Wire rope basic knowledge
Wire rope basic knowledge
Wire rope is a type of rope
which consists of several strands of metal wire laid (or 'twisted') into
a helix. Initially wrought iron wires were used, but today steel is the
main material used for wire ropes.
Historically wire rope
evolved from steel chains which had a record of mechanical failure.
While flaws in chain links or solid steel bars can lead to catastrophic
failure, flaws in the wires making up a steel cable are less critical as
the other wires easily take up the load. Friction between the
individual wires and strands, as a consequence of their twist, further
compensates for any flaws. This method of minimising the effect of flaws
may also be seen in Damascus steel, employing multiple folding or
laminations.
History and materials
Modern wire rope was
invented by the German mining engineer Wilhelm Albert in the years
between 1831 and 1834 for use in mining in the Harz Mountains in
Clausthal, Lower Saxony, Germany. It was quickly accepted because it
proved superior to ropes made of hemp or to metal chains, such as had
been used before.
Wilhelm Albert's first ropes
consisted of wires twisted about a hemp rope core, six such strands
then being twisted around another hemp rope core in alternating
directions for extra stability. Earlier forms of wire rope had been made
by covering a bundle of wires with hemp.
In America wire rope was
later manufactured by John A. Roebling, forming the basis for his
success in suspension bridge building. Roebling introduced a number of
innovations in the design, materials and manufacture of wire rope.
Manufacturing a wire rope is
similar to making one from natural fibres. The individual wires are
first twisted into a strand, then six or so such strands again twisted
around a core. This core may consist of steel, but also of natural
fibres such as sisal, manila, henequen, jute, or hemp. This is used to
cushion off stress forces when bending the rope.
This flexibility is
particularly vital in ropes used in machinery such as cranes or
elevators as well as ropes used in transportation modes such as cable
cars, cable railways, funiculars and aerial lifts. It is not quite so
essential in suspension bridges and similar uses.
Wire rope is often sold with
vinyl and nylon coatings. This increases weather resistance and overall
durability, however it can lead to weak joints if the coating is not
removed correctly underneath joints and connections.
Lay of wire rope
Left-hand ordinary lay (LHOL) wire rope (close-up). Right-hand lay strands are laid into a left-hand lay rope.
Right-hand Lang's lay (RHLL) wire rope (close-up). Right-hand lay strands are laid into a right-hand lay rope.
The lay of a wire rope
describes the manner in which either the wires in a strand, or the
strands in the rope, are laid in a helix
Left and right hand lay
Left hand lay or right hand
lay describe the manner in which the strands are laid to form the rope.
To determine the lay of strands in the rope, a viewer looks at the rope
as it points away from them. If the strands appear to turn in a
clockwise direction, or like a right-hand thread, as the strands
progress away from the viewer, the rope has a right hand lay. The
picture of steel wire rope on this page shows a rope with right hand
lay. If the strands appear to turn in an anti-clockwise direction, or
like a left-hand thread, as the strands progress away from the viewer,
the rope has a left hand lay. (The rope in the left hand lay photo shows
one left hand lay rope from left to right and top to bottom, with 5
right hand lay strands, and part of a sixth in the upper left. It is not
5 right hand lay ropes adjacent to each other.)
Ordinary, Lang's and alternate lay
Ordinary and Lang's lay
describe the manner in which the wires are laid to form a strand of the
wire rope. To determine which has been used, first identify if left or
right hand lay has been used to make the rope. Then identify if a right
or left hand lay has been used to twist the wires in each strand. (On
ordinary lay, the outer wires approximately follow the alignment of the
rope: with Lang's lay they are cross at an angle of about 45°.) Lang's
laid rope is able to flex over sheaves easier (with less damage) but it
has the disadvantage of having a high torque tendency (it tends to
untwist when tension load is applied) compared with ordinary laid rope.
Untwisting can be dangerous with a steel-cored rope: load is shed from
the strands and may cause the core to fail as it becomes higher loaded.
For this reason, swivel termination units can be dangerous.
AULONE