Page 12 - Wire Rope News & Sling Technology - December 2018
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continued from page 10
         veloped a series of patents for coating
         ferrous metal by dipping articles in a
         bath of molten zinc. As a result of his
         success (which he called “galvanizing”)
         this method of protecting wire was
         adopted  very quickly  in Europe and
         America for protecting wire from rust.
           Hot dip  galvanizing  is  called a  sac-
         rificial  process.The  corrosion  caused
         by the  atmosphere (which  cannot  be
         stopped) attacks the zinc coating first.
         The surface of the steel is protected by
         the preferential attack on the coating
         until it has been eaten away. Com-
         mon sense makes it seem obvious the
         thickness of the coating is a consider-
         ation we must take into account when
         estimating how long the zinc protec-
         tion will last. When the cable wire was
         purchased there was no engineering
         standard for galvanizing by the hot dip
         method.  Obviously,  no  thickness  was
         specified.  If  any  of  the  internal  wires
         has failed because the coating is gone
         and pitting has begun, we have no way
         of knowing about that, either.
         The Brady Splices
           The Brooklyn  Bridge is  the only
         bridge ever built with Brady splices.
         Those little couplings are the most ex-
         perimental factor in this giant metal
         fatigue experiment. Never used before
         or  since,  there  is  no track  record for
         comparison. At each splice there are
         circular  circumferential  V-grooves cut
         into the wire by the coupling. In the
         fundamental principles  of  mechanical
         engineering being taught today, such
         grooves are well known as stress-rais-
         ers. When the main cables were being
         made, the  Brady splices  were  an  un-
         tried solution devised by Washington
         Roebling and his engineering  staff as   Lore Croghan of the Brooklyn Daily Eagle newspaper says: “Many of the people who swarm
         a way to save time. There was no de-  to the iconic span’s promenade to stroll and snap selfies ignore the painted white lines that
         bate with anyone about their decision   separate its bike and pedestrian lanes. Because signs posted on the bridge about the walk-
         in 1877. Today most people are com-  ing and biking paths are written solely in English, they’re unreadable for some visitors… The
                                            pedestrians don’t pay attention. They treat the bike lane as their own.”
         pletely unaware of the numerous little   Brooklyn Eagle photo by Lore Croghan
         splices everywhere in the cables.
           Roebling’s father had his own meth-  Brooklyn machine shop, Brady Manu-  cables. The state if the art at that date
         od for splicing wires end-to-end. Two   facturing  Company. We are fortunate   limited the lengths of the wire used
         wire ends were tapered with a ham-  that the owner,  James Brady,  pro-  to about 600 feet…these wires were
         mer and scratched to a rough surface   vided the story in a piece he wrote for   made  practically  continuous  by  the
         with  a  file.  The  two  roughened  sur-  American Machinist  magazine March   use of the couplings.”
         faces were overlapped and the connec-  16, 1905. He had been called into court   (When I’m reading Brady’s explana-
         tion was secured with a wrapping of   to testify in  a patent infringement   tion, I find it difficult to keep in mind
         fine  wire.  At  a  rope  factory,  it  was  a   lawsuit. The technique  he used for   he is talking about cutting two internal
         process that could be done all the time   couplings was known as a “vanishing   threads in thousands of little chunks of
         without any interruptions but it was   thread on a cone”. He began his expla-  metal smaller than the USB flash drive
         utterly unsuitable for a huge project   nation of the contract by emphasizing   I’m using to record this essay.)
         like the big bridge where the cost pre-  how the use of crucible cast steel wire   “The threads were of course right and
         diction of the project had been grossly   severely limited the wire lengths:   left and the abutting ends of the wires
         underestimated by John A. Roebling   “I secured a contract for furnishing   were cut obliquely so that they inter-
         the original designer of the bridge.  the couplings (about 200,000) required   locked as shown when drawn together,
           The  splices  were  created by a  tiny   in connecting the wires for the bridge      continued on page 14

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