70811_41.pdf

CHAPTER 41

EQUIPMENT DESIGN

Karl A. Sweitzer

Charles A. Hull

Allan G. Piersol

INTRODUCTION

Equipment is defined here as any assembly of parts that form a single functional unit

for the purposes of manufacturing, maintenance, and/or recordkeeping, e.g., an elec-

tronic package or a gearbox. Designing equipment for shock and vibration environ-

ments is a process that requires attention to many details. Frequently, competing

requirements must be balanced to arrive at an acceptable design. This chapter

guides the equipment designer through the various phases of a design process, start-

ing with a clear definition of the requirements and proceeding through final testing,

as illustrated in Fig. 41.1.

ENVIRONMENTS AND REQUIREMENTS

The critical first step in the design of any equipment is to understand and clearly define

where the equipment will be used and what it is expected to do. The principal environ-

ments of interest in this handbook are shock and vibration (dynamic excitations), but

the equipment typically will be exposed to many other environments (see Table 20.1).

These other environments may occur in sequence or simultaneously with the dynamic

environments. In either case, they can adversely affect the dynamic performance of the

materials used in a design. For example, a thermal environment can directly affect the

strength, stiffness, and damping properties of materials. Other environments can also

indirectly affect the dynamic performance of an equipment design. For example, ther-

mal environments can produce differential expansions and contractions that may suf-

ficiently prestress critical structural elements to make the equipment more susceptible

to failure under dynamic loading.

The preceding example illustrates the need to understand all of the design

requirements, not just the dynamic requirements. A comprehensive set of require-

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EQUIPMENT DESIGN

ments (or equipment specifications) must be developed so that no aspect of the

design’s performance is left uncontrolled. Unfortunately, different types of require-

ments often lead to difficult design tradeoffs that must be resolved. Priorities must

be established in these situations. For example, a low-cost weak material may be pre-

ferred over a more expensive stronger material if the operational stresses can be

kept low. This example reflects the fact that many requirements are not purely tech-

nical. Cost, schedule, and safety issues are additional requirements that are always

on the mind of project management. Still other requirements can be more emotional

(e.g., aesthetic appeal).

The approach to equipment design presented in this chapter is the systems engi-

neering concept of minimizing the life cycle cost, where the life cycle is defined as all

activities associated with the equipment from its initial design through its final dis-

posal after service use. Stated simply, the design process should consider and mini-

mize the costs incurred over the complete life of the equipment. Extra effort put

forth early in the design phase can often have a large payoff later in the life of the

equipment. For example, the cost of correcting a problem in manufacturing can be

many times greater than the cost of making the correction during the design phase.

Additional costs, such as disposal and recycling of the equipment after it has passed

its useful life, can be minimized with proper attention early in the design phase.

DYNAMIC ENVIRONMENTS

Shock and/or vibration (dynamic) environments cover a wide range of frequencies

from quasi-static to ultrasonic. Examples of different dynamic environments and the

frequency ranges over which they typically occur are detailed in the various chapters

and references listed in Table 28.1. The classification of vibration sources and details

on how measured and predicted data should be quantified are presented in Chap. 22.

From a design viewpoint, dynamic excitations can be grouped as follows.

Quasi-Static Acceleration. Quasi-static acceleration includes pure static acceler-

ation (e.g., the acceleration due to gravity) as well as low-frequency excitations. The

range of frequencies that can be considered quasi-static is a function of the first nor-

mal mode of vibration of the equipment (see Chap. 21). Any dynamic excitation at a

frequency less than about 20 percent of the lowest normal mode (natural) frequency

of the equipment can be considered quasi-static. For example, an earthquake excita-

tion that could cause severe dynamic damage to a building could be considered

quasi-static to an automobile radio.

Shock and Transient Excitations. Shock (or transient) excitations are character-

ized as having a relatively high magnitude over a short duration. Many shock exci-

tations have enough high-frequency content to excite at least the first normal mode

of the equipment structure, and thus produce substantial dynamic response (see

Chap. 8). The transient nature of a shock excitation limits the number of response

cycles experienced by the structure, but these few cycles can result in large displace-

ments that could cause snubbing, yielding, or tensile failures if the magnitude of the

excitation is sufficiently large. Frequent transients can also result in low-cycle fatigue

failures (see Chap. 34).

Periodic Excitations. Periodic excitations are of greatest concern when they

drive a structure to respond at a normal mode frequency where the motions can be

dramatically amplified (see Chap. 2). Of particular concern is the repetitive nature

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of the response that can accumulate enough cycles to cause fatigue failures at exci-

tation levels less than those required to cause immediate yielding or fracture. The

most basic form of a periodic excitation is the sinusoidal excitation caused by rotat-

ing equipment. However, other periodic excitations may include strong harmonics

that might be damaging, e.g., the vibrations produced by reciprocating engines

and gearboxes (see Chap. 38). All harmonics of the periodic excitation must be

considered.

Random Excitations. Random excitations occur typically in environments that

are related to turbulence phenomena (e.g., wave and wind actions, and aerodynamic

and jet noise). Random excitations are of concern because they typically cover a

wide frequency range. All natural frequencies of the structure within the frequency

bandwidth of a random excitation will respond simultaneously. Assuming the struc-

ture is linear, the response will be approximately Gaussian, as defined in Chap. 11,

meaning that large instantaneous displacements, as well as damaging fatigue

stresses, may occur.

Mixed Periodic and Random Excitations. Mixed excitations typically occur

when rotating equipment induces periodic excitations that are combined with exci-

tations from some flow-induced source. An example would be a propeller airplane,

where the periodic excitation due to the propeller is superimposed on the random

excitation due to the airflow over the fuselage (see Chap. 29, Part III). It is important

to compute the stresses in the equipment due to both excitations applied simultane-

ously. The same is true of shock excitations that may occur during the vibration

exposure.

OTHER ENVIRONMENTS

Other environments may have an effect on material properties and/or help define

what materials and finishes can be used during the design and construction of the

equipment. The more important environments that should be considered are as fol-

lows.

Temperature. Material properties can change dramatically with temperature. Of

particular concern for dynamic design are the material stiffness changes, especially

in nonmetallic materials such as plastics and elastomers (see Chap. 33). Most plastics

show a dramatic reduction in stiffness at higher temperatures. Material strength and

failure modes will also change with temperature. Some metals will exhibit high-

strength ductile behavior at room temperature, and then shift to low-strength brittle

behavior at low temperatures (see Chap. 34). Thermal strains can also induce

stresses and deformations in structures that need to be considered as part of the

design process. A thorough understanding of the expected operating and nonoper-

ating temperatures, plus the amount of exposure time in each temperature range, is

required when designing equipment structures for dynamic environments.

Humidity. Humidity can have an effect on material properties, especially plastics,

adhesives, and elastomers (see Chap. 33). Some nonmetallic materials can swell in

humid environments, resulting in changes in stiffness, strength, and mass. Humid

environments can also lead to corrosion in some materials that ultimately produce

lower strengths.

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EQUIPMENT DESIGN

Salt/Corrosion. Ocean and coastal environments are of particular concern because

the corrosion they commonly produce can lower the strength of a material. Corrosion

and oxidation can also cause clogging or binding in flexible joints. Protective finishes,

seals, and naturally corrosive resistant materials are needed when equipment is

designed to withstand long durations in ocean and coastal environments. Corrosive

environments can also occur in power plants and chemical processing industries.

Other. Other environments might affect the dynamic performance of equipment.

Two such examples are vacuum and electromagnetic fields. Vacuum environments

(e.g., space vehicles or aircraft at high altitudes) can cause pressure differentials in

sealed structures, which produce static stresses that are superimposed on the stresses

due to dynamic responses. Vacuum environments also lack the damping provided by

the interaction of the structure with the air. Electromagnetic fields can interfere with

the functional performance of electronic subassemblies, and sometimes induce

vibration of steel panels.

LIFE-CYCLE ANALYSIS

Dynamic design typically concentrates on the service environment, but there are

other conditions during the life of a product that may require special consideration.

The definition of all of the different conditions (environment magnitudes and dura-

tion) that the equipment will be exposed to during its total life, from manufacture to

disposal, is commonly referred to as a life-cycle analysis.

Manufacturing Conditions. The life of equipment typically begins when it is

manufactured. Manufacturing-induced residual stresses and strains due to plastic

deformations, excessive cutting speeds, elevated adhesive cure temperatures, or

welding can adversely affect the initial strength of materials. Understanding the

material properties after manufacturing-induced excitations (and possible rework)

is a critical first step in a life-cycle analysis.

Test Conditions. Equipment often undergoes factory acceptance or environmen-

tal stress screening tests (see Chap. 20) before it is put into service. These test envi-

ronments can induce initial stresses and strains that reduce the resultant strength.

An example is a pull test of a wire bond. The test should produce failure in a poor

bond, but may also cause permanent plastic deformation in the ductile wire. When

predicting the overall fatigue life of an item of equipment, any initial tests must be

considered as excitations that will accumulate damage.

As discussed in Chap. 20, at least one sample item of any new equipment must

pass a qualification test to verify that it can survive and function correctly during

its anticipated shock and/or vibration environments. This qualification test gener-

ally represents the most severe dynamic environment the equipment will experi-

ence, and hence the equipment must be designed for this test environment.

However, since the sample item used for the qualification test is not delivered for

service use, the qualification test does not have to be included in the life-cycle

analysis.

Shipping and Transportation. Once an equipment item is manufactured, it

probably will be transported to its operating destination. This transportation envi-

ronment can often induce excitations that will not be seen in service use. Examples

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