MAINTENANCE ENGINEERING

UNIT I PRINCIPLES AND PRACTICES OF MAINTENANCE PLANNING

Basic Principles of maintenance planning – Objectives and principles of planned maintenance

activity – Importance and benefits of sound Maintenance systems – Reliability and machine

availability – MTBF, MTTR and MWT – Factors of availability – Maintenance organization –

Maintenance economics.

INTRODUCTION

Maintenance Engineering is the discipline and profession of applying engineering concepts to the optimization of equipment, procedures, and departmental budgets to achieve better maintainability, reliability, and availability of equipment.

Maintenance, and hence maintenance engineering, is increasing in importance due to rising amounts of equipment, systems, machineries and infrastructure.

Since the Industrial Revolution, devices, equipment, machinery and structures have grown increasingly complex, requiring a host of personnel, vocations and related systems needed to maintain them.

A person practicing Maintenance Engineering is known as a Maintenance Engineer.

OBJECTIVES AND PRINCIPLES:

  • Analysis of repetitive equipment failures.
  • Estimation of maintenance costs and evaluation of alternatives.
  • Forecasting of spare parts.
  • Assessing the needs for equipment replacements and establish replacement programs when due application of scheduling and project management principles to replacement programs.
  • Assessing required maintenance tools and skills required for efficient maintenance of equipment.
  • Assessing required skills required for maintenance personnel.
  • Reviewing personnel transfers to and from maintenance organizations assessing and reporting safety hazards associated with maintenance of equipment.

Reliability may be defined in several ways:

  • The idea that an item is fit for a purpose with respect to time.
  • In the most discrete and practical sense: "Items that do not fail in use are reliable" and "Items that do fail in use are not reliable".
  • The capacity of a designed, produced or maintained item to perform as required over time.
  • The capacity of a population of designed, produced or maintained items to perform as required over time.
  • The resistance to failure of an item over time.
  • The probability of an item to perform a required function under stated conditions for a specified period of time.
  • In line with the creation of safety cases for safety, the goal is to provide a robust set of qualitative and quantitative evidence that an item or system will not contain unacceptable risk.
  • The basic sorts of steps to take are to:
  • First thoroughly identify as many as possible reliability hazards (e.g. relevant System Failure Scenarios item Failure modes, the basic Failure mechanisms and root causes) by specific analysis or tests.
  • Assess the Risk associated with them by analysis and testing.
  • Propose mitigations by which the risks may be lowered and controlled to an acceptable level.
  • Select the best mitigations and get agreement on final (accepted) Risk Levels, possible based on cost-benefit analysis.

AVAILABILITY

  • A Reliability Program Plan may also be used to evaluate and improve Availability of a system by the strategy on focusing on increasing testability & maintainability and not on reliability.
  • Improving maintainability is generally easier than reliability. Maintainability estimates (Repair rates) are also generally more accurate.
  • However, because the uncertainties in the reliability estimates are in most cases very large, it is likely to dominate the availability (prediction uncertainty) problem; even in the case maintainability levels are very high.
  • When reliability is not under control more complicated issues may arise, like manpower (maintainers / customer service capability) shortage, spare part availability, logistic delays, lack of repair facilities, extensive retro-fit and complex configuration management costs and others.
  • The problem of unreliability may be increased also due to the "Domino effect" of maintenance induced failures after repairs.
  • Only focusing on maintainability is therefore not enough. If failures are prevented, none of the others are of any importance and therefore reliability is generally regarded as the most important part of availability.

One of the most important design techniques is redundancy.

RELIABILITY THEORY

Reliability is defined as the probability that a device will perform its intended function during a specified period of time under stated conditions.

ACCELERATED TESTING:

The purpose of accelerated life testing is to induce field failure in the laboratory at a much faster rate by providing a harsher, but nonetheless representative, environment.

In such a test, the product is expected to fail in the lab just as it would have failed in the field—but in much less time.

The main objective of an accelerated test is either of the following:

  • To discover failure modes.
  • To predict the normal field life from the high stress lab life.

Software reliability is a special aspect of reliability engineering. System reliability, by definition, includes all parts of the system, including hardware, software, supporting infrastructure (including critical external interfaces), operators and procedures. Traditionally, reliability engineering focuses on critical hardware parts of the system. Since the widespread use of digital integrated circuit technology, software has become an increasingly critical part of most electronics and, hence, nearly all present day systems. Despite this difference in the source of failure between software and hardware, several software reliability models based on statistics have been proposed to quantify what we experience with software: the longer software is run, the higher the probability that it will eventually be used in an untested manner and exhibit a latent defect that results in a failure (Shooman 1987), (Musa 2005), (Denney 2005). As with hardware, software reliability depends on good requirements, design and implementation. Software reliability engineering relies heavily on a disciplined software engineering process to anticipate and design against unintended consequences. There is more overlap between software quality engineering and software reliability engineering than between hardware quality and reliability. A good software development plan is a key aspect of the software reliability program. The software development plan describes the design and coding standards, peer reviews, unit tests, configuration management, software metrics and software models to be used during software development.

  • Define objective and scope of the test
  • Collect required information about the product
  • Identify the stresses
  • Determine level of stresses
  • Conduct the accelerated test and analyze the collected data.

MEAN TIME BETWEEN FAILURES

Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a system during operation. MTBF can be calculated as the arithmetic mean (average) time between failures of a system.

FORMAL DEFINITION OF MTBF

By referring to the figure above, the MTBF is the sum of the operational periods divided by the number of observed failures.

If the "Down time" (with space) refers to the start of "downtime" (without space) and "up time" (with space) refers to the start of "uptime" (without spMean time betMean time between failuresween failuresace), the formula will be:

The MTBF is often denoted by the Greek letter θ, or

The MTBF can be defined in terms of the expected value of the density function ƒ(t)

where ƒ is the density function of time until failure – satisfying the standard requirement of density functions –

The Overview

For each observation, downtime is the instantaneous time it went down, which is after (i.e. greater than) the moment it went up, uptime. The difference (downtime minus uptime) is the amount of time it was operating between these two events. MTBF value prediction is an important element in the development of products. Reliability engineers / design engineers, often utilize Reliability Software to calculate products' MTBF according to various methods/standards.

However, these "prediction" methods are not intended to reflect fielded MTBF as is commonly believed. The intent of these tools is to focus design efforts on the weak links in the design

MTTR

MTTR is an abbreviation that has several different expansions, with greatly differing meanings. It is wise to spell out exactly what is meant by the use of this abbreviation, rather than assuming the reader will know which is being assumed. The M can stand for any of minimum, mean or maximum, and the R can stand for any of recovery, repair, respond, or restore. The most common, mean, is also subject to interpretation, as there are many different ways in which a mean can be calculated.

  • Mean time to repair
  • Mean time to recovery/Mean time to restore
  • Mean time to respond
  • Mean time to replace

In an engineering context with no explicit definition, the engineering figure of merit, mean time to repair would be the most probable intent by virtue of seniority of usage.

It is also similar in meaning to the others above (more in the case of recovery, less in the case of respond, the latter being more properly styled mean "response time").

UNIT II

MAINTENANCE POLICIES – PREVENTIVE MAINTENANCE

Maintenance categories – Comparative merits of each category – Preventive maintenance,

maintenance schedules, repairs cycle - Principles and methods of lubrication – TPM.

The maintenance is defined as follows: “the work of keeping something in proper condition; upkeep.” This would imply that maintenance should be actions taken to prevent a device or component from failing or to repair normal equipment degradation experienced with the operation of the device to keep it in proper working order. For example, equipment may be designed to operate at full design load for 5,000 hours and may be designed to go through 15,000 start and stop cycles. The wear-out period is characterized by a rapid increasing failure rate with time. In most cases this period encompasses the normal distribution of design life failures.

The design life of most equipment requires periodic maintenance. Belts need adjustment, alignment needs to be maintained, proper lubrication on rotating equipment is required, and so on. In some cases, certain components need replacement, (e.g., a wheel bearing on a motor vehicle) to ensure the main piece of equipment (in this case a car) last for its design life. Anytime we fail to perform maintenance activities intended by the equipment’s designer, we shorten the operating life of the equipment. But what options do we have? Over the last 30 years, different approaches to how maintenance can be performed to ensure equipment reaches or exceeds its design life have been developed in the United States. In addition to waiting for a piece of equipment to fail (reactive maintenance), we can utilize preventive maintenance, predictive maintenance, or reliability centered maintenance.

Reactive Maintenance

Reactive maintenance is basically the “run it till it breaks” maintenance mode. No actions or efforts are taken to maintain the equipment as the designer originally intended to ensure design life is reached. Studies as recent as the winter of 2000 indicate this is still the predominant mode of maintenance in the United States. The referenced study breaks down the average maintenance program as follows:

  • >55% Reactive
  • 31% Preventive
  • 12% Predictive
  • 2% Other.

Note that more than 55% of maintenance resources and activities of an average facility are still reactive.

Advantages to reactive maintenance can be viewed as a double-edged sword. If we are dealing with new equipment, we can expect minimal incidents of failure. If our maintenance program is purely reactive, we will not expend manpower dollars or incur capital cost until something breaks. Since we do not see any associated maintenance cost, we could view this period as saving money. Our labour cost associated with repair will probably be higher than normal because the failure will most likely require more extensive repairs than would have been required if the piece of equipment had not been run to failure. Chances are the piece of equipment will fail during off hours or close to the end of the normal workday. If it is a critical piece of equipment that needs to be back on-line quickly, we will have to pay maintenance overtime cost. Since we expect to run equipment to failure, we will require a large material inventory of repair parts. This is a cost we could minimize under a different maintenance strategy.

Advantages

•Low cost.

•Less staff

Disadvantages

•Increased cost due to unplanned downtime of equipment.

•Increased labour cost, especially if overtime is needed.

•Cost involved with repair or replacement of equipment.

•Possible secondary equipment or process damage from equipment failure.

•Inefficient use of staff resources.

Preventive Maintenance

Preventive maintenance can be defined as follows: Actions performed on a time- or machine-run-based schedule that detect, preclude, or mitigate degradation of a component or system with the aim of sustaining or extending its useful life through controlling degradation to an acceptable level.

While preventive maintenance is not the optimum maintenance program, it does have several advantages over that of a purely reactive program. By performing the preventive maintenance as the equipment designer envisioned, we will extend the life of the equipment closer to design. This translates into dollar savings. Preventive maintenance (lubrication, filter change, etc.) will generally run the equipment more efficiently resulting in dollar savings. While we will not prevent equipment catastrophic failures, we will decrease the number of failures. Minimizing failures translate into maintenance and capital cost savings.

Advantages

•Cost effective in many capital-intensive processes

•Flexibility allows for the adjustment of maintenance periodicity.

•Increased component life cycle.

•Energy savings

•Reduced equipment or process failure

•Estimated 12% to 18% cost savings over reactive maintenance program.

Disadvantages

•Catastrophic failures still likely to occur.

•Labour intensive.

•Includes performance of unneeded maintenance.

•Potential for incidental damage to components

Predictive Maintenance

Predictive maintenance can be defined as follows: Measurements that detect the onset of system degradation (lower functional state), thereby allowing causal stressors to be eliminated or controlled prior to any significant deterioration in the component physical state. Results indicate current and future functional capability.

Basically, predictive maintenance differs from preventive maintenance by basing maintenance need on the actual condition of the machine rather than on some preset schedule. You will recall that preventive maintenance is time-based. Activities such as changing lubricant are based on time, like calendar time or equipment run time. For example, most people change the oil in their vehicles every 3,000 to 5,000 miles travelled.

The advantages of predictive maintenance are many. A well-orchestrated predictive maintenance program will all but eliminate catastrophic equipment failures. We will be able to schedule maintenance activities to minimize or delete overtime cost. We will be able to minimize inventory and order parts, as required, well ahead of time to support the downstream maintenance needs. We can optimize the operation of the equipment, saving energy cost and increasing plant reliability. Past studies have estimated that a properly functioning predictive maintenance program can provide a savings of 8% to 12% over a program utilizing preventive maintenance alone. Depending on a facility’s reliance on reactive maintenance and material condition, it could easily recognize savings opportunities exceeding 30% to 40%. In fact, independent surveys indicate the following industrial average savings resultant from initiation of a functional predictive maintenance program:

•Return on investment: 10 times

•Reduction in maintenance costs: 25% to 30%

•Elimination of breakdowns: 70% to 75%

•Reduction in downtime: 35% to 45%

•Increase in production: 20% to 25%.

Advantages

•Increased component operational life/availability.

•Allows for pre-emptive corrective actions.

•Decrease in equipment or process downtime.

•Decrease in costs for parts and labour.

•Better product quality.

•Improved worker and environmental safety.

•Improved worker morale.

•Energy savings.

•Estimated 8% to 12% cost savings over preventive maintenance program.

Disadvantages

•Increased investment in diagnostic equipment.

•Increased investment in staff training.

•Savings potential not readily seen by management.

Reliability Centred Maintenance

Reliability centred maintenance (RCM) magazine provides the following definition of RCM: “a process used to determine the maintenance requirements of any physical asset in its operating context.”

The following maintenance program breakdowns of continually top-performing facilities would echo the RCM approach to utilize all available maintenance approaches with the predominant methodology being predictive.

•<10% Reactive

•25% to 35% Preventive

•45% to 55% Predictive.

Because RCM is so heavily weighted in utilization of predictive maintenance technologies, its program advantages and disadvantages mirror those of predictive maintenance. In addition to these advantages, RCM will allow a facility to more closely match resources to needs while improving reliability and decreasing cost.

Advantages

•Can be the most efficient maintenance program.

•Lower costs by eliminating unnecessary maintenance or overhauls.

•Minimize frequency of overhauls.

•Reduced probability of sudden equipment failures

•Increased component reliability.

Disadvantages

•Can have significant start-up cost, training, equipment, etc.

•Savings potential not readily seen by management.

PLANNED PREVENTIVE MAINTENANCE

Planned Preventive Maintenance('PPM') or more usual just simplePlanned Maintenance(PM) orScheduled Maintenanceis any variety of scheduled maintenance to an object or item of equipment. Specifically, Planned Maintenance is a scheduled service visit carried out by a competent and suitable agent, to ensure that an item of equipment is operating correctly and to therefore avoid any unscheduled breakdown and downtime.

Together withCondition Based Maintenance, Planned maintenance comprisespreventive maintenance, in which the maintenance event is preplanned, and all future maintenance is pre-programmed. Planned maintenance is created for every item separately according to manufacturer’s recommendation or legislation. Plan can be based on equipment running hours, date based, or for vehiclesdistance travelled. A good example of a planned maintenance program iscarmaintenance, where time and distance determine fluid change requirements. A good example ofCondition Based Maintenanceis the oil pressure warning light that provides notification that you should stop the vehicle because failure will occur because engine lubrication has stopped.