Page 12 - Wire Rope News & Sling Technology - December 2019
P. 12

continued from page 10             in frictional lost than it adds in me-  sides of an equalizer sheave – friction
           The rope under the equalizer sheave   chanical advantage!           stops at the sheave.
         must be visible to inspect, not always   In  a  twenty  part  boom  luffing  ropes   Fatigue at the end connections is
         easy. Case #1: It’s easy to inspect the   that connect the boom dead end to the   caused by the forces  on the rope that
         rope on a 20-ton Whiting Bridge Crane.   boom drum is a continual reeved system.   abruptly halt.  About eighteen  rope
         Lower  the  load  block  to  the  floor  and   The mechanic advantage of the reeving   diameters are needed for  the rope to
         continue  paying-off and pulling down   is 20 to 1 in the stationary position, and   stretch and absorb shock loading, fric-
         on one side of the rope that was in con-  each part is holding 1/20th of the load.   tional cycling, and vibration loading
         tact with the sheave hits the floor. To   However,  when  moving,  each  sheave   within its elastic range. When these
         test for lubrication and brittle fatigue,   adds a frictional force via bending the   forces  approach the end-connection,
         bend the rope backward 180 degrees,   rope 180 degrees around the sheave and   the rope gets too short to absorb the
         listen  for  “cracking”  noise  and  falling   rotating the sheave on its axle. How   stretching;  thus,  the rope reluctantly
         wires.  Some experienced inspectors   much frictional build-up depends on the   accepts the weakening cycles. In some
         “open” the rope using a “marlinspike”   stiffness of the rope and the efficiency   cases, owners  swap boom pendants,
         to inspect  the rope’s core  for  broken   of the sheave bearings. Rope and sheave   end-to-end to shift the fatiguing cycles.
         wires. If any of these conditions exist -   lubrication is critical to the efficiency of   The  elastic  range  is  defined  as  after
         REPLACE THE ROPE.                  the system, maintenance.           stretching  under  load,  and  the  load  is
           Fig. 4 (previous page) shows a distor-  The rope’s “dead-end” isn’t dead; it   removed, the wire returns to the origi-
         tion of the rope over an equalizer that   cycles!  Friction  build-up and follows   nal length. The amount of stretch is de-
         was pulled down to ground level, which   the direction of the rope. If the rope   pendent on the alloy of the wire. When
         shows deformities which must be hand   is  heading  toward  the  end connection   a rope is deformed, it is due to some of
         straighten and lubricated. If hand re-  (booming-down), the frictional  load is   the wires in the “rope lay” area that are
         forming can’t be accomplished, the rope   the  maximum  –  stretching  the  rope   stretched and hold the rope in the “bend”
         is  permanently deformed (stretched);   at the  socket.  Reversing  the  direction   position, (again Fig. 4). Test show that
         REPLACE THE ROPE.                  (booming-up, in-hauling), the frictional   repeated stretching reduces the strength
           In Case #2: Inspection on this crane is   load vanishes, and the socket assumes   of the wire and causes breaks. Fatigue
         difficult  due  to  access  and  guards  over   it’s  1/20th  of  the  load.  This  pulsation   breaks are distinguished by a noticeable
         sheaves, Fig. 5 (below). What it takes to   force  causes  fatigue. The  build-up of   type of wire breaks, (Fig. 6, next page).
         inspect this crane are hours, an experi-  friction  also applies to ropes on both      continued on page 14
         enced maintenance crew, come-a-longs,
         an aerial lift, limit switches bypassed,   Fig. 5
         and hand tools to do the work. If a crane
         certifier  asks  a  supervisor  if  the  work
         was  performed  properly, and  the  an-
         swer is yes, he has two choices. Require
         the operation be performed in his pres-
         ence, so he can verify, (best if planned)
         and risk upsetting the owner. Or, docu-
         ment the areas that were not inspected
         due to its location. Which one is selected
         depends on the certifier’s familiarity and
         knowledge of the owner’s operations.
           The rope under the equalizer sheave
         gets a quadruple “whammy” load force
         cycling, bending stress, moisture ac-
         cumulation, and neglect due to their
         location. Some owners don’t know how
         long the ropes have been on the crane.
         When asked for records located by the
         office staff, some maintenance super-
         visors are shocked by how long the
         ropes were on the crane, and no wire
         rope certificates.

         FRICTION;
           The reason for two separate reeving
         systems and an equalizer is friction.
         On some heavy-lift cranes, there may
         be four separate reeving systems with
         two equalizer sheaves in one hoisting
         or boom luffing function. The friction-
         al multiplies with sheave, increasing
         rope loading that reaches a maximum
         at the dead end. About 24 parts of
         rope are maximum on today’s cranes.
         After this, the next sheave adds more

         12     Wire Rope News & Sling Technology   December 2019
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