The motors and drives for conveyors played a major role in the decision to reopen the Bull Mountain coal mine in Montana. They were crucial for efficiently delivering up to a million tons of coal per month. Siemens Industry Inc., Johnson City, Tenn., supplied the gearboxes and drives, which could keep the 60-in.-wide belts carrying 5,000 tons/hr moving without overtensioning or inducing dynamics waves in the belt.
In one of the mine’s conveyors, a Siemens Flender right-angle alignment-free 24:1 ratio gearbox takes power from four 500-hp motors with air-cooled variable-frequency drives through a PLC. Operators monitor and control the belt using two control panels that keep them updated on the belt’s tension, torque, speed, faults, trends, and alarms. Sensors throughout the mine connect to the panels through fiber optics. The gearbox and motors are supported by a pulley shaft through a rigid coupling with a single torque arm for vertical support. This makes it simple to move the power units, which happens regularly when seams get tapped out and operations move to another portion of the mine. For example, the alignment-free drive alone cuts reinstallation from three days to half a shift.
The gearbox is also quieter than similar units, so operators know the gearboxes are functioning smoothly and not ready to breakdown. And the gearbox’s reduced noise levels make for a safer and more-comfortable working environment for miners. A ribbed housing and fan lets the compact gears run 30% cooler.
Over the past year, the mine’s 300 employees have relied on the conveyors to move more than a million tons of coal per month with 98% uptime, exceeding the industry target of 95%. The plan is to use the conveyors and motors for the next 25 years at the newly renamed Signal Peak mine as it yields over 300 million tons of coal, less than a third of the mine’s total capacity.