Vibrating screens: how to avoid failures in these assets?

Vibrating screens are complex assets used to separate materials into different particle sizes. They are widely used in mining, but are also found in processes in the construction, recycling, chemical and food industries.

Simply put, vibrating screens work by promoting mechanical movement while the materials pass through metal meshes, being separated according to their volume: the smaller ones pass through the mesh and the larger ones remain on the surface. In this way, screening takes place on a large scale and, with this asset, it is possible to obtain materials in different sizes.

Vibrating screens can have some variations in their structure depending on the type of process they are involved in. They can have different levels of inclination, number of screens or mesh density. They can also have different deck configurations, with variations in the opening and type of mesh. In some cases, it is possible to combine several screens, discharging the material onto one screen and directing it to another, and so on.

The screening stage carried out by this type of equipment is crucial for other stages within the same production line. This is because separating the solids into homogeneous fractions simplifies transportation, storage and even the proper mixing of the raw material into the correct granulometry for other processes. It is therefore common to regularly monitor the performance of this equipment, as it can become a bottleneck in production.

Within the agribusiness sector, screens can take part, for example, in seed processing operations, where they separate the grains by size, roughness and density of the seeds, as well as cleaning and classifying the seeds in the storage and processing stages.

Vibrating screen components  

Three sets of basic components make up screens. They are:

• The screening mesh, the component responsible for separating the screened material according to particle size;
• The suspension system, which dampens the vibration of the asset;
• The motor-coupling-exciter assembly, which induces vibratory movement in the equipment structure.

Screening mesh  

As already mentioned, screening meshes are the components that separate the materials according to their size suitability for the process. To do this, they can have different opening geometries, although rectangular screens predominate. It is the industry segment and the type of material processed that defines the type of screen.

Generally, the mesh composition of vibrating screens is a superimposition of metal meshes made of steel. Nowadays, other types of materials are also used in the meshes, such as a mixture of steel coated with polyurethane and rubber. Metal screening mesh tends to be more expensive to maintain and have a shorter lifespan, but the latter options offer lower costs and can increase the lifespan of conventional metal mesh by up to 12 times.

Polyurethane meshes are more resistant to wear and tear and can be manufactured in different hardnesses. In addition, another function of this type of mesh is to cover the wire mesh for protection and increased durability. In this way, the polyurethane enables good sliding and is resistant to abrasion. Mesh made of rubber, on the other hand, in addition to varying thicknesses, offers much more elasticity than other types.

Note the percentage of open area when considering the screening mesh. This is defined by dividing the sum of all the open areas on the screen by the total area of the screening mesh. In order to maintain the mesh spacing while maintaining the percentage of open area, it is possible to reduce the diameter of the metal wire that makes up the mesh. This results in an increase in the screening speed, but at the same time the mesh becomes more fragile.

Suspension system  

The suspension system of the vibrating screens, made up of coil springs, dampens the vibration generated by the equipment. In addition, this system can withstand high loads of material dumped onto the screen.

For efficient operation, the suspension system needs to provide good damping and maintain elastic force during the equipment’s period of activity. If this component loses its elastic coefficient, its damping capacity is compromised, which can lead to misalignment and other failures, affecting the equipment’s efficiency.

Motor-coupling-exciter assembly  

The role of this set of components is to generate vibratory movement in the screen. The motor converts electrical energy into mechanical energy to carry out the movement of the asset. The motors most commonly used in vibrating screens are 8-pole electric motors with a speed of 750 to 900 rpm. In the case of high-frequency screens, it is common to find 4-pole motors with speeds of up to 1800 rpm. The motors are connected to the unbalance exciters via common couplings, cardan shafts or pulley-belt systems.

The common coupling is the connection system that allows power to be transmitted between the motor shaft and the unbalance exciter shaft, keeping them together and synchronized. The cardan shaft transmits the force from the motor to the vibrating mechanism. It consists of two tubular shafts coupled to the motor and unbalance exciter shafts.

Finally, the pulley-belt system, whose function is to drive the vibrating mechanism, consists of a belt that connects the motor and exciter shafts by means of pulleys attached to each of these shafts. This type of drive makes it possible to control the drive speed of the vibration system.

Vibration mechanisms  

There are three main types of vibrating mechanisms in screens: 

Long shaft mechanism 

The long shaft mechanism has masses that rotate at the ends of the shafts and a heavy piece along the shaft that connects the two sides of the screen. The shafts are supported by large bearings covered by a flange. The counterweights are placed outside the structure and protected by a cover for safety. They are mounted precisely to ensure that the masses are aligned correctly, and each shaft is driven by its own motor.

Box-type mechanism 

In the box-type mechanism, the masses are placed in a single module called a box. Inside this box, there is a gear system that connects the two masses, allowing them to rotate at the same time with just one motor. The gearbox is completely filled with oil to prevent wear and tear and keep it running in sync for longer. This type of vibrating mechanism can be installed directly in the desired location, following structural and safety criteria, and is generally positioned on the upper side of the screen.

Bag-type mechanism  

The bag-type mechanism has a counterweight that can move and another that is fixed. The moving counterweight rotates when actuated and has a different mass to the fixed counterweight. They are mounted at an incline so that there is a force related to the weight of the fixed counterweight, reducing the centrifugal force of the moving counterweight. When the counterweights are aligned correctly, the resulting force is maximized. This system is usually installed in pairs at the bottom of the screen.

Main faults in vibrating screens

The most common failures in vibrating screens are often related to incorrect assembly of the components, contamination of the lubrication, accumulation of material and overloads.

Faults in the screening mesh  

The screening mesh of the processed material is crucial and its integrity directly affects repair costs and time when damaged. Damage to the screen can cause additional problems such as overloading the springs, unbalance in the screening movement, unbalance of the screen itself and premature wear of the suspension system and deck. 

Faults in springs 

Springs are crucial to the operation of vibrating screens, as they support the weight of the equipment and help absorb vibrations. Problems such as broken, weakened or poorly adjusted springs can cause unbalance and inadequate vibration.

Motor or Transmission System Faults

The electric motor and transmission system are crucial to the operation of vibrating screens. Common problems include broken belts, faulty bearings and electrical unbalance.

Faults in unbalance exciters  

In the case of box-type excitation mechanisms, frequent failures are associated with worn or misaligned gears, broken teeth and bearing faults. Bag-type mechanisms, on the other hand, generally show irregular wear on the bearings, while the raceways, rolling elements and cages tend to fail. During the dynamic monitoring of these subassemblies, the synchronous failure frequencies of the motor, gearing and bearings are mainly observed in order to assess the severity and mode of failure during operation.

Most common types of failure in vibrating screens

• Bearings locking and breaking;
• Bearings and motor overheating;
• Screws breaking and loosening;
• Pulley breakage;
• Belt wear or misalignment;
• Misalignment between pulleys and belts;
• Structural cracks in the screening deck or mesh;
• Breakage of the screening mesh;
• In box-type mechanisms, broken transmission teeth;
• Fatigue of the suspension springs;
• Feed overloads.

Check out Dynamox’s Vibrating Screen Failure Guide

Solution for monitoring vibrating screens

For safe and more efficient maintenance, count on the Dynamox Solution for reliable, up-to-date data on the health of the vibrating screens in your asset plant.

Dynamox has wireless triaxial sensors that can be used to monitor motors, bearings and bases. Sensor collection can be automated via Gateways, allowing predictive monitoring of fault symptoms that can be identified, monitored and corrected by collecting trends, spectra, waveforms and temperatures. It is recommended to install HF+ type sensors for motors and bearings.

For bases, the TcAs sensor can be used, since a high frequency value is not required for monitoring this type of component. In addition, it is worth remembering that Dynamox sensors are IP 66, 68 and 69 certified, which ensures sufficient resistance to be applied in environments with a lot of particulates, high temperature and humidity.

To monitor the electrical components of screens, Enging allows electrical analysis to be added to vibration and temperature analysis. In this way, using exclusively electrical variables, the solution develops non-invasive, real-time monitoring solutions that enable early and accurate detection of faults via the web platform.   

Asset Pro – Vibrating Screens  

In order to continuously monitor the performance and health indicators of vibrating screens, the Dynamox Solution also includes Asset Pro, a specialist dashboard for this type of equipment.

Vibrating screens are highly complex assets, with unique characteristics and particularities that make monitoring them a challenging task. Therefore, the aim of Asset Pro is to support maintenance teams in monitoring indicators and identifying faults.

This tool allows for the centralization of various pieces of information about the equipment, such as reports, condition monitoring, automated detection and other specific metrics that provide a broad view of strategic data about the asset’s condition, ensuring more assertive maintenance decisions.

The specialist dashboard generated by Asset Pro makes it possible to visualize the positioning of the sensors on the asset in a similar way to real equipment. This tool makes it easier to identify inefficiencies, whether in one step, on one side or in the screen as a whole, especially by comparing orbits.

Asset Pro is an ideal tool for increasing the assertiveness of asset maintenance and operation decisions:

• Advanced information about the equipment is centralized in the integrated, cloud-based platform;
• Operates based on specific indicators for each asset;
• Maximizes the operational efficiency of the asset.

Monitoring screen performance is fundamental to understanding possible productivity losses and ensuring that the asset is operating in the best possible way.

Australian mining company identifies fault in vibrating screen thanks to Dynamox Solution

Using the Dynamox Solution to monitor its plant’s vibrating screens, one of the world’s largest mining companies in Australia identified a fault in the asset’s bearing when it received an A2 alert from the platform. The asset was in a location that was difficult to access and install sensors.

After the alert, the asset’s Waveform Envelope and Spectral Envelope were analyzed, showing a bearing fault related to looseness due to inadequate interference between the bearing and housing and/or shaft.

Using the autocorrelation tool, the circular form showed that there are 2 predominant points of looseness, with a 180º lag.

The asset was maintained after all the data provided by the Dynamox solution had been analyzed. The envelope waterfall shows that the graphs have returned to normal.

Want to know more about the Dynamox Solution for vibrating screens? Check out our portfolio.