Handtool design
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The human hand is able to perform a large
variety of activities, ranging from those
that require fine control to others that
demand large forces.
Introduction-factors effecting
operating effectiveness
Environment
Capabilities of
Operator
Anthropometry
Anatomy
Physiology
Practice
Training
Control body linkage
Extremity -hand
Type of grip
Type of coupling
Static, dynamic coupling
Clothing restrictions
Design variables
Shape Material
Size Surface
Operating effectiveness
Performance, stress, strain, safety
Control task
Position axis
Resistance
Accuracy
Speed frequency
Continuous -discrete
DOF
Direction, extent path of motion
Outline
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Hand task activities
Accurate and fast movements
Forceful exertions
Couplings
Designing hand tools
Design rules for handtools
Injury reduction
Force and gloves
Lab
Hand task activities
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The following are various hand work activities.
Fine manipulation of objects with little
displacement and force.
Fast movements toward an object, requiring
moderate accuracy but a fairly small force.
Frequent movements between targets usually
with some accuracy, but little force.
Forceful activities with little or moderate
displacement.
Forceful activities with large displacements.
Accurate and fast movements
Speed and accuracy can best be described
by Fitt’s Law.
 MT = a +b Log2 2A
W
 MT = movement time, A is amplitude, W
is target size, and a and b are constants.
 Fitts found when precision of the target
was fixed, motion time increased with the
logarithm of distance.
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Forceful exertions
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Exerting force with the hands is complex:
The thumb is the strongest digit – the little finger the
weakest.
The gripping and grasping strengths of the whole hand are
larger than any digit alone.
The forearm can produce fairly large twisting torques.
Large torques are produced with the elbow at right angles.
Torque about the elbow depends upon the angle of the
elbow.
The strongest pulling or pushing forces toward or away
from the shoulder can be exerted with an extended arm.
Couplings
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See handout
Designing hand tools
Hand tools need to fit the contours of the
hand.
 They need to be held securely with a
straight wrist and suitable arm posture.
 The posture must utilise strength and
energy capabilities, without overloading
the body.
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Design rules for handtools
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Push or pull in the direction of the forearm, with
the handle directly in front of it: keep the wrist
straight.
Provide good coupling between hand and handle
by shape and friction.
Avoid pressure spots or pinch points.
Round edges and pad surfaces.
Avoid tools that transmit vibrations to the hand.
Do not operate tools frequently and forcefully by
hand.
Injury reduction techniques
Frequency of forceful hand exertions.
 Peak grip forces during manual exertions,
 Awkward postures during hand exertions,
 Vibration of power tools.
 Cold temperature.
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Force
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For an eight-hour workday, the force applied whether
in gripping or pinching, should be less than 30% of
MVC.
The duration of the task needs to be reduced for each
increase in force exerted above 30% MVC.
For non-repetitive operations (a few times a day) 50%
MVC is acceptable.
For repetitive operations 20%, MVC is acceptable.
For continuous static force requirements, force should
be limited to 15%.
Gloves
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Gloves are used in operations that require forceful
hand exertions.
The type of glove worn is dependant upon the task
undertaken to ensure:
Good traction
A good fit
Size of object being handled
Orientation of the forces of the hand.
Laboratory
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Using the handouts and video clips, redesign the work tools to comply with the
theoretical principles covered in class.