Modern Mining March 2020

VIBRATING SCREENS

manufacturing a screen that offers optimal cost of ownership. For example, the correct choice of isola- tors is significant. “The screen stands on an isolator and it’s an important factor when designing a vibrat- ing screen. It needs to be sized according to the size of screen and type of building structure” says Schoepflin. There are three basic isolators used in vibrating screens – coil spring, rubber buffer and a torsional spring. A coil spring typically gives the best isola- tion because its stiffness is linear across the stroke range. A rubber buffer, although non-linear, offers a better load handling and damping capacity. A tor- sional spring can be mounted on the support frame and that completely prevents sideways movements and keeps the machine in line and stable during start-up and stopping. Starting and stopping of a vibrating screen needs to be carefully considered in the design process. In the tests done by Kwatani on a specific screen, a coil spring takes 50 seconds to stop, a rubber buf- fer stops at 21 seconds, while the torsional spring stops within 12 seconds. “For cost-effectiveness, if the machine needs to be stopped in a certain man- ner, we can adopt a hybrid approach because a torsional spring is a bit expensive. We can provide the required stability by combining a torsional spring and a coil spring to get the cost element in line – and this has a vast impact on the total cost of ownership,” says Schoepflin. Another important parameter is the testing element. Every Kwatani unit is tested before it’s shipped and all units are commissioned by the company’s experienced and qualified technicians. This process ensures that the unit meets the design parameters that it was designed and fabricated for. “This is done to make sure that the unit meets the desired quality and robustness,” says Schoepflin, adding that in terms of quality, the OEM is one of the only 5% of local manufacturers of its size to be ISO9001:2015 certified. Reiterating the company’s testing capabilities, Schoepflin makes special mention of the company’s variable speed drive that has been connected to the test bench with specialised monitoring software. This allows Kwatani to experiment with the gearbox at dif- ferent speeds. “This allows us to be more accurate when specifying the required drive for the vibrating screen or feeder, so that we can specify the right size and capacity in a more scientific way,” she says. “For instance, we may find that a smaller drive can provide the motion required without necessar- ily needing a larger drive, allowing the customer to save on energy costs. We are able to size the drive to the need, because often a larger drive means big- ger upfront costs, and higher energy consumption.” Uptime and maintenance costs Schoepflin says in a sales situation, often customers

cost more than the price of a new screen. Therefore, there is need to consider where the screen will go in the existing plant setup, its impact on the exist- ing infrastructure, for example, power requirements, available headroom, weight restrictions and existing shoots, among others. The information is then disseminated into the mechanical design process where the equip- ment is designed with the application and existing infrastructure in mind. When designing, Kwatani con- siders many factors that affect the life of the screen, efficiency and performance. The aim is always to produce a durable, long-life screen that does the job properly. “These factors have a massive effect on the total cost of ownership equation, and they have to be taken into account, while at the same time meeting the tonnage and process efficiency requirements. It is always important to design a solution that can give the most optimal output at the given infrastructure,” she adds. Correct choices Schoepflin also reasons that the choice of various components of the screen, depending on the appli- cation requirements, is important in designing and

Kwatani has seen mines gradually embrace the value of maintenance contracts to avoid costly downtime.

Key takeaways  The design and engineering of a vibrating screen has a great bearing on total cost of ownership  Knowing the exact type of machine that should be inserted in a plant, understanding the requested function and developing a machine that best suits these needs is very important  Reliable vibrating screen designs are dependent on the proper marriage of the manufacturer’s capabilities and the understanding of requirements of the design  The choice of various components of the screen, depending on applica- tion requirements, is important in designing and manufacturing a vibrating screen that offers optimal cost of ownership

30  MODERN MINING  March 2020