1Chapter 14Hand tools1. Special-purpose vs. general purpose toolsAn SP tool Performs better for tasks that are repeated many times/week, quality of task is improved, usually more costly, needs variety of tools in the inventory. A general purpose tool can do variety of tasks, cost is low, but quality, speed etc are compromised.A mechanic wants a special wire-stripping tool ($35); it will be used once per day. It will save about 30 seconds per use. Should it be purchased? Whether a tool is expensive depends on a detailed analysis, not just capital cost. Benefits include savings beyond just labor savings(capability, quality, cost of safety, pain etc.)2. Design tool to be used by either hand• Preferred hand has more gripping force, more dexterity, and better performance. • 10% workforce is left handed• The penalty for non preferred hand is minimal for simple movement and tasks.Two handed tools have two benefits• Left handed workers will be proficient. In sports both handed products are available.• May be used by the other hand when, the preferred hand is doing something else or resting.3. Power with motors more than muscleAdvantages• Motor power is cheap compared to muscle power.• Motor extend human capability• Larger percent of workforce will be able to do the jobDisadvantages• When motor powered – potential for vibration problems• May associate cold exhaustTypes of Grips – Power grip• Fingers are closed with the thumb and the tool handle perpendicular to the forearm axis. Provides maximum gripping force.• Direction of force:– Parallel to forearm – balance is important– At an angle to forearm – wrist deviation should be minimized, bent hammer handles.• Force generates a torque about forearmTypes of Grips – Semi-power Grip• Fingers act as a group but thumb position changes.– Oblique grip: thumb aligned along tool axis, improves precision.– Pinch grip: held between thumb and other fingers, more stress on finger muscles– Hook grip: fingers wrap around tool, thumb is passive2Types of Grips – Precision Grips• Has about 20% strength of a power grip.• Internal precision grip (e.g. table knife)• External precision grip (e.g. pencil)• Forearm may need to be supported.For heavy tools held in hand, keep the CG of tool close to the handle, this will reduce the holding torque on wrist.For same weight and cutting force, which of the two hand drills will produce less torque on the wrist: (i) When held in hand? (ii) When drilling?Grip design - CG of the toolCGMoment arm for CGMoment arm for cutting forceFc(1)Moment arm for cutting forceCGMoment arm for CGFc(2)Hand Size & Grip• Hand Sizes– Are larger with gloves, may reduce torque – Tend to be larger for men than for womenGrip diameter• For power grip, 35 mm to 45 mm is optimal.• For power grip thumb must close other fingers. Custom fit size is 10 mm less than grip circumference• For precision grip, use 9.5 mm to 12.7 mm.Grip Shape: Section Perpendicular to Grip Axis• If rotation is not a problem, cylindrical shape is most forgiving, because no sharp edges.• To prevent rotation and slippage use noncircular cross-section, e.g. elliptical, ovel as seen in the hammer handles. This will reduce gripping force.• Non circular sections such as square, will increase contact pressure• For positive prevention of rotation provide thumb conoid.Thumb conoidGrip Shape:Section Along Tool Grip Axis• Varying diameter keeps tool from moving forward and backward in hand and allows force exerted along tool axis to be greater.• Can act as a shield in the front to prevent injury for accidental slippage• Avoid finger grooves along the length of the handle – do not increase contact area but increase pressure point for hand sizes for which it is not designed.• A pommel can increase the force when the tool is pulled toward the body.Pommel3Grip Length• For power grip, all fingers must make contact – wide enough, consider glove • For internal precision grip, tool must extend past palm but not as far as wrist. Tissue pressure on palmer side may be an issue.• For external precision grip, shaft must be supported, long enough – try to write with a small IKEA pencilsOther geometric and material characteristics• Larger handle diameter increases work torque thus smaller gripping force is required –a smaller screw driver may not need to have proportionally smaller handle diameter.• Rubberized material to increase coefficient of friction – less gripping force to produce torque• Avoid ridges along the handle length –increases localized tissue pressure or contact stressGrip Surface• Make the Grip Surface Smooth, Compressible, and Nonconductive– Smoothness reduces pressure points.– Compressibility increases contact area and thus minimizes pressure on the hand tissue– Non-conductivity reduces heat and cold problems and risk of electric shock.Tools for grinding, polishing which are held in hand for extended periodThe gravitational force due to weight of the tool itself and/or operational force give rise to static loading in hand, arm, back.Trigger force that needs to be maintained by fingers can also give rise to static loading and fatigue in weak finger muscles.Static muscle load resulting in muscle fatigue and sorenessIn non-neutral posture, a joint is comparatively weak. Hence, in terms of percent of MVC, muscle fatigue ensues quicker for the same external force. Back/neck/shoulder – during bent posture, moment arm of the upper body segments increases, causing increase in static load levels of the low back and neck muscles –back pain, neck pain. Arm – Extended elbow with forceful movement, tennis elbow. Keep elbow at about 90o.Arm abduction or elevation of elbow causes static load due to weight of the arms– give rise to static loading in shoulder/neck muscles.Awkward joint positions- back, neck and armBent wrist, high activation force-carpal tunnel syndrome – bend the tool, tilt the workplace to correct the situation.Straight and bend handle drilling machines, bent handle pliers, bent handle scissors designed to promote better wrist postures.Awkward joint angles in hand, wrist, fingers4If the trigger needs to be held, reduce activation force to reduce forces on fingers, or provide a power grip switch or a bar – instead of a single finger switch. Static muscle load from triggerSpring loaded return for a two handled tool, eg. pliers – finger extensor