   the same diameter as the running sheave.
         noted some were  nearing  their maxi-  bending moments, causing  increasing   The standards have changed accordingly.
         mum range. Conditions as these must   fatigue and wire breaks. Early stan-  The sheave can adjust the length of
         be noted on the inspection report. This   dards required that equalizer sheaves   the rope easy enough so long as sheaves
         is one small example of the equipment   need only be one-half the diameter of   are properly maintained and free to ro-
         design affecting the ease or difficulty of   the “running” (rotating) sheaves. Less   tate. The reeving is accomplished using
         maintaining a crane.               support by the sheaves increases wire   a single rope with both ends anchored
                                            loading and weakening due to the   on the drum(s) and the sheave in the
         EQUALIZER SHEAVES;                 smalled D/d ratio support.         middle  acting as dead-ends  to form
           Equalizer sheaves introduced new    After a study by the Aluminum In-  the two reeving systems. This adjust-
         problems. The rope ends are now bent   dustry  showed  that  significant  rope  life   ing activity is “assumed” to stop after
         over  the  curvature  of  a sheave,  in-  improvements occurred when equalizer   a short time in service, necessary only
                                                                               during  the  “construction”  stretch  or
                                                                               break-in period of the new rope is com-
                                                                      Fig. 4
                                                                               plete. Then the sheave is considered
                                                                               stationary, most sheaves don’t even
                                                                               have grease fittings, as evidence of the
                                                                               design’s appraisal of their function.
                                                                                 There are other advantages of using
                                                                               a sheave rather than a beam to produce
                                                                               two separate reeving systems. A big
                                                                               benefit  was  in  reducing  the  time  and
                                                                               the simplicity of installing one length
                                                                               of rope versus  two. The  end of a new
                                                                               rope is long spliced to the old rope, in-
                                                                               haul,  and the  old rope  pulls  the  new
                                                                               rope through a complex reeving system
                                                                               automatically as the sheave rotates.
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         10     Wire Rope News & Sling Technology   December 2019
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