Page 45 - HUB-4 Magazine Issue 97

Aggregate Washing & Screening
Shaking out the details
on Screen Vibration
By Susie O. Bartoli / Business Development Specialist-Vibration / Martin Engineering
From quarrying and mining to recycling and agriculture, a
multitude of industries rely on screen vibration for various
purposes, but they often encounter similar problems. If the
screening rack is properly engineered to accommodate the
weight, volume, and type of motion, then the support structure
will rarely require maintenance. However, vibrators continually
combat resisting forces, as well as the harsh industrial
environment of bulk handling, which reduces the equipment
life. [Fig.1]
Figure 1 – Screen vibrators are wear parts, so ergonomic access is
important for workplace safety.
The period between potential breakdowns depends on several
factors, including the operating environment, centrifugal force
requirements, the condition of incoming power, lubrication
type and intervals, and the vibrator's design and build quality.
Volumes and Vibratory Screening
Since the early 20th century, the demand for mineral resources
has steadily grown, forcing bulk handlers and material
recyclers to continually increase production volumes.[1]
Simply increasing the volume or speed of throughput might
require the material to undergo several passes to achieve
proper separation. [Fig.2]
Factors operators need to consider in vibratory screening
applications:
• Efficiency – The volume of the material stream determines
the amount on the screen, which can influence how well it
is separated.
• Material – The speed, volume, and desired output of
screening are dependent on the properties and angle of
repose of the application.
• Angle of repose – The angle of repose is how the material
spreads naturally across the surface. The loading method
and type of vibratory motion assist in even distribution
across the screen.
• Screen design – First, the screen must be able to withstand
the additional weight. Then, whether dewatering or
separating, the grid pattern, permeability, and type of
vibratory motion need to promote the application’s
separation.
• Effective Slope – The angle of applied vibration determines
the speed at which the material moves down the screen.
Gravity, amplitude, and frequency are factors in the screen’s
effectiveness.
• Vibratory amplitude and frequency – Increasing either the
amplitude or the frequency will increase the volume of
material moving through the system.
• Vibratory motion – There are two general vibratory
motions, linear (back-and-forth or up-and-down), and
elliptical (rotational).
Failure Mechanisms in Screen Vibration
When a screening rack goes out of service, it can significantly
impact production. Simply adjusting the volume of material
fed to the remaining units accelerates wear and tear, which
raises the likelihood of additional breakdowns. This downtime
increases labor costs, increases exposure to safety risk, and
elevates the cost of operation.
Failure mechanisms are separated into five distinct
categories:[2]
1. Design Deficiency – Inadequate engineered design either in
the screen structure or the vibrator. [Fig.3]
Figure 2 – The steady increase in demand requires managers of older
screen systems to increase production beyond the original specs.[1]
Figure 3 – A well-designed screening system is extensively tested in many
applications to find deficiencies.
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