Ergonomic Concerns of Hand Tool Design

"What are the major ergonomic concerns of a hand tool design?

Weight of the tool
Ideally, a worker should be able to operate a tool with one hand. Therefore the weight of the tool, especially for repetitive use, should not exceed 1 kg (2.2 lb.). It is also important that the centre of gravity be aligned with the centre of the gripping hand.

In other words, tools should feel "easy" to hold either in an upright position or in the position it will be used (ie. pointing down). For example, drills that are "front-heavy" will require effort (especially in the wrist and forearm) to hold in a usable position and should be avoided. The exception to this principle is a power hand tool, such as a grinder, that has to be heavy in order to reduce the force that the worker has to exert while using it. Tools heavier than 1 kg or poorly balanced tools should be supported by counter-balancers.

Diameter
Handles should be cylindrical or oval in cross section, with a diameter of between 30 mm and 45 mm. For precision work the recommended diameter for handles is between 5 mm and 12 mm. For a greater torque large screwdrivers should have a handle diameter up to 50-60 mm.

Length
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. Tool handles longer than 100 mm (preferably 115-120 mm) will reduce the negative effects of any compression exerted. Rounded handles will minimize palm compression on the palm still further. Keep in mind that the use of gloves requires longer tool handles.

Separation between handles
Crushing, gripping or cutting tools such as pliers or tongs are equipped with two handles. The recommended distance separating handle is between 50 mm and 65 mm. Such a range will fit both male and female users. Tools with larger or smaller spans will reduce one's maximum grip strength and may contribute to the onset of carpal tunnel syndrome.

Materials and texture of handles
To ensure a good grip on a handle, sufficient friction must exist between the hand and the handle. This is particularly important where a considerable force must be applied with a sweaty hand. Hand tools should be made of non-slip, non-conductive and compressible materials. For example, 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. Glossy coatings and highly polished handles should be avoided. The electrical and heat insulation properties of the handles are important for power hand tools. Handles made of plastics or compound rubber are recommended. Sharp edges and contours can be covered with cushioned tape to minimize lacerations. "

Available at http://www.ccohs.ca/oshanswers/ergonomics/handtools/tooldesign.html (accessed on 09/06/09).