Work Tool Design
Work Tool Design Grip Strength and Endurance Grip strength is the maximum momentary squeeze force exerted on a hand dynamometer (Bazar, 1978) Endurance is the length of time a person can exert a specified force (Bazar, 1978) A hand dynamometer is a device which measures the force an individual exerts through the hand grip The following factors affecting grip strength and endurance should be considered in hand tool design: Size of grip span. The tool handle should fit the hand of the user Type of grasp/grip. Power grip strength is about four times greater than the maximum pinch strength Age. Maximal muscle strength is reached at the age of about 20 for men and a few years earlier for women (Rodahl, 1989) Gender. The grip strength of men is greater than that of women (Falkel, 1985) Handedness. The grip strength and endurance of the dominant hand are higher than those of the non-dominant hand Vibration. The grip strength requirements for grasping and controlling vibrating tools are higher than those required by non-vibrating tools Wrist posture. Grip strength is reduced with the deviation of the wrist from its neutral posture Gloves. Gloves increase grip strength requirement Surgery. Grip strength is reduced after wrist surgery to relieve cts Development in Tool Handle Design A slightly bent handle better fits the natural contour of the hand and minimizes the need for a tight grip to maintain the tool. John Bennet introduced a handle for push brooms with a 19 degree bend at its stem (Emanuel et al, 1980) This angle as the basis for design to keep the user’s wrist straight Bennet has patented the 19 ± 5 degree angles on virtually all tool handles and named them BIOCURVE Tool Ergonomics Principles for Hand-Tool Design and Evaluation Avoid rigid, form-fitting handles with grooves for each finger. Such handles do not improve the grip strength and function unless they are sized to a particular user’s hand Avoid hand tools which require awkward movement, or cannot be operated effectively with neutral wrist posture and low force Tool handles should be designed so that the user can maintain the hand in line with forearm Ergonomics Principles for Hand-Tool Design and Evaluation-cont’ Avoid tool handles with sharp corners, edge, or pinch point. Tool handles should be either round or oval. All pinch points should be eliminated The tool-handle surface should be compressible, nonconductive, and smooth. Handles should have enough coefficient of friction to minimize hand-gripping force required for tool control Ergonomics Principles for Hand-Tool Design and Evaluation-cont’ Avoid hand tools that impose concentration pressure over the soft tissues of the hand which impair circulation and the nerve function. More comfortable Tool handles should fit the hand, include the proper thickness, shape, and length so that the stress-bearing area of the hand is as large as practical. The optimal grip with fingers, palm, and the hand is achieved by a span size (diameter) : 6.25 cm and 9.00 cm Ergonomics Principles for Hand-Tool Design and Evaluation-cont’ Choose or design tools that can be used by either hand. The handles should be designed so that the stabilizing handle can be adjustable to either side of the tool to accommodate both right-handed and left handed individuals. Choose or design tools that can be used effectively by both man and women Ergonomics Principles for Hand-Tool Design and Evaluation-cont’ Choose or design special-purpose tools for specific task. Consider the angles of the grip, forearm, and tool to minimize deviation Consider provisions for tool safety Avoid tool handles that place concentrated pressures on the pressuresensitive areas overlying the blood vessels and nerves in the base of the palm of the hand The handles should be modified to transmit the force a tougher area of the hand-the area between the thumb and index finger More comfortable Provide vibration dampening for vibrating hand tools Use anti vibrating glove Choose ergonomically designed tools to maintain a more natural position of the wrist and to insure better distribution of grip forces during task performance Use long-handles screwdrivers. Avoid hand tools with fluted handles surface. They concentrate stress over a small surface in the hand Provide workers with tools and machine control which can be operated most effectively, with neutral body part postures and low force Use power grips for power, and use precision grips for precision Substitute power tools for hand tools that require high force levels. Power tool should be properly designed, have adequate grip size, not develop excessive vibration, be light enough to handle easily, not require excessive trigger action and be easy to use. Design guidelines for hand tool design: For power grip Grip with entire hand Grip thickness 50-60 mm Grip length minimum 125 mm Grip force maximum 100 N Grip shape no cylindrical, preferably triangular with 110 mm periphery Tool weight maximum 2.3 kg, preferably about 1.2 kg Trigger activated by thumb with locking mechanism For precision grip Grip between thumb and finger Grip thickness 8-13 mm Grip length minimum 100 mm Tool weight maximum 1.75 kg Trigger activated by distal phalanges of finger(s) with fast release Biomechanical Principles for Tool Design The force required for operating a tool should be sufficient to provide proper sensory feedback to the musculoskeletal system, and particularly to the tactile surface of the hand. Therefore, the required force must be optimized The tool should provide a precise and optimal amount of stress concentrated at a specific location on the work-piece. The tool should provide a contact surface area between its handle and the user’s hand large enough to avoid concentration of a high compressive stress Guidelines - Summary Description Tool shape Direction of force is in-line with forearm and wrist (typically horizontal) Direction of force is perpendicular to forearm and wrist (typically vertical) Separation distance between handles (for crushing, gripping or clipping tools such as pliers or tongs) Handle length Guideline Slightly contoured Bent handle Reason Easy grip Minimal wrist deviation Straight handle Minimal wrist deviation 50 - 65 mm Fits both men and (separation distance) women with maximum grip strength > 100 mm (ideally 115 Keep contact out of - 120 mm) palm 30 - 45 mm Greater force and stability 50 - 60 mm Greater torque Handle diameter (power grip) Handle diameter (greater torque for screwdriving tasks) Handle diameter 5 - 12 mm Greater control (precision task) Material and texture Non-slip nonFor comfort and of handles conductive materials reduces effort required to use tool Tool weight (< 1 kg) One handle 1 hand use Tool weight (> 1 kg) Two handles or tool balancer 2 hand use
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