Ergonomics often is described as matching human capabilities (physical, psychological, physiological and biomechanical) to the demands of a specific task. When there is a mismatch in some of these requirements, one of the consequences may be the development of musculoskeletal injuries.
Musculoskeletal disorders (MSDs), also called cumulative trauma disorders or repetitive stress injuries, are injuries that occur over a period of time due to repeated exposure to risk factors. Examples of MSDs include tendonitis, carpal tunnel syndrome, back injuries, thoracic outlet syndrome, ganglion cysts and De Quervains.
Common occupational risk factors for the development of an MSD are the application of excessive force, high repetition, awkward postures, long task durations and static loading of the muscles. The level of injury risk depends on both the frequency and severity of risk factors present in a task. The risk-injury relationship often is described as an exponential relationship. Using ergonomics principles to reduce job risk factors not only decreases the injury risk but often increases the efficiency and quality of a job and improves the overall productivity.
Hand tools are an extension of one's hands. In a manufacturing or small assembly setting, MSD risk factors are significantly influenced by the type of hand tools selected. This article will review some important factors to consider when selecting hand tools.
Pistol or In-line
The shape of the hand tool and its handle design largely determine the way that the hand tool will be used. For most powered and pneumatic hand tools, there are two common shapes: pistol grip and inline. Pistol grip hand tools have a handle that is bent at approximately 90 degrees to the tool tip. Inline tools have a handle that is in-line with the end of the hand tool.
The goal in handle orientation selection is to maintain a neutral wrist posture while the hand tool is being used. A neutral wrist posture is one that is straight, not bent in any direction. Neutral wrist postures are important because as the wrist posture moves away from neutral (straight), the amount of grip force that a person can apply to a hand tool decreases from maximum. For example, when the wrist is moved from neutral to 45 degrees, ulnar deviation (bending wrist toward the little finger), the grip force changes from a maximum of 100 percent to three-quarters of maximum (75 percent), or a 25 percent decrement (Putz-Anderson, 1994). This means to get a task done at a particular force, a user would have to exert a higher percentage of his/her maximum grip force when working in the bent wrist posture.
Besides the type of handle, the height and orientation of the work piece should also be determined as that will influence the neutral wrist posture. For instance, a pistol grip tool would be the best choice for a task requiring screws to be fastened into a horizontal plate located at a height between the knees and hips. If the same task was completed at waist or elbow height, an inline tool would be appropriate. If the screws were driven into a plate that was overhead, a pistol grip hand tool would again be the best choice. Inline hand tools are good for horizontal work surfaces at about elbow height and vertical surfaces at knuckle height. There are also handles that are positioned at a variety of different angles in between the pistol grip and the inline. These tools should be selected when the work surface is variable and when it assists in maintaining a neutral wrist posture.
In addition to the shape of the handle, the tool weight and center of gravity (CG) can play a significant role in the amount of force needed to activate the tool. The tool's weight should be minimized and, when possible, should be less than 3 pounds (Fernandez and Marley, 1998). When a hand tool greater than 3 pounds must be used, counterbalance the weight by suspending it or using a tool balancer. When heavy tools such as saws are counterbalanced, an increase in tool accuracy often is observed. This is because the user does not have to spend time fighting with the tool to position it and can, instead, very easily get ready for the next cut.
One mistake that can be made when balancing tools is to balance them to a zero weight. Balancing the weight to zero often eliminates the feedback or "feel" of the hand tool, which sometimes can lead to poorer work quality. Therefore, it may be necessary to balance the tool weight to a minimum so that the operator can feel the hand tool and the work being performed. This small compensation is enough to preserve the quality of the work being performed and minimize the weight being supported by the user to an acceptable level.
Not only should the overall tool weight be considered, but the location of the CG also should be investigated. The CG of the tool should be as close to the hand location as possible. The further the CG is away from the hand, the more force is required to maintain control of the tool.
The use of some powered hand tools produces vibration that can be transmitted through the handle of the tool to the hand, wrist and forearm of the user. Vibration is measured in terms of frequency (in hertz) and amplitude (in Root Mean Squared, RMS). Particular frequencies are known to be associated with the development of MSDs. When possible, use hand tools that minimize the amplitude of vibration or are recoilless. When such hand tools are not available, the use of vibration-dampening gloves or vibration-dampening tool handle wraps should be considered to reduce the amplitude of vibration.
Tool Handle Interface
Having an effective interface between the tool handle and the hand of the user is important in decreasing the injury risk, and optimizing tool efficiency. When determining the appropriateness of a tool handle, it is important to keep in mind the actual tool user. Is the hand size small, medium or large? Is the user male or female? Will the tool be used by one person or multiple people? In what work environment will the tool be used? Are oils or chemicals present? Are electrical hazards present? Are gloves used? These questions impact tool selection characteristics such as handle length; handle span; handle shape and material; and location of pneumatic tool exhaust.
The length of the tool handle is one of the most important handle design considerations. The handle should stretch across the full length of the palm from the thumb to the lower edge of the hand, about 4 inches. If the handle is too short, it may cause compression of tissues on the palm. To accommodate both the largest and smallest hand sizes, select hand tools with a handle length between 3.75 and 4.2 inches.
The diameter of the handle should be considered based upon the type of grasp required. When a power grip (similar to grabbing a baseball bat) is used, the diameter should be 1.25 to 2 inches. More precision tasks (like holding a pencil or pen) require that the diameter be 0.3 to 0.6 inches. Hand tool diameters that are larger than recommended require a higher gripping force than those with the correct diameter. The greater the gripping force requirement, the quicker the user will experience fatigue.
In the case of tools such as scissors or pliers that have two handles, the distance between the two handles (span) should be 2 to 2.4 inches for female users and 2.2 to 2.7 inches for male users. Often it is not possible to have different tools for males and females, so in general, span size in the range of 2 to 2.7 inches will accommodate both male and female hands. In addition to looking at handle span, hand tools that must be opened for activation should be equipped with a spring. Repeated finger movement to open pliers or scissors increases MSD risk factors such as awkward postures, high force application and high repetition.
Tool handles come in different cross-sectional shapes: round, triangular or square. The shape of the handle should allow for the even distribution of forces over the largest area of the hand possible. Tools with deep "flutes" or "ridges" should be avoided because they may cause excessive pressure on the soft tissue of the palm. Handles that have bumps tend to spread the fingers further apart. When this occurs, the efficiency of force application is lost and the ability to effectively operate the tool may be impaired. Form-fitting tools are not appropriate in most cases because they are made for only one hand size and while they may be helpful for a few, they are not appropriate for most of the user population. The most preferred handle shape is cylindrical.
The material of the handle should be non-slip, non-porous and non-conductive to increase the ease of gripping, minimize the likelihood of chemical contamination and increase electrical safety. Glossy paints and highly polished surfaces should be avoided on all handles in order to reduce slipping. Rubber or vinyl sleeves usually are effective at minimizing the absorption of chemicals and are resistant to small chips and grit, while providing good thermal and electrical energy insulation and providing enough friction for a good grip.
When selecting a pneumatic hand tool, the location of the air exhaust should be directed away from the hand. Air exhaust is usually cold and can decrease the temperature of the tool handle or blow cold air directly on the hand of the user. Exhaust air that is directed through the handle toward the hand causes vasoconstriction of the blood vessels in the hand, which in turn can impair hand function.
Working with Hand Tools
In addition to the orientation of the tool handle and the dimensions related to tool handle design, a few other things should be considered to ensure that the hand tool selected is appropriate for the user and maximizes productivity. The type of activation lever, the force required to activate the handtool, the use of gloves and the location of tool feed lines such as power or air can impact the ease of use as well as the tool efficiency.
Choose the positioning and type of activation lever so that the tool can be used by either the right or left hand. Power tools should be activated by a lever or strip that is at least 2 inches long so that it can be activated using two or more fingers. When an activation button is desired, it should be activated using the thumb and located so that either hand can use the hand tool. Activation switches that only use a single finger contribute to high force application (greater than the capacity of one finger), high repetition and long task duration, which are all risk factors in the development of MSDs.
Not only should the location and type of activation be considered, but also the force required to activate a hand tool. Generally speaking, for an 8-hour daily exposure, force exerted for either gripping or pinching should be less than 30 percent of the maximum voluntary contraction (MVC) in order to delay the onset of fatigue. MVC is the maximum force that can be exerted by the user while in a particular working body posture. When the 30 percent MVC criteria cannot be met, the duration of exposure (use) should be decreased. For non-repetitive operations (a few times per day), a force application of up to 50 percent MVC is acceptable. For extremely repetitive operations, 20 percent MVC is acceptable. When a continuous, static force is required, such as holding an object, the force should be limited to 10 percent MVC.
In some situations, the use of gloves is required due to contact with chemicals, weather or other factors. When this is the case, additional clearances should be provided between the handle and other obstructions to ensure the hand will fit. Gloves also reduce the range of motion of the hand, the dexterity and the hand force capability of the user. To compensate for the decreased range of motion, be sure that all access points are easily accessible and require minimal bending of the gloved hand. Because dexterity is decreased, ensure that small parts are placed so that they can be easily retrieved. Additionally, ensure that the type of gloves selected account for the decreased force capacity. Rubber gloves can reduce MVC by 19 percent, cotton gardening gloves by 26 percent and asbestos furnace gloves by as much as 38 percent. When gloves are required for safety, the gloves with the minimum thickness and highest hand-to-tool friction possible should be selected.
The location of the power cord and other tool feeds (such as air or water) not only impact the injury risk factors but also can impact the tool efficiency and work quality. The lines can get in the way during use or cause the operator to fight with the tool, requiring an excessive amount of force and awkward postures. Many times, fighting with a cord leads to more quality errors and higher re-work rates. Hand tools should be selected so that when plugged and connected to a power source, the cords will not interfere with the interface between the operator and the tool or the tool and the work piece.
One last consideration is the noise that is produced by hand tools. Some hand tools, like impact tools and pneumatic drivers, produce sound levels that are higher than is acceptable based on the exposure duration (85 dBA for 8-hour day). To conform to proper noise exposure level requirements, consider hand tools (with comparable performance specifications) with acceptable noise levels or utilize PPE such as ear plugs or ear muffs.
Selecting the appropriate hand tool for a task is not easy. User and task characteristics need to be evaluated prior to hand tool selection. Hand tool characteristics such as orientation, handle specifications (size, span, material, etc.), activation methods and required applied forces should be considered during the selection process, in order to reduce the risk factors for MSD while striving to increase work quality and productivity.
Sidebar: Top 30 Safety Hand Tool Practices
1. Use each tool only for the job it was designed to do.
2. Discard damaged or abused tools promptly.
3. Buy several versions or sizes of the same tool.
4. Inspect for distortion, cracks, chips, wear or mushrooming.
5. Keep all tools clean and in working order.
6. Be sure handles are fixed firmly to a tool's working end.
7. Be sure tools and work mate properly to avoid slippage.
8. Handles are made for the tool; never use extensions.
9. Confine impact forces to striking and struck tools.
10. Hold work in a clamp or vise, not in your hand.
11. Start off slowly when engaging the tool and the work.
12. Shut current off before using a tool near electricity.
13. Make sure the handle sits securely in your hand.
14. Keep moving parts lightly lubed; avoid lube leakage.
15. Wear approved safety goggles when using hand tools.
16. Keep hands away from sharp edges.
17. Pull, don't push, a wrench handle for more leverage.
18. Position your body securely while working with the tool.
19. Keep jaw teeth, cutters and blades sharp for better results.
20. Keep tool's moving parts properly cleaned and tightened
21. Use steady pressure on jaws and cutters; don't rock the tool.
22. Support long, overhanging work in a vise at the far end.
23. Use pads in the jaws to protect soft or crushable work.
24. Use a tool close to the vise or clamp.
25. Hold work in a clamp or vise with sufficient pressure.
26. Keep clamped assemblies away from vibration and bumping.
27. Discard a tool instead of repairing it by welding or brazing.
28. Keep tools from excessive heat.
29. For continuous work, use comfort grips or gloves.
30. Follow instructions on the tool and/or package.
Source: Hand Tools Institute
References and Resources
Fernandez, J.E. and Marley, R.J. (1998). Applied Occupational Ergonomics: A Textbook. Kendall/Hunt, Dubuque, Iowa.
National Institute for Occupational Safety and Health (NIOSH) (2004). Easy Ergonomics: A Guide to Selecting Non-Powered Handtools. DHHS Publication No. 2004-164. (www.cdc.gov/niosh/docs/2004-164/default.html)
Putz-Anderson, V. (1988). Cumulative Trauma Disorders: A Manual for Musculoskeletal Diseases of the Upper Limbs. Taylor and Francis, Bristol, Pa.
Jeff Fernandez is a principal consultant at JFAssociates Inc. (www.jfa-inc.com) and can be reached at [email protected] and (703) 395-6934. Brandy Ware is a managing consultant at JFAssociates Inc. (http://www.jfa-inc.com) and can be reached at [email protected]