cooperation for solving problems associated with WMSDs. The feasibility and creation
of such a consortium is dependent on the cooperation of tool manufacturers, industrial
tool users, and those involved in power hand tool research and testing. To elicit
cooperation, those having access to specific resources must be willing to discuss a
proposed strategy for action.

Conferences could be held at regular intervals with plenary papers, presenters, and
specific workshops to improve communication and the transfer of applicable techno-
logical ideas. Also, during these conferences constant updating of information on the
availability of specific research and intervention concepts could occur.

Potential Roles Within the Consortium

Several groups, including manufacturing companies, union-management safety and
health joint groups, tool manufacturers, NIOSH, or others may have resources that could
be applied to support this endeavor of cooperation. The allocation of such resources
may include co-funding, or facilitating a conference or discussion session(s), and
suggesting facilities available for research and development.

As an example, NIOSH can act as a partner for conferences while simultaneously being
one of the major forces behind the development and support of a “clearinghouse” for
information, translation, and dissemination. In taking this approach, NIOSH can provide
the leadership and coordination that is necessary for initiation of such a strategy.

Although other funding and information sources may be needed, enough may be known
already for this process to begin. The potential use of becoming involved with existing
partnerships such as those offered by the University of Wisconsin-Madison Industrial
Hand Tool and Ergonomics Research Consortium and the National Center for
Manufacturing Sciences (NCMS) should be considered. These partnerships are
examples of associations that may be addressing some of the current concerns and issues
involving power hand tool ergonomic research.

The inability to transfer up-to-date, pertinent, power hand tool information to industry
may indicate a core problem with the process of acquisition and dissemination of power
hand tool ergonomic information. This does not entirely result from a lack of concern,
or limited resources, but rather a lack of an appropriate vehicle for information
acquisition and dissemination. Such a lack hampers the identification of problems and
possible solutions even though potential solutions to particular problems or other
established preventive strategies may be known. In addition, reported information may
not be available in a form that is immediately usable by individual's seeking assistance
with design, selection, installation, and/or use of power hand tools. Therefore, it is

9


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30


operator the longest to reaction torque. The speed of the shutoff mechanism controls
exposure to peak reaction force for automatic shutoff tools. Consequently, automatic
shutoff tools have the shortest torque reaction time since these tools cease operation
immediately after the desired peak torque is achieved. One study found ground reaction
forces were smallest for operators running right-angle nutrunners having fast torque
shutoff mechanisms, and handle displacement was greatest when delayed shutoff
mechanisms were used (Kihlberg et al. 1993). Another study found that perceived
exertion ratings increased as torque impulse increased (Freivalds and Eklund 1991).

Handles

Power tool handles are often categorized as in-line, pistol-grip, angle, and accessory
handles. The particular handle shape selected depends on the magnitude of reaction
torque, the location and orientation of the work, the location of the operator, and the
accessibility of the work. When experienced tool operators were asked to rate power tool
handle size on continuous scales between 0 to 10, all tools having handle circumferences
less than 12 cm were rated “just right.” When provided with a simulated power tool
having a continuously adjustable handle span, the preferred handle size increased as
hand size increased, suggesting that tool handles available in different sizes are preferred
over one-size handles. When pistol-grip nutrunner operators were allowed to adjust the
handle span to any size they preferred, handle size increased proportionally to hand
length (Oh and Radwin 1993).

The shape of the tool handle can affect the mechanical advantage that the operator has
for reacting against tool forces. A nonspherical handle can provide an improved reaction
arm for in-line power tools. The most common nutrunner configurations are in-line
(straight), pistol grip, and right angle. Right-angle nutrunner operation often requires
use of both hands, especially when larger tools are operated, however, only one hand
usually is affected by reaction torque. The hand holding the distal handle is used for
reacting against the torque reaction force and providing tool support, while the other
hand produces push force.

Work Location and Orientation

A series of psychophysical studies examined perceived exertion and postural discomfort
during power hand tool operation (Ulin et al. 1992a; Ulin et al. 1993a; Ulin et al. 1993b).
Subjects were asked to drive screws using small (3.2 Nm) power screwdrivers under
various workstation conditions, and then were asked to rate perceived exertion using
Borg’s 10 point scale. When working on a horizontal surface, the operators’ perceived
exertion levels increased as horizontal distance increased. The most comfortable vertical
location for tool use was between 102 and 153 cm. The most comfortable horizontal

26


3 ENGINEERING CONTROL AND DESIGN CONSIDERATIONS

This section addresses research in the area of control technology necessary for preventing
or controlling musculoskeletal stress factors. This research includes research tool design,
component design, joint fastening engineering and subsequent manufacturing
methodology, and testing as it relates to physiological stress.

Engineering Controls

Engineering controls use engineering principles in equipment design and retrofitting for
preventing, controlling, and decreasing the occurrence of physical hazard exposures.
Engineering controls applied to power hand tools involves the consideration of tool,
component, and workplace technologies. The objective is to control recognized hazards
and prevent adverse effects resulting in fatigue, discomfort, and work-related
musculoskeletal disorders.

Tool and Workplace Factors as they Relate to Controls

The following is a list of areas recognized for consideration, analysis, and implemen-
tation of controls for WMSDs:

© Posture

e Exertion/Force

Contact pressure

¢ Handle friction

¢ Gloves

© Center of gravity (COG)

© Tool Location

© Tool activation and throttle

¢ Reaction Torque

¢ Balancer and Suspension

© Vibration

Posture

It is often recommended to avoid tools that require working with the wrist to be flexed,
or hyper-extended, to have or ulnar or radial deviation when a specific job is performed.

35


WORKSHOP ORGANIZATION

A workshop on ergonomic aspects of power hand tools in the automotive manufacturing
industry was held on January 13 and 14, 1994, at NIOSH to address the issue of
ergonomics and power hand tool design. The workshop was entitled “A Strategy for
Industrial Power Hand Tool Ergonomic Research—Design, Selection, Installation, and
Use Within the Automotive Manufacturing Industry.” It consisted of a day and a half
of panel discussions between representatives from the automotive manufacturing
industry, the tool manufacturing industry, academia, government, and the consulting
field. The workshop addressed the following issues:

¢ Areas of concern involving power hand tool design and usage
¢ Research and development

e Elimination of research and development redundancy

¢ Necessary research priorities

On the first day, the workshop was separated into three concurrent sessions for focused
discussions: (1) epidemiology, (2) problem source identification, and (3) engineering
controls/design/ manufacturing. Before the workshop, panel members received a draft
of Dr. Robert Radwin’s paper entitled Industrial Power Hand Tool Ergonomic Research:
Current Research, Practice, and Needs. This paper, which is a substantial attachment to this
report, discusses these issues:

© Workers at risk

® Jobs and tasks with power hand tool usage

® Tools used for the job

® Biomechanical and physiological concerns

© Body segments affected

© Available industry specific injury rates

© Tool mechanical parameters, operational characteristics, and ambient conditions
contributing to musculoskeletal stress

© Control technology available for preventing or controlling musculoskeletal
stress

© Tool design, component design, fastening technology, and manufacturing
methodology and testing as they pertain to biomechanical and physiological
stress


Radwin RG, Armstrong TJ, Chaffin DB [1987]. Power hand tool vibration effects on grip
exertions. Ergon 30(5):833-855.

Radwin RG, VanBergeijk E, Armstrong TJ [1989]. Muscle response to pneumatic hand
tool torque reaction forces. Ergon 32(6):655-673.

Radwin RG, Armstrong TJ, Chaffin DB, Langolf GD, Albers JW [1990a]. Hand-arm
frequency-weighted vibration effects on tactility. Int J Ind Ergon 6:75-82.

Radwin RG, Armstrong TJ, VanBergeijk E [1990b]. Vibration exposure for selected power
hand tools used in automobile assembly. Am Ind Hyg Assoc J 51(9):510-518.

Radwin RG, Oh §S, Jensen TR, Webster JG [1992]. External finger forces in submaximal
static prehension. Ergon 35(2).

Riley MW, Cochran DJ, Schanbacher CA [1985]. Force capability differences due to
gloves. Ergon 28:441-447.

Rothfliesch S, Sherman D [1978]. Biomechanical aspects of occupational occurrence and
implications regarding surgical management. Orthopedic Rev 7:107-109.

Schlegel RE, Willcoxon R [1989]. An ergonomic evaluation of powered hand tool use in
the automobile industry. In: Mital A, ed. Advances in industrial ergonomics and safety
1. Philadelphia, PA: Taylor & Francis, p. 267-274.

Silverstein BA, Fine LJ, Armstrong TJ [1987]. Occupational factors and carpal tunnel
syndrome. Am J Ind Med 11:343-358.

Sperling L, Kadefors R, Kilbom A [1991]. Tools and hand function: The cube model—a
method for analysis of the handling of tools. Queinnec Y, Daniellou F, eds. Designing
for everyone. London, England: Taylor & Francis, pp. 176-178.

Sperling L, Dahlman S, Wikstrém, Kilbom A, Kadefors R [1993]. A cube model for the
classification of work with hand tools and the formulation of functional requirements.

Appl Ergon 24(3):212-220.

Streeter H [1970]. Effects of localized vibration on the human tactile sense. Am Ind Hyg
Assoc J 31:87-91.

45


BLS [1993]. Workplace injuries and illnesses in 1992. In: News—Bureau of Labor
Statistics. Washington, D.C.: U.S. Department of Labor, Bureau of Labor Statistics,
USDL-93-553.

Cannon LJ, Bernacki EJ, Walter SD [1981]. Personal and occupational factors associated
with carpal tunnel syndrome. J Occup Med 23:255-258.

Carlsé6 S, Mayr J [1974]. A study of the loads on joints and muscles with a pneumatic
hammer and bolt gun. Work-Environ-Health 11:32-38.

Cederqvist T, Lindberg M [1993]. Screwdrivers and their use from a Swedish con-
struction industry perspective. Appl Ergon 24(3):148-157.

Fothergill DM, Grieve DW, Pheasant ST [1992]. The influence of some handle designs
and handle height on the strength of horizontal pulling action. Ergon 35(2):203-213.

Fraser TM [1980]. Ergonomics principles in the design of hand tools. International Labor
Office, Report No. 44, Geneva.

Freivalds A, Eklund J [1991]. Subjective ratings of stress levels while using powered
nutrunners. In: Karwowski W, Yates JW, eds. Advances in industrial ergonomics and
safety II. Bristol, PA: Taylor & Francis, pp.379-386.

Grant KA, Habes DJ, Steward LL [1992]. The influence of handle diameter on manual
effort in a simulated assembly task. In: Kumar S, ed. Advances in industrial ergonomics

and safety IV. Bristol, PA: Taylor & Francis, pp.797-804.
Greenberg L, Chaffin DB [1977]. Workers and their tools: A Guide to the Ergonomic
Design of Hand Tools and small presses. Midland, MI: Pendell Publishing Company.

Hamilton AA [1918]. A Study of Spastic Anemia in the Hands of Stonecutters: Effects
of the air hammer on the hands of stonecutters. Bulletin 236. Washington, D.C.: U.S.
Department of Labor, Bureau of Labor Statistics, Accident and Hygiene Series. Report
No. 19.

Hasan J [1970]. Biomedical aspects of low-frequency vibration: a selective review. Work,
Environ-Health 6:19-45.

Hertzberg HTE [1955]. Some contributions of applied physical anthropology to human
engineering. Ann NY Acad Sci 63:616-629.

43


reduced by the placement of plates against fuselage panels held in place with suction
pads.

Tool manufacturers are being challenged to provide new power hand tool designs that
minimize musculoskeletal stresses. Several manufacturers have already incorporated
new technologies based on current ergonomics knowledge regarding power hand tools.
A notable example is provided by power hand tools that are designed to produce
reduced vibration. Examples of specific tool properties that are affected through design
are listed in Table 4. As new knowledge becomes available, tool manufacturers need a
means of getting information so they can produce state-of-the-art tools based on the

latest technologies.

Table 4. Power hand tool design properties affecting physical stress factors

Physical factor Design objectives

Tool load Use of light weight and composite materials
Optimum load distribution
Optimum handle location

Handle size/shape Optimum size handle
Optimum handle shape
Adjustable handle size

Handle orientation Optimum angle
Adjustable orientation
Work location Optimum location for tool load, handle size,
and handle orientation
Work material Tool speed
Bits and blades
Sound level Motor
Housing and suspension
Mufflers
Fasteners Fastener head design
Torque Shutoff mechanism
Vibration Motor mounting

Tool balance and load distribution

41


— muscle, tendon or ligament tear, bone fractures, etc.,
— chronic musculoskeletal disorders,

— cumulative trauma disorders,

— vascular disorders,

— vibration white finger,

— hearing impairments, and

— respiratory disorders

A survey of state and federal agencies found that hand tool use was associated with 9%
of all work-related compensable injuries (Aghazadeh and Mital 1987). The majority of
workers were injured when they were struck by or were struck against hand tools, or
when they overexerted themselves (Mital and Kilbom 1992). Power hand tool vibration
exposure has been associated with vascular disorders and neuromuscular disturbances
(NIOSH 1989). Vibration has also been cited as an etiologic factor of chronic nerve and
tendon disorders, including carpal tunnel syndrome and tendinitis (NIOSH 1989). Jobs
involving highly repetitive hand tool use have been associated with physical stress
factors for musculoskeletal disorders (Cannon et al. 1981; Rothfliesch and Sherman 1978;
Silverstein et al. 1987). A surveillance study by NIOSH found that musculoskeletal
injuries accounted for 24% of all hand tool injuries (power and non-power hand tools)
reported in the Supplementary Data System (SDS) (Myers and Trent 1988). These
injuries included disorders of the soft tissues, such as the tendons, nerves, muscles, and
connective tissues. The same study found that for operations involving power hand
tools, inflamed wrist joints were more common than wrist lacerations and other acute

injuries.

Power Hand Tools Used in Manufacturing

Power hand tools in manufacturing include portable power hand tools generally used
throughout industry for fabricating, assembling and disassembling, and forming
material. Portable power hand tools used in manufacturing are listed in Table 1.

2 POWER HAND TOOL ERGONOMIC TOPICS

This section addresses ergonomic research in the identification of mechanical parameters,
operational characteristics, and ambient conditions that contribute to musculoskeletal

stress for the operator.

13


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31


FOREWORD

The National Institute for Occupational Safety and Health (NIOSH) conducts research
to identify, measure, analyze, and control occupational hazards. One NIOSH research
priority is the control of work-related musculoskeletal disorders. An important step in
controlling these disorders is to understand the dose-response relationship between the
activity and the injury or illness. This document presents the proceedings of a NIOSH
workshop that addressed duplication of power hand tool research, possible research
gaps, and a cohesive research strategy for the automotive manufacturing industry.
These proceedings contain a summary of conclusions from each workshop session,
recommendations for effective information gathering, a possible research strategy, and
an updated review of current research knowledge. Although the workshop emphasized
the automotive manufacturing industry, other industries should find the session results,
information gathering outline, and strategy useful when addressing topics related to
power hand tools and work-related musculoskeletal disorders.

Foie. (anne.

Linda Rosenstock, M.D., M.P.H

Director, National Institute for
Occupational Safety and Health

Centers for Disease Control and Prevention

iii


the trigger is released, often making a spring mechanism necessary (ANSI 1975). The
spring action also means that the fingers must work against a spring force, in addition
to working against the force needed to activate the tool. Some tools allow for multiple
finger activation to distribute load among fingers (Oh and Radwin 1993). However,
when precise trigger control is necessary, such as in variable speed tools, four finger
control is not feasible due to the necessary stabilization of the tool handle from other

fingers.

Feed and Reaction Force

High feed forces are associated with power tool operation requiring sustained exertions,
such as drills. Power screwdriver feed force may be affected by the fastener head used
(Cederqvist and Lindberg 1993). The type of fastener head and its orientation affect the
amount of force and posture the operator must assume to accomplish the task.
Self-tapping screws require more force than pre-tapped holes. Feed force requirements
increase as torque level increases for cross recess screws. Material hardness is also a
feed force factor for self-tapping screws.

Nutrunners account for almost 75% of the power hand tool inventory of a typical
automobile assembly plant, of which there may be as many as 4,500 tools (VanBergeijk
1987). Nutrunner torque output can range from less than 0.8 Nm to more than 700 Nm.
Forces acting on the hand when operating a nutrunner is operated include (1) push or
feed force, (2) tool support or holding force, and (3) torque reaction force. Feed force
is necessary for starting a fastener and keeping the bit or socket engaged during the
securing cycle, and it is affected by the work material and design of the fastener.
Holding force is the force necessary for supporting a nutrunner, and it is dependent on
the tool mass, its center of gravity, the length of the tool, and possible air hose
attachments. Reaction torque is produced by spindle rotation and is affected primarily
by the spindle torque output and tool length.

As torque is applied to a threaded fastener, it rotates at a relatively low spindle torque
until the clamped pieces come into intimate contact. This torque can approach zero when
free running nuts are used, or it can be significant when locking nuts, thread interference
bolts, or thread forming type fasteners are used. This is called the run-down phase.
After the fastener brings the clamped members of the joint into initial, intimate contact
it continues to draw the parts together until they form a solid joint. When the joint
becomes solid, continued turning of the nut results in a proportionally increasing torque.
This is the elastic portion of the cycle and is the time when reaction torque forces are
produced. Torque build-up, and consequently torque reaction force, continues rising at
a fixed rate until peak torque is achieved, which is the clamping force of the joint. These
forces require a reaction from the operator to control the tool and keep it stable.

24


perhaps necessary to develop a system allowing individuals from various activity fields
to communicate openly and exchange ideas and concepts in the prevention of
musculoskeletal stress to the operator of power hand tools.

The creation of a central location for the objective, nonproprietary information gathered
from government, industry, and academia would increase the accessibility for all those
interested, and possibly decrease redundancy in research and engineering. A central
location or “clearinghouse” would serve as a vehicle for those seeking pertinent data and
assistance in dealing with concerns and issues concentrated on power hand tool
ergonomics. This concept of a “clearinghouse” could also include, if necessary, a process
for digesting information and disseminating it in a user friendly form for ready access.

REFERENCE

BLS (1993). Occupational injuries and illnesses in the United States by industry, 1991-92.
Washington, D.C.: U.S. Department of Labor, Bureau of Labor Statistics.

10


repeatedly making the same movements and exertions over again. The risk of
developing a hand or wrist disorder is significantly increased for workers performing
highly repetitive and forceful exertions (Silverstein et al. 1987).

Contact stresses are produced when parts of the body come into contact with objects,
resulting in forces transmitted through the skin to underlying structures, such as
muscles, tendons and nerves. Some areas of the body are better suited for bearing
contact stress than others. The skin on the back of the hand, for example, is much
thinner than the palmar side and less suited for withstanding loads. Contact stresses are
related to the force and area of contact, described by the pressure exerted against the
skin. Pressure is the force exerted over a given area, which increases when contact area
decreases. Therefore a repeated force or load over a small surface area of the palm may
result in pain and detrimental stress to the hand.

Symptoms in the arm and hand associated with prolonged and repeated vibration
exposure are collectively referred to as hand and arm vibration syndrome. These include
vascular disorders, joint deformations, soft tissue damage, and neurological disturbances
(Hasan 1970). Vibration white finger, is a vascular disorder, resulting in episodic
vasospasm in the fingers of occupational origin, causing them to appear white when
exposed to a cold environment. Vibration has also been cited as a factor in chronic
nerve disorders, such as carpal tunnel syndrome (Armstrong et al. 1987; Cannon et al.
1981; Rothfliesch and Sherman 1978).

Other risk factors for work-related musculoskeletal disorders can adversely affect
vibration transfer to the tool operator’s hand. Highly repetitive work, for instance can
affect vibration exposure through accumulated doses of repeated vibration exposure.
Also, repetitive operations increase vibration exposure time, and forceful exertions,
which improve coupling between the handle and hand, increase vibration transmission.
Work standards are additional factors affecting the work environment in which hand
tools are used. One study found the prevalence of vibration syndrome was greater
among incentive workers than among hourly workers (Wasserman et al. 1981). The
report suggested that the intensity of incentive work increased grip exertions and
improved coupling between the hand and the power tool, thus resulting in increased
vibration transmission. A direct relationship was observed between repetitiveness and
vibration exposure (Radwin and Armstrong 1985). Workers exposed to greater repetitive
stresses were also subject to greater exposures of vibration.

Fingers and hands can be exposed to cold when work is in a cold environment, or when

air blows across the hands, arms, or face from pneumatic power tools exhaust vents.
Handles made of conductive materials can conduct heat away from the hands,

16


1 INTRODUCTION

Scope of the Problem

There is a crucial need for research concerning power hand tool design, selection,
installation and use, and the consequences of power hand tools for operators. The high
incidence of musculoskeletal injuries in manufacturing has prompted concern that
workers be protected from excess physical stress arising from exposure to repeated and
sustained motions and exertions, work requiring awkward postures, contact stress, cold,
and vibration. Both management and labor have come to recognize that because of the
widespread use of power hand tools, improvements in the interface between the human
operator and the tools frequently employed for repetitive manual work can affect a large
number of jobs. Consequently, power hand tool manufacturers are being challenged by
customer demands for products that help minimize physical stress. The lack of specific
information has prompted industry to call on ergonomics researchers to provide
knowledge about the complex relationships between physical stress and power hand tool
operation, which can be applied to prevent musculoskeletal stress disorders.

Background of Injuries Associated with Power Hand Tool Use

At the end of the nineteenth century an American inventor named MacCoy developed
an implement called a pneumatic tool, powered by compressed air and designed to be
held in the hands (50 and 100 years . . . . 1989). It did not take long until these tools
were used for rock breaking and demolition. By 1918 Hamilton, studying Indiana
stonecutters who worked extensively with air hammers, concluded that the combination
of vibration, cold temperatures, and the manner in which the tools were held and used
were responsible for numerous symptoms, including numbness, vasospasms, and
manual dexterity deficits (Hamilton 1918). Air-powered tools were limited to chipping
and stone cutting.

The assembly line brought highly specialized and repetitive work into the factories.
Rather than using a variety of tools for completing an entire project, a worker used a
relatively small number of tools to repeatedly accomplish a specific task. In fitting with
this task procedure, power tool use has become widespread in manufacturing.
Furthermore, workers who use power hand tools often spend a large portion of the
workday holding and operating them.

There have been numerous reports of injuries and disorders associated with jobs
involving use of power hand tools. Some disorders associated with power hand tool use
include

— acute musculoskeletal injuries,

12


Forearm flexor muscle activity, measured by root-mean-square electromyography
(RMS-EMG) while operating an automatic air shutoff right-angle nutrunner during the
torque-reaction phase, was more than four times the muscle activity used for holding the
tool, and two times the run-down phase (Radwin et al. 1989). Forearm flexor EMG
activity during the torque-reaction phase increased for tools having increasing peak
spindle torque.

Threaded fastener joint stiffness ranges from hard to soft. Hard joints are formed when
two solid and fairly rigid objects are brought together, such as when a pulley is attached
to acrankshaft. Soft joints involve objects having more elastic properties, such as when
a body mount is attached. Torque Rate is used for quantifying joint stiffness and is
defined as the angular rate of torque build-up to the resistance of tightening. A high
torque rate joint is defined by the International Standards Organization (ISO) as one
where the torque increases from 50% to 100% of the peak torque in less than 30° of
angular displacement. A low torque rate joint increases from 50% to 100% of peak
torque in more than 360° displacement (ISO 1981).

The tightening process for high torque rate joints is extremely fast. Reaction forces
transmitted to the tool operator peak in a very short time, determined by the time
necessary to reach the installed torque level and the delay of the tool torque control
mechanism. The actual relationship between installed torque and the reaction torque is
affected by the mass of the tool, the size of the operator, and the way the tool is held by
the operator. The reaction torque and, consequently, the reaction force can be greater
or less than the target torque (ISO 1981). One study showed that muscle response was
affected by torque build-up time (Radwin et al. 1989). Average EMG activity was
greater for 0.5 s torque build-up time than for a 2 s torque build-up time for right-angle
nutrunners, ranging in torque between 30 Nm and 100 Nm.

There is a tradeoff for high torque/high feed force tools when posture is used as one of
the criteria for tool selection. Although an in-line power tool maintains a neutral wrist
posture when working on a horizontal surface, high torque in-line screwdrivers require
high grip force. Consequently, less force may be needed for a pistol-grip tool than for
an in-line tool. Use of a pistol-grip power tool, however, requires relocation of or
reorientation of the work so that a stressful wrist posture is not assumed.

The three major operating modes for nutrunners are (1) mechanical clutch, (2) stall, or
(3) automatic shutoff. Although clutch tools limit reaction torque, ratcheting clutch tools
can expose workers to significant levels of vibration if used frequently. When a stall tool
is used, maximum reaction torque time is directly under operator control by a release
of the throttle, which can last as long as several seconds, Stall tools tend to expose an

25


include the identification of areas and criteria for tool research. The following example
outline illustrates a conceivable procedure for the initial process of effective information
gathering:

1. Identify power hand tools utilized in industry.

e Identify tool specifications most often reported.

¢ Define typical power sources.

© Identify and categorize types of tools under development.
2. Identify problems associated with power hand tool use.

e Highlight case studies and long-term epidemiological studies illustrating
biomechanical and physiological effects of power hand tool use.

© Categorize tool mechanical properties for ergonomic consideration. These
properties may include

— tool weight and load distribution,
— tool grip size, shape, and texture,
— torque reaction,

— torque scatter performance, and
— vibration

This categorization process should be flexible enough to allow for the inclusion of
environmental and work technique interaction variables.

3. Identify research applicable to the area of adverse biomechanics and pathophysiol-
ogy.

¢ Who is performing the research?
@ What is their research concentration area?
© Where is the research being done?

© How is the research being done?


ACKNOWLEDGMENTS

This document was prepared by the Division of Physical Sciences and Engineering
(DPSE), Dennis M. O’Brien, Ph.D., Director. The contributions from the panel
moderators, session recorders, panel members, and steering committee is gratefully
acknowledged. In addition, we wish to thank Dr. Robert Radwin for developing the
“white paper.”

We also wish to thank the following NIOSH employees for providing advice, logistical
support, and assistance in the preparation of the workshop and proceedings: Rosalynd J.
Kendall, Heather K. Houston, Maggie A. Ivory, Debra A. Lipps, Bernice L. Clark, Deanna
L. Elfers, Dennis M. O’Brien, Phillip A. Froehlich, Amy Beasley-Spencer, Gary S. Earnest,
Ronald M. Hall, Daniel R. Farwick, Paul A. Hentz, Roger Wheeler, Anne Votaw, Vanessa
Becks, and Anne Stirnkorb.

viii


Manual Dexterity and Motor Capabilities

There is a tradeoff between the increased mechanical advantage provided by a long tool,
versus its weight and size, and the ability for an operator to manipulate and handle it
(Sperling et al. 1991). A tool that is too large may be too difficult to use for performance
of precision work. Operations that require low force and precise movements may
require a tool handle that permits precision gripping and pinching to maximize the fine
motor control capabilities of small muscles in the hands and arms. The type of grip
suitable for a manual tool operation is often limited by the size of the tool. Although
a power grip can provide greater strength than a pinch grip, a pinch grip provides
greater control of precision movements. Consequently, tool selection should optimize
the proportion of strength that an operator must exert with the ability to make necessary
movements with speed and precision. One method used for muscle driven tools
involves time, force, and precision considerations (Sperling et al. 1993).

Workstation and Task Factors

Power hand tool characteristics and operational requirements for hand tools may
include:

¢ Tool weight and load distribution
© Triggers

© Feed and reaction force

¢ Handles

© Work location and orientation

© Tool accessories

¢ Vibration

¢ Noise

Tool Weight and Load Distribution

Musculoskeletal injuries may arise simply from the act of lifting and manipulating power
hand tools if they are too heavy or if manipulation involves awkward postures (Myers
and Trent 1988). The actual time workers spend operating power hand tools on
automobile assembly lines is only a small portion of the overall cycle time, and the
majority of the time is spent handling and manipulating power hand tools (VanBergeijk

22


e Air pressure (kPa)

¢ Precision and tolerance (mm)

Bits, blades, and abrasives

© Power source (air, electric, hydraulic)

Human Operator Capabilities

Another important consideration in hand tool selection is how these engineering
requirements will affect the tool operator at the “operator end” of the tool. For example,
a power hand drill operation requiring a certain diameter hole drilled in a specific
material may require a minimum feed force (the amount of force needed for the drill bit
to work against the material). This force requirement at the drill bit directly affects the
hand tool operator’s exertion because the operator must apply force against the handle
to push the drill against the work material. Furthermore, the type of exertion may be
different if the drill has a pistol-grip handle, rather than an in-line handle.

The feed force necessary for accomplishing a task is only one type of force affecting the
operator. Power hand tools also generate forces, which in turn can act against the
operator. For instance, the torque at a power screwdriver spindle results in a reaction
force at the tool handle. The human operator must react against this force by exerting
an equal and opposite force to hold onto the tool handle as the tool functions. The
amount of force generated and the manner in which that force is directed back to the
operator is influenced by the power hand tool capacity, its dimensions, and the handle
shape. Additional force demands on the operator’s hands may be introduced due to the
posture an operator must adopt while holding the tool and the manner in which he/she
must grip the tool.

Furthermore, the posture assumed when a particular tool is held for a task can affect the
operator's ability to use that tool by affecting his/her strength capabilities. The handle
size and shape can also limit an operator's ability to produce forces. Gloves can further
affect the force available for gripping handles. Understanding and controlling these
ergonomic factors are important for limiting physical stress to the tool user and for
producing optimum work performance.

Typically, power hand tools utilize an auxiliary power source external from the human

body. Common power sources include pneumatic or air pressure, electric current,
hydraulic pressure, gasoline motors, or a power explosion. The output from power hand

19


Problem Source Identification Session

Moderator

Vern Putz Anderson, Ph.D.
Section Chief
Psychophysiology and
Biomechanics Section
Applied Psychology and
Ergonomics Branch
Division of Biomedical and
Behavioral Sciences

Recorder

Katharyn Grant, Ph.D.
Industrial Engineer
Psychophysiology and
Biomechanics Section
Applied Psychology and
Ergonomics Branch
Division of Biomedical and
Behavioral Sciences

Panel Members

Thomas Armstrong, Ph.D.

Professor

The University of Michigan

Center for Ergonomics

Industrial and Operations
Engineering

William Marras, Ph.D.

Professor

Ohio State University

Industrial and System Engineering
Department

Steve Meagher, M.D.
Consultant

Robert Radwin, Ph.D.
Associate Professor
University of Wisconsin
Department of Industrial
Engineering

Donald Wasserman, MSEE, MBA

Human Vibration Consultant

Engineering Control, Design, and
Manufacturing Session


WORKSHOP PARTICIPANTS

Workshop Chair

Stephen S. Smith, M.S.

Mechanical Engineer

Engineering Control Technology
Branch

Division of Physical Sciences
and Engineering

National Institute for Occupational
Safety and Health

Workshop Co-Chair

James H. Jones, C.I-H.

Acting Deputy Director

Division of Physical Sciences
and Engineering

National Institute for Occupational
Safety and Health

Epidemiology Session

Moderator

Marie Haring Sweeney, Ph.D.
Assistant Chief for Special Projects
Industrywide Studies Branch
Division of Surveillance, Hazard
Evaluations, and Field Studies

Recorder

Bruce Bernard, M.D.

Supervisory Medical Officer

Division of Surveillance, Hazard
Evaluations, and Field Studies

Panel Members

Gordon Reeve, Ph.D.
Corporate Epidemiologist
Ford Motor Company

iv

Sheryl Ulin, Ph.D.

Research Fellow

The University of Michigan

Center for Ergonomics

Industrial and Operations Engineering
Problem Source Identification Session


4. Determine areas in need of additional research.

© Identify strategies for filling existing research gaps. This may require
prioritizing research.

— Determine if research gap(s) is worth filling.

— Determine if current research can be extended to encompass power hand
tool design topics not being investigated.

— Determine necessary resources for accomplishing established goals.
¢ Discuss possibility of coordinating research efforts between interested facilities:
— Identify essential components for coordination.

— Design the coordination process to allow for the variability and uniqueness
in operation of each facility.

© Assess needs for the development of medical surveillance programs to identify
potential problem areas.

By providing a cohesive overall plan and coordinating the various parties, a mechanism
avoiding duplication and promoting cooperation would be developed. This should lead
to more efficient development of needed research or implementation of existing
solutions.

Also based on session discussions and experiences summarized by participants during
the workshop, there was a strong desire for improved communication and collaboration
between the fields represented at the workshop. Although some research and design
information may be available, workshop participants considered the lack of appropriate
channels of communication to be a significant hindrance to adequate dissemination of
sufficient information. A clearinghouse of such information would assist those involved
in research and design, as well as individuals in the field, in seeking new, pertinent
information. Panel members, and audience participants felt there was a substantial need
for a forum or forums in which ideas, issues, and concerns could be objectively
addressed.

STRATEGY PLAN

The establishment of a consortium including representatives from concerned parts of
industry, academia, and government may well set the stage for discussion and possible


Workshop Steering Committee

Heinz Ahlers, J.D.

Chief, Group II

Document Development Branch

Division of Standards Development
and Technology Transfer

Steve Hudock, M.S.

Safety Engineer

Education Resource Development
Branch

Division of Training and
Manpower Development

Jeffrey E. Fernandez, Ph.D.
Assistant Professor
Department of Industrial
Engineering
The Wichita State University,
(on assignment with EID, NIOSH)

Sheila Krawczyk, Ph.D.

Visiting Researcher

Engineering Control Technology
Branch

Division of Physical Sciences and
Engineering

Larry Fine, M.D.

Director

Division of Surveillance, Hazard
Evaluations, and Field Studies

James McGlothlin, Ph.D.

Research Industrial Hygienist

Engineering Control Technology
Branch

Division of Physical Sciences and
Engineering

Dan Habes, M.S.

Engineer

Applied Psychology and
Ergonomics Branch

Division of Biomedical and
Behavioral Sciences

Aaron Schopper, Ph.D.

Chief

Protective Technology Branch
Division of Safety Research

vii


Table 1. Manufacturing power hand tool categories and examples

Type of power tool Examples

Thread fastener driving tools Screwdrivers and nutrunners (pistol grip, angle, in-line)
Impact wrenches
Ratchet wrenches
Tube nut wrenches

Abrasive material displacement Sanders (rotary, orbital, belt, drum, disc, oscillating, etc.)
tools Grinders (angle, die or straight, horizontal, vertical)
Buffers
Polishers

Reciprocating and rotary files
Wire brush tools

Portable cutting tools Reciprocating saws
Panel saws
Trim saws
Shears and nibblers

Hole preparation tools Drills (angle, in-line, pistol grip)
Reamers
Tapers

Linear motion securing tools Staplers

Hog ringers and clinchers

Pressed-in or driven insert fastener tools
Nail drivers

Pin drivers

Percussion tools Riveting tools
Chipping hammers
Scalers
Engraving tools

Special purpose tools Cable binding tools
Clip squeezers
Electrode dressers
Lint or roll pickers
Multiple spindle tools
Tab set tools
Tube rollers
Wire wrapping and unwrapping tools
Wire strapping tools

14


CENTERS FOR DISEASE CONTROL,
‘AND PREVENTION

Proceedings of a NIOSH Workshop

A Strategy for Industrial Power Hand Tool
| Ergonomic Research — Design,
| Selection, Installation, and Use in
Automotive Manufacturing
|

egy
s %

: a U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
& Public Health Service

% Centers for Disease Control and Prevention

om National Institute for Occupational Safety and Health



result in greater effort and even loss of tool control. The amount of friction depends
upon the coefficient of friction between the hand and the material of the object grasped.
Some materials have greater coefficients of friction and, consequently, better frictional
characteristics than others (Buchholz et al. 1991). Coefficient of friction of palmar skin
is complex, but is not independent of normal force, and can decrease with increasing
force. When surfaces are coated with oil and other contaminants, palmar skin coefficient
of friction may decrease, and coefficient of friction is affected by material texture (Bobjer

et al. 1993).

Gloves

Hand tool operators often wear gloves to protect their hands. Gloves affect the force
available for gripping handles. While gloves may increase friction, they do negatively
affect hand forces by adding to the force necessary to oppose the resistance of the glove.
A decrease in grip strength by as much as 15% - 20% has been observed when gloves
are worn (Hertzberg 1955; Lyman 1957). On the other hand, a 20% to 30% increase in
the ability to prevent objects from rotating or sliding out of the hand was reported when
gloves were worn (Riley et al. 1985). The potentially adverse effects from glove usage
can be minimized by the provision of properly fitting gloves, and ones that protect only
as much of the hand as necessary.

Center of Gravity (COG)

As a general rule, it is recommended that the center of gravity of a hand tool should be
aligned with the center of the grasping hand so that the hand will not have to overcome
rotational moments, causing the tool to rotate the operator's wrist and arm. It is not too
difficult to determine the center of gravity of a hand tool. The center of gravity is the
intersection of plumb lines dropped when the tool is suspended from two different
points. The center of gravity of the tool should be determined with the tool fitted with
all its attachments, including air hose, couplings, and sockets, Forces introduced from
unbalanced tool installations can sometimes be eliminated by changed extensions and
adapters, relocated attachments, or suspended airlines. If the tool center of gravity is not
aligned with the hand, a hand tool can be suspended from a counterbalance.

Tool Location

A power hand tool should be installed so the operator can lay it on the ground, hang
it from the ceiling, or attach it to some fixed supporting structure, rather than holding
it in the hand. If a fixed support cannot be obtained, a belt, instead of the hand, should
support the tool. If a heavy power hand tool requires support during operation, then
it is considered better to let one hand do the holding, while the other does the operating,

37


WORKSHOP SUMMARY

BACKGROUND

The Engineering Control Technology Branch (ECTB) of the National Institute for
Occupational Safety and Health (NIOSH) initiated a project entitled the “Ergonomic
Study of Power Hand Tools in the Automotive Manufacturing Industry” to address the

issue of

upper extremity work-related musculoskeletal disorders (WMSDs)*. The

following are some reasons for focusing on the association between WMSDs and power
hand tool use in manufacturing:

The use of power hand tools in modern industry for repetitive, manual work
is widespread.

The best tool for a specific task is often not obvious, because there is such a
large variety of power hand tools to choose from.

Automation is usually not an alternative to work performed with power hand
tools.

Work involving power hand tools often combines several WMSDs risk factors
(Le., vibration, force, posture, contact stress, and repetitive motion).

Workers who use power hand tools often spend a large portion of the
workday holding and operating them.

Some power hand tools are capable of producing very high forces.

There is a high incidence of WMSDs in manufacturing industries today,
particularly in industries using power hand tools.

‘The

term work-related musculoskeletal disorders (WMSDs) _ includes

musculoskeletal, neuromuscular, as well as neurovascular tissue injury and illness.


CONTENTS

Foreword 2.6... ce eee eee teen eee
Workshop Participants .........-.0.-+2 +000

Epidemiology Session ............--

Problem Source Identification Session . .

Engineering Control/Design/Manufacturing Session ............- vi
Workshop Steering Committee ... 2... 0... eee eect ene vii
Acknowledgments ©. 0.0... 6. cece cette eee tee eens viii
Contents 20... ee eens ix
Workshop Summary ©... 0.6.6 c eee eee etter eee eens 1

Background .............2-0e e000 1

Workshop Organization ............ 3

Session Findings ............... 4

Summary and Recommendations . . 6

Strategy Plan . 0.6... eee eee tenes 8
Industrial Power Hand Tool Ergonomics Research: Current

Research, Practice, and Needs—Robert Radwin, Ph.D. ..........- 11

ix


Table 3. Power hand tool mechanical properties affected by certain tool parameters

Tool properties Tool parameters affecting tool properties
Load Center of gravity location
Tool mass
Use of counterbalance or articulating arm
Power line installation
Handle size Type of grip needed (power, pinch)
Handle shape Type of grip needed (power, pinch)

Handle orientation

Feed force

Sound level

Reaction torque

Vibration.

Type of grip needed (power, pinch)
Distribution of load
Work location

Type of fastener head
Type of fastener tip or drill bit
Work material

Tool speed
Power

Work material
Tool location

Spindle torque

Tool and handle length
Stiffness of joint
Torque reaction bar

Tool weight
Work material
Abrasive material
Tool speed

Tool power
Handle location
Moment of inertia

33


Grip strength depends on the type of grip used for holding a handle. The hand is less
capable of performing precise movements and exertions when using a power grip
because of the large muscles recruited. It is the large muscles that provide the high
strength in power gripping.

Handle shape can affect hand and arm strength (Fothergill et al. 1992). Handle size
affects grip strength. If an object is too large or too small, the strength of the hand is
greatly compromised. Numerous studies have demonstrated the effects of handle size
on grip strength. When these data guide the selection of tool handles, it is important to
know not only the handle span and handle circumference the data represent, but also

the handle shape.

A particular finger, or combination of fingers, can also affect grip strength (Radwin et
al. 1992), The thumb, index, and middle fingers are the strongest fingers and should be
utilized for producing the most grip force. The ring finger and small finger are less
capable of producing forces and should be primarily used for stabilizing handles.

Some pistol-grip power tool designs permit the thumb and index to assume a precision
grip, but also provide a handle for reacting against spindle torque. Tools designed for
one type of grip may sometimes be used in a different manner. Handle dimensions
should differ, depending on whether they are being used for precision tasks, power
manipulations, or for carrying.

Increasing the surface area of contact between the handle and the hand increases the
amount of torque that can be generated. Grip strength, however, has been shown to
decrease when handles with diameters greater than 50 mm are used (Pheasant and
O'Neill 1975).

Anthropometry

The population of workers that will use the tools is another important consideration.
Strength decreases for aging populations (Fraser 1980). Hand size and strength are
related to gender.

Worker anthropometry can affect the posture a worker assumes when operating a hand
tool, The ideal work location for tall workers may be too high for short workers. The
location, orientation, and tool design should all be considered together along with the
stature of the worker (Armstrong 1986a; Armstrong et al. 1986b). In situations where
the work location and orientation cannot be adjusted to suit the worker, it may be
possible to select another tool. In cases where the work specifications determine the tool
design, it may be possible to change the location or orientation of the work.

21


as opposed to letting one hand simultaneously hold and operate the tool. Power and
air cabling should not interfere with the operation and/or manipulation of a tool.

Tool Activation and Throttle

Push-to-start activated power hand tools free the operator from having to squeeze a
trigger or lever, but they can increase feed force requirements. The level of feed force
required can be addressed by selecting tools that minimize the activation force needed
for operating push-to-start tools. Tool throttles should not concentrate forces, but
distribute forces in the finger segment and among several fingers. Lever triggers should
not produce contact stresses or pinch points. Levers should take the shape of the handle
when fully pressed. Levers should be at least as long as the hand.

Reaction Torque

Methods for minimizing reaction torque include use of long reaction arm tools, such as
a right-angle nutrunner rather than a pistol-grip nutrunner. Installation of torque
reaction bars and use of torque absorbing suspension systems also minimize reaction
torque. Reaction torque for in-line handles are absorbed by an operator directly
grasping the tool. Increased grip force is needed to prevent the tool from rotating in
hand. High friction handle surface can help reduce grip force demand. Short length
pistol-grip handles that move the hand closer to the vertical center of gravity help
reduce wrist torque when the tool is carried or positioned; however, the mechanical
advantage offered by a longer handle for reacting against reaction torque is also reduced.
Accessory handles can provide a grip for a second hand to hold a tool.

The Ford Motor Company has power hand tool operation guidelines that specify torque
reaction bars on in-line power tools for torque levels greater than 3.2 Nm (VanBergeijk
1987). Torque reaction bars are required for pistol-grip nutrunners operating at torque
levels greater than 6.8 Nm. Ford Motor Company practices limit a maximum measured
operator reaction force of 220 N, or a maximum measured impulse of 220 Ns.
Automatic shutoff tools are substituted for stall tools when torque is greater than
40.7 Nm.

Alternative methods for limiting reaction torque include (1) use of torque reaction bars,
(2) installation of torque absorbing suspension balancers, (3) provision of tool mounted
nut holding devices, and (4) use of tool support reaction arms. A torque reaction bar
can sometimes transfer loads back to the work piece. Tools that are equipped with a
stationary reaction bar adapted to a specific operation so that reaction force can be
absorbed by a convenient solid object can eliminate completely reaction torque from the
operator’s hand. These bars can be installed on pistol-grip , in-line, and angle tools. The

38


tools is often many times greater than what can be produced by a bare hand, even if
manual hand tools were used. The hand, however, must react against the increased
forces and torque produced by power hand tools. Since the energy provided from
power hand tools is normally greater than the energy produced manually, the potential
for injuries is greater. These forces can be reduced through the mechanical advantage
provided by handles and external suspension systems.

When selecting a hand tool, the following hand tool operator characteristics should be
considered:

¢ Strength

¢ Anthropometry

© Manual dexterity and motor capabilities

Strength

Safe power tool operation requires that an operator possess the ability to adequately
support the tool in a particular position, while reacting against forces produced by the
power tool. The capacity to produce forces is usually referred to as strength. Strength
is therefore the maximum force an individual can produce. The ability for hand tool
operators to produce the required forces should be taken into account when hand tools
are selected. An operator normally cannot use a tool for long without becoming fatigued
if required to exert force at maximum capacity.

The ratio of the required force to an individual’s strength approaches unity as the force
requirement approaches an operator's strength capability. This proportion is related to
the ability to sustain a given exertion, and to the onset of localized muscle fatigue. The
greater the proportion of strength that a repeated exertion requires, the quicker the
operator will be unable to sustain that exertion, and the more rapid the onset of fatigue.
Tools should therefore be selected so that this proportion is minimized. This can be
accomplished either by selecting tools requiring less force or by using tools in ways that
maximize an operator's strength.

Strength is affected by body position, the direction of exerted force relative to the body,
the type of grip used for holding the tool, and the handle size. Operator strength
capabilities change as the position of limbs and body changes. For instance, arm
muscles are strongest when pushing and pulling in a direction towards and away from
the body (i.e., push and pull), as opposed to moving across the front of the body (ie.,
left and right) (Greenberg and Chaffin 1977).

20


Engineering Control/Design/Manufacturing Session

Moderator

Hongwei Hsiao, Ph.D.
Research Industrial Engineer
Protective Technology Branch
Division of Safety Research

Recorder

Chery] Estill, M.S.

Industrial Engineer

Engineering Control Technology
Branch

Division of Physical Sciences and
Engineering

Panel Members

Walter Boryczka

Power Tool and Engineering
Specialist

Advance Manufacturing Operations

Chrysler Corporation

Robert Bruno
Design Manager
Group Four Design

Harold Josephs, P.E.
Director Fastener Research Center
Lawrence Technical University

Stephan Konz, Ph.D.

Professor

Department of Industrial
Engineering

Kansas State University

vi

Bo Lindquist

Manager R & D Applied Ergonomics
Product Safety and Liability

Atlas Copco Tools

Anil Mital, Ph.D., P.E.

Professor

Ergonomics and Engineering Controls
Research Laboratory

Industrial Engineering

University of Cincinnati

Ed Mohr, C.S.P.
Ergonomics Advisor
Manufacturing Engineer
North American Operations
General Motors Corporation


Industrial Power Hand Tool Ergonomics Research

Current Research, Practice, and Needs

Robert G. Radwin, Ph.D.
Associate Professor
Department of Industrial Engineering
University of Wisconsin-Madison

and

Stephen S. Smith, M.S.

Engineering Control Technology Branch
Division of Physical Sciences and Engineering
National Institute for Occupational Safety and Health
Centers for Disease Control and Prevention
Public Health Service
U.S. Department of Health and Human Services


considerable vibration from the wobbling action of the rotating spindle. Tools requiring
maintenance or those becoming unbalanced are also potential sources of vibration. Some
tools are specifically designed to minimize vibration and may be fitted with isolation
attachments.

Handle location and the type of tool can have a dramatic effect on the level of vibration
transmitted to the operator. Vibration measurements made from a large chipping
hammer running at full throttle indicated the acceleration ratio between the chisel end
and the rear handle was approximately 78:1 (Wasserman et al. 1981). The same study
also found rear handle acceleration levels for a small stone chipper were as high as 2.5:1,
apparently because of very little mass damping for this light weight tool.

Vibration may affect the force exerted in repetitive manual tasks. Hand tool vibration
can introduce disturbances in neuromuscular force control resulting in excessive grip
exertions when an operator holds a vibrating handle (Radwin et al. 1987). A study on
pneumatic hammer recoil observed a stretch reflex and muscular contractions in the
elbow and wrist flexors (Carlséé and Mayr 1974). Since forceful exertions are a
commonly cited factor of chronic muscle, tendon, and nerve disorders of the upper
extremities, vibrating hand tool operation may increase the risk of work-related
musculoskeletal disorders through increased grip force. The results of Carlsé6 and
Mayr’s study demonstrated that grip exertions increased with tool vibration. The
magnitude of this increase in grip force was similar to the increase in grip force for a
two-fold increase in load weight.

Vibration has been shown to produce short-term sensory impairments (Radwin et al.
1990a; Streeter 1970). Recovery time is exponential and can require more than 20
minutes (Kume et al. 1984). Tasks requiring a high degree of tactile sensitivity, such as
inspecting for smoothness or rough edges, should avoid the involvement of tools
producing vibration in the 160 Hz range since sensory performance, and thus work
performance in manual inspection task will be affected. Sanding and grinding operations
are two examples of tasks requiring inspection for smoothness. Workers often sand or
grind a surface and periodically inspect their work using tactile inspection to determine
if the surface was sanded to the desired level of smoothness. Diminished tactility may
result in a surface feeling smoother than it is, while actually being rougher surface than

desired.

Unbalanced grinding wheels can generate periodic forces at a frequency equal to the
rotational speed and at multiples of this frequency. Usually the frequency of the
rotational speed dominates over the higher frequency vibration. When grinding wheel
imbalance causes vibration, there is little to be gained by a machine that produces less

28


vibration. The best method of reducing this type of vibration is by elimination of the
imbalance and by isolation of the operator. Increasing the tool moment of inertia makes
the handle less sensitive to unbalanced wheels. This is accomplished by distributing the
tool mass as far from the center of gravity as possible. Moving the center of gravity as
close to the grinding wheel as possible will decrease the wobbling motion. Oftentimes
impulse tools are considered because they can produce high torque levels without
producing high reaction forces. The tradeoff is low reaction torque for increased

vibration.

Noise

Loud noises can be produced by power hand tool motors and from vibration of moving
objects, such as a grinding wheel against a grinding surface. Pneumatic vane motors
generate sound when a major change in airflow occurs due to pressure differences and
compression of air when the vanes rotate. The sound has a fundamental frequency
equivalent to the motor rotation speed times the number of vanes. Vibration in the tool
housing can also transmit noise to the surrounding air.

Airflow noise is another common source of noise. This is caused by the formation of
turbulence inside the tool. If air passes sharp corners or edges, airflow noise is
enhanced. The dominant source of the airflow noise is strongly dependent on the speed
of air leaving the housing of the tool.

The Relationship Between Tool Mechanical Properties and
Musculoskeletal Stress

Power hand tools, in conjunction with the specific work and environment in which they
are used, can potentially introduce factors that are associated with musculoskeletal
stress. Some of these potential conditions are summarized in Table 2. Physical stress
factors may be reduced or eliminated by changing the tool, task, or work environment.
Stress factor reduction involves identifying properties of the specific power hand tool
needed for a task.

Specific power hand tools can introduce varying levels of musculoskeletal stresses,
depending on certain properties of those tools. For example, the force needed for
carrying and positioning a power hand tool often depends on the tool load. Conse-
quently a heavy tool requires greater exertions than a less heavy tool. Although an
exertion of less force is the desired outcome, for certain tasks a heavy tool is more
advantageous for the operator than a lighter one. There are several parameters
associated with specific power tools that affect these mechanical properties. Tool load
is affected by tool mass, mass distribution, use of an external support mechanism, and

29


ISO [1981]. Hand-held pneumatic assembly tools for installing threaded
fasteners—reaction torque and torque impulse measurements. Geneva, Switzerland:
International Standards Organization, ISO 6544.

Kihlberg S, Kjellberg A, Lindbeck L [1993]. Pneumatic tool torque reaction forces,
displacement, muscle activity and discomfort in the hand/arm system. Appl Ergon 24(3):
165-173.

Kume Y, Maeda S$, Hashimoto F [1984]. Effect of localized vibration in work environment
on organic functions at finger tip for surface roughness. In: Mathews ML, Atwood DA,
eds. Proceedings of the 1984 International Conference on Occupational Ergonomics.
Toronto, Ontario, Canada: Human Factors Conference, Inc. pp. 457-461.

Lyman J [1957]. The effects of equipment design on manual performance in protection
and functioning of the hands in cold climates. Production and Functioning of the Hands
in Cold Climates. (Fisher RR, ed.) National Academy of Sciences, National Research
Council, Washington D.C., pp. 86-101.

Mital A, Kilbom A [1992]. Design, selection and use of hand tools to alleviate trauma of
the upper extremities: part II—the scientific basis (knowledge base) for the guide. Int J
Ind Ergon 10:7-21.

Myers JR, Trent RB [1988]. Hand tool injuries at work: a surveillance perspective. J Safety
Res 19.

NIOSH [1989]. NIOSH criteria for a recommended standard: occupational exposure to
hand-arm vibration. Cincinnati, OH: U.S. Department of Health and Human Services,
Public Health Service, Centers for Disease Control, National Institute for Occupational
Safety and Health, National Institute for Occupational Safety and Health, DHHS
(NIOSH) Publication No. 89-106.

Oh S, Radwin RG [1993]. Pistol grip power tool handle and trigger size effects on grip
exertions and operator preference. Hum Factors 35(3):551-569.

Pheasant S, O'Neill D [1975]. Performance in gripping and turning—a study in
hand/handle effectiveness. Appl Ergon 6(4):205-208.

Radwin RG, Armstrong TJ [1985]. Assessment of hand vibration exposure on an
assembly line. Am Ind Hygiene Assoc J 46(4):211-219.


particularly when the tool is used in cold environments or when air passes through the
pneumatic hand tool handles. Cold hands can affect strength, manual dexterity, and
tactual sensitivity. Allowances should be made for those effects when power tools are
selected for use in cold work environments.

Tool Selection

Tool selection should be made within the context of the specific job. By considering the
ergonomic aspect of tool application for a specific job, the adverse effects of using the
wrong tool can be prevented. It is generally agreed that physical stress, fatigue, and
WMSD can be prevented or reduced by selecting the proper tool for the task. Usually
tools regarded as best for the job are the ones that minimize physical stress by producing
low demands of force on the hand; that are not awkward to hold and handle; and that
minimize shock, recoil, and vibration. Physical stress on the tool operator is often
considered as a result of how a particular tool is used for a specific task, rather than just
its use. Therefore, properly using the correct tool for the task should also be considered
in the context of tool selection.

Currently, there are tools being marketed as “ergonomic.” Many of these tools have
features that may have ergonomic benefits when used correctly. Although some of these
tools may have desirable properties, ergonomics experts concur that selecting a specific
tool without considering the task requirements, work station design, and setup may not
prevent adverse effects, and in some cases can have potentially detrimental
consequences. Ergonomic principles, along with tool design, should therefore guide the
selection, installation, and use of the right tool for the right application.

Hand tools should be selected based on physical process engineering requirements and
ergonomic criteria. When both of these aspects are considered, the appropriate hand
tool for a particular job should:

© Maximize work performance.
e Enhance work quality.
© Minimize operator stress.
® Prevent fatigue onset.
Engineers often specify power hand tool requirements for a specific job in terms of the

manufacturing process, such as the drill bit speed needed to make a specific hole in a
material, or the torque needed to tighten a screw for a threaded fastener joint. Although

17


DISCLAIMER

Sponsorship of this workshop and these Proceedings by NIOSH does not constitute
endorsement of the views expressed or recommendation for the use of any commercial
product, commodity or service mentioned. The options and conclusions expressed in
the plenary paper are those of the authors and not necessarily those of NIOSH.

The research recommendations are not to be considered as final statements of NIOSH
policy or of any agency or individual who was involved. They are intended to be used
in advancing the knowledge needed for worker protection.

This document is in the public domain and may be freely copied or
reprinted. Copies of this and other NIOSH documents are available from

Publication Dissemination, EID
National Institute for Occupational Safety and Health
4676 Columbia Parkway
Cincinnati, Ohio 45226

Fax number: (513) 533-8573

To order NIOSH publications or to receive information about occupational
safety and health problems, call 1-800-35-NIOSH

DHHS (NIOSH) Publication No. 95-114


Tools that require the forearm to be rotated while the wrist is bent are also not
recommended. Neither should tools require operators to work with the elbow above
mid-torso height or require work with the hands down behind the shoulder line in the
sagittal plane. Generally, it is recommended that to the elbow remain at the side of the
body to avoid extreme rotation of the forearm, to avoid extreme deviation of the wrist
side to side, and to avoid palmar wrist flexion (towards the palm of the hand) or
extreme wrist extension (towards the back of the hand). Awkward postures can often
be avoided by selecting the appropriate tool for the particular work location.

Exertion/Force

When frequent exertions are necessary, the required forces should be reduced.
Sometimes reducing the force requirements has the adverse effect of increasing the
number of repetitions necessary for accomplishing a task. For example, if a small, light
die grinder is used, the load is reduced, but the lighter grinder requires more force and
repetitions than would a heavier angle grinder. When decreasing the time required to
perform an operation, it is important not to increase the number of repetitions made in
a work cycle.

Force can also be reduced by elimination of weight from tools and handled parts, or by
provision of mechanical aids for handheld tools, parts, and materials. Reaction torque
is affected by the types of fasteners and tool power settings. Increasing friction between
the hand and objects grasped can also reduce forces. If force is required for
simultaneously gripping while pushing or pulling, a flange at the end of the tool should
be provided to reduce slippage and possibly to decrease the grip force requirements.

Contact Pressure

Acute contact pressure, and hence mechanical stress concentrations in the hand, can be
reduced by increased handle sizes, elimination of sharp edges, and use soft compliant
coverings on handles. Handles should be as large as possible, while permitting
comfortable fit in the hands, but not so large as to diminish strength. The size of the
handle depends on the force and dexterity requirements of the task, A smaller handle
is necessary for a light, precision, manipulative task than for a forceful one requiring
gross movements.

Handle Friction

Sufficient friction should be present between the handle and the hand to provide a
secure grip and prevent slippage. The frictional characteristics of the handle affect the
grip force needed to maintain control of the tool, and the ability needed to exert torque.
Surfaces that do not provide adequate friction require greater grip force, which may

36


(a) F,

(b)

Figure 1, Hand forces associated with alternate power hand tool configurations; (a) in-
line, (b) pistol grip, and (c) right angle.

34


Panel members discussed these and other topics of concern. The moderators, having
input from panel members, provided a half-day summary of each session's discussions.
The results of the discussion sessions were summarized and used for modifying and
refining Dr. Radwin’s paper. The final version is contained in these proceedings.

SESSION FINDINGS

The following section is a summary of the discussions that occurred in each session.
Panel members were asked to provide comments and suggestions based on their activity
and expertise in a particular area of work, research, and design.

Epidemiology

This session examined existing epidemiological research and discussed future research
needs. The following concerns were highlighted during the discussion:

© Lack of epidemiological data relating hand tool use to specific workers or
worker populations.

© Lack of good exposure/response data.

© Limited data on the relationship between power tools use and risk factors for
WMSDs, including repetition, force, posture, vibration, contact stresses, etc.

The employee populations considered most often exposed to musculoskeletal stress
associated with power hand tool use were the skilled workers, who constitute
approximately 20% of the automotive industry work force. These workers may be
involved in either nonstereotypic, nonrepetitive, or in repetitive work in structured jobs.

There was consensus on the need for more epidemiological data in the investigation of
operator and power hand tool interaction. There was also consensus on a need for a
better system of identification and integration of injury data and tool and job description
data. An improved engineering data system containing, among other things, typical
mechanical characteristics of the tool and material specifications would assist in
identifying WMSD risk factors. Also, an improved epidemiological data system could
assist in determining how to reduce risk in the following ways:

1. Evaluate exposure/response relationships between tool use and work-related
musculoskeletal disorders.

2. Provide useful information for improving tool design.


Tichauer ER, Gage H. [1977]. Ergonomic principles basic to hand tool design. Ergonomic
guides. Am Ind Hyg Assoc J 38(11): 622-634.

Ulin SS, Snook SH, Armstrong TJ, Herrin GD [1992a]. Preferred tool shapes for various
horizontal and vertical work locations. Am Occup Environ Hyg 7(5).

Ulin $S, Armstrong TJ, Snook SH, Franzblau AF [1993a]. Effect of tool shape and work
location on perceived exertion for work on horizontal surfaces. Am Ind Hyg Assoc J
54(7):383-391.

Ulin SS, Armstrong TJ, Snook SH, Keyserling MW [1993b]. Perceived exertion and
discomfort associated with driving screws at various work locations and at different
work frequencies. Ergon 36(7):833-846.

Ulin $S, Armstrong TJ [1992]. Development of guidelines for the use of powered hand
tools. In: Hagberg M, Kilbom, eds. Arbete Och Halsa. Proceedings of the International
Scientific Conference on Prevention of Work-Related Musculoskeletal Disorders.

Stockholm, Sweden pp. 293-295.

VanBergeijk E [1987]. Selection of power tools and mechanical assists for control of
occupational hand and wrist injuries. In: American Conference of Governmental
Industrial Hygienists Staff, eds. Ergonomic interventions to prevent musculoskeletal
injuries in industry. Chelsea, MI: Lewis Publishers.

Wasserman DE, Reynolds DD, Behrens V, Taylor W, Samueloff S [1981]. Vibration white
finger disease in U.S. workers using pneumatic chipping and grinding hand tools.
Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service,
Centers for Disease Control, National Institute for Occupational Safety and Health,
DHHS (NIOSH) Publication No. 82-101.

46


8. More epidemiological data, including theories and models.

Engineering Controls/Design/Manufacturing

The intent of this session was to discuss control technology necessary to prevent or
control WMSD etiological factors. The discussions focused on current needs in the area
of tool design, component design, fastener engineering, and manufacturing methodology
and testing, taking into account biomechanical and physiological stress. The panel
members agreed that there was a need for more research on WMSD risk factors, acute
injuries, fastener technology, and tool testing. However, information gathering, access,
and dissemination to support tool design, selection, and installation were the immediate
needs. The following list consists of such information needs highlighted during this
session:

1. The development of a systematic flowchart to understand the relationship among
elements within the manufacturing process (computerized expert system).

2. The facilitation of more and improved communication among ergonomists,
engineers, and designers for a more effective, systematic approach to the
manufacturing process.

The lines of such communication could be augmented by the development of a better
information translation and dissemination process. Some panel members and audience
participants expressed a desire to have research information be more readily accessible
to other researchers and individuals in the field. They also felt guidelines specific to
industry should be established; however, these guidelines should focus on methodology
and not on specific standards.

SUMMARY AND RECOMMENDATIONS

There was consensus across sessions that epidemiological research of power hand tools
and operator interaction was needed. This need coincides with the apparent lack of
epidemiological data pertaining to power hand tools, industrial operations, and rate of
specific injuries. In each session, participants felt that more epidemiological information
would augment other efforts in musculoskeletal stress identification and prevention. As
an example, during the summary session it was indicated that more epidemiological data
was needed to understand the magnitude of WMSDs. More worker characteristics and
job specific data needs to be collected as epidemiological information to assist in
obtaining an historical perspective and tracking workers on the job. Such
epidemiological information should reveal power hand tool ergonomic research needs.
However to adequately catalog research needs, an epidemiological procedure should


Proceedings of a NIOSH Workshop

“A Strategy for Industrial Power Hand Tool Ergonomic
Research—Design, Selection, Installation, and
Use in Automotive Manufacturing”

January 13-14, 1994
Cincinnati, OH

U.S. Department of Health and Human Services
Public Health Service
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health

August 1995


location for using tools was within 38 cm of the worker. Driving screws at a 114 cm
vertical work location had the minimum perceived exertions for right-angle, pistol-grip,
and in-line tools. When the screws were driven on a vertical surface, the minimum
perceived rating for the pistol-grip tool occurred for 114 cm and 140 cm, and the lowest
ratings for the in-line and right-angle tools occurred for 191 cm. When a horizontal
work surface was mid-chest or elbow height, right-angle and in-line tools received the
best ratings, but the pistol-grip tool received the best ratings for screws driven near

mid-thigh height.

The above mentioned psychophysical studies agreed with mannequin predictions, which
indicated an optimal workstation height for these power hand tools between 100 cm for
a fifth percentile female and 121 cm for a ninety-fifth percentile male (Armstrong et al.
1986b). A survey of power hand tool operators also had similar results (Armstrong et
al. 1989), Operators rated vertical heights between 102 cm and 153 cm, and horizontal
distances within 38 cm as the most comfortable.

Tool Accessories: Balancers, Articulating Arms, and Other Accessories

Spring or air balancers are available for counterbalancing tool loads in the hands.
Articulating arms and hoists will reduce the weight of heavy power tools. Special
attention may be required to install balancers so that minimal effort is needed when
holding and using the tools in the desired work location. Spring counterbalances
produce a force that opposes gravitational forces so the tool weight is reduced. If these
balancers are not installed correctly, however, they can actually have the reverse effect
of increasing the opposing force necessary to keep the tool in place.

Vibration

Vibration is an intrinsic property of power hand tool operation and can be a by-product
of its operation, or it may be the desired action. Vibration levels depend upon hand tool
size, weight, method of propulsion, and the tool drive mechanism. Continuous vibration
is inherent in reciprocating and rotary power tools. Impulsive vibration is produced by
tools operating by shock and impact action, such as impact wrenches or chippers. The
tool power source, such as air, electric, or hydraulic, can also affect vibration. Vibration
is also generated at the tool-material interface by cutting, grinding, drilling, or other such
actions. The parameters of the generated vibration can be affected by work material
properties, disk abrasives, abrasive surface area, and fastener type.

Accessory attachments may become a source of vibration if they fit improperly or cause
a tool to become unbalanced. An example is a loosely fitting extension shaft on a

nutrunner, which causes a tool usually not considered a vibrating hand tool to produce

27


Problems Associated with Power Hand Tool Use

Power hand tool operation has been associated with exposure to a variety of physical
stress factors. The relative contribution of physical stress factors associated with work
involving hand tools has been difficult to separate from power hand tool use because
many manufacturing jobs involving power hand tools also involve extensive use of the
upper limbs. For instance, power hand tool operators may need to assume awkward
postures dictated by a specific tool handle location and/or work piece orientation. The
same tool may be used repetitively for short-cycle time tasks. Some power tools can also
introduce vibration and have triggers that may cause contact stress from sharp edges
against the fingers or palm. The hands may at the same time be exposed to cold air
produced from pneumatic tool exhaust outlets.

Physical Stressors

Physical stressors associated with hand tool operation include awkward postures,
forceful exertions, repetitive motion, contact stress, and vibration. The level of physical
stress associated with chronic work-related musculoskeletal disorders may be so small
in magnitude that any single occurrence seems harmless, but repeated exposure over
several weeks, months, or even years may lead to an illness. The objective when
selecting, installing, and using power hand tools is to minimize operator exposure to
each physical stress factor. All of these physical stress factors need to be considered in
relation to one another, because reducing exposure to one factor, may have the adverse
effect of increasing exposure to another factor.

Posture refers to the position that the body assumes for a specific task. While it is not
possible, or even desirable, to immobilize the body in one “ideal posture,” some postures
are more stressful than others and should be avoided. Handling a power tool while
flexing or extending the wrist and rotating the forearm may irritate tendon attachments
at the epicondyle. An example is provided by arm position during operation of a right
angle nutrunner where the wrist is flexed while the forearm is pronated. An extended
wrist and supinated forearm operating a pistol-grip drill on the underside of a horizontal
surface at eye level is another example. Posture is affected by work location, work
position, tool shape, and worker size.

The force requirements for a job are often related to the weight of the tools being
handled. The distribution of tool weight can also affect how force is transmitted to the
user. Force demands that exceed an operator's strength capabilities may cause loss of
control, leading to an unintentional injury and poor work quality.

Repetitiveness refers to the number of times that similar movements are repeated over
time, or how often certain exertions are made. Repetitive operations often require

15


Delivering on the Nation's promise:
Safety and health at work

For all people
Through research and prevention

DHHS (NIOSH) Publication No. 95-114



advantages from tool mounted reaction devices include the following: (1) all reaction
forces are removed from the operator, (2) one hand operated pistol-grip and in-line
reaction bar tools can be used rather than right-angle nutrunners which usually require
two hands, (3) reaction bar tools can be less restricting on the operator's posture, (4) tool
speed and weight are improved over right-angle nutrunners in most tool sizes, (5)
reaction bars can improve tool performance. The disadvantages are that reaction bars
must be custom made for each operation and the combination of several attachments for
one tool can be difficult. Torque reaction bars also add weight to the tool and can make
the tool more cumbersome to handle. Providing tools with torque reaction bars or using
torque absorbing suspension systems can eliminate torque reaction effects completely,
although these interventions are not always practical, especially when there is limited
accessibility, manipulation restrictions, or no surfaces for reaction bars to contact. A
shorter tool, however, can be used if a reaction bar is provided. Acceptance and use
of these devices varies greatly. Use of impulse tools can help reduce reaction torque, but
such tools also produce vibration and noise, which must also be considered.

Balancer and Suspension

Balancer spring tension should be adjusted so the operator does not have to counter
more force than necessary. Balancers should be adjusted so that tool aligns as closely
to the work area as possible to prevent unnecessary reaching. The counterbalance
should not lift the tool when it is released so that the operator has to elevate the
shoulder to reach the tool. Also situations where operators tend to work ahead or behind
the assembly line should be avoided. A trolley and rail system should be installed if a
tool is to be moved horizontally. Special attention may be required to assure that the
balancer is attached directly above the work.

Overhead suspension devices are best suited for assembly operations that can use a
spring balancer or air balancer. Floor or side mounted articulating reaction and support
devices are also available. Articulating arms help support the tool weight, absorb
reaction torque, and free the hands for other activities. Articulating arms, however,
restrict freedom of motion and may require an operator to manipulate a greater mass
and consequently react against greater inertial forces.

Vibration

Besides considering alternative tools that produce less vibration, proper maintenance on
current tools can help prevent vibration caused from worn bearings, malfunctions, or
inadequate lubrication. Unbalanced rotating shafts and disks generate excessive
vibration. Tool speed and power settings can also affect a tool’s vibration characteristics.
Tool operators should avoid using long, loose extension shafts on rotating spindles.

39


1987). One study found that the frequency of plant medical visits were significantly
related to the mass of the tools operated (Schlegel and Willcoxon 1989).

An operator's exertions can be affected by the specific type of external power. A large
electric motor, for instance, may carry more weight than a comparable air-powered tool.
External power accompanies additional ergonomic considerations. These include the
added weight from having a motor, handles for supporting power tools, triggers for
activation, a feed and reaction torque, use of balancers, torque reaction bars and other
accessories, a required posture, vibration, and acoustical noise. Also the required use
of tool accessories such as air hoses, power lines, couplings, and adapters contribute an
often overlooked addition of weight that the tool operator must hold and carry.

Psychophysical experiments have provided some insight into the load that power tool
operators prefer using. When experienced hand tool operators were asked to rate the
mass of the power tools they operated on continuous scales between 0 to 10, tools
weighing 0.9 to 1.75 kg mass were rated “just right” (Armstrong et al. 1989). Another
psychophysical experiment showed that perceived exertion for a tool mass of 1 kg was
significantly less when compared to tools with masses of 2 kg and 3 kg (Ulin and
Armstrong 1992b). A study investigating muscle activity of the hands and forearms
observed a significant correlation between tool mass and grip exertions (Grant et al.

1992).

There can be a tradeoff between the selection of a lightweight tool and the benefit of the
added weight for performing operations that require high feed force. For example, the
power available for a grinding task increases with the increasing mass of a grinder.
Reducing the weight of a grinder may increase the amount of feed force and time
necessary for the operator to accomplish the task, consequently, also increasing an
operator's exposure time to other variables such as vibration.

Triggers

Triggers extending far away from the handle make it necessary to use the distal finger
segments. This has been associated with a stenosing tenosynovitis of the volar finger
tendons. Use of the distal phalanx to actuate a tool trigger contributes to trigger finger
(Tichauer 1977). Trigger finger derives its name from the snapping sensations and
movements resulting from stenosis of the tendon sheaths.

Throttle force is the force needed to overcome the resistance of a valve, or to close an

electrical switch when a power hand tool is activated. Standards developed by the
American National Standards Institute (ANSI) specify that a tool should shutoff when

23


the manner in which the power line or air line is attached. Some mechanical tool
properties and tool-related parameters affecting these properties are listed in Table 3.
Engineering control strategies often make use of these properties for mitigating
potentially stressful conditions. Mitigation of potentially stressful conditions can often
be accomplished through tool design, selection, and installation, while considering the
task and the conditions in which the task is performed.

Tradeoff Between Different Physical Stresses in Tool Selection

Power hand tools having similar operating specifications are sometimes available in
alternative configurations. In the case of nutrunners, this often means choosing between
in-line, pistol-grip, and right angle configurations that perform similar functions. These
alternative configurations are illustrated in Figure 1. Often process engineering
requirements make no distinction between which configuration is needed. Selection
sometimes in practice is a function of availability rather than ergonomics. Awkward
postures can sometimes be avoided by selecting alternative power hand tool

configurations.

Although there is research available with respect to posture and perceived exertion
associated with operating these power hand tools at various locations, there is little
information concerning use of these power tools when considering the tradeoff between
posture and force. An example is provided by the selection of an in-line nutrunner
rather than a pistol-grip nutrunner for operation on a horizontal work surface. The
tradeoff between additional grip force necessary for operating an in-line power tool in
one posture and the reduced hand force offered by the pistol-grip tool in another posture
is not known.

Torque developed at an in-line power hand tool spindle (T,) must be overcome by
tangential shear forces between the hand and the handle (F;). These shear forces are
proportional to the compressive grip force (F,) and the coefficient of friction between the
hand and the handle (See Figure 1). Alternatively, hand force (F,) needed to react
against a pistol-grip nutrunner torque reaction force is proportional to the ratio between
spindle torque (T,) and the tool handle length (L;). The tradeoff between the force
necessary for operating a pistol-grip power hand tool and an in-line hand tool for the
same torque (T,) is not known. The hand force for a right-angle nutrunner is
pre-positional to the ratio between spindle torque (T,) and to length (Lg). Since (Lg) is
often greater than (Ly), a right-angle nutrunner will require less hand force than a
pistol-grip nutrunner for the same torque level. There is little data available that
considers the tradeoff between hand force and posture associated with each tool

configuration.

32


these specifications affect the quality and performance of the physical process, they often
fail to take into account the physical capabilities of the hand tool operator performing
the work. If not considered, improper use of certain power hand tools may lead to
operator fatigue or to increased risk of musculoskeletal injury.

The physical process task requirements and the capabilities of the tool operator often
interact. If the task requirement exceeds an operator's capabilities, task performance can
be compromised. If an operator becomes fatigued, work performance and quality will
suffer. Power hand tool selection for a particular task should be based on (1) process
engineering requirements, (2) human operator capabilities, and (3) workstation and task
factors.

Process Engineering Requirements
Power hand tool selection first involves finding a specific tool capable of performing the

physical task. For instance, if the operation involves tightening a screw in an assembly
task, process engineering requirements undoubtedly include choosing a tool capable of
producing a specific torque, rotating the spindle at a particular speed, and having the
correct spindle and socket size. These manufacturing process requirements are often
based on the product design and parameters needed for accomplishing the task quickly
and reliably, at the desired quality level. Process engineering requirements for hand
tools may include specifications for the following:

Speed (rpm)

¢ Dimensions (cm)

¢ Torque (Nm)

© Feed force (N)

© Power (W)

© Weight (N)

¢ Activation (trigger, push-to-start)

© Spindle and chuck diameter (cm)

@ Noise Level (dBA)

18


|
|

© An understanding of fundamental ergonomic aspects of power hand tool use
is needed.

© Power hand tool manufacturers and industries using power hand tools are
beginning to recognize the importance of ergonomics in power hand tool
design, selection, installation, and use.

Industries involved with manually intensive assembly tasks present an increased risk of
WMSDs to the labor population. According to the Bureau of Labor Statistics (BLS) 1992
data, manufacturing industries accounted for two-thirds of all newly reported
occupational illnesses. A majority of those cases were disorders associated with repeated
trauma (approximately 282,000). Typically, many of these manufacturing tasks include
power hand tools.

In automotive manufacturing, the need to complete a high number of precision tasks in
a timely manner often involves the extensive use of power hand tools. Also, in the 1991
BLS report, the automotive manufacturing industry had one of the greatest illness
incidence rates of new cases of disorders associated with repeated trauma at 558 per
10,000 full-time workers, third only to meat and poultry processing (BLS 1993).

As a result of these problems, automotive manufacturers and tool manufacturers have
allocated considerable resources to the study of ergonomics and power hand tool use.
Consequently, automotive manufacturers are searching for objective information relating
power hand tool use to effects upon the operator. Because of their familiarity with the
topic and magnitude of their concern, this workshop focussed upon tool use in the
automotive manufacturing industry.

This project centered around the apparent need for more ergonomic research in the
development, selection, and utilization of industrial power hand tools. Initial meetings
with automotive representatives and tool experts indicated that they possessed
information on the availability of tools. However, they lack extensive information on the
operator interaction with specific tool mechanical properties. They need this information
to understand and control potentially adverse biomechanical and physiological effects
from power hand tools.

The consensus among representatives was a desire to see a more focused emphasis on
tool design and operation. Therefore, the workshop attempted to address the following
issues: (1) initiate a process of identifying specific concerns involving power hand tool
design and use, (2) acknowledge what essential research and development has and has
not been done, (3) coordinate activity to alleviate research and development redundancy,
and (4) provide a focus for categorizing and prioritizing necessary research.

2


These can introduce vibration in tools not normally considered vibrating hand tools.
Modifications in work methods may affect vibration exposure levels, such as redesigned
production processes the reduction or elimination of vibrating hand tools, redistribution
of the work among workers, and use of external tool support devices for reducing grip
force or eliminating the need to hold tools when in use.

Although vibration levels should be measured using accelerometers mounted near the
location where the tool is handled, sometimes dominant frequencies can be roughly
estimated. Experience has shown spectra for rotating power tools, such as sanders,
grinders, and polishers, had large, distinct, dominant fundamental frequencies
corresponding closely to 88% of the tool free speed (in rpm), divided by 60 (Radwin et
al. 1990b). Harmonics were also present, but the magnitudes of the harmonics were far
less than the fundamental frequency magnitudes. Manufacturer-supplied, free-running
tool speeds may be practical for initially estimating the frequency where most of the
vibration energy is contained for these rotary action tools. This may be useful in
practice, for example, when vibration isolation gloves and accessories for specific power
hand tools are selected. The fundamental frequency of the power tool may be compared
with the frequency range provided by the protective device in isolation. This method,
however, is not recommended to take the place of direct hand tool vibration
measurement for worker vibration exposure assessment, as specified in hand-arm
vibration exposure standards.

Vibration may be reduced by using resilient mounts on handles. Vibration isolation for
limiting vibration transmission from power tools to the hands and arms has been made
difficult by the vibration frequencies involved. It is possible for these mounts to resonate
and amplify vibration rather than attenuate. Since attenuation occurs only when the
vibration spectrum falls above the resonant frequency of the isolation system or material,
or when the vibration frequency is less than the resonant frequency of the isolating
material, the handle acts as a rigid body and no vibration is attenuated. If the vibration
frequency is approximately equivalent to the isolator resonant frequency, the system will
actually intensify vibration levels. Weaker suspension systems with lower resonant
frequencies are often impractical because such a system is usually too flexible for the
heavily loaded handles of tools like grinders. Handles loaded with high forces must be
very rigid. Grinding tools typically run at speeds near 6000 rpm (100 Hz) making it
difficult to have a resilient vibration isolating handle.

Silencers are effective in attenuating the high frequency noise caused by turbulent
airflow. Compound grinding wheels are being developed that have a noise damping
layer between two thin wheels to attenuate high frequency vibration and noise. High
noise levels associated with riveting in an airplane parts plant, for example, can be

40


4 SUMMARY/CONCLUSIONS

The information summarized in this report attempts to highlight the available research
and current practices regarding ergonomics and power hand tool use. The identification
of future research needs requires a consensus among researchers, power hand tool
manufacturers, and industrial power hand tool users.

The proper design and selection of hand tools is a complex process of analyzing the
work station, materials, and methods; of accounting for worker characteristics; and of
considering all of the variations that may result from the process or the individual. To
accomplish this it is necessary to identify and have an adequate understanding of the
interrelationship of most, if not all, of the process variables. This information must be
integrated to critically evaluate the physical characteristics of the tool and the location
of its application.

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Armstrong TJ [1986a]. Ergonomics and cumulative trauma disorders. Hand Clinics
2(3):553-565.

Armstrong TJ, Radwin RG, Hansen DJ, Kennedy KW [1986b]}. Repetitive trauma
disorders: job evaluation and design. Human Factors 28(3):325-336.

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3. Help optimize tool application (i.e., optimize tool choice).
4. Provide improved criteria for tool purchasing and acquisition.

The description of tool selection and use should include details of the tool operation
(what the tool should be used for), workstation design, and other activities associated.
with the job. Epidemiological data should also include previous job information, such
as particular job duration and time on job.

Problem Source Identification

This session discussed the extent to which current research knowledge identifies
mechanical parameters, operational characteristics, and ambient conditions contributing
to musculoskeletal stress. In this session, three interacting job design variables were
identified as (1) worker, (2) tool, and (3) task attributes. Corresponding typical task risk
factors were identified as static exertions, repetitive exertions, forceful exertions, contact
stress, awkward postures, low temperatures, and vibration. In addition, mechanical risk
factors related to tool design were considered to be load (mass), handle size, handle
shape, handle orientation, feed force, torque, and vibration. Apparent research needs
established during this session were:

1. Extensive literature search and analysis to highlight what is known and to identify
research needs.

2. A strategy for optimizing interactions between worker, tool, and task attributes
identified previously as dynamic interacting variables.

3. More emphasis on the entire job design with corresponding operational character-
istics.
4. More dose/response data for a better understanding of tool-operator interaction and

subsequent biomechanical and physiological reaction.

5. Fast, accurate, and practical methods for obtaining biomechanical, physiological, and
psychophysical measurements.

6. Improved models of worker-tool-task interactions built on the realization that what
has been considered acceptable may not be necessarily safe.

7. Better transfer of technological information among those involved in power hand
tool research, design, test, usage, and procurement activities.