25.2 Machinery Failure

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Most machinery is required to operate within a relatively close set of limits. These limits, or operating

conditions, are designed to allow for safe operation of the equipment and to ensure equipment or system

design specifications are not exceeded. They are usually set to optimize product quality and throughput

(load) without overstressing the equipment. Generally speaking, this means that the equipment will

operate within a particular range of operating speeds. This definition includes both steady-state

operation (constant speed) and variable speed machines, which may move within a broader range

of operation but still have fixed limits based on design constraints. Occasionally, machinery is required

to operate outside these limits for short times (during start-up, shutdown, and planned overloads).

The main reason for employing machine condition monitoring and fault diagnostics is to generate

accurate, quantitative information on the present condition of the machinery. This enables more

confident and realistic expectations regarding machine performance. Having at hand this type of reliable

information allows for the following questions to be answered with confidence:

* Will a machine stand a required overload?

* Should equipment be removed from service for maintenance now or later?

25-2 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

* What maintenance activities (if any) are required?

* What is the expected time to failure?

* What is the expected failure mode?

Machinery failure can be defined as the inability of a machine to perform its required function. Failure

is always machinery specific. For example, the bearings in a conveyor belt support pulley may be severely

damaged or worn, but as long as the bearings are not seized, it has not failed. Other machinery may not

tolerate these operating conditions. A computer disk drive may have only a very slight amount of wear or

misalignment resulting in noisy operation, which constitutes a failure.

There are also other considerations that may dictate that a machine no longer performs adequately.

Economic considerations may result in a machine being classified as obsolete and it may then be

scheduled for replacement before it has “worn out.” Safety considerations may also require the

replacement of parts in order to ensure the risk of failure is minimized.

25.2.1 Causes of Failure

When we disregard the gradual wear on machinery as a cause of failure, there are still many specific

causes of failure. These are perhaps as numerous as the different types of machines. There are, however,

some generic categories that can be listed. Deficiencies in the original design, material or processing,

improper assembly, inappropriate maintenance, and excessive operational demands may all cause

premature failure.

25.2.2 Types of Failure

As with the causes of failure, there are many different types of failure. Here, these types will be subdivided

into only two categories. Catastrophic failures are sudden and complete. Incipient failures are partial and

usually gradual. In all but a few instances, there is some advanced warning as to the onset of failure; that

is, the vast majority of failures pass through a distinct incipient phase. The goal of machine condition

monitoring and fault diagnostics is to detect this onset, diagnose the condition, and trend its progression

over time. The time until ultimate failure can then hopefully be better estimated, and this will allow plans

to be made to avoid undue catastrophic repercussions. This, of course, excludes failures caused by

unforeseen and uncontrollable outside forces.

25.2.3 Frequency of Failure

Anecdotal and statistical data describing the

frequency of failures can be summarized in what

is called a “bathtub curve.” Figure 25.1 shows a

typical bathtub curve, which is applicable to an

individual machine or population of machines of

the same type.

The beginning of a machine’s useful life is

usually characterized by a relatively high rate of

failure. These failures are referred to as “wear-in”

failures. They are typically due to such things as

design errors, manufacturing defects, assembly

mistakes, installation problems and commissioning

errors. As the causes of these failures are found and corrected, the frequency of failure decreases.

The machine then passes into a relatively long period of operation, during which the frequency of

failures occurring is relatively low. The failures that do occur mainly happen on a random basis.

This period of a machine’s life is called the “normal wear” period and usually makes up most of the life of

a machine. There should be a relatively low failure rate during the normal wear period when operating

within design specifications.

Failure

Rate

Time In Service

Wear In Normal Wear Wear Out

FIGURE 25.1 Typical bathtub curve.

Machine Condition Monitoring and Fault Diagnostics 25-3

© 2005 by Taylor & Francis Group, LLC

As a machine gradually reaches the end of its designed life, the frequency of failures again increases.

These failures are called “wearout” failures. This gradually increasing failure rate at the expected end of a

machine’s useful life is primarily due to metal fatigue, wear mechanisms between moving parts,

corrosion, and obsolescence. The slope of the wearout part of the bathtub curve is machine-dependent.

The rate at which the frequency of failures increases is largely dependent on the design of the machine and

its operational history. If the machine design is such that the operational life ends abruptly, the machine

is underdesigned to meet the load expected, or the machine has endured a severe operational life

(experienced numerous overloads), the slope of the curve in the wearout section will increase sharply

with time. If the machinery is overdesigned or experiences a relatively light loading history, the slope of

this part of the bathtub curve will increase only gradually with time.

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