March 2016
|
World Coal
|
53
THE
RIGHT
TRAIL
Proper trailing cable selection and
management can power greater
underground coal mine productivity.
Gary Mostyn, Nexans AmerCable,
USA, explains.
W
ith coal and metal prices hovering near
10-year lows, underground coal mines are
looking for new ways to get the greatest
productivity from each shift. While many
mines think equipment will provide the best boost, an often
overlooked cost-saving and increased output strategy
involves power cables. The appropriate cables, properly
managed and maintained, can greatly increase mine
production, which, in turn, improves profitability.
While static application cables are an important part of
mining infrastructure, they rarely require replacement or repair
once properly installed. Dynamic cables, generally referred to
as ‘trailing cables’, are used to power mobile or dynamic
motion equipment, such as shuttle cars, drills, scoops,
continuous miners, road headers and longwall systems.
Constant operational movement, handling and redeployment
to different sections means these cables have a shorter service
life and must be frequently repaired or replaced.
Cost-effective trailing cables are difficult to design and
manufacture because they must remain extremely flexible
and durable when in an energised, constant motion state. The
core feature of every high-quality trailing cable is a conductor
made of flexible, tin-coated copper. Some lower-cost cables
only use bare copper conductors, but they sacrifice flexibility
and endurance. As the cable is reeled up and out in typical
mining applications, all of its elements, such as insulation,
shielding, grounds and overall jacket, must move and flex
homogeneously to combat the harsh physical stresses of
bending, stretching, torsion and heat. At the same time, high
dielectric strength must be maintained.
Externally, constant motion exposes the cable jacket to
rough, sometimes sharp, rock surfaces that can ultimately
shorten the cable’s service life and possibly compromise safety.
Abrasion, tears and cuts all conspire to shorten a trailing
cable’s anticipated service life, so the jacket compound and
manufacturing method are key to its overall life span.
Independent laboratory testing and real-world mining data
indicate the most durable, flexible jacket manufacturing
process is mould-cured rubber. This process is a complicated
one in which a long length of completed cable is encased in an
extruded metal mould or sleeve, placed into a large autoclave
and heated to a temperature that crosslinks the rubber. The
metal mould fosters consistent heat transfer throughout the
cable’s entire length and squeezes the expanding rubber
during vulcanisation. This creates a smooth, extra-dense
surface that is extremely resistant to abrasion and tears, as well
as having excellent tensile strength.
Another method of cross-linking rubber jackets is
continuous vulcanisation (CV). CV-cured jackets are extruded
onto the cable assembly and pulled through a long tube that
is typically filled with high-temperature steam. Direct
exposure to steam permeates the raw rubber, which
negatively affects the jacket’s abrasion and tear resistance, as
well as its tensile strength.
In today’s budget-restrained market, coal mines are faced
with difficult strategic cable purchasing decisions. CV-cured
