Mining Mirror May 2017 - Page 26

Mining in focus Kwatani screen used in manganese extraction, hoisted and prepared for transportation to the customer. buildings where the concrete floor may have disintegrated over time. “If one corner of the vibrating screen drops by only 10mm on a weak concrete floor, the vibrating screen runs in a twisted state, which increases the possibility of inefficient performance and eventually downtime,” says Mayhew-Ridgers. “This can easily be prevented by inserting a spacer plate between the floor and the affected pedestal to get the surface level back at the correct height again,” he adds. Even if one or two pedestals point inwards or outwards, the isolators stand at an angle, which could create problems. Another possible cause of breakdown is if the cardan shaft spline, which connects the motor base with the exciter gearbox, doesn’t have enough play on both sides. If there is insufficient play, the spline of the cardan shaft could bottom out and [24] MINING MIRROR MAY 2017 cause damage to the bearings on the exciter gearbox as well as the motor base. Alternatively, if the gap in the spline is too big, the spline can come loose when the vibrating screen stops and disconnects the screen from the motor base. According to Mayhew-Ridgers, operators should also make sure that the cable entry point into the terminal box of unbalanced motors (an alternative exciter solution to an exciter gearbox) should be from the bottom up, especially in wet applications. When the entry point is exposed and susceptible to water infiltration, especially if the terminal box lid is open, or if the O-ring or gaskets are not in place, water will enter and wreck the electrics. “These are the small details that artisans, normally in a rush, tend to forget,” says Mayhew-Ridgers. The danger of tightening bolts Another issue that could cause damage if not properly managed, is tightening the bolts. “Whether it is replacing the entire vibrating screen or just replacing the exciters, torqueing is of the utmost importance. Artisans tend to flog the bolts when they refit exciter gearboxes or unbalanced motors. They have a flogging spanner and hit it with a hammer. The torque on most machines are of a high value — sometimes more than 1 000–1 500 newton meters — and flogging it with a hammer will not get the bolts to the required torque. The exciter gearbox or unbalanced motor could loosen. This in turn can then cause damage to the mounting plate underneath the exciter. “If the top surface of the mounting plate is not planed to make sure it is perfectly flat, the exciter will continue coming loose, as the surface onto which it should be