Trail Mountain Mine Works for 99% Conveyor Availability with New Alignment Free Drive™
Setting a Goal
One of 11 drives at the
Trail Mountain Site |
In January of 1994, when Energy West Mine Superintendent John Elkin was charged
with getting the Trail Mountain Mine's longwall mining operation up and running he set a goal: to
achieve 99% conveyor availability on the mainline conveyors.
As any mine manager will tell you, conveyor availability is the number one maintenance concern.
In an industry where conveyors can run 24 hours a day, seven days a week, downtime translates
into thousands of lost dollars. Conveyor availability is critical in Elkin's case because the Trail
Mountain Mine will be supplying coal to local power plants after its sister mine, the Cottonwood
Mine depletes its reserves next year.
Currently, Energy West's Cottonwood Mine provides coal for PacifiCorp's, Hunter, Utah power
plant at the rate of approximately 17,000 tons per day. This plant supplies power to Utah, and
other western states.
The Trail Mountain Mine will ship its coal via an enclosed, 400-foot (122 m) overland tube
conveyor that will span the Cottonwood Canyon and connect with the Cottonwood Mine's five
miles of belt line. Conveyors linking Trail Mountain's mining face to the processing facility at the
Cottonwood Mine will total 7.5 miles (12 km). Next, the coal will be processed for primary
sizing at Cottonwood and then shipped by truck to the power plants where it will undergo final
processing.
The Trail Mountain Mine, approximately 3.8 million tons (3.45 million metric tons) of
sub-bituminous coal will be mined per year with its longwall and continuous miner operations.
The rugged desert property near Orangeville, Utah has been mined in one form or another since
the 1950s. Before being purchased in October 1992, by the Portland, Oregon-based
PacifiCorp, the mine was mined and owned by ARCO Coal Company.
Getting it Right
The gear drive assembly is the lifeline of a mine's conveyor systems. Specifying and selecting the
proper drives and couplings will be crucial to the Trail Mountain Mine achieving 99% conveyor
availability.
Also involved with Trail Mountain's drive system selection was Interwest Mining Company's
Vice President Dan Baker. Interwest Mining Company is a Salt Lake City-based company that
manages PacifiCorp's mining operations.
"Coal haulage from the longwall operation is as vital as any part of the longwall system," noted
Baker. "Without the entire longwall system operating at its budgeted production level, production
requirements will not be maintained. Interruptions in longwall coal production add directly to coal
production costs and to the total mine cost."
Designing the conveyor systems and specifying drive equipment was especially important
because Trail Mountain had to be redeveloped to accommodate the new, larger, 60-inch (1.524
m) belt lines that will be used in the new longwall operations. Previously, 42-inch (1.07 m) belts
were in place.
Getting the order to design Trail Mountain's conveyor systems was the mine equipment and
engineering firm, Long-Airdox. Energy West had been using Falk® gear drives and couplings at
its Deer Creek and Cottonwood Mines and achieved 99% conveyor availability. So, when
Long-Airdox suggested the new Falk Alignment Free Drive gear drives for the six mainline
conveyors at Trail Mountain, Superintendent Elkin had no qualms about trying the new design.
"We base our decisions on past experience. We had 99% conveyor availability with Falk drives
and couplings at our other mines. We had been looking into the alignment-free set-up quite a bit,
so when Long-Airdox suggested to try the new Falk system, we trusted them. Falk
drive's track
record at our other mines speaks for itself," said Elkin.
The Drive
Designed specifically for the rugged requirements of high horsepower underground and surface
belt conveyors, the new alignment-free technology allows the gear drive to be mounted
directly to the conveyor eliminating the need for a foundation and a conventional low-speed shaft
coupling. The bolt-on design of the drive also lends itself to fast installations requiring no
alignment. The low-speed shaft bolts directly to the conveyor shaft by means of connecting
flanges on the two shafts. Simple, registered fits and bolt-together construction eliminate the need
for time-consuming alignment of both input and output shafts during installation and periodic
maintenance.
"We knew the alignment-free drives were going to be advantageous for us to try," said Elkin.
"The ease of installation was one of the biggest things we were looking for. Because we are
redeveloping this mine to supply coal when our sister mine runs out of reserves, we had to
complete the drive installations as soon as we could," said Elkin. "The drive installation was as
easy, if not easier than Falk claimed. It had to be twice as fast as a conventional drive
installation," noted Elkin. "It was just a matter of taking a forklift, picking up the gear case and
drive motor, sticking it on, fitting in the bolts and tightening them up."
The Need for Soft Conveyor Start-ups
Trail Mountain also needed drive systems that included couplings on the high-speed or
motor-connected side that were capable of extended soft starts under peak-load conveyor
conditions. Elkin noted that soft starts are crucial in reducing equipment stress and extending
equipment service life.
Accomplishing
soft starts should be a goal on every heavy-duty conveyor application. Because
AC motors can exceed 250 percent of full-load torque during start-up, the loads generated can
damage and overstress equipment.
"The motor current can remain at up to 600 percent of full load during a substantial portion of
acceleration. These high currents build up heat that can drastically shorten the
motor's life expectancy." Excessive shock loading that occurs during
start-up can surpass conveyor belt design limits resulting in damaged belts, take-up problems,
not to mention the high potential of material spillage. Fluid couplings offer a "soft start" option for
conveyor systems designed to prevent such equipment damage and loss.
Under both loaded and unloaded conditions, fluid couplings are designed to provide smooth
conveyor acceleration by keeping initial starting torque as low as 40 percent of full load,
controlling acceleration and dampening shock and vibration.
Energy West chose to connect all eleven of the mine's alignment-free drives on the section belt
and mainline conveyors with fluid couplings. The third west conveyor is currently 1,000 feet(303 m)
long but will eventually be extended to 4,000 feet (1220 m) to reach down to the longwall.
Each of the two right angle alignment-free drives on the third west conveyor is powered by a
350 hp (261 kW) AC electric motor (1780 rpm). Both drive through Falk's new 1000 Series
enhanced start fluid coupling to the Falk Alignment Free gear drive (34.25:1 ratio). The total
output torque for the drive system is 863,525 inch-pounds (97,492 Nm).
To prevent roll back of the conveyor during stop sand shutdowns, a Falk NRT®
backstop is mounted on the low speed shaft of the conveyor. Like all of the conveyors at Trail
Mountain, the third west 60-inch stop sand runs at 570 ft/min (174 m/min), carrying up to 3,000
tons (2,722 m/tons) of coal per hour.
Total conveyor systems will contain over 2.5 miles (4 km) of conveyor belts including a 12-foot
diameter enclosed, 400-foot (122 m) overland tube conveyor to carry the coal to the Cottonwood's processing facility. Other conveyors at the mine include three other tandem drive
systems like the aforementioned third west, and a 6,000-foot (1829 m), 60-inch (1.52 meter), mainline conveyor with three drives powered by 400 hp (299 kW) DC electric motors coupled
with Falk Steelflex® couplings. The speed and torque are controlled through a PLC and load
cells to provide constant conveyor belt tension under dynamic loading conditions. All of the
mine's conveyors use Falk Alignment Free Drive gear drives. Since shaft misalignment is a
leading cause of equipment failure and downtime, the alignment-free design will most likely find a
welcome place with mine maintenance personnel.
The Alignment Free Drive is alignment-free in two respects: the drive doesn't require parallel
or angular alignment in relation to the conveyor. This is a significant time savings considering
traditional, foot-mounted drive alignment can take two or three men up to 16 hours to complete
drive-to- conveyor and motor-to-gearbox alignment. This labor intensive, time consuming
activity can take even a veteran crew up to 32 man-hours to complete.
The alignment-free design also reduces the amount of maintenance time. Elkin noted that his
maintenance people like the alignment-free design because no lubrication is needed on the low
speed shaft-side of the drive. "Because there is no coupling like we had on our foot mounted
drives, we don't have to spend the time to grease it," said Elkin. "It also saves us money because
with one less component on the system, maintenance costs are further reduced and difficult fluid
coupling alignments are eliminated."
Another design feature that attracted Elkin to the alignment-free design was that it was free of the
high initial expense associated with conventional drive designs. "We wanted something that
would be dependable, required less maintenance and less initial cost," said Elkin. "The
alignment-free design seemed to be the answer."
Because the new drive design doesn't need its own foundation, engineering and materials costs
will be significantly reduced. All that is required is an anchored torque-arm support forth drive.
Also reducing initial cost is the use of a fluid coupling. Fluid
couplings, a less expensive alternative to complex soft-start devices on
fixed length conveyors, allow a motor sized for the running load, not the
starting torque to be used. "Using a fluid coupling saves the customer money
because many times a less expensive motor and starter can be used."
Elkin added that because the alignment-free drive has a symmetrical housing design, it
contributes to increasing conveyor availability. The drive can be used in either the right-hand or
left-hand assembly with only an oil dip-stick and torque arm change required.
"With this design, we can save money by stocking spares that can be used for a right or left-hand
configuration," said Elkin.
The Alignment Free Drive is available in five sizes, can be flange or hollow shaft mounted,
accepts "alignment-free" Nema or I.E.C. flange mounted AC or DC motors thru 800 hp (630
kW), and can use fluid couplings or other flexible shaft couplings for the motor connection.
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