Commonly a tool comprises a head and a handle, with sometimes a shaft, or, in the case of the power tool, a body. Since the tool must meet the requirements of multiple users, basic conflicts can arise which may have to be met with compromise. Some of these conflicts derive from limitations in the capacities of the user, and some are intrinsic to the tool itself. It should be remembered, however, that human limitations are inherent and largely immutable, while the form and function of the tool are subject to a certain amount of modification. Thus, in order to effect desirable change, attention must be directed primarily to the form of the tool, and, in particular, to the interface between the user and the tool, namely the handle.
The Nature of Grip
The widely accepted characteristics of grip have been defined in terms of a power grip, a precision grip and a hook grip, by which virtually all human manual activities can be accomplished.
In a power grip, such as is used in hammering nails, the tool is held in a clamp formed by the partially flexed fingers and the palm, with counterpressure being applied by the thumb. In a precision grip, such as one uses when adjusting a set screw, the tool is pinched between the flexor aspects of the fingers and the opposing thumb. A modification of the precision grip is the pencil grip, which is self-explanatory and is used for intricate work. A precision grip provides only 20% of the strength of a power grip.
A hook grip is used where there is no requirement for anything other than holding. In the hook grip the object is suspended from the flexed fingers, with or without the support of the thumb. Heavy tools should be designed so that they can be carried in a hook grip.
For precision grips, recommended thicknesses have varied from 8 to 16 millimetres (mm) for screwdrivers, and 13 to 30 mm for pens. For power grips applied around a more or less cylindrical object, the fingers should surround more than half the circumference, but the fingers and thumb should not meet. Recommended diameters have ranged from as low as 25 mm to as much as 85 mm. The optimum, varying with hand size, is probably around 55 to 65 mm for males, and 50 to 60 mm for females. Persons with small hands should not perform repetitive actions in power grips of diameter greater than 60 mm.
Grip Strength and Hand Span
The use of a tool requires strength. Other than for holding, the greatest requirement for hand strength is found in the use of cross-lever action tools such as pliers and crushing tools. The effective force in crushing is a function of the grip strength and the required span of the tool. The maximum functional span between the end of the thumb and the ends of the grasping fingers averages about 145 mm for men and 125 mm for women, with ethnic variations. For an optimal span, which ranges from 45 to 55 mm for both men and women, the grip strength available for a single short-term action ranges from about 450 to 500 newtons for men and 250 to 300 newtons for women, but for repetitive action the recommended requirement is probably closer to 90 to 100 newtons for men, and 50 to 60 newtons for women. Many commonly used clamps or pliers are beyond the capacity of one-handed use, particularly in women.
When a handle is that of a screwdriver or similar tool the available torque is determined by the user’s ability to transmit force to the handle, and thus is determined by both the coefficient of friction between hand and handle and the diameter of the handle. Irregularities in the shape of the handle make little or no difference to the ability to apply torque, although sharp edges can cause discomfort and eventual tissue damage. The diameter of a cylindrical handle that allows the greatest application of torque is 50 to 65 mm, while that for a sphere is 65 to 75 mm.
Shape of handle
The shape of a handle should maximize contact between skin and handle. It should be generalized and basic, commonly of flattened cylindrical or elliptical section, with long curves and flat planes, or a sector of a sphere, put together in such a manner as to conform to the general contours of the grasping hand. Because of its attachment to the body of a tool, the handle may also take the form of a stirrup, a T-shape or an L-shape, but the portion that contacts the hand will be in the basic form.
The space enclosed by the fingers is, of course, complex. The use of simple curves is a compromise intended to meet the variations represented by different hands and different degrees of flexion. In this regard, it is undesirable to introduce any contour matching of flexed fingers into the handle in the form of ridges and valleys, flutings and indentations, since, in fact, these modifications would not fit a significant number of hands and might indeed, over a prolonged period, cause pressure injury to the soft tissues. In particular, recesses of greater that 3 mm are not recommended.
A modification of the cylindrical section is the hexagonal section, which is of particular value in the design of small calibre tools or instruments. It is easier to maintain a stable grip on a hexagonal section of small calibre than on a cylinder. Triangular and square sections have also been used with varying degrees of success. In these cases, the edges must be rounded to avert pressure injury.
Grip Surface and Texture
It is not by accident that for millennia wood has been the material of choice for tool handles other than those for crushing tools like pliers or clamps. In addition to its aesthetic appeal, wood has been readily available and easily worked by unskilled workers, and has qualities of elasticity, thermal conductivity, frictional resistance and relative lightness in relation to bulk that have made it very acceptable for this and other uses.
In recent years, metal and plastic handles have become more common for many tools, the latter in particular for use with light hammers or screwdrivers. A metal handle, however, transmits more force to the hand, and preferably should be encased in a rubber or plastic sheath. The grip surface should be slightly compressible, where feasible, nonconductive and smooth, and the surface area should be maximized to ensure pressure distribution over as large an area as possible. A foam rubber grip has been used to reduce the perception of hand fatigue and tenderness.
The frictional characteristics of the tool surface vary with the pressure exerted by the hand, with the nature of the surface and contamination by oil or sweat. A small amount of sweat increases the coefficient of friction.
Length of handle
The length of the handle is determined by the critical dimensions of the hand and the nature of the tool. For a hammer to be used by one hand in a power grip, for example, the ideal length ranges from a minimum of about 100 mm to a maximum of about 125 mm. Short handles are unsuitable for a power grip, while a handle shorter than 19 mm cannot be properly grasped between thumb and forefinger and is unsuitable for any tool.
Ideally, for a power tool, or a hand saw other than a coping or fret saw, the handle should accommodate at the 97.5th percentile level the width of the closed hand thrust into it, namely 90 to 100 mm in the long axis and 35 to 40 mm in the short.
Weight and Balance
Weight is not a problem with precision tools. For heavy hammers and power tools a weight between 0.9 kg and 1.5 kg is acceptable, with a maximum of about 2.3 kg. For weights greater than recommended, the tool should be supported by mechanical means.
In the case of a percussion tool such as a hammer, it is desirable to reduce the weight of the handle to the minimum compatible with structural strength and have as much weight as possible in the head. In other tools, the balance should be evenly distributed where possible. In tools with small heads and bulky handles this may not be possible, but the handle should then be made progressively lighter as the bulk increases relative to the size of the head and shaft.
Significance of Gloves
It is sometimes overlooked by tool designers that tools are not always held and operated by bare hands. Gloves are commonly worn for safety and comfort. Safety gloves are seldom bulky, but gloves worn in cold climates may be very heavy, interfering not only with sensory feedback but also with the ability to grasp and hold. The wearing of woollen or leather gloves can add 5 mm to hand thickness and 8 mm to hand breadth at the thumb, while heavy mittens can add as much as 25 to 40 mm respectively.
The majority of the population in the western hemisphere favours the use of the right hand. A few are functionally ambidextrous, and all persons can learn to operate with greater or less efficiency with either hand.
Although the number of left-handed persons is small, wherever feasible the fitting of handles to tools should make the tool workable by either left-handed or right-handed persons (examples would include the positioning of the secondary handle in a power tool or the finger loops in scissors or clamps) unless it is clearly inefficient to do so, as in the case of screw-type fasteners which are designed to take advantage of the powerful supinating muscles of the forearm in a right-handed person while precluding the left-hander from using them with equal effectiveness. This sort of limitation has to be accepted since the provision of left-hand threads is not an acceptable solution.
Significance of Gender
In general, women tend to have smaller hand dimensions, smaller grasp and some 50 to 70% less strength than men, although of course a few women at the higher percentile end have larger hands and greater strength than some men at the lower percentile end. As a result there exists a significant although undetermined number of persons, mostly female, who have difficulty in manipulating various hand tools which have been designed with male use in mind, including in particular heavy hammers and heavy pliers, as well as metal cutting, crimping and clamping tools and wire strippers. The use of these tools by women may require an undesirable two-handed instead of single-handed function. In a mixed-gender workplace it is therefore essential to ensure that tools of suitable size are available not only to meet the requirements of women, but also to meet those of men who are at the low percentile end of hand dimensions.
The orientation of a tool handle, where feasible, should allow the operating hand to conform to the natural functional position of the arm and hand, namely with the wrist more than half-supinated, abducted about 15° and slightly dorsiflexed, with the little finger in almost full flexion, the others less so and the thumb adducted and slightly flexed, a posture sometimes erroneously called the handshake position. (In a handshake the wrist is not more than half-supinated.) The combination of adduction and dorsiflexion at the wrist with varying flexion of the fingers and thumb generates an angle of grasp comprising about 80° between the long axis of the arm and a line passing through the centre point of the loop created by the thumb and index finger, that is, the transverse axis of the fist.
Forcing the hand into a position of ulnar deviation, that is, with the hand bent towards the little finger, as is found in using a standard pliers, generates pressure on the tendons, nerves and blood vessels within the wrist structure and can give rise to the disabling conditions of tenosynovitis, carpal tunnel syndrome and the like. By bending the handle and keeping the wrist straight, (that is, by bending the tool and not the hand) compression of nerves, soft tissues and blood vessels can be avoided. While this principle has been long recognized, it has not been widely accepted by tool manufacturers or the using public. It has particular application in the design of cross-lever action tools such as pliers, as well as knives and hammers.
Pliers and cross-lever tools
Special consideration must be given to the shape of the handles of pliers and similar devices. Traditionally pliers have had curved handles of equal length, the upper curve approximating the curve of the palm of the hand and the lower curve approximating the curve of the flexed fingers. When the tool is held in the hand, the axis between the handles is in line with the axis of the jaws of the pliers. Consequently, in operation, it is necessary to hold the wrist in extreme ulnar deviation, that is, bent towards the little finger, while it is being repeatedly rotated. In this position the use of the hand-wrist-arm segment of the body is extremely inefficient and very stressful on the tendons and joint structures. If the action is repetitive it may give rise to various manifestations of overuse injury.
To counter this problem a new and ergonomically more suitable version of pliers has appeared in recent years. In these pliers the axis of the handles is bent through approximately 45° relative to the axis of the jaws. The handles are thickened to allow a better grasp with less localized pressure on the soft tissues. The upper handle is proportionately longer with a shape that fits into, and around the ulnar side of, the palm. The forward end of the handle incorporates a thumb support. The lower handle is shorter, with a tang, or rounded projection, at the forward end and a curve conforming to the flexed fingers.
While the foregoing is a somewhat radical change, several ergonomically sound improvements can be made in pliers relatively easily. Perhaps the most important, where a power grip is required, is in the thickening and slight flattening of the handles, with a thumb support at the head-end of the handle and a slight flare at the other end. If not integral to the design, this modification can be achieved by encasing the basic metal handle with a fixed or detachable non-conductive sheath made of rubber or an appropriate synthetic material, and perhaps bluntly roughened to improve the tactile quality. Indentation of the handles for fingers is undesirable. For repetitive use it may be desirable to incorporate a light spring into the handle to open it after closing.
The same principles apply to other cross-lever tools, particularly with respect to change in the thickness and flattening of the handles.
For a general purpose knife, that is, one that is not used in a dagger grasp, it is desirable to include a 15° angle between handle and blade to reduce the stress on joint tissues. The size and shape of handles should conform in general to that for other tools, but to allow for different hand sizes it has been suggested that two sizes of knife handle should be supplied, namely one to fit the 50th to 95th percentile user, and one for the 5th to 50th percentile. To allow the hand to exert force as close to the blade as possible the top surface of the handle should incorporate a raised thumb rest.
A knife guard is required to prevent the hand from slipping forward onto the blade. The guard may take several forms, such as a tang, or curved projection, about 10 to 15 mm in length, protruding downwards from the handle, or at right angles to the handle, or a bail guard comprising a heavy metal loop from front to rear of the handle. The thumb rest also acts to prevent slippage.
The handle should conform to general ergonomic guidelines, with a yielding surface resistant to grease.
The requirements for hammers have been largely considered above, with the exception of that relating to bending the handle. As noted above, forced and repetitive bending of the wrist may cause tissue damage. By bending the tool instead of the wrist this damage may be reduced. With respect to hammers various angles have been examined, but it would appear that bending the head downward between 10° and 20° may improve comfort, if it does not actually improve performance.
Screwdrivers and scraping tools
The handles of screwdrivers and other tools held in a somewhat similar manner, such as scrapers, files, hand chisels and so on, have some special requirements. Each at one time or another is used with a precision grip or a power grip. Each relies on the functions of the fingers and the palm of the hand for stabilization and the transmission of force.
The general requirements of handles have already been considered. The most common effective shape of a screwdriver handle has been found to be that of a modified cylinder, dome-shaped at the end to receive the palm, and slightly flared where it meets the shaft to provide support to the ends of the fingers. In this manner, torque is applied largely by way of the palm, which is maintained in contact with the handle by way of pressure applied from the arm and the frictional resistance at the skin. The fingers, although transmitting some force, occupy more of a stabilizing role, which is less fatiguing since less power is required. Thus the dome of the head becomes very important in handle design. If there are sharp edges or ridges on the dome or where the dome meets the handle, then either the hand becomes callused and injured, or the transmission of force is transferred towards the less efficient and more readily fatigued fingers and thumb. The shaft is commonly cylindrical, but a triangular shaft has been introduced which provides better support for the fingers, although its use may be more fatiguing.
Where the use of a screwdriver or other fastener is so repetitive as to comprise an overuse injury hazard the manual driver should be replaced with a powered driver slung from an overhead harness in such a manner as to be readily accessible without obstructing the work.
Saws and power tools
Hand saws, with the exception of fret saws and light hacksaws, where a handle like that of a screwdriver is most appropriate, commonly have a handle which takes the form of a closed pistol grip attached to the blade of the saw.
The handle essentially comprises a loop into which the fingers are placed. The loop is effectively a rectangle with curved ends. To allow for gloves it should have internal dimensions of approximately 90 to 100 mm in the long diameter and 35 to 40 mm in the short. The handle in contact with the palm should have the flattened cylindrical shape already mentioned, with compound curves to reasonably fit the palm and the flexed fingers. The width from outer curve to inner curve should be about 35 mm, and the thickness not more than 25 mm.
Curiously, the function of grasping and holding a power tool is very similar to that of holding a saw, and consequently a somewhat similar type of handle is effective. The pistol grip common in power tools is akin to an open saw handle with the sides being curved instead of being flattened.
Most power tools comprise a handle, a body and a head. Placement of the handle is significant. Ideally handle, body and head should be in line so that the handle is attached at the rear of the body and the head protrudes from the front. The line of action is the line of the extended index finger, so that the head is eccentric to the central axis of the body. The centre of mass of the tool, however, is in front of the handle, while the torque is such as to create a turning movement of the body which the hand must overcome. Consequently it would be more appropriate to place the primary handle directly under the centre of mass in such a way that, if necessary, the body juts out behind the handle as well as in front. Alternatively, particularly in a heavy drill, a secondary handle can be placed underneath the drill in such a manner that the drill can be operated with either hand. Power tools are normally operated by a trigger incorporated into the upper front end of the handle and operated by the index finger. The trigger should be designed to be operated by either hand and should incorporate an easily reset latching mechanism to hold the power on when required.