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Handtool design. 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. Control body linkage Extremity -hand Type of grip Type of coupling

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handtool design
Handtool design
  • 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
Introduction-factors effecting operating effectiveness

Control body linkage

Extremity -hand

Type of grip

Type of coupling

Static, dynamic coupling

Clothing restrictions


Control task

Position axis



Speed frequency

Continuous -discrete


Direction, extent -

path of motion

Capabilities of







Design variables

Shape Material

Size Surface

Operating effectiveness

Performance, stress, strain, safety

  • 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
Hand task activities
  • 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
Accurate and fast movements
  • Speed and accuracy can best be described by Fitt’s Law.
  • MT = a +b Log2 2A


  • 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.
forceful exertions
Forceful exertions
  • 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.
  • See handout
designing hand tools
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.
design rules for handtools
Design rules for handtools
  • 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
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.
  • 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 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.
  • Using the handouts and video clips, re-design the work tools to comply with the theoretical principles covered in class.