CRANE CRITIQUE
CONSTRUCTION NEWS BULLETIN
HOIST BLOCK SPIN What causes it and how to prevent it
A monthly crane and rigging informative column for all personnel directly or indirectly involved with crane safety. Each month we will attempt to explain a different technical issue pertaining to crane operations here on Guam, addressing the sometimes over-looked or misunderstood topics by management and operators alike. By Dave Barnhouse Since cranes were first used, one problem has been prevalent during multi-part line lifts – rotation of the hoist block. This is called block spin or cabling, the point at which the block spins to entangle the hoist line. A formula exists that will predict the length at which cabling will occur but for the most part here on Guam if and when it will happen is discovered when it actually occurs. This can be at best inconvenient but otherwise may have disastrous results. If a block spins uncontrollably after a load is lifted to a point where it would be impossible to lower the load to the ground by booming down, it may present quite a problem to lower the load as once the cable is tightly wound the load weight may not be sufficient to overcome the cable friction. If spinning the load is possible and the block spins with the hook and load, this may alleviate the problem long enough to complete the lift. If an attempt is made to hoist up or down while the hoist line is twisted, severe damage or parting of the wire rope may result. If cabling persists and is uncontrollable, a temporary fix is to lock the hook with the block allowing control of block spinning by controlling the load. This is not advisable as a permanent cure as the block hook must be free to swivel. Another fix that usually alleviates the problem is to disconnect the dead end and allow the wire to untwist a number of times, run the hoist the full length as possible and repeat if necessary. What causes block spin is the characteristics of the cable design. With 6 x 19 IWRC wire for example, the outer stands and the core strands are laid in the same direction. Thus, whenever a load is applied, both the rope and the core tend to unlay in the same direction. 26 | JULY2017
Rotation resistant wire rope tends to twist much less but is not usually used on multi-part lines for a couple of reasons. Some steps to prevent or reduce cabling: 1) Ensure wire rope is installed on the drum correctly. 2) Use short as possible wire rope lengths, keep boom length to a minimum. 3) Use IWRC wire only, Fiber Core wire ropes www.guamcontractors.org
have a higher torque factor than IWRC. 4) Eliminate odd-part reeving, even number of parts are more stable than odd. 5) Increase the spread between the wire rope falls. Dead end the wire rope away from the falls if possible. When should free-spinning swivels be used on a hoist reeving system? There seems to be much disagreement and confusion of exactly Like Us On Facebook