In the last five years, grinding units have increased considerably in capacity,
resulting in a trend toward fewer comminution machines per grinding line.
Today, 40 ft. diameter SAG mills and 30 ft. diameter ball mills, both with motor
powers of more than 22 MW, are commonplace as mine operators look for
even larger equipment to achieve greater economy of scale.
Mills with these larger drive capacities are driven exclusively by a “wrap
around” ring motor (gearless mill drive). It is due to this development that
powerful braking systems have become necessary. A typical braking system
consists of two or more spring-applied, hydraulically-released brake calipers
mounted onto two pedestals, positioned symmetrically on each side of the mill.
In an emergency or power failure, the brakes are used to stop a rotating
mill from full speed in approximately 10 seconds to protect the bearings, or
simply to hold the mill when it is out of balance for maintenance, including liner
replacement. In all cases, the brakes provide complete static holding control of
the mill, even at angles of up to 45 degrees.
These braking requirements demand very high static torque ratings, with the
largest systems producing 51 million Nm of torque.
RELIABLE BRAKING TECHNOLOGIES FOR GEARLESS AND GEARED MILLS
Leading grinding mill OEMs and mine operators rely on Twiflex Limited for
efficient and cost-effective hydraulic caliper braking solutions for ball and
In static operation, the braking system is used to
hold the mill during liner replacement and general mill
maintenance. For dynamic operation, the system can
operate in two modes: stopping the rotating mill from full
speed in an emergency or providing inching/creeping
functions in the event of bearing lubrication problems or
power failures. For the first, a controlled application of
the brakes is required. For the second, the brakes are
operated quickly to give accurate stops needed by the
Twiflex VMS-DP brake calipers (with pedestals) plus a
hydraulic power pack installed on a 22.5 MW SAG mill,
11.6 m dia. x 13.7 m long, at a copper mine in Sweden.
The 8-off VMS-DP brakes produce 38 MNm braking
torque and can stop the mill with the 1375 Mg full
process charge in less than 2 seconds.
Calipers are controlled by a hydraulic power unit
designed to meet each individual application. The
HPU offers an advanced and flexible means of brake
control and monitoring, providing both local and remote
operation for inching and creep modes of mill control,
interfaced through the main PLC.
TWIFLEX GRINDING MILL BRAKES ARE ENGINEERED
FOR LONG-LASTING RELIABILITY
The Twiflex VMS-DP spring-applied, hydraulicallyreleased
floating caliper brakes are widely used on
gearless ball and SAG mills around the world. The brakes
provide an adjustable braking force from 590 to 737 kN
at 2mm air gap and a friction coefficient of 0.4.
The friction material for the VMS-DP has been
specifically chosen to meet mill emergency stopping
duty requirements. The material provides excellent friction
and wear properties.
For geared grinding mills, Twiflex offers its popular VKSD
spring-applied, hydraulically-released brake caliper which
produces up to 119 kN braking force at 2mm air gap and
0.4 coefficient of friction.
Geared mills can also be direct-driven using a VFD motor
to drive the pinion. In these cases, the monospring
VKSD-FL (floating version) is commonly used to act on a
brake disc fitted to the torque limiter. The floating option
allows for movement of the disc during mill start up.
In addition to protecting the drive, the brakes are also
used to prevent the mill from rocking during a power
failure due to the unbalanced load.
Pairs of Twiflex VKSD-FL brakes (with pedestals) for
installation on a 24 ft. dia. dual-pinion, direct-drive
geared ball mill at a copper/gold mine in Canada. The
VKSD-FL (floating) calipers act on a brake disc fitted to
a torque limiter. The brakes provide 454 kNm on the mill’s
If the mill is allowed to rock, having lost drive from the
motor, there is a high risk of bearing damage. Under
these conditions the brakes are used to provide a
controlled stop between 30 and 60 seconds. Similar
to the gearless mill requirements, the brakes are also
designed for static holding of the mill charge.
Both VMS-DP and VKSD-FL brakes incorporate Twiflex’s
unique “Park Off” feature that allows the brakes to be set
to remove all stored energy, making it completely safe
for pad replacement and maintenance. This feature also
permits brake installation without the need for hydraulic
Twiflex provides complete mill braking packages,
including brake pedestals/stands, fully-assembled brakes
with hydraulics and power packs.