Brakes and Maintenance
Twiflex, which is part of the Heavy-Duty Overrunning Clutch and Brake
division of Altra Industrial Motion, supplies braking systems for both
geared and gearless grinding mills. Twiflex told IM that their main focus
over the last 12 months has been the development of a large mono spring
(floating) spring applied, hydraulic release disc brake for the dual pinion
direct drive geared mills. The result has been the introduction of the
VMS-FL which is based on Twiflex’s popular VMS range commonly used
on mine hoists. The VMS-FL has a braking force rating of 460 kN which
assumes a coefficient of friction of 0.4 and 2 mm pad air gap, and the
design incorporates Twiflex’s unique safety ‘park-off’ feature meaning the
brake can be adjusted so when depressurized there is zero stored energy for
maximum safety.
The VMS-FL has a braking force rating of 460 kN which assumes a coefficient of friction of 0.4 and 2 mm pad air gap.
The VMS-FL is used in conjunction with a torque limiter fitted
between the motor and gearbox. During maintenance when the motor is
shut down and the torque limiter is disengaged, the brake is activated to
hold the mill drive stationary and prevent the mill from moving due to the
unbalanced load. The requirement of the brakes is to hold the mill charge at
an angle of up to 45°.
In normal operation, after the mill controller receives the stop
command from the customer’s DCS it starts to ramp down the speed. When
reaching zero speed the drive system slowly starts in the reverse direction to
roll back the mill until zero torque is remaining in the system and the motor
creates a positive torque to just hold the mill with the charge unbalanced. By
slightly reducing the torque the direction of rotation is changes and the mill
is gently rolled back until the charge is balanced.
During a power failure the motors are disabled so the mill slows to
a rocking mode due to the unbalanced charge. This is a critical condition
whereby lubrication to the mill pad bearings is limited by the number of
accumulators in the system. To prevent equipment damage the brake is used
to provide a controlled stop (between 10 and 30 seconds).
One of two VMS-DP brake stations supplied for the 12.2m diameter fearless SAG mill.
Twiflex and US distributors Hilliard have recently completed the
installation on both geared and gearless mills at the Mount Milligan mine
in British Columbia, Canada. The scope of supply for the
40 ft. gearless SAG mill included two brake stations each with
3-off VMS-DP brakes and a hydraulic power unit to provide
controlled braking functions. For the two 24 ft dual pinion
direct drive geared mills, Twiflex provided VKSD-FL (floating)
brakes on each mill to provide 454 kNm on the high speed
shaft. The brakes act on a 1.778 m diameter disc fitted to a
torque limiter.
Steve Powell, Product Manager at Twiflex said: “The
increase in the size of grinding mills has become a major
challenge to the suppliers of drive chain components but
particularly to brake manufacturers. A good example is
Twiflex’s braking system on the 11.6 m gearless grinding mill
at the Boliden Aitik copper mine in Northern Sweden which is
required to produce a massive 38 MNm torque. The 8-off
VMS-DP dual piston spring applied hydraulic released brakes
each weighing 1,850 kg act on a 13 m diameter flange and can
stop the mill with a full process charge of 1,375 kg in less than
two seconds.”
VKSD-FL brake stations utilized on two geared ball mills.
Whilst the number of brakes and their size have increased
to cope with the bigger mills the limiting factor is still the
friction material. Twiflex have installed a dedicated test facility
at their Bedford facility to investigate materials both for static
and dynamic duty with this development in mind.
Powell concluded by saying “Twiflex focus is on
developing and introducing new products to the mining
industry and the company have recently launched new spring
applied, hydraulically released brakes (VBS, VCSD and VKSD
large pad) for this market. We are currently working on low
temperature versions of these brakes in addition to on-going
product development and improvements. Our intention is to
develop a larger VMS-DP for gearless mills.”