Shaking out the details on Screen Vibration

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]

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 20^th 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]
2. Manufacturing and Assembly Process Deficiency – Structural adjustments post-installation are common and can change the center of gravity or increase the machine's weight, which reduces efficiency.
3. Operational Deficiency – Increasing volumes that exceed the recommended limits or the miscalibration of vibrators that put an undue strain on units are common operational deficiencies.
4. Inadequate Maintenance – Neglecting to monitor pads and springs on screening structures, not following lubrication procedures, and not addressing rust promptly are common and costly failure mechanisms.
5. Wear and Deterioration – These factors are unavoidable, but proper installation and maintenance of quality equipment can extend the system’s lifespan.

Vibratory Amplitude and Frequency

Amplitude refers to the magnitude or intensity of the vibration, indicating how far an object moves from its resting position.  On the other hand, frequency indicates how many cycles of vibration occur per unit of time (measured in Hertz).  Higher frequency can increase volume but has the potential to increase screen wear and stress on the frame if force outputs are not considered.

Vibrator Motion and Placement

The placement and angle of the vibrators influence how the motion occurs.  With the correct slope and amplitude calculated, gravity will assist the material down the screen. 

Linear side-to-side motion is effective for sectors such as mining and recycling, as it allows material to spread across the screen.  Mounting vibrators on the sides of the rack causes larger materials to shift and roll to each side. 

Linear up-and-down motion is highly effective for sand biofiltration and particulate dewatering.  With vibrators mounted on rails above the material flow, the internal weights lift and drop a specialized screen made of durable fine mesh. [Fig.4]

Elliptical rotational motion is prominent in agriculture for the threshing process, but also in recycling and bulk processing. Set in front of or behind the screen, the action causes a “cheese grater” effect.

Electric and Hydraulic Screen Vibrators

The weight and volume of the bulk handling environment require a powerful vibrator to do the job effectively.  While electric screen vibrators are common, hydraulic vibrators are more applicable for portable screen applications where no electricity is available.  [Fig.5]

Industrial electric vibrators for material screening should use inverter-duty rated and Class H insulated windings to prevent overheating in continuous-duty environments. Martin® Engineering’s Screen Vibrators produce up to 16,500 lbs (7,483 kg) of centrifugal force. The adjustable eccentric weights tailor performance for 3- or 4-panel screens, featuring low-maintenance, greaseable, long-lasting cylindrical roller bearings.

An example of a hydraulic screen vibrator is the Martin® Hydraulic Screen Vibrator, which provides up to 8,300 lbs (3,855 kg) of centrifugal force for efficient material separation.  The IP66 enclosure ensures a dry and grit-free internal environment and uses the same mounting bolt patterns as electric vibrators. 

Conclusion

A well-constructed system, along with proper calibration and placement of vibrators for the application, is essential for efficient and optimized screen separation. To further extend the equipment’s life, these systems should be easy to maintain and accessible, ensuring both workplace safety and lower operational costs. 

Research

[1] B. Ramatsetse, K. Mpofu, O. Makinde, et al; Design And Structure Optimization Of A Reconfigurable Vibrating Screen For The Mining And Mineral Processing Industries. Tshwane University of Technology. New technology and innovation in the Minerals Industry Colloquium, Southern African Institute of Mining and Metallurgy, Emperors Palace, June 2016.

[2] T. Linhares , A da Silva Scari , C. Vimieiro; Causes Of Failures In Vibrating Screens: A Literature Review. Minerals Engineering, Volume 218, November 2024