Technical Article

Predictive vs. Preventive Maintenance

May 11, 2020 by Muhammad Asim Niazi

This article covers how predictive and preventive maintenance work, the types of each form of maintenance, and applications in which each is used.

Predictive and preventive maintenance both belong in any maintenance strategy, as they serve the purpose of finding, detecting, and eliminating faults before they happen.

While both are important and are equally used, there is a difference in the application and methodology of each type. One cannot be given importance over the other.

 

What is Predictive Maintenance? 

Predictive maintenance is maintenance carried out after predicting any type of part failure/performance reduction which results in reduced output or breakdown of a machine. 

 

How does Predictive Maintenance work?

Depending on the complexity and design of the system, there may be many parts of predictive maintenance, but most commonly it works in three different steps:

  1. Essential data
  2. Data storage
  3. Data processing

 

Let’s look at the role of each of these three steps.

 

Essential Data 

The first step in implementing predictive maintenance requires the gathering of relevant data. This data is collected by different sensors installed at critical and important parts of the system. Also referred to as condition monitoring, this data helps to analyze the condition of the machine. The data can be collected during operation, called Online Monitoring, or when there is no operation, called Offline Monitoring.

 

Data Storage

The data collected from sensors must be stored in a reliable platform which makes it impossible to change. At the same time, it also makes it available when it’s required by authorized personnel at a desired station or location.

 

Data Processing

The data collected and stored must be processed to extract the required information. This processing is done according to predefined algorithms. These algorithms use current data along with historical trends to predict possible malfunctions and the type of malfunction at a pinpoint location. The algorithm is developed according to the needs and expectations of an individual organization or company.

 

Types of Predictive Maintenance

There are quite a few types of predictive maintenance used in different facilities. They include:

  • Vibration analysis
  • Oil analysis
  • Motor analysis
  • Temperature measurement
  • Ultrasonic Monitoring

 

Vibration Analysis

Vibration analysis is used for rotating/moving mechanical parts. Normally any rotating or moving part creates vibration, but any change in the levels of vibration is not a good thing. This maintenance technique can be used to indicate problems with the bearing, misalignment between rotating components, or issues with the shaft in a rotating mechanism.

 

Oil Analysis

Oil is an important component of machinery like engines, compressors, pumps, and gearboxes. So the analysis of oil can greatly help maintain the condition of the equipment. Tests such as viscosity measurement, moisture analysis, and contamination levels indicate the health of the equipment.

 

Motor Analysis

Parameters of motors such as current, voltage, and resistance can indicate the health and condition of the motor. 


Figure 1. All industrial motors must be checked regularly to ensure they stay in the best working condition.
 

The condition of the motor, in turn, provides information about the attached mechanical assembly like a blower or a gearbox.

 

Temperature Measurement

Temperature is the most common indicator of the condition of electrical, mechanical, or HVAC systems. An Increase in temperature levels can be due to an increase in friction levels due to blockage or jamming of a mechanical part. Also, it can relate to electric-related failures. 

 

Ultrasonic Monitoring

Ultrasonic monitoring uses high-frequency sound waves to detect faults in a system. Any ultrasonic changes can be used to detect changes in the behavior of the system, which results from a variety of faults. This technique can be used both for electrical and mechanical systems.

 

Examples of Predictive Maintenance

To better understand the types of predictive maintenance, let’s look at a couple of examples of predictive maintenance out in the field.

 

Temperature Measurement of Electrical Panels 

Temperature measurements of electrical panels can be performed to detect the uneven distribution of load, improper connections, or under or overrated installed components. All these are represented by the uneven distribution of temperature value displayed on the device. More commonly, this test is performed using thermal imagers. The major advantage of temperature measurements is they can be performed without dismantling the electrical panel.

 

Figure 2. Thermal imaging of an electrical component. Image courtesy of Fluke Corporation.

 

 The major advantage of temperature measurements is they can be performed without dismantling the electrical panel.

 

Oil Analysis for Power Generators 

Oil is used in power generators for a variety of purposes: to reduce wear and tear between different moving parts, cooling purposes, as a cleaner, to prevent corrosion, and more. Analyzing oil helps to detect changes in its basic composition by performing tests like moisture analysis and viscosity measurements.

Analyzing oil also helps in the detection of contamination and the possibility of debris from any mechanical part. It gives important information about the actual condition of the internal parts that aren’t visible to the human eye.


What is Preventive Maintenance?

Preventive maintenance is the term for maintenance carried out according to a predefined schedule, irrespective of the condition of the equipment or part. 

The main idea behind preventive maintenance is to look for any possible cause of equipment failure that would result in a breakdown. The planning of equipment can be done according to the usage of equipment or the time elapsed. 

 

What are the Steps of Preventive Maintenance?

The key to preventive maintenance lies in following a set of steps.

  • Planning of Preventive Maintenance: It starts with the development of a schedule, more often at the beginning of the year. Maintenance is planned in monthly, quarterly, semi-annually, or yearly groups.
  • Checklist: A checklist of maintenance procedures is prepared to make sure all the necessary and important maintenance procedures are performed.
  • Approval of the Schedule: The schedule is reviewed and approved by the respective heads of the department i.e., production and engineering. 
  • Distribution of the Schedule: After approval, the schedule is distributed to all concerned departments so they are informed about the maintenance frequency and can set production plans accordingly.
  • Execution of the Schedule: When the time comes for scheduled maintenance, the engineering team informs the operator in charge of the machine, and the maintenance is executed.
  • Documentation and Record-Keeping: After the preventive maintenance is completed, a checklist is filled out by the tester which is then verified by both engineering and production heads. If a discrepancy is found, it is recorded and further action is carried out to fix the issue.
  • Handing Over: In the last step, the machine is handed over to production by make sure that the machine is in running condition.

 

Areas Where Preventive Maintenance is Required

Electrical Panels

In an electrical panel, the state of the electrical components are checked. This includes tightening of screws, checking the panel fan, removing dirt, and moisture cleaning. 

 

Electrical panels must be maintained on a regular schedule.

 

Special attention should be paid to power incoming and earth connections.

DC Power Supplies 

Since the majority of components run on DC power supply, it’s important to make sure that the output of the power supply is within range.

VFD and Motor

VFD and motor maintenance include checking for correct parameterization of the VFD, condition of the VFD connections, the motor fan (if installed), condition of the terminal box of the motor, visual inspections of the motor, and correct current rating of the motor.

Field Instruments and Wiring 

For field instruments and wiring, the condition of the wiring should be checked for any damage and proper insulation must be maintained. Sensors should be properly fixed and free from any loosing.

Lubrication

Lubrication maintenance is required for all machinery that uses oil. The required oil quantity should be up to the fill mark and the correct type of oil should be used. Also, there should be no leakage of oil throughout the system.

Mechanical Assemblies

Mechanical assemblies should be fixed and free from any loosing. Also, check for any unusual noise and movement.

Moving Parts

Moving parts should be properly aligned with each other. If belts are used, make sure that the condition of the belt is satisfactory and properly aligned.

Gear Box 

Check for oil level, the condition of the mechanical seal, any noise or vibration, and overheating of the gearbox.