Range of Motion and Strength ( Physical Ergonomics )

A number of measures are required to describe the capacity of an individual (or population) to achieve task performance (e.g., reach, lift, pull). Joint range-of-motion (ROM, also called mobility or flexibility) and joint (or muscle) strength begin to describe capacity and are especially relevant for tasks performed briefly or infrequently. Additional information will be required for highly demanding, prolonged, or frequent tasks, as well as additional types of measures (e.g., fatigue and environmental stress as described below).

Range of Motion

Joint ROM refers to the limits of joint motion and is represented as rotations about a given joint or of body segments (e.g., torso flexion). Two different forms of ROM are commonly measured. The first, passive (or assisted), involves external sources of force or moment to achieve joint motion. Examples include the use of gravity during a squat, to assess knee flexion, or forces /moments applied by an experimenter or device. The second, active ROM, requires muscle contraction to achieve joint motion and is associated with narrower motion limits than passive. In practice, the relevant type of ROM is determined by task requirements.

Measuring ROM from individuals is possible using a variety of equipment, from lowcost goniometers (for measuring included angles) to high-cost and sophisticated marker tracking systems. More often, population ROM data are obtained from a number of accessible sources (often in conjunction with anthropometric data). A number of factors can be expected to have an influence on ROM. Although ROM decreases with age, the changes are usually minimal in healthy individuals until the end of typical working life (i.e., 65). Women generally have higher ROM ranges, although gender differences are typically 10%. Little association has been found between anthropometry and ROM, although ROM does decrease with obesity. In simple cases, such as those involving one joint, application of ROM data is straightforward and follows similar methods described in anthropometry (e.g., using percentiles). When multiple joints are involved, it is common to use human modeling software to assess the potential limitations due to ROM.

Maury A. Nussbaum

Industrial and Systems Engineering

Virginia Polytechnic Institute and State University

Blacksburg, Virginia

Jaap H. van Diee¨n

Faculty of Human Movement Sciences

Vrije Universiteit

Amsterdam, The Netherlands

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Centrifugal CASTING

Principle: Pouring molten metal into a rotating mold and molten metal due to centrifugal force will be compressed so that the workpiece is obtained without disabilities.

The casting is used extensively for casting plastics, ceramics, concrete and all metals.

Advantage Centriugal Casting:

1. Riser is not required

2. Squiggly Items can be processed with good surface quality

3. small dimensional tolerances

4. uniform thickness of workpiece

Centrifugal Casting Disadvantage:

1. Prices expensive equipment

2. Expensive maintenance costs

3. Low production rate

4. One product in one mold

5. Large centrifugal

Centrifugal casting can be divided into 2 types, namely:

A. Horizontal Centrifugal Casting

B. Vertical Centrifugal Casting

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Engineering Plastics

Many thermoplastics are now accepted as engineering materials and some are distinguished by the loose description engineering plastics. The term probably originated as a classification distinguishing those that could be substituted satisfactorily for metals such as aluminium in small devices and structures from those with inadequate mechanical properties. This demarcation is clearly artificial because the properties on which it is based are very sensitive to the ambient temperature, so that a thermoplastic might be a satisfactory substitute for a metal at a particular temperature and an unsatisfactory substitute at a different one.

A useful definition of an engineering material is that it is able to support loads more or less indefinitely. By such a criterion thermoplastics are at a disadvantage compared with metals because they have low time-dependent

moduli and inferior strengths except in rather special circumstances. However, these rather important disadvantages are off-set by advantages such as low density, resistance to many of the liquids that corrode metals and above all, easy processability . Thus, where plastics compete successfully with other materials

in engineering applications it is usually because of a favourable balance of properties rather than because of an outstanding superiority in some particular respect, although the relative ease with which they can be formed into complex shapes tends to be a particularly dominant factor. In addition to conferring the

possibility of low production costs, this ease of processing permits imaginative designs that often enable plastics to be used as a superior alternative to metals rather than merely as a tolerated substitute.

Currently the materials generally regarded as making up the engineering plastics group are Nylon, acetal, polycarbonate, modified polyphenylene oxide (PPO), thermoplastic polyesters, polysulphone and polyphenylene sulphide. The newer grades of polypropylene also possess good basic engineering performance and this would add a further 0.5 m tonnes. And then there is unplasticised polyvinyl chloride (uPVC) which is widely used in industrial pipework and even polyethylene, when used as an artificial hip joint for example, can come into the reckoning. Hence it is probably unwise to exclude any plastic from consideration as an engineering material even though there is a sub-group specifically entitled for this area of application.

In recent years a whole new generation of high performance engineering plastics have become commercially available. These offer properties far superior to anything available so far, particularly in regard to high temperature performance, and they open the door to completely new types of application for plastics.

The main classes of these new materials are

(i) Polyarylethers and Polyarylthioethers

polyarylethersulphones (PES)

polyphenylene sulphide (PPS)

polyethernitrile (PEN)

polyetherketones (PEK and PEEK)

(ii) Polyimides and Polybenzimidazole

polyetherimide (PEI)

thermoplastic polyimide (PI)

polyamideimide (PAI)

(iii) Fluompolymers

fluorinated ethylene propylene (FEiP)

perfluoroalkoxy (PFA)

A number of these materials offer service temperatures in excess of 200°C and

fibre-filled grades can be used above 300°C.

PLASTICS
ENGINEERING
Third Edition
R.J. Crawford, BSc, PhD, DSc, FEng, FIMechE, FIM
Department of Mechanical, Aeronautical
and Manufacturing Engineering
The Queen’s University of Belfast
l E I N E M A N N
OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS
SAN DlEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

Butterworth-Heinemann
An imprint of Elsevier Science
Linacre House. Jordan Hill, Oxford OX2 8DP
225 Wildwood Avenue, Woburn, MA 01801-2041
First published 1981
Second edition 1987
Reprinted with corrections 1990. 1992
Third edition 1998
Reprinted 1999.2001, 2002
Copyright 0 1987, 1998 R.J. Crawford. All rights reserved
The right of R.J. Crawford to be identified as
the author of this work has been asserted in
accordance with the Copyright. Designs and
Patents Act 1988

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Anthropometry in Design

Anthropometric data are often estimated using predictive formulas or standardized manikins.Most often, these approaches are intended as indicators of the ‘‘average’’ human. As such,its utility can be limited in that there is no individual who is truly average across multipledimensions, and relationships between measures may not be linear or the same betweenpeople. For example, a person with a 50th percentile arm length likely does not have a 50thpercentile leg length (it may be close or quite different). Further, many anthropometric tablesonly present average values (e.g., for center-of-mass location), making estimates of individualdifferences impossible.

A second limitation in the application of anthropometry arises from potential biases. As noted above, most of the larger datasets were derived several decades ago, thus not account2 Physical Ergonomic Analyses 767

ing for general and nontrivial secular trends toward larger body sizes across all populations. Many of these studies were also performed on military populations, and questions arise as to whether the values are representative in general. Additional biases can arise due to ethnic origins, age, and gender. Overall, application of anthropometric data requires careful attention to minimize such sources of bias.

Three traditional approaches have been employed when using anthropometry in design. Each may have value, depending on the circumstances, and differ in their emphasis on a portion of a population. The first, and most straightforward, is design for extremes. In this approach, one ‘‘tail’’ of the distribution in a measure is the focus. In the example above for door height, the tall males were of interest, since if those individuals are accommodated, then all shorter males and nearly all females will as well. Alternatively, the smaller individual

may be of interest, as when specifying locations where reaching is required: If the smallest individual can reach it, so will the larger ones.

The second approach, design for average, focuses on the middle of the distribution. This has also been termed the ‘‘min–max’’ strategy, as it addresses the minimal dimension needed for small individuals and the maximal dimensions for large individuals. A typical nonadjustable seat or workstation is an example of designing for the average. In this case, both the smallest and largest users may not be accommodated (e.g., unable to find a comfortable posture).

Design for adjustability is the third approach, and this seeks to accommodate the largest possible proportion of individuals. For example, an office chair may be adjustable in height and/or several other dimensions. While this approach is generally considered the best among the three, with increasing levels or dimensions of adjustability comes increasing costs. In practice, designers must balance these costs with those resulting from failure to accommodate some users.

In all cases, the design strategy usually involves a goal or criterion for accommodation. Where the large individual is of concern (e.g., for clearance), it is common practice to design for the 95th percentile males. Similarly, the 5th percentile female is used when the smaller individual is of concern (e.g., for reaching). When the costs of failure to accommodate individuals is high, the tails are typically extended. From the earlier example, it might be desirable to ensure that 99.99% (or more) of the population can fit through a doorway.
Application of anthropometry in the design process usually involves a number of steps.

Key anthropometric attributes need to first be identified, then appropriate sources of population data (or collect this if unavailable). Targets for accommodation are usually defined early (e.g., 99%) but may change as costs dictate. Mock-ups and/or prototypes are often built, which allow for estimating whether allowances are needed (e.g., for shoe height or gait in the doorway example). Testing may then be conducted, specifically with extremes of the population, to determine whether accommodations meet the targets.


Maury A. Nussbaum
Industrial and Systems Engineering
Virginia Polytechnic Institute and State University
Blacksburg, Virginia
Jaap H. van Diee¨n
Faculty of Human Movement Sciences
Vrije Universiteit
Amsterdam, The Netherlands

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.

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