Selecting the Right Tool For the Job, Part 1

There are several ergonomic design guidelines that have been developed to assist with choosing the correct tool for the job.  In Part 1 of this Ergo Tip, we will explore the attributes of the tool and review general ergonomic guidelines that should be considered when selecting a tool for a particular task.  In Part 2 (upcoming issue), we will explore how the tool interacts in the environment that it is used.

Non-powered and powered hand tools are used in a variety of industries including construction, manufacturing, and agriculture.  The advancement in technology has resulted in a complex interaction between the worker and the tools they use.  Improper hand tool design or selection can result in injuries, diminished quality of work, and decreased productivity.  The purpose of this Ergo Tip is to provide general design guidelines that should be considered when selecting a tool for a particular task.  Proper tool design and selection can decrease the risk of injury, limit awkward postures while performing the task, and improve efficiency through reduced force exertion.   

There are many challenges involved in selecting or designing the right tool for the job.  The goal when selecting or designing a tool is to eliminate or at least minimize the following risk factors:

• Awkward wrist and hand postures
• Static muscle loading
• Excessive force to locate and drive tool
• Vibration
• Noise
• Torque reaction
• Temperature
• Pinch points

There are several ergonomic design guidelines that have been developed to assist with selecting the right tool for the job: 

Tool Weight and Distribution

The weight of the tool and the distribution of the load can affect the way the worker holds the tool and may impact the degree of control that they have over the tool.  Tools that are too heavy to repetitively lift and manipulate can lead to the development of musculoskeletal injuries as they quickly tire out the muscles in the arm.    

The maximum acceptable weight for a tool depends on:

• the type of grip the worker uses on the tool (power grip vs. pinch grip),

• the type of work they are performing (precision work vs. high force),

• the length of time they are required to hold the tool,

• whether they are holding the tool with one hand or two hands,

• the posture required to use the tool (overhead vs. in front of the body),

• where the center of gravity of the tool is

The ideal weight for a hand tool that is operated repetitively with one hand is 2.2 lbs (Canadian Centre for Occupational Health and Safety).  It is also important that the center of gravity of the tool is aligned with the center of the hand.  If the center of gravity of the tool is far from the wrist, then the maximum weight should be reduced.   For precision work where the small muscles of the hand support the tool, the weight should be far less.  If you do have a large heavy tool that your worker will be using, it is possible to implement balancers and articulating arms which takes the weight of the tool out of the workers hands, and then all they are required to do is guide the tool into position.    

Handle Design:  Diameter Handles should be cylindrical in shape, and allow the worker to use a power grip to hold the tool.  For tools requiring a power grip, the optimal handle diameter is 1.5", with a minimum of 1.2" and a maximum of 2" (East & Sood, 2005).  For tools where a pinch grip is used, the optimal diameter for the tool is 0.4" ((East & Sood, 2005).  It is beneficial to have handles of varying sizes to accommodate workers who have smaller or larger hands.
Handle Design:  Length The preferred handle length is 5.5" (East & Sood, 2005).  A handle that is too short can end in the palm of the hand which can result in damage to the soft tissues in the palm of the hand.  You want the handle to extend across the entire width of the palm, and they should be padded to reduce contact stress during use.
Handle Design:  Grip Span Tools used for crushing, gripping or cutting such as pliers, have two handles that are held between the span of the workers hand.  If the span of the tool is too large or too small, it will reduce the worker's maximum grip strength.  The optimal span for these types of tools is 3" (East & Sood, 2005).

Handle Design: Shape Tool handles are often categorized as pinch, in-line, pistol-grip, and right angle.  The handle shape that is selected is dependant on the reaction torque produced, the location and orientation of the work, and the type of work they are performing.  When performing precision work, such as electrical circuitry, a pinch grip would be appropriate as it would provide the most control.  You want to select a handle shape that maintains the wrist in a neutral wrist posture – no wrist flexion, extension or ulnar or radial deviation.
Pinch Grip: Precision tasks Low-force tasks	Inline Grip: Lower torque tasks Part is secured in the horizontal plane
Pistol Grip: Moderate torque tasks	Right Angle Grip: High torque tasks
Handle Material The material coating the handle should be made of a non-slip, non-conductive and compressible material.  Sufficient friction must exist between the worker's hand and the tool so that the worker can use the tool effectively and efficiently, and it does not slip during use.  For power tools, it is important to consider the need for electrical and heat insulation in addition to the other requirements for handle material (East & Sood, 2005).
Triggers Triggers should be designed to allow for multiple finger activation in order to distribute the load of activating the trigger among several fingers instead of just one.  This can reduce the incidence of developing stenosing tenosynovitis (Trigger Finger), which is a condition where the motion of the tendon that opens and closes the finger is limited, causing the finger to lock or catch as the finger is extended (Wikipedia).

References

"Tool Design."  Canadian Centre for Occupational Health and Safety. 

http://www.ccohs.ca/oshanswers/ergonomics/handtools/tooldesign.html.  (April 8, 2010).

"Easy Ergonomics:  A Guide to Selecting Non-Powered Hand Tools"  California Department of Industrial Relations.

http://www.dir.ca.gov/dosh/dosh_publications/handtools.html.  (April 8, 2010)

East, J. & Sood, D., 2005.  Ergonomic Guidelines for Selecting Hand and Power Tools.  EHS Today.

http://ehstoday.com/health/ergonomics/ehs_imp_37964/  (April 8, 2010)

"Your Health and Safety at Work:  Ergonomics."  International Labour Organization.

http://actrav.itcilo.org/actrav-english/telearn/osh/ergo/ergonomi.htm.  (April 8, 2010).

Proceedings of a NIOSH Workshop:  A Strategy for Industrial Power Hand Tool Ergonomic Research – Design, Selection, Installation, and Use in Automotive Manufacturing

http://origin.cdc.gov/niosh/pdfs/95-114.pdf

http://www.hand-tools-manufacturers.com/handtools-buying-guide.html

http://en.wikipedia.org/wiki/Trigger_finger

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