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Your next big brake...
Controlling loads in the heaviest engineering applications can require brakes capable of generating torque measured in the millions of newton meters. Tony George of Twiflex explains how these giant brake systems are engineered and looks at design considerations.
As the quest to improve efficiencies across all markets and industries continues, so demands for higher production rates, bigger loads and faster speeds have a knock-on effect on the design of equipment and its associated safety systems, including brakes.
Take mining for example, the winders used to hoist people and materials thousands of metres can require braking systems with a capacity of 10 million Nm or more. One recent project, for a double drum winder at a copper mine in Zambia, needed a braking solution for parking/holding and emergency-stop duties on a 47.5 tonnes payload. Twiflex supplied calipers derived from its existing VMS3-SPS unit, with increased pad area to deal with the high thermal loads generated during dynamic stopping. The brakes operate on two 6.8m diameter discs and produce a total torque of 12.2MNm.
The very largest brakes are found elsewhere in the mining sector. Grinding mills, which reduce the particle size of crushed ore for subsequent processing, require brakes to perform several critical functions. They are used to hold the mill for maintenance, to stop rotation in the event of power failure, and to inch out-of-balance loads to a neutral position. The biggest gearless mill designs have a brake disc profile around a drum that may exceed 12m in diameter. In such applications, typical scope of supply may include six or eight of the largest VMS-DP calipers, generating torques in excess of 50MNm.
The most common brakes for large applications are spring applied, hydraulically released designs, capable of holding the load in the event of a power failure or loss of hydraulic pressure. The braking force – the tangential force acting on the disc – generated by each caliper is a function of the spring pack that creates the clamping effort and the lining materials that generates the friction with the disc. The overall braking torque is determined by the braking force per caliper, the number of calipers employed and the diameter of the disc.
As a result, the specific torque requirements of any given application can usually be achieved in several ways. A key role for the application engineering teams at manufacturers like Twiflex is to work with its customers to offer an optimum commercial solution. That would be a solution that meets the technical needs of the application, while taking account of the installation environment (including space constraints, and the working conditions); and one which considers the whole lifecycle of the equipment, including installation and commissioning, operation and maintenance.
Where brakes see high cyclic duties, the frequent application and retraction of the brake can affect the life expectancy of the springs that generate the clamping force. In these applications, it is necessary to calculate the fatigue life of these springs in consideration of the expected brake operations, and to balance the need for a high fatigue life with the physical size of the resulting spring module.
Similarly, extreme operating temperatures will influence the choice of materials used in the brake. Twiflex brakes are typically cast in a ductile iron, which is suitable for dynamic braking at temperatures down to –20°C. If the application demands it, low-temperature material grades permit some brakes to be manufactured and specified for use at operating temperatures as low as –40°C.
While some specific application requirements may require the development of a new caliper design, most braking systems are based on combinations of standard, modular components. This modular approach offers numerous benefits for end users: simplifying the design process, controlling purchases costs and ensuring the availability of parts and service support over an operating life that can extend into decades.
All Twiflex modular brakes can be installed, set and maintained, including replacing linings, using standard hand tools and without dismounting the caliper. The weight of the caliper itself often precludes its removal; the VMS-DP for example approaches 1,900kg per brake. The modular nature, however, means that removal of the whole assembly is rarely required, with parts or sub-assemblies easily handled.
The newest design in the modular brake range is the recently-launched type VSD caliper. This unit replaces the proven Twiflex “VS” caliper that has been in service in heavy-duty applications around the world for several decades. The VSD follows the design-language of the smaller VKSD brake, and fills the gap in braking force range between this and the larger VMS unit.
Now based on a split caliper/two-module design, the VSD can inherently accommodate any disc thickness from 25 mm upwards. Unlike the VS, this unit is also available as a floating brake – a single spring module and a ‘reactive half’ that, when assembled on a dedicated bracket, can move with any axial misalignment of the brake disc.
The VSD also includes the ‘parked-off’ feature which is a development from Twiflex. A unique maintenance feature, the parked-off state means that hydraulic pressure may be dropped with the brake pads fully retracted while, at the same time, the springs are at their free length. This is a completely benign condition that permits the brake to be set, adjusted or maintained with no potential energy left in the caliper.
The demands placed on major industrial assets continue to rise. Mines are getting deeper, for example, and processing equipment must be built to handle ever higher volumes and faster production speeds. Delivering the higher braking forces required by those applications will create significant engineering challenges. By working with a manufacturer with extensive knowledge and long experience of the most difficult braking applications, equipment designers can be sure of solutions that deliver those forces consistently, reliably and cost-effectively.
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