The performance of a machine or equipment depends not only on its design and manufacturing operating environment, usage, and so on but also on the maintenance carried out since proper functioning over an exteded time period requires effective industrial maintenance.
Definition Of Preventive Maintenance
“Preventive maintenance actions are carried out according to prescribed criteria of time, usage, or condition and are intended to reduce the probability of failure or the functional degradation of an Asset or Machine.“
Preventive maintenance is an absolute requirement of asset reliability and effective management of asset life cycle costs. An effective maintenance organization must dedicate a portion of its craft workforce, as well as planning and supervisory support with timely execution of preventive maintenance activities.
Preventive Maintenance actions increase the lifetime or reliability of the manufacturing machine or asset and keep them into the fully functional stage.
The aims of Preventive Maintenance actions are to:
- Prevent failure.
- Detect the onset of failure: Whilst we may not be able to prevent a failure, frequently we do know how to detect the onset of failure. The knowledge of how to do this is increasing every day, through Machine history records & condition monitoring technologies.
- Find hidden failures: Check to see if a failure has occurred before equipment is called into service.
Preventive Maintenance includes both short downtime activities, such as visual inspection, lubrication, testing, planned replacement of parts or components, and also a major amount of downtime activities such as overhaul that require proper planning and resources.
Types of Preventive Maintenance
Preventive Maintenance actions can be grouped into three following categories:
- Predetermined Maintenance
- Condition‐based Maintenance
- Opportunistic Maintenance
Predetermined Preventive Maintenance
The Predetermined Preventive Maintenance activities are carried out at discrete time instants by some predetermined rule. This measurement of timing involves either by a clock or by the usage.
Preventive maintenance is a maintenance activity repeated at a predetermined frequency. The frequency may be based on calendar time or other occurrences such as service hours or the number of operations. For example, someone may change the oil in a car every 3000 miles or every 3 months. Predetermined Preventive Maintenance actions can be further divided into two following subcategories:
Clock Based Maintenance Actions
Clock based maintenance actions may define three types of clocks and, as a result, we have three further subcategories of predetermined maintenance actions.
Calendar clock: This is the familiar clock with a fixed starting time – the start of a new year. As such, maintenance actions are carried out at predetermined time instants based on this clock. One such policy is the block replacement used for a fleet of identical items (such as Road lights in a State Highways) where all of them are replaced periodically.
Age clock: Here, a familiar clock which is set to zero when an item is put to use. Preventive Maintenance actions are carried out at predetermined time instants using this clock. In other words, the Preventive Maintenance actions are based on an item reaching some age.
The above two clocks run continuously, and they are appropriate for items in continuous operation over time. The next clock is appropriate for items used intermittently.
Usage clock: Here, the age clock which stops when the item is not in use (either due to idling, or undergoing Preventive Maintenance or Corrective Maintenance action). An example of this is the usage clock which clocks the number of hours run for a Railway engine.
Usage‐Based Maintenance Actions
Preventive Maintenance actions are based on the usage of the item. Usage can be measured in different ways such as output (number of take‐offs and landings for landing gear, the number of strokes produced by a stamping press tool, etc.). This is appropriate for items from various sectors (such as manufacturing, transport, etc.) that are used either continuously or intermittently.
Predetermined Preventive Maintenance actions based on a calendar clock are easier to administer than those using age‐ or usage‐based clocks. However, the latter reflects the degradation in a more sensible manner and hence is preferred in many cases.
Condition‐based Maintenance Actions
When Preventive Maintenance activity is done by the parameter measure with the degradation over time and the onset of failure, these preventive maintenance activities known as conditional-based maintenance actions. These time instant parameter may also be defined by the data collected using appropriate condition‐monitoring techniques.
The parameter may be monitored continuously or at discrete points in time. In the case of monitoring at discrete points in time, the monitoring frequency needs to be determined.
The most important factor is the ability to identify a measurable parameter that provides a correlation between the measurement and the level of degradation. A combination of the understanding of failure mechanisms, availability of the appropriate measurement technology, and analysis tools is vital for Condition Based Maintenance implementation.
CBM actions can be further divided into two categories. In both categories, the data are analyzed to assess the condition or state of the item. Both categories are as follows :
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Functional tests:
The focus is on deciding whether to take action (such as to repair or not) immediately or later.
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Condition monitoring and inspection:
Extrapolating the degradation into the future (through trend analysis) and predicting when action might be needed. This allows some time for proper planning of maintenance activities and the necessary resources needed. A proper understanding of the failure mechanisms is critical for CBM, and proper modeling is needed to predict the residual life to plan effective maintenance strategies.
Predetermined Preventive Maintenance actions are unable to avoid many spare parts failures because they do not take into account the condition of the item. The only way of reducing the probability of breakdown is through early replacements.
Opportunistic Maintenance Actions
Opportunistic maintenance actions are carried out at convenient moments which are unpredictable and they can be categorized broadly into the following two groups:
Internal to the object:
Failure of a component providing an opportunity to carry out Preventive Maintenance actions on some of the non‐failed components.
External to the object:
The object being in the idle state due to external factors (such as production ceasing due to running out of inputs or excess inventory resulting in a temporary shutdown of the plant) provides an opportunity to carry out Preventive Maintenance actions.
Preventive Maintenance Checklist
Inspection:
Check for conformity by measuring, observing, testing, or gauging the relevant characteristics of an item. Generally, the inspection can be carried out before, during, or after other maintenance activities.
Compliance test:
The test used to show whether or not a characteristic or a property of the item complies with the stated specification.
Monitoring:
Monitoring is distinguished from inspection in that it is used to evaluate any changes in some parameters of the equipment with time. Activity performed either manually or automatically which is intended to observe the actual state of the item. Monitoring may be continuous, over an interval of time, or after a given number of operations. Monitoring is usually carried out in the operating state.
Routine maintenance:
Routine maintenance may include, for example, cleaning, tightening of screws & nuts, tightening the electrical wire connections, checking liquid level, lubrication, and so on.
Overhaul:
A comprehensive set of examinations and actions carried out in order to maintain the required level of availability and safety of the equipment. An overhaul may be performed at prescribed intervals of time or after a number of operations and may require a partial or complete dismantling of the item.
Rebuilding:
The objective of rebuilding is normally to provide the item with a useful life that may be greater than the lifespan of the original equipment. Action following the dismantling of the item and the repair or replacement of those components that are approaching the end of their useful lives should be replaced regularly. Rebuilding differs from overhaul in that the actions may include improvements and modifications, understood as follows:
Improvement:
Combination of all technical, administrative, and managerial actions intended to make better the dependability of the item without changing its required function.
Modification:
Combination of all technical, administrative, and managerial actions intended to change the required function of the item. Modification, in fact, is not a maintenance action but concerns changing the original function of the item to a new required function. The changes may have an influence on the dependability or on the performance of the item, or both.
Repair:
Physical action is taken to restore the required function of faulty equipment. Within a repair we can normally find the following actions:
Fault diagnosis:
Actions are taken for fault recognition, fault localization, and fault isolation at the appropriate indenture level and cause identification.
Fault correction:
Actions are taken after fault diagnosis to put the item into a state in which it can perform a required function.
Function check‐out:
Action taken after maintenance actions to verify that the item is able to perform the required function.
Turnaround maintenance:
The periodic shutdown of the manufacturing unit to perform overhaul and repair activities and to inspect, test, and replace process materials. Turnarounds are expensive, both in terms of lost production (duration of several weeks or months) and direct costs for the labor, tools, heavy equipment, and materials used to execute the project.
Preventive Maintenance Plan
Preventive maintenance actions must be performed systematic & routinely well planned in order to avoid breakdowns or failures. The key point of preventative maintenance is to perform it before the system or equipment fails. If you don’t do it routinely and on time, it won’t be preventive in nature. It will, rather, transform into corrective, or reactive maintenance.
The goal of preventative maintenance is to minimize failures by detecting and fixing them before they lead to costly downtimes. An essential part of preventative maintenance is defining when maintenance needs to be performed. It could be based on time or usage, and once an equipment’s Preventive Maintenance is due, it needs to be carried out immediately.
General steps for Preventive Maintenance Planning as follows :
Step #1 Identify the Asset
Preventive Maintenance must perform on all the machines & equipment that have directly or indirectly impact the manufacturing or production system. A simple rule is to select all those assets that the manufacturing unit cannot survive without. An asset that, if fails, will result in downtime should be in the top priority.
Step #2 Create a Preventive Maintenance schedule or Plan
After identification of assets for preventive maintenance or scheduled maintenance, the next step is to make a schedule for All assets as per the priorities. it is also very important that Preventive maintenance should be performed in the spare time of that assets.
Identification of appropriate interval time for each asset in terms of calendar clock, Age clock, or usage clock. For example, some machine needs preventive maintenance once in a year whereas other needs it twice a year and another need preventive maintenance after 2 lakhs operations of production. Fig. Shows a preventive maintenance plan for stamping press tools.
In this Preventive Maintenance schedule, Preventive Maintenance must perform as per scheduled time or number of strokes whichever is earlier. For example, Schedule due date of the Forming tool of HDR MOD “C” December 2019 but if that tool produces 100000 strokes before December 2019( Let us assume in October), Preventive Maintenance must perform in October.
Step #3: Perform Preventive Maintenance
Preventative maintenance must perform as soon as its due date. Preventive Maintenance actions must perform by the maintenance team and ensure everything is done as explained above in the Preventive maintenance checklist.
Step #4: Repeat the cycle
After identifying the appropriate interval or usage, repeat the preventive maintenance cycle. Like the Trimming tool of HDR MOD “C” a Preventive maintenance plan is taken in July 2019 after four-month or 75000 strokes.
