October 26, 2011—
Since the early 1990’s grinding mills have become larger to offer greater economy of scale with increased throughput. It is not unusual to find SAG mills up to 40 feet in diameter and ball mills of 26 feet. Motor power is ever increasing with current designs producing upwards of 20 MW. For these large drives the solution has been to introduce low speed ring motors “wrapped” around one end of the mill. Such designs however, whilst giving great flexibility in the control and drive system, do limit the options for safety and static control in the event of total power failure for example. The only option has been to consider fitting a braking system around the opposite end of the mill. The brakes can be used to stop a mill from full speed in just a few seconds, to protect the bearings, or simply to support the offset weight of the charge following a crash stop. In all cases such brakes offer complete control of the static holding of the mill, even at holding angles of up to 40 degrees. These requirements demand a static torque of many million Newton-meters. The largest systems installed can produce a static torque of 24 million Nm. The normal control modes for a mill achieve these functions electrically using the flexibility and power of the motor. The braking system can be used in conjunction with the motor but does offer a back up safe mode in the event of power fail. Altra Industrial Motion offers a system that includes the brake pedestals, brake calipers, hydraulic power pack and control logic. With over a dozen systems now installed around the world the new floating disc brake caliper has proved that modular braking on gearless drives is worth consideration when commissioning new grinding mill equipment.

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