PRODUCT DESIGN

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

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

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

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.

HOT CHAMBER DIE CASTING

In this process, the diluent metal furnace together with cylinder injection molding machine and submerged in liquid metal. Injection cylinder pneumatic or hydraulic driven. In general, these types of Die Casting cicik only to deng, white tin, lead and alloys.

On this machine has a main component: the cylinder plunger, neck geese (goose neck) and the nozzle.

The liquid metal is pressed into the mold cavity pressure is maintained forever freezing occurs. Goose neck that liquid metal submerged position when the plunger on top. Then, molten metal is injected into the mold cavity very quickly

COLD CHAMBER DIE CASTING

In this machine, stove apart from the engine. Machine requires greater pressure to close the mold and filling the mold cavity.

The way these machines work, starting from liquid metal melting and then poured into the plunger is adjacent to the mold, there was a hydraulic presses. This process is generally suitable for metals that have high melting temperatures, such as aluminum and magnesium.

Anthropometric Data and Use

Several large-scale anthropometric studies were conducted in the 1960s and 1970s, mostly in industrialized countries. Contemporary studies are typically of smaller scope, with the exception of the ongoing CAESAR (Civilian American and European Surface Anthropometry Resource) project (http: / / store.sae.org/ caesar / ). Anthropometric data are generally presented in tabular form, with some combination of means, standard deviations, and population percentiles. A normal statistical distribution is usually assumed, a simplification which is reasonable in most cases, though which also leads to larger magnitudes of errors at extremes

of populations (e.g., the largest and smallest individuals).

Standard statistical methods can be employed directly for a number of applications. If, for example, we wish to design the height of a doorway to allow 99% of males to pass through unimpeded, we can estimate this height from the mean ( ) and standard deviation ( ) as follows (again assuming a normal distribution). Male stature has, roughly, 175.58 and 6.68 cm. The standard normal variate, z, is then used along with a table of cumulative normal probabilities to obtain the desired value:

where zA is the z value corresponding to a cumulative area A and Y is the value to be estimated. Here, z0.99 2.326, and thus Y 191.1 cm, or the height of a 99th percentile male. Clearly, however, further consideration is needed to address a number of practical issues. These include the relevance of the tabular values, whether this static value is applicable to a functional situation, and if /how allowances should be made for clothing, gait, etc.

Percentile calculations, as given above, are straightforward only for single measures. With multiple dimensions, such as several contiguous body segments, the associated procedures become more involved. To combine anthropometric measures, it is necessary to create a new distribution for the combination. In general, means add, but variances (or standard deviations) do not. Equations are given below for two measures, X and Y (a statistics source should be consulted for methods appropriate for n 2 values):

where indicates addition if measures are to be added and subtraction otherwise, cov is the covariance, and r is the correlation coefficient. As can be seen from these equations, the variance ( 2) of the combined measure reduces to the sum of the individual variances when the two measures are independent, or cov(X,Y) rXY 0. Human measures are generally moderately correlated, however, with r on the order of 0.2–0.8 depending on the specific measures.

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

Anthropometry

Fundamentals of Anthropometry and Measurement Anthropometry is the science that addresses the measurement and/or characterization of the human body, either individually or for populations. Engineering anthropometry is more application oriented, specifically incorporating human measures in design Examples include placement of a control so that most individuals can reach it, grip sizing for a hand-held tool, and height of a conveyor. Within ergonomics, anthropometric measures can be classified as either static or functional. The former are fundamental and generally fixed measures, such as the length of an arm or a body segment moment of inertia. Such static data are widely available from public and commercial sources. Functional measures are obtained during performance of some task or activity and may thus depend on several individual factors (e.g., training, experience, motivation). These latter measures are specific to the measurement situation and are hence relatively limited. Despite the availability of static measures, it is the functional measures that are directly relevant in design. The remainder of this section provides an overview of applied anthropometric methods. Results from anthropometry will also be critical in subsequent sections that address mechanical loading during task performance.

Static anthropometric measures are of four types: linear dimensions (e.g., body segment lengths), masses or weights, mass center locations, and moments of inertia. Linear dimensions can be obtained quite simply using tape measures or calipers, with more advanced recent approaches using three-dimensional (3D) laser scanning. A key issue with respect to linear dimensions is the differentiation between surface landmarks and underlying joint centers of rotation. The former are easily located (e.g., the lateral and medial boney ‘‘knobs’’ above the ankle joint), and methods have been developed to translate these to estimates of underlying joint centers that are required for biomechanical modeling (Section 2.4).

Mass (and/or volume) measures are often obtained using liquid immersion, though as

noted above, recent scanning methods are also being employed. Locations of segment (or whole-body) center of mass can also be obtained using liquid immersion and a number of segmental balance methods. Segment moments of inertia are usually obtained using dynamical tests, where oscillatory frequencies are obtained during natural swinging or following a quick release. Representative geometric solids (e.g., a truncated cone) can also be used to model body parts and obtain analytical estimates.

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

PHYSICAL ERGONOMIC ANALYSES

Overview

In this section we present several of the more common ergonomic methods, tools, and procedures. A focus is maintained on material that is directly relevant to design. In each section, a review of the underlying theory is given, followed by exemplary approaches (e.g., empirical equations or models). In some sections, sample applications to occupational tasks or scenarios are given. Sources of additional information are provided at the end of the chapter.

Anthropometry

Fundamentals of Anthropometry and Measurement Anthropometry is the science that addresses the measurement and/or characterization of the human body, either individually or for populations. Engineering anthropometry is more application oriented, specifically incorporating human measures in design Examples include placement of a control so that most individuals can reach it, grip sizing for a hand-held tool,and height of a conveyor. Within ergonomics, anthropometric measures can be classified as either static or functional. The former are fundamental and generally fixed measures, such as the length of an arm or a body segment moment of inertia. Such static data are widely available from public and commercial sources. Functional measures are obtained during performance of some task or activity and may thus depend on several individual factors (e.g., training, experience, motivation). These latter measures are specific to the measurement situation

and are hence relatively limited. Despite the availability of static measures, it is the functional measures that are directly relevant in design. The remainder of this section provides an overview of applied anthropometric methods. Results from anthropometry will also be critical in subsequent sections that address mechanical loading during task performance.

above the ankle joint), and methods have been developed to translate these to estimates of underlying joint centers that are required for biomechanical modeling (Section 2.4). Mass (and/or volume) measures are often obtained using liquid immersion, though as noted above, recent scanning methods are also being employed. Locations of segment (or whole-body) center of mass can also be obtained using liquid immersion and a number of segmental balance methods. Segment moments of inertia are usually obtained using dynamical

tests, where oscillatory frequencies are obtained during natural swinging or following a quick release. Representative geometric solids (e.g., a truncated cone) can also be used to model body parts and obtain analytical estimates.

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

Disciplines Contributing to Physical Ergonomics

As noted earlier, ergonomics is inherently a multidisciplinary field. Indeed, it can be argued that ergonomics is essentially the intersection of several more primary fields, both receiving basic information from them and providing applications, tools, and procedures to them. Among the primary disciplines contributing to ergonomics and in which the ergonomist must have a fair degree of knowledge are anatomy, physiology, and mechanics. As will be seen in the subsequent presentation of methods, all are needed at some level to adequately address the measurement and matching problems earlier noted.

Ergonomics in the Design Process

Where are ergonomists and how do they contribute to the design process? As with any profession, the answer is necessarily diverse. In smaller enterprises, if any individual has responsibility for ergonomics, it is often along with safety and other related topics. In larger enterprises, there may be one or more persons with specific ergonomics training and expertise. In the largest, such as automotive manufacturers, whole departments may exist. Ergonomics consultants are also widely employed, as are ergonomists in academic centers.

Methods by which ergonomics is part of the design process are similarly quite varied. Broadly, ergonomics in design is either proactive or reactive. In the former, procedures such as those described below are applied early in a product or process life cycle, such as in the conceptual or prototype phases. In reactive situations, ergonomics methods are employed only after a design problem has been identified (e.g., a control cannot be reached, required torques cannot be generated, or work-related injuries are prevalent).

A natural question arises as to the need for ergonomics in design. Justification comes from a range of documented case studies, formal experiments, and economic evaluations. Though somewhat oversimplified, ergonomic design attempts to optimize design by minimizing adverse physical consequences and maximizing productivity or efficiency. It should be noted that these goals are at times in conflict, but ergonomic methods provide the tools whereby an optimal balance can be achieved.

An important justification for ergonomics is provided by the high costs of mismatches between demands and capabilities. Occupational musculoskeletal illnesses and injuries (e.g., sprains, strains, low-back pain, carpal tunnel syndrome) result in tens of billions of dollars (U.S.) in worker compensation claims, additional costs related to worker absenteeism and turnover, and unnecessary human pain and suffering. Increasingly, there are legal requirements and expectations for ergonomics in design (e.g., national standards or collective bargaining contracts). With respect to economics, it has been well documented (e.g., Refs. 3 and 4) that occupational ergonomic programs have led to reductions in overall injuries and illnesses and work days missed, with concurrent improvements in morale, productivity, and work quality. Further, ergonomic controls usually require small to moderate levels of investment and resources and do not drastically change jobs, tasks, or operations.

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

Basis of Physical Ergonomics

At its most basic level, PE is concerned with two aspects of physical effort: (1) the physical demands placed on the human and (2) the physical capabilities of the human in the situation where the demands are present. Simply, the goal is to ensure that demands do not exceed capacity, as is typical for the design of any mechanical system.

A major challenge in PE is the measurement of both demands and capacity in the wide variety of circumstances where human physical exertions are performed (or required, as in the occupational context). Physical requirements vary widely and include dimensions such as force, torque, repetition, duration, posture, etc. Similarly, human attributes vary widely across related dimensions (e.g., strength, endurance, mobility).

Along with the measurement problem is the related problem of matching. Given the variability in both demands and capacity, potential mismatches are likely (and prevalent in many cases). Occupationally, poor matching can lead to low productivity and quality, worker dissatisfaction and turnover, and in many cases musculoskeletal illness and injury.

Ongoing research is being widely conducted to address both the measurement and matching problems. In the material presented below, we have given examples of contemporary approaches, but the reader is advised that more advanced methods are currently available and the technology is rapidly advancing.

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

WHAT IS PHYSICAL ERGONOMICS?

Definitions and Brief History Ergonomics is derived from the Greek ergon (‘‘work’’) and nomos (‘‘principle’’ or ‘‘law’’). Original attribution is debated but is usually given to the Polish professor Wojciech Jastrzebowski in a treatise published in 1857 and intended to represent the ‘‘science of work.’’ The same term seems to have been reinvented in 1949 by the British professor K. F. H. Murrell with the same general intended meaning. In the past decades ergonomics as a scientific and/ or engineering discipline has seen a dramatic increase in research and application as well as attention from the general public. As such, the original term has, in cases, lost some of its original meaning. Returning to these origins and to provide a focus for this chapter, the following definition does well to encompass the current state of ergonomics as having a theoretical and multidisciplinary basis, being concerned with humans in systems and ultimately driven by real-work application (Ref. 1): Ergonomics produces and integrates knowledge from the human sciences to match jobs, products, and environments to the physical and mental abilities and limitations of people. In doing so it seeks to safeguard safety, health and well being whilst optimising efficiency and performance. As a recognized professional domain, ergonomics is quite young, becoming formalized only after the World War II. Related efforts were certainly conducted much earlier, such as Borelli’s mechanical analysis of physical efforts in the late-seventeenth century and the identification of work-related musculoskeletal illnesses by Ramazzini in the early-eighteenth century. It was only in the 1950s, however, that researchers with expertise in engineering, psychology, and physiology began to come together to realize their common goals and approaches. Ergonomics continues to be an inherently multidisciplinary and interdisciplinary field, which will be evident in the remaining material presented in this chapter.

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