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A Check-List for Handle Design
Department of Surgery
The Royal Adelaide Hospital, South Australia
Patkin, M. (2001). A checklist for handle design. Ergonomics Australia On-Line, 15
This checklist was first developed and published for surgical workshops in Sydney,
Australia, in 1969 and has been revised since then at conferences in Israel,
Australia, and the People's Republic of China.
A definitive version of this checklist, with line diagrams, was included in the Proceedings of the 1985 Victorian (Australia) Occupational Health & Safety Convention. A html version was published in Ergonomics Australia On-Line 11(2), 1997.
Permission is given by me to reproduce it freely so long as it is done in its complete form here, including these prefatory remarks, and not for profit.
Correspondence is welcome. Address it to firstname.lastname@example.org or to Michael Patkin, Archer Street, North Adelaide SA 5006, Australia.
WHY HANDLE DESIGN?There will always be a need for well-designed hand tools and hand operated controls despite newer technology. Good handle design is important at work and in all kinds of daily activities for items that are efficient to use, safe, and attractive to buy.
Anything that can picked up by the human hand or which the body comes in contact with is in some sense a handle. All these need some of the same features, whether it is a door or a door-handle pushed open by the body, or a book or glass or box, or any of a hundred thousand other items.
The author of children's books about Peter Rabbit, Beatrix Potter, insisted her publisher made her books small, to fit the small hands of children, at the same time as the typeface was large to make reading easy. Like many expert and intelligent people she was an excellent intuitive ergonomist.
Newer demanding activities such as micro- and keyhole surgery have put more critical demands on hand-work than in the past. There is an even greater need to apply principles of good design to handles and understanding better how they are to be used.
There are other reasons for looking at the ergonomics of handle design for products. They are more likely to sell better when competing internationally with established manufacturers.
This article starts by looking at the common types of hand-grip, their features, and how they hold and use items. Most of the article consists of more than 50 criteria against which the design of a handle or hand-held item can be compared, and it includes ways of checking some features of handle design.
THE TROUBLES WITH HANDLESHandles are often too small, too stiff, sharp, awkwardly placed, and confusing to use. Often only a little thought is needed to design or buy better tools and equipment. A simple check-list for handle design ought to be used by designers, buyers, and users of equipment of all kinds.
Despite the change in our society from an industrial to an information base, and from human to machine power, humans still communicate with machines by applying their hands or fingers to controls or keys of some kind. (Voice recognition and other modes for control have limitations including band-width, which will not be considered further here).
Even in the most advanced workplace, there are still many times each day when items have to be picked up and shifted, or handled in some other way. Too often the contact between hand and equipment is awkward, inaccurate, or unsafe. Bulky items like refrigerators and heavy office equipment still have to be gripped or pushed by human hand. The innards of a washing machine, or a motor engine, must have space in and around them for hands (and lines of illumination and sight) as well as tools.
For those more interested in robots than the problems of humans, some of the lessons learned from this aspect of the human-machine interface will help plan or improve the working relationship between robots and materials, a bionic approach useful elsewhere.
THE GENERAL HAND-MACHINE PROBLEMThe simple act of gripping a hand-rail to support the body can suggest the desirable thickness, length, and position of a general purpose handle, and perhaps some other criteria as well. However, individuals vary in how they grip according to their habit and size, and the same individual may vary the grip according to posture, force needed, other constraints, or for no obvious reason at all.
The problem of allowing for different grips can be solved by tailoring handles to different users and situations. This makes it even more important to have a general list of factors for handle design, which can be applied to different types of work and other activities.
It is necessary to begin by looking at ways in which humans grip handles. The two main ways are the power grip, for large heavy items, and the pinch grip, for small light items. Variations of these two make it worth considering five common basic grips, which follow.
TYPES OF HAND GRIPHere these are considered as 6 types (there are other classifications, see eg., Mackenzie and Iberall 1994.
• Power grip
• External precision grip
• Internal precision grip
• Ulnar storage grip
• Other Power grip Power grip The fingers are bunched firmly around an object and overlapped by the thumb. The handle is thick enough to separate the finger-tips from the palm. (In this situation the forearm muscles have shortened half-way through their available range of contraction, and they are at their most efficient, because of the mechanics of the line of pull).
There should be a large area of contact, with no spots of local high pressure to prevent strength of grip being inhibited by discomfort. (This is like not being able to put your full body weight on the foot if there is a pebble in the shoe).
A common variant of the power grip is having the thumb out straight along the back of a handle. This is a 'power grip with a precision component'.
Fig 1 Power grip - thumb can be straightened as a precision component.
In holding a handle firmly in this kind of grip, movements arc carried out by the powerful muscles of the forearm, upper arm and shoulder, and not by the fine and delicate muscles in the palm of the hand. The positions of the finger joints are fixed by the shape of the grip, which further fixes the small hand muscles. There is none of the accuracy and control of fine movement which is available with the pinch or precision grips described below.
This grip between the thumb and the side of the index finger is used for picking up small objects, but not for manipulating them accurately, which needs the next grip to be described. Variants of the pinch grip include a flat grip for the edge of a dinner-plate, and many other finger postures which shade into one another. Small objects have to be gripped mechanically, with tweezers or forceps, or stuck onto handles such as the 'dops' used by diamond-polishers.
Fig 2 Flat variation of the pinch grip - stronger as it moves from the tip of the finger External precision grip.
This grip is for fine work such as writing. It starts off with a pinch grip, but has the two extra components of support for the instrument in the cleft of the thumb and support for the whole hand along its medial edge. It is of special importance for microsurgery and micro-electronics (Patkin, 1977).
Ulnar storage grip.
I have applied this clumsy expression to a grip in which an item is stored in the little and ring fingers (for example the cork of a bottle) when the other three digits carry out some separate task (such as holding a glass). Simple examination of one's own hand shows that the end of something like a glass laboratory reagent bottle stopper should be 2 or 3 cm. long, about the same thickness, and also fulfil some of the other criteria in the long list that follows later.
Many types of hand grip can be described, such as the 'inter-digital grip' for a cigarette, several double grips, and dozens of others suggested by daily observation or ordinary activities. Other grips are used in highly skilled activities, for example the 'suture storage grip' of some surgeons, the various palming manoeuvres in conjuring, and keying manoeuvres in music.
What follows now is a general-purpose handle check-list for all kinds of hand tools, and for all kinds of items commonly picked up. It is based mostly on the power grip of the hand. It is easy to modify for the other hand-grips mentioned, and this will be demonstrated, as already mentioned, for instruments used in microsurgery and micro-electronics. Small changes in detail will be needed for different items.
THE ULTIMATE CHECK LIST FOR HANDLE DESIGNThere is no ultimate checklist. There are always more aspects of human activity to discover. However at any one time there has to be a solid interim foundation to work from. More than fifty criteria for handle design are grouped under 13
Validating design 13.
• Length at least 10 to 15 centimetres, to fit the width of the palm, longer for large-handed population, shorter if the butt end of the handle is to fit into the palm, when it should be rounded. Allow for the thickness of working gloves.
The length of the shaft of the tool has to be looked at separately.
• Thickness, to allow the thumb to just cover the end of the index and middle fingers. For maximum power in an adult male, it should be 3 or 4 cm. in diameter (Drury, 1980) Fig 4 Handle diameter can be specified for strongest grip.
• Cylindrical, if the grip is to twist round the handle, for example a one-piece rolling pin. If the rolling pin has an axle inside it, the handle will give a more secure grip if it is a little flattened.
• Uniform diameter and smooth surface along the length, to allow sliding, for example on the back of an axe handle.
• Thickened centrally, if there is a need to secure against sliding. An example is a sheep-shearing hand-piece. This is not shifted within the palm, but there is much wrist and arm movement. Sweat and lanoline would make the grip less secure otherwise.
• Flattening for the thumb to straighten, and press on, and guide, as a precision variant of the power grip, e.g. a fine mallet or a fencing sword.
• Flattening for the thumb and fingers, to prevent unwanted twisting, for example a saucepan handle.
• No sharp edges or high spots in the area of grip. These decrease comfort, strength, and security of grip to an extent which can be measured. They may cause injury. However an edge or raised area is useful on the end of the nongrip area of the handle, for example away from the hot part of a frying-pan, to act as a guide to the safe position of the hand, and as a warning (see 'hilt' below).
• Shape coding for function, for example the controls on aeroplanes.
(McCormick & Sanders, 1983, Chapter 9, Controls).
• Smoothness, mentioned earlier for sliding or rotating the handle within the hand. A smooth surface is better if it is non-reflective, to avoid glare in brightly-lit work. This is a common problem with surgical instruments.
• Roughness may be deliberate, for example towelling on a tennis racket.
However knurling is often designed carelessly, so that it is ineffective or overdone as on the tops of some bottles and jars.
• Skin damage - allergy to nickel or nylon affects some people. Blistering and cuts may be a sign of bad design or overuse, but repeated use may be followed by the protective calluses typical of some occupations.
• Replaceability of the handle or cover, as wear occurs.
• Safety - insulation against heat, vibration, and electricity, and padding against sudden jolts.
Fig 5 Switch designed to be turned off by people insensitive to heat
4. SECURITY AGAINST SLIP
• Pommel - an enlargement of the butt end, for security against slip, which would occur with momentary relaxation of grip.
• Hilt - a protection against sliding onto a sharp blade, or onto the shaft of a hot soldering iron.
• Central thickening (mentioned above, under 'shape') to allow the handle to be trapped in the palm when it is sweaty and slippery, for example a sheepshearing handpiece.
• Gentle finger grooving, in some cases.
• Non-rounded (also mentioned above under 'shape') to stop a handle rolling off a sloping work-surface.
5. STIFFNESS (including resistance, weight and rotational inertia)
• The force needed to use a handle occasionally should be less than one-third of average possible maximum for the user population.
• Recommended values are found in standard textbooks of ergonomics or human factors, which should be on a handy shelf for anyone with a serious interest in design or use of handles.
• Typical values recommended area pull of 8 kg. on an arm lever on heavy equipment, 50 to 100 grams for the keys on an electronic keyboard, and about 2 kg. to close the ratchet on artery forceps. Pistol shooters in serious competitions have a detailed knowledge of how stiff triggers should be on different types of gun.
• Controls should be stiff enough to avoid accidental activation, by light brushing or by the force needed to pick it up. Too slack a control can be bad, for example the 'reset' key on older Apple computers, when it was next to the shift key. Accidentally pressing this could mean the loss of several hours of programming, with the exasperating need to repeat it.
• As additional safeguards, handles may be shielded by a cover, hinged bar, or protected by an additional trigger mechanism needing one or more fingers to work it.
• No slack, unless intentional.
• Clicking 'detent' positions, if appropriate, for controls.
• Planned 'feel' for rotational inertia, by concentrating the mass more at the middle or the ends of the tool.