What are Eccentric Shafts in Vibrating Screens?

October 11, 2018

In mining equipment, all vibrating screens use some form of kinetic excitation. Linearly casting ore up and forward, freely vibrating screens use a simple excitation mechanism, plus gravity to move material, at which point the undersize is processed and the screening operation continues. Sets of springs reinforce the linear movement. Opting out of this linear sieving mode, eccentric shafts in vibrating screens provide a more rounded vibrational profile.

Eccentric Shafts: Circling the Square

Lines and sharp angles have always been favoured by engineering principles. Even computers prefer lines over arcs and shapely curves. Having said that, mining equipment is expected to process substantial quantities of natural material. Sometimes, working with nature, with rocks and aggregate material, a circular screening mode is considered more effective than the freely vibrating solution. To deliver that circular screening momentum, engineers fit their mining equipment with eccentric shafts. Fabricating the shaft so that it’s mounted off-centre or out-of-balance, the formerly rotating motion, provided as normal by a powerful electric motor, is transformed into reciprocating action. Basically, the screening deck moves in a circle while also moving backwards and forwards in an adjustable line. Let’s look more closely at that linear component.

Adjustable Reciprocating Action

Pegging issues (aggregate particles stuck in the screening apertures) are solved by tilting the screening deck to a moderate 12° inclination. That angle shouldn’t exceed 20°. As this inclination angle on the screening equipment increases, the motor-to-drive coupling drive shafts use their eccentrically shaped rods to expedite circular sieving motion. But now the circle is stretching. It’s becoming elliptical. To really give this elliptical shape definition, equipment designers add a second, perhaps even a third eccentric shaft to the deck’s architecture. Two shafts are responsible for the long axial movements while the third takes care of the shorter axis. Clearly, then, just by adjusting the three eccentric shafts, we can adjust sieving efficiency and reshape the screening ellipse so that it suits a specified aggregate profile.

There are, of course, other solutions to this aggregate “shoving” issue. There are counterweights and specially shaped shaft couplers. Small half-moon shaped components lock against each other and provide vibrational energy. But eccentric shafts in vibrating screens use fewer moving parts, so they’re easier to maintain than those irregularly interlocking shaft couplings. Furthermore, elliptical drive systems are easier to control, more resilient when extreme loads are on the move, and they’re clearly a more versatile mechanism. Used in the mining industry with a large oscillating configuration, those material sieving oscillations can be adjusted to suit other applications, including the pharmaceutical sector.

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