These advances in tools also have created new challenges involving the complex interactions between users and their tools. Improper hand tool selection has been known to result in injuries, variable quality of work and decreased efficiency and productivity.
Using the wrong tool or using the right tool incorrectly can cause work-related musculoskeletal disorders (WMSDs). WMSDs are injuries of the muscles, tendons, joints and nerves that usually manifest over a period of time and can affect a variety of body parts. Risk factors associated with the use of hand tools that can increase the risk of WMSDs are awkward wrist and hand postures, static muscle loading, mechanical stress, vibration, noise, torque, temperature and pinch points. When you eliminate or at least minimize exposure to these risk factors, you reduce the risk of WMSDs for the worker.
For this reason, including ergonomic guidelines in tool design has received considerable attention in the past few years. Ergonomic design can help optimize human performance by ensuring that the hand tools support the task needs as well as the human capacity. Specifically, ergonomic design guidelines have been developed that ensure job demands do not exceed human capabilities.
Begin by gathering information to answer the following questions:
- What will be the function of the tool? (What are the specific task requirements, like torque magnitude?)
- Where will the tool be used? (Determine the work surface orientation.)
- Who will be using the tool? (Gather information about the user's gender, anthropometry,etc.)
Once you've answered those questions, use this list of ergonomic design guidelines for tool characteristics to reduce ergonomic risk associated with hand tools.
The weight of the tool and distribution of the load within the tool affect the way the operator holds the tool; whether one or both hands are required to stabilize the tool; the amount of time an operator can hold the tool; and the precision with which it can be manipulated. It is best to limit the weight of the tool to 3 pounds (1.4 kg) or less for tools operated with one hand. For precision operations, tools should weigh less than 1 pound (.5 kg).
The distribution of weight in the tool also should facilitate comfortable gripping in the orientation that helps align the tool's center of gravity with the center of the gripping hand. For example, drill tools that are front-heavy require more effort to balance while in use. When the tool weight cannot be reduced or if the tool is poorly balanced, use a tool balancer. Other controls include using tool holders and articulating arms, or adding microbreak straps to hand tools to allow the operator to relax his/her grip when the tool is not in use.
Tools should be designed so they can be held using a power grip. A power grip requires the operator to align the fingers such that they work in conjunction with, rather than against, each other to maximize the hand capacity. Exceptions to this are tools used for precision work. A pinch grip may be more appropriate for precision work to allow better finger control for minute manipulations with the tool.
Handle shape is an important consideration affecting wrist and arm postures. Determine handle shape after considering the type of task, orientation and layout of the task, and the workplace. Select the handle so that the tool does not require wrist flexion, extension or ulnar or radial deviation, allowing the operator to maintain a neutral wrist posture.
Pistol grips are preferred when the force is exerted in a straight line in the same direction as the straightened forearm and wrist, especially when the force must be applied horizontally. Tools with straight handles are a good choice for tasks in which the force is exerted perpendicular to the straightened forearm and wrist, for instance, when the force must be applied vertically. Bent handles are effective when most tasks are performed in the same plane and height as the arm and hand.
Handles should be cylindrical or oval in shape, and the preferred diameter for tools used with a power grip (for example, screwdrivers) is 1.5 inches (3.8 cm). The recommended minimum is 1.2 inches (3 cm), and the recommended maximum is 1.8 inches (4.6 cm). The preferred diameter for tools used with a pinch grip (for example, tweezers) is 0.4 inches (1 cm). The recommended minimum is 0.3 inches (0.8 cm), and the recommended maximum is 0.5 inches (1.3 cm).
A handle that is too short can cause unnecessary compression in the middle of the palm. It should extend across the entire breadth of the palm. The preferred handle length is 5.5 inches (14 cm), with a recommended minimum of 4 inches (10.2 cm). In addition, rounded padded handles will reduce any soft tissue compression that may exist. A longer handle length may be required if work is performed wearing gloves.
Crushing, gripping or cutting tools such as pliers or tongs are equipped with two handles. The preferred handle span for one-handed tools with two handles (for example, pliers) is 3 inches (7.6 cm). The recommended minimum is 2 inches (5.1 cm), and the recommended maximum is 4 inches (10 cm) to fit both male and female users. Tools with larger or smaller spans will reduce the user's maximum grip strength.
To ensure a good grip on a handle, sufficient friction must exist between the hand and the handle. Hand tools should be made of non-slip, non-conductive and compressible materials. Textured rubber handles provide a good grip, reduce the effort needed to use the tool effectively and prevent the tool from slipping out of the hand. Avoid glossy coated and highly polished handles.
The electrical and heat insulation properties of the handles are important for power hand tools. Handles made of plastics or compound rubbers are recommended.
Where possible, power tools should replace hand tools that normally require the exertion of frequent and repetitive force to do the job. The greater the force exerted with a hand tool, and the more the hand must twist to use it, the greater the risk. Power tool triggers should be at least 1 inch (2.5 cm) in length to allow more than one finger to activate them.
The goal is to create as much distance as possible between the vibration-generating tool and the user. Use tool covers and anti-vibration gloves to achieve this distance. Proper tool maintenance also may reduce the vibration.
Mechanical stress or pressure can be transmitted to the palm and the fingers during hand tool use, especially when large forces must be exerted. Forces should not exceed 22 pounds/inch2 (10 kg/cm2). Force exerted by the finger (e.g., to activate a trigger or a slide switch, or to steady a heavy tool) can put pressure on the tissues.
Due to the wide range of available alternatives, the process of finding hand tools that support good ergonomics can be difficult and complicated. In addition, lack of objectivity when purchasing ergonomically designed hand tools may create situations in which the product intended to reduce the risk of injury may either have no effect, or in some cases, even increase the risk of injury. The copious use of the term "ergonomics" in marketing products with limited or no support for associated reduction of physical stress or improved human performance may add to confusion for the buyer.
Following the guidelines offered here should help streamline the tool selection process. Applying these guidelines in tool design can help maximize human performance on the job by making the job easier for the worker, improving safety and decreasing injuries.
Sidebar: Take-Away Tips for Tool Selection
- Use the right tool for the job, and the right tool for the user.
- "Bend" the tool, not the wrist. Use tools with angled or "bent" handles when appropriate.
- Avoid high contact forces and static loading.
- Reduce excessive gripping force or pressure.
- Avoid extreme and awkward joint positions.
- Avoid twisting hand and wrist motion by using power tools rather than hand tools.
- Avoid repetitive finger movements, or at least reduce their number.
- Minimize the amount of force needed to activate trigger devices on power tools.
- Avoid thumb triggers.
- Use two- or three-finger triggers for power tools; use four-finger triggers only when the tool is balanced.
- Maximum grip force for a trigger should not exceed 4 pounds (1.8 kg).
- Choose tools with handles that have added friction such as compressible rubber or closed-cell foam, with slightly etched surfaces.
- Handles should be 4.5 to 5.5 inches (11.4 to 14.0 cm) long; add .5 inches (1.3 cm) of length, minimum, if wearing gloves.
- Select tools with 1.25- to 1.75-inch (3.2- to 4.4-cm) handle diameter, 1.5-inch (3.8-cm) recommended. For precision operations, use .3- to .6-inch handle diameter (.8- to 1.5-cm).
- Use torque reaction bars on tool balancers.
- Use tools with auto-shutoff clutch to prevent "kick."
- Use pulse tools.
- Replace torque-producing hand tools with automatic torque drivers mounted in fixtures.
- Straight or in-line power tools with a torque output exceeding 28 inches/pound (33 cm/kg) should be equipped with a reaction torque-limiting device.
- Pistol grip tools with torque output exceeding 60 inches/pound (69 cm/kg) should be equipped with a reaction torque-limiting device.
- Right-angle power tools with more than 442 inches/pound (509 cm/kg) of torque should be mounted on an articulating arm, use multiple spindles or have other torque-limiting devices.
- Add micro-break straps to allow users to relax their grip between tasks.
- In general, specify in-line tools to work on horizontal surfaces, pistol-grip tools to work on vertical surfaces.
- Use a dyna-swivel attachment on pneumatic tools to reduce pressure force.
- Avoid or limit vibration; select impact tools that minimize vibration (e.g., impulse tools) rather than mechanical impact tools.
- Select pneumatic tools that do not allow air exhaust to vent on the hand, wrist, face or other exposed skin areas.
James East, CPE, is a managing consultant and ergonomics engineer and Deepti Sood is an associate consultant and ergonomics engineer for Humantech. Since 1979, Humantech has assisted companies throughout North America with successful ergonomics initiatives in a variety of workplace settings, including production and assembly, offices and laboratories. Through a clear focus on low-cost, high-impact workplace improvements that produce quantifiable savings, Humantech clients benefit from health and safety improvements and substantial business results. For additional information visit www.humantech.com.