The Ormeau plant uses 8 Malco vibrating screens. Vibrating screens are driven by 7.5KW electric motors running at 1040rpm, the speed is reduced through pulley ratios; the drive is transferred to the screen through dual drive V belts. The V belts turn the main shaft of the screen, attached to the shaft is an offset counterweigh. As the screen shaft turns the counterweigh causes a cyclic weight imbalance, and the screen moves in an orbital motion.
The orbital motion together with the screen declination angle gives the material ‘throw’ and causes the material to flow down the screen. The motion is accommodated by the springs which are located on all 4 corners of the screen body. The vertical motion allows for the vital concept of ‘Separation’ which either causes material to be retained on a screen deck or screened though the screen deck (Malco 1986). The process is continuous except for screen 1, which is turned on and off by the operator to control the amount of material going into the primary jaw crusher .
Malco vibrating screen working principle
Screen 1 instead of using a direct drive from the drive V belts uses a fluid coupling. The fluid coupling allows for a soft start and also as a load dampener. Since screen 1 is stop started approximately 30 times in an hour with material on the screen, to not overload the motor, it needs to be soft started and also an amount of drive slippage between the motor and the screen.
The fluid coupling is oil filled. Once the electric motor is started the impeller begins to turn. Oil is thrown outward strikes the vanes of the turbine causing the turbine to rotate. Therefore power is transferred through the fluid between the input and output. The turbine is connected directly to the driver v belt pulleys. The fluid coupling is not to be confused with a torque converter. The difference being the angle of the turbine vanes.
Malco vibrating screen can be seen how the vibrating screen operates. Material, which consists of a mixture of sizes, is feed into the screen. The mixture of size feed material is vital, as oversized material is needed to hold undersized and near size material close to the screen cloth so that it can be separated and pass through the screen matt. Screening efficiency is affected by many things like: deck motion, speed and throw, frequency of vibration, screen area, retention time, etc. A vibrating screen is generally not 100% efficient in separating materials.
VIBRATING SCREENS PROTECTION SYSTEMS
There are no protection systems for the vibrating screens that will switch the electric motor off in the event of a failure, but each of the screens are connected to a sequence interlock switch. The sequence interlock switch will not let a component start if the screen or conveyor further down the production process is not running first. The advantage of this is that a component that is not running will not be filled with material. Once the component gets filled with material, it must be removed by hand.