Handtool design

1. 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.

2. Introduction-factors effecting operating effectiveness Control body linkage Extremity -hand Type of grip Type of coupling Static, dynamic coupling Clothing restrictions Environment Control task Position axis Resistance Accuracy Speed frequency Continuous -discrete DOF Direction, extent - path of motion Capabilities of Operator Anthropometry Anatomy Physiology Practice Training Design variables Shape Material Size Surface Operating effectiveness Performance, stress, strain, safety

3. Outline • Hand task activities • Accurate and fast movements • Forceful exertions • Couplings • Designing hand tools • Design rules for handtools • Injury reduction • Force and gloves • Lab

4. 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.

5. 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.

6. 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.

7. Couplings • See handout

8. 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.

9. 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.

10. 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.

11. Force • 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%.

12. Gloves • 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.

13. Laboratory • Using the handouts and video clips, re-design the work tools to comply with the theoretical principles covered in class.