25.3 Basic Maintenance Strategies

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Maintenance strategies can be divided into three main types: (1) run-to-failure, (2) scheduled, and (3)

condition-based maintenance. Each of these different strategies has distinct advantages and

disadvantages, which will be described below. Specific situations within any large facility may require

the application of a different strategy. Therefore, no one strategy should be considered as always superior

or inferior to another.

25.3.1 Run-to-Failure (Breakdown) Maintenance

Run-to-failure, or breakdown maintenance, is a strategy where maintenance, in the form of repair work

or replacement, is only performed when machinery has failed. In general, run-to-failure maintenance is

appropriate when the following situations exist:

* The equipment is redundant.

* Low cost spares are available.

* The process is interruptible or there is stockpiled product.

* All known failure modes are safe.

* There is a known long mean time to failure (MTTF) or a long mean time between failure (MTBF).

* There is a low cost associated with secondary damage.

* Quick repair or replacement is possible.

An example of the application of run-to-failure maintenance can be found when one considers the

standard household light bulb. This device satisfies all the requirements above and therefore the most

cost-effective maintenance strategy is to replace burnt out light bulbs as needed.

* Generally (outside of start-up and shutdown) machinery is required to operate at constant

speed and load.

* Machinery failure is defined based on performance, operating condition, and system

specifications.

* Machinery failure can be defined as the inability of a machine to perform its required

function.

* Causes of machinery failure can be generally defined as being due to deficiencies in the

original design, material or processing, improper assembly, inappropriate maintenance, or

excessive operation demands.

* The frequency of failure for an individual machine or a population of similar machines can

be summarized using a “bathtub curve.”

25-4 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

Figure 25.2 shows a schematic demonstrating the relationship between a machine’s time in service, the

load (or duty) placed on the machine, and the estimated remaining capacity of the machine. Whenever

the estimated capacity curve intersects with (or drops below) the load curve, a failure will occur. At

these times, repair work must be carried out. If the situation that exists fits within the “rules” outlined

above, all related costs (repair work and downtime) will be minimized when using run-to-failure

maintenance.

25.3.2 Scheduled (Preventative) Maintenance

When specific maintenance tasks are performed at set time intervals (or duty cycles) in order to maintain

a significant margin between machine capacity and actual duty, the type of maintenance is called

scheduled or preventative maintenance. Scheduled maintenance is most effective under the following

circumstances:

* Data describing the statistical failure rate for the machinery is available.

* The failure distribution is narrow, meaning that the MTBF is accurately predictable.

* Maintenance restores close to full integrity of the machine.

* A single, known failure mode dominates.

* There is low cost associated with regular overhaul/replacement of the equipment.

* Unexpected interruptions to production are expensive and scheduled interruptions are not

so bad.

* Low cost spares are available.

* Costly secondary damage from failure is likely to occur.

An example of scheduled maintenance practices can be found under the hood of your car. Oil

and oil filter changes on a regular basis are part of the scheduled maintenance program that most

car owners practice. A relatively small investment in time and money on a regular basis acts to

reduce (but not eliminate) the likelihood of a major failure taking place. Again, this example shows

how when all, or most, of the criteria listed above are satisfied, overall maintenance costs are

minimized.

Figure 25.3 shows a schematic demonstrating the relationship between a machine’s time in-service,

the load (or duty) placed on the machine and the estimated remaining capacity of the machine when

scheduled maintenance is being practiced. In this case, maintenance activities are scheduled at regular

intervals in order to restore machine capacity before a failure occurs. In this way, there is always a

margin between the estimated capacity and the actual load on the machine. If this margin is always

present, there should theoretically never be an unexpected failure, which is the ultimate goal of

scheduled maintenance.

Machine Duty (Load)

Estimated

Capacity

and

Load

Time In Service

Machine Capacity (Est.) Failures

Maintenance

Activities

FIGURE 25.2 Time vs. estimated capacity and actual load (run-to-failure maintenance).

Machine Condition Monitoring and Fault Diagnostics 25-5

© 2005 by Taylor & Francis Group, LLC

25.3.3 Condition-Based (Predictive, Proactive, Reliability Centered,

On-Condition) Maintenance

Condition-based maintenance (which is also known by many other names) requires that some means of

assessing the actual condition of the machinery is used in order to optimally schedule maintenance, in

order to achieve maximum production, and still avoid unexpected catastrophic failures. Conditionbased

maintenance should be employed when the following conditions apply:

* Expensive or critical machinery is under consideration.

* There is a long lead-time for replacement parts (no spares are readily available).

* The process is uninterruptible (both scheduled and unexpected interruptions are excessively

costly).

* Equipment overhaul is expensive and requires highly trained people.

* Reduced numbers of highly skilled maintenance people are available.

* The costs of the monitoring program are acceptable.

* Failures may be dangerous.

* The equipment is remote or mobile.

* Failures are not indicated by degeneration of normal operating response.

* Secondary damage may be costly.

An example of condition-based maintenance practices can again be found when considering your car,

but this time we consider the tires. Regular inspections of the tires (air pressure checks, looking for cracks

and scratches, measuring the remaining tread, listening for slippage during cornering) can all be used to

make an assessment of the remaining life of the tires and also the risk of catastrophic failure. In order

to minimize costs and risk, the tires are replaced before they are worn out completely, but not before

they have given up the majority of their useful life. A measure of the actual condition of equipment is

used to utilize maintenance resources optimally.

Figure 25.4 shows a schematic drawing that demonstrates the relationship between a machine’s time in

service, the load (or duty) placed on the machine, and the estimated remaining capacity of the machine

when condition-based maintenance is being practiced. Note that the margin between duty and capacity

is allowed to become quite small (smaller than in scheduled maintenance), but the two lines never touch

(as in run-to-failure maintenance). This results in a longer time between maintenance activities than

for scheduled maintenance. Maintenance tasks are scheduled just before a failure is expected to occur,

thereby optimizing the use of resources. This requires that there exists a set of accurate measures that can

be used to assess the machine integrity.

Each of these maintenance strategies has its advantages and disadvantages and situations exist where

one or the other is appropriate. It is the maintenance engineer’s role to decide on and justify the use

of any one of these procedures for a given machine. There are also instances where a given machine

will require more than one maintenance strategy during its operational life, or perhaps even at one

Machine Duty (Load)

Estimated

Capacity

and

Load

Time In Service

Machine Capacity (Est.)

Maintenance

Activities

Margin

FIGURE 25.3 Time vs. estimated capacity and actual load (scheduled maintenance).

25-6 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

time, and situations where more than one strategy is appropriate within a particular plant. Examples

of these situations include the need for an increased frequency of monitoring as the age of a machine

increases and the likelihood of failure increases, and the scheduling of maximum time between

overhauls during the early stages of a machine’s useful life, with monitoring in between looking for

unexpected failures.

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