Category: Applications and good monitoring practices articles

Vibrating screens are used to separate materials according to their size and are especially important in mining processes. This machine basically consists of a motor, exciter mechanisms, bearings, screen mesh and vibration attenuators. It works by means of a precise vibratory movement with a suitable inclination angle, which provides benefits such as saving energy and reducing costs in the screening process.

Various components of a vibrating screen can fail, leading to repairs, downtime and various costs. The implementation of a continuous monitoring system for failure detection and other predictive maintenance strategies are fundamental for increasing the safety and reliability of this type of machinery.

Although the entire system has several vibration loads, which are inherent to the process or its components, it is still possible to relate these loads to specific parts of the equipment or the process that the equipment goes through. For example, harmonic loads are related to the vibrational isolation present in vibrating screens, or even transient vibrations can arise due to a loading impact/impulse on the screen.

In order to assertively monitor vibrational behavior, the installation of vibration sensors can become an important ally. However, when wired systems are installed in this type of machinery, they often result in high maintenance costs for cables and infrastructure. Thus, there are huge advantages to using wireless sensors, such as little or no maintenance costs, easy installation, automated and continuous monitoring (with the help of a gateway), among others.

To obtain good results with wireless vibration sensors, certain precautions must be taken, such as choosing a suitable sensor, mounting it correctly on the components to be monitored and setting an appropriate operating configuration.

As a practical example, for a screen like the one in the figure below, driven by exciter boxes, we recommend installing HF+ sensors, measuring up to 13kHz (blue DynaLogger), for motors, bearing housings and exciters. For the spring bases, it is recommended to install TcAs sensors of up to 2.5kHz (green sensor).

Figure: Indication of sensor mounting on vibrating screens

 

Location on the screen	Number of DynaLoggers	DynaLogger
Motor	2	HF+
Bearing housings	2	HF+
Exciter 1	2	HF+
Exciter 2	2	HF+
Spring basis	4	HF+ or TcAs
TOTAL	12

In general, when monitoring motors, we recommend using two sensors, one for the drive end (DE) and one for the non-drive end (NDE). For the exciter mechanisms, we recommend using a sensor for each bearing housing. A DynaLogger HF+ is recommended per exciter cell. In the case of external bearings, HF+ sensors are also used for each bearing housing. Below are some photos of the installation on these components.

Motors

Circular exciter cells, exciters and bearing housings

In circular exciter cells, due to the casing, it is often not possible to mount the sensor directly on the component, so it is mounted as close as possible to the casing.

Spring bases

For the springs, it is recommended to install TcAs or HF+ sensors. Each of the spring bases should have a sensor installed in order to monitor the movement of the screen and compare the right and left sides, as well as the loading and unloading parts, in addition to the diagonal direction.

Field photos of sensors mounted at these locations:

It is worth mentioning that Dynamox’s sensors are IP 66 and IP 68 certified, guaranteeing sufficient robustness to be applied in environments with large amounts of particulates, high temperature and humidity, without compromising the quality of the data generated and without the need for maintenance or human intervention in collection.

Conveyor belts are widely used in the transportation and material movement in diverse production processes. Generally, the equipment has a constant flow, moving materials from one point to another of the production process. The equipment is basically composed of motors, gearboxes, pulleys and their respective bearing casings.

The motor is responsible for activating the machinery that, due to its low operation speed and high loads, requires a coupled gearbox. Motors and gearboxes have already been specifically addressed before, so we will give more attention to the conveyor belt pulleys in this article.

     

Figure 1: Positioning and sensor model  indication for motors and gearboxes
 

The pulleys are responsible for activation, return, stretching and belt back, depending on their position on the equipment.  All pulleys are fixed to the equipment structure by means of bearing housings. The bearing housings, in turn, consist basically of bearings, bushings and nuts, and can be manufactured with a variable number of rolling elements, sizes, formats, etc. 

Keeping  bearing casings in good operating condition is an essential task for safety  and good performance of the production process.  The most common bearing failure modes are linked to the following causes:

• Handling and transport;
• Assembly errors;
• Excessive stresses and loads;
• Misalignments;
• Inefficient lubrication;
• Inefficient sealing.

These faults can generate defects in the cage, balls or rollers, inner and outer races.

These defects are identified through predictive maintenance, using vibration analysis. Two sensor models can be used to successfully monitor pulley bearings: 1) DynaLogger TcA+ with maximum frequency of 1KHz and 2) DynaLogger HF with maximum freq. of 6.4 KHz. Both models are IP66 and IP68 certified, being impermeable to dust and resistant to liquids, making it possible to install near the belt.  The sensor must be placed parallel to the rotation axis, and avoiding flexible bases and covers.

Figure 2: Conveyor belt and bearing housing of the drive pulley in detail





Figure 3: Conveyor schematic with application of DynaLoggers HF sensors in pulley bearing housing
 

In order to demonstrate the good use of sensors in conveyor belt pulleys let’s take a brief example.

Example: Fault monitoring appearing in the frequency of the outer race (highlighted in the green lines) of the 23152 CCK/W33 bearing with 84 RPM.

Figure 4: Fault spectra on bearing outer race

How to monitor electric motors with the DynaPredict Solution:

Electric motors are responsible for the operation of the majority of industrial equipment to generate movement. Failures in this component may cause a halt in production as well as accidents.

Monitoring operational conditions is an important activity in the search for reliability of these assets.

Dynamox’s sensors allow for the online monitoring of parameters such as vibration and temperature which allow for the identification of potential defects and planned interventions, thereby reducing production downtime and increasing the efficiency of the production process as a whole.

Regarding the position of sensors at an electric motor, it is recommended the monitoring be done either from the drive end (DE) or non-drive end (NDE) as shown in the image below.

OBS: the NDE spot may offer certain challenges to the sensor mounting, because of the protective cover. If necessary, consider drilling a hole or cutting a notch in the cover to allow access to a rigid mounting location. Never install sensors on the motor terminal or cooling fins, which are parts that emit high levels of vibration and make fault analysis difficult.

Regarding this equipment, two types of sensors can be used:

•  DynaLogger HF+. A sensor with a maximum frequency of 1300 Hz. This sensor is recommended for motors coupled to equipment of most importance. Due to its high frequency range, it is possible to identify early stage bearing or lubrication defects, as well as faults in the rotation frequency of the machine, such as with the TcAs.
• DynaLogger TcAs . A Sensor with a maximum frequency around 2500Hz, which is recommended for low to medium criticality machines that do not present a history of bearing or lubrication related failures, since these defects may usually appear at higher frequencies. These sensors allow for the detection of faults in the rotational frequency of the machine, such as unbalance, misalignment, looseness and others. Besides,  this sensor can be useful to detect bearing defects in the later stage.

 

For the DynaLogger mounting, there are two main alternatives: studor adhesive mounting. See more details here under item 4.

Regarding defects,  the following root causes related to the machine problems, in terms of the axis and its characteristic frequencies, are indicated below:

The following sections will present best practices in the positioning of Dynamox vibration and temperature sensors for typical industrial machinery, such as motors, pumps, compressors, exhaust fans, etc.

The objective of the guide is to assist the user in choosing a suitable location for installation of the sensors, helping to obtain reliable and quality data that allows an effective predictive analysis.

The guide will also indicate which sensor best applies to each machine, focusing on the following models:

DynaLogger HF + : Triaxial temperature and vibration monitoring with a maximum frequency of 13000 Hz, performing telemetry and spectral analysis
DynaLogger TcAs : Triaxial vibration and temperature monitoring with frequency up to 2500 Hz, performing telemetry and spectral analysis
DynaLogger TcAg : Triaxial vibration and temperature monitoring with a maximum frequency of 2500 Hz. 

NOTE: The information contained in these sections are only recommendations and should not be taken as positioning rules.