Brakes shut down 1,000 tonne mill in two seconds 09 August 2011

Mount Milligan gold and copper mine in British Columbia, Canada – due to commence production in 2013 – will be relying on safety braking systems from Twiflex for its new 22MW SAG mill.

Metso Minerals was commissioned to supply the primary crusher, two ball mills and a SAG mill, and turned to Twiflex for its gearless mill drives – in this case, two stations with Twiflex VMS-DP brakes, plus an electro-hydraulic power pack.

VMS-DP units are spring applied, hydraulically released safety brakes designed to operate in the most arduous conditions, where reliable operation all year round is paramount.

Unlike calliper brakes, these units do not need guarding. Moreover, they are about one third smaller than calliper units, providing a more compact assembly, and one that reduces cost, both in terms of the size of hydraulic power pack and the volume of oil required to release the brakes.

The SAG mill measures 12.2m in diameter and is 7.31m long. In static operation, the VMS-DP brakes will be used to hold the mill during liner replacement and general mill maintenance.

For dynamic operation the system can operate in two modes: stopping the mill from full speed in an emergency, or enabling inching and creeping operations in the event of bearing lubrication problems or power failure.

For the first, a controlled application of the brakes is required and, for the second, the brakes are operated quickly to give accurate stops as required by the mill operator.

The braking system at Mount Milligan will generate up to 30MNm braking torque, acting on a 13.48m mill flange diameter. The VMS-DP calipers weigh 1.85 tonnes each and are able to deliver 940kN clamping force. With a full process charge of 1,043 tonnes, the braking system is able to stop a mill in less than two seconds.

To achieve this, the friction material – a blend of non-asbestos friction modifying elements held together with a binder to give a friction coefficient of 0.4 at 300C – has been chosen for grinding mill installations.

The VMS-DP brakes are mounted across the mill flange symmetrically and held in place by two half brackets bolted either side of the pedestal vertical member, ensuring the braking force is equally distributed. Each brake has a spring module with two spring-applied, hydraulically retracted pistons. The module is bolted to a reactive backplate, featuring two cast bosses, one either side of the brake assembly.

VMS-DP is a floating brake (±10mm) with an adjustable clamping force from 735 to 920kN. The brake can be used on installations with a disc or extended mill flange diameter of at least 4.5m and thickness of 117mm to 130mm, but the braking path needs to be 300mm minimum on the radius.

The maximum disc diameter is 20m. The hydraulic power pack offers versatile brake control as it allows both local and remote operation for inching and creeping duties.

Brian Tinham

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