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Moving loads manually
Applied Ergonomics 1972, 3.4, 190- 194
Moving loads manually B.T. Davies. Senior Lecturer, Department of Engineering Production, University of Birmingham.
Many aspects that must be considered in the manual handling of loads are mentioned. These include legal aspects, recommended loads, techniques of lifting and carrying, dress, pulling and pushing, and pace of work. A new film on lifting and handling is noted and a plea is made for industry to consider training one or two workers as instructors in good techniques of physical moving of loads. An idea frequently expounded is that hard physical work is rarely encountered in industrialised countries and that it has been replaced by mechanical power. While, no doubt, many such examples can be quoted, enquiries continue to be received from industry on such questions as 'how much can we expect a worker to lift and carry?', 'how often can we expect a woman operator to lift a quoted weight?', 'how can we reduce the number of back injuries in this organisation?'.
There are Statutory Regulations dealing with the lifting of heavy loads in the Woollen and Worsted Textiles, Jute & Pottery Industries, as follows:
The question of the physical moving of loads is likely to remain with us for many years and, if the report of the Chief Inspector of Factories (1970) is any guide, the number of injuries attributed to this act will remain around a steady 25% or more of all industrial accidents. Figures published in this report show that of a total of 304 595 reported accidents, 83 187 were categorised under the 'handling of goods'. If we include 'falls on the same level' (24 051)and, without making the connection too tenuous, add 'stepping on or striking against objects or persons' (27 241 ) and 'struck by falling objects' (21 810), we have a total of 156 289. If the number of days lost by accidents due to the faulty lifting and handling of material could be eliminated, a marked contribution to the wellbeing of the country would be made and a great deal of pain and misery removed.
Man
68 kg(150 lb)
54.4 kg ( 120 lb)
Woman over 18 years
29.5 kg(65 lb)
22.6 kg (50 lb)
Female under 18 years
22-6 kg(50 lb)
18.1 kg (40 Ib)
Male under 16 years
22-6 kg(50 Ib)
t8.1 kg (40 lb)
The maximum weight a person can be expected to lift has never been satisfactorily resolved, although much attention has been devoted to this point. The maximum permissible load would appear to be of interest only in exceptional cases; the most physiologically economic or subjectively acceptable load seems to be of more realistic value to industry. Before dealing with these it is perhaps wise to look at the legal aspects of the physical moving of loads. The Factories Act of 1961 states: 'No one must be employed to lift, carry or move any load so heavy as to be likely to cause injury'. Legal interpretation of the Factories Act includes, as quoted by Fife and Machin (1972), the following: 1. "Employed to lift" - a person is not employed to lift if he has been told to ask for help which is readily available. 2. "Likely to cause injury" - Court of Appeal held that a weight of 66 kg (145 Ib) was not likely to cause injury to a man of experience.
190
AppliedErgonomics December
1972
1. The Woollen & Worsted Textiles (Lifting of Heavy Weights) Regulations, 1926. Persons employed
Compact or
Maximum weight not
rigid
compact or rigid
2. Pottery (Health & Welfare) Special Regulations, 1958. A woman or young person must not carry more than 18-1 kg (40 lb) more than 45.7 metres (50 yd) on a level surface. Women shall not carry a sagger weighing more than 13.6 kg (30 lb) or in conjunction with another person more than 22-6 kg (50 lb) or raise a sagger to a height of more than 1.3 m (4ft 6in). 3. The Jute (Safety, Health & Welfare) Regulations, 1948. The Jute Regulations are the same as the Woollen & Worsted Textiles Regulations, except that no maximum weights are mentioned for men, and that young persons under 16 years should lift only 18-1 kg (40 Ib) rigid and 15.8 kg(35 Ib) not rigid. 4. Agriculture (Lifting of heavy weights} Regulations, 1959. (SI 1959 No 2120). The maximum weight of any load consisting of a sack or bag with contents which may be lifted by one worker unaided is 81.5 kg (180 lb) These then are the legal aspects of the lifting and carrying of weights in this country. Fairly comprehensive coverage of the maximum permissible loads in other countries is given in the ILO publication (1964).
We can now look at some figures of suggested weights that workers can be expected to lift. For many years it has been thought that a weight is 'heavy' when it reaches 35% of body weight. If for an 'average' man body weight is considered to be 68 kg (150 lb) and for women 61-21 kg (135 lb), the weight considered heavy is, for men 24 kg (51 lb), and for women 21.3 kg(47 lb). The relevant figures for the 5th and 95th percentiles are:
5th percentile
95th percentile
19-8 kg (44 lb)
27-8 kg (61 lb)
Women 15.7 kg (35 Ib)
27.0 kg (60 lb)
Men
The CIS Information Sheet No 3 (ILO, 1962) suggests that the following weights can be lifted without imposing a critical strain on the lumbar intervertebral discs: men 400 kg (887 lb) With a straight back women 240 kg (555 lb) This is obviously a theoretical value and omits to take into account other important factors. However, when discussing reasonable weight limits for occasional lifting by any method, and taking account of the relationship between efficiency and age, it shows:
Men
Age (years) 14-16 1 6 - 18 18-20 20-35 35-50 over 50
Women
15 kg(33 lb) 19" 42" 2 3 " 51 " 25" 55" 21" 45" 16" 36"
l0 kg (22 lb) 12" 2 6 " 14" 31" 15" 3 3 " 13" 29" 10" 22"
Fig 1 Unstable starting position of lift
It suggests that where lifting is frequent, these figures should be reduced by 25% to take account of the fatigue involved. Compare the large difference between the above figures and those of Poulsen & J6rgensen (1971), who refer to normal, healthy individuals having a normal correlation between back strength and strength of other relevant muscle groups:
From floor to table height Women 25 yrs old.
Height of Lift 150 cm
175 cm
Max range
38kg(851b)
Permissible single lift
27 kg(60 lb) 44 kg (98 lb)
Permissible repeated lift
19kg(421b) 31kg(691b)
63kg(1381b)
Men 25 yrs old.
Max range
80 kg (175 lb) 107 kg (235 lb)
Permissible single lift
56 kg (123 lb) 75 kg (164 lb)
Permissible repeated lift
40kg(891b)
53 k g ( l l 8 1 b )
'Single' here means once or twice per hour and 'repeated' means up to about six times per minute over a short period of a working day. These figures can be further compared with those of Damon et al, (1966), who state that the maximum weight
Fig 2 A good stable starting position for a lift
Applied Ergonomics December 1972
191
that can be lifted without strain by 99% of the young, healthy male population is as follows: To knee level
28.1 kg (62 lb)
To waist level
17-2 kg (38 lb)
To shoulder or eye level
12-2 kg (27 lb)
However, in another section they state that with the load near the floor, the lifting force may be only 75 to 88% of that at the best height. Snook, Irvine, and Bass (1969) investigating maximum acceptable weights while lifting, quote for males: Floor to knuckle
16"7 kg (37.3 lb)
Knuckle to shoulder
15"1 kg (34.4 lb)
Shoulder to arm reach
13"0 kg (29-4 lb)
The marked variation in acceptable or permissible weight to be lifted can thus be seen. It will be appreciated that these results are often obtained from particular experiments under certain specified conditions; extrapolation to general industrial situations must be made with great care. Other aspects of lifting and carrying should be considered. Snook and Irvine (1966) point out that the 20 to 50 year old age group forms 68% of the total labour force and represents 75% of the high cost manual handling accidents. Therefore, attention to the abilities and even training of this group is of major importance.
Dress The pamphlet 'lifting in Industry' (Anon, 1966) points out the importance of the correct clothing for tasks involving lifting and carrying. 'Clothing should be such that it allows easy movement of arms and legs and will not catch in machinery or on a load. Safety shoes or boots, properly laced, with a non-skid sole should be worn. For certain tasks special protective clothing should be worn, ie, industrial aprons, goggles, masks, shoulder pads, and where provided they should always be worn'. Although such advice may sound superfluous, it is a common occurrence for people inappropriately dressed to attempt to avoid soiling their clothes by lifting or carrying objects by holding them away from the body. The simple mechanics of the increasing leverage is often such that the task becomes increasingly difficult and dangerous. Whitney (1958) has shown that the important factor in determining the ability to lift is the distance of the feet from the grasping axis.
Techniques of lifting and carrying Glover and Davies (1961) describe as an objective ot'a manual handling and lifting programme which they introduced into a 6000 employee factory, 'the raising of the status of lifting in men's minds from that of a menial drudge to one of almost a semi-skill, which if thought about can be made easier and if neglected can cause pain and danger'. Only slowly is industry realising that workers often need instruction in the correct methods of lifting and handling. Some forward-looking firms are now taking advantage of a scheme introduced by the author whereby selected workers
192
AppliedErgonomics December
1972
are sent on a week's course where they are trained as instructors in the many techniques of correct lifting and handling. Publicity is often given to a technique of lifting which involves full flexion of the hips and knees with the feet either together or parallel and slightly apart (Grandjean 1969; U.S. Dept. of Labour 1965). This is an unsatisfactory starting position for most lifting tasks. It imposes some strain on the joints and is an extremely unstable position from which to lift. It is a starting position for a lift which cannot be considered safe. The correct techniques are well documented m the the pamphlet "Lifting in Industry" and in a film "A new way to lift" (Anon, 1970), but the main points are repeated here: I. Position of the feet. Good lifting posture starts with the feet. The feet must be far enough apart to give a balanced distribution of the weight: as a general rule the leading foot should point in the direction of the movement. 2. Hips, knees and back. The knees and hips should be bent, and the back kept as straight as possible, with the chin tucked in. 3. Arms. The arms should be held as near to the body as possible. Holding the load as close to the body as possible allows friction between load and clothing to help sustain the load. 4. Grasps. While these vary for different tasks, generally the broader the gripping surface the better. Wherever possible the whole of the hand should be used, not just the fingers. 5. Pace. Lifts should be made smoothly, no jerks or snatches should occur. Two further points must be emphasised when considering techniques of lifting. It has been shown thal a bend and twist, (ie, flexion and rotation of the spine), during a lift, is a common cause of spinal injury and this should be avoided. Davis (1959) has shown that the common 'bend-over' lift predisposes to hernia.
Teamwork in lifting When more than one person is revolved in a lifting task, it is preferable that they should all be of roughly the same height, that they should all lift in accordance with the principles of good lifting, and that the lift should begin on an agreed signal from the senior worker. The final resting place and the route to the resting place of the object should be clearly understood. Dashier (1935) quotes the occurrence of a "Coordination loss' when more than one person is involved in a pulling task, and the same reasoning would seem to apply in lifting. It is unlikely that each person in the team would exert his maximum force at exactly the same time. As a result, the total that can be pulled or lifted is not the sum of each person's capacity. The total reduces quite markedly as the number participating increases. "One person could on average pull 63 kg (140 tb), two persons managed, not 126 kg(280 lb) but t 18 kg (261 lb), a trio pulled 160 kg (345 lb) not 189 kg (420 lb) and eight people pulled 248 kg (558 lb) not 504 kg(1 120 lb)". Although space is often at a premium in mdustly, enough space and clean floors are essential for safe lifting
and handling. What 'enough space' may be for a particular task is something for ergonomists to define. It seems likely that many accidents in the categories mentioned earlier, ie, 'falls on the same level', 'stepping on objects' etc, may be due to lack of clear working space.
Yokes and harnesses These are often advocated to facilitate the moving of loads and have been shown to reduce energy expenditure in many tasks. From a safety point of view, however, they are not to be recommended. If for any reason the load gets out of control, it is often difficult for the worker to disengage promptly from the load.
A comprehensive investigation carried out by Frederick (1959) has shown that the best area for manual lifting is between 1 and 1.5 m (40 and 60 in) from floor level and that the best weight unit for lifting within the recommended area is about 18 kg (40 lb). This article is extensively quoted here because it is felt that with the work of Snook et al (1969) this is the most practical and useful article appearing in the literature on the physical handling of loads. The compounded results of Frederick's experiment are shown in Fig 3. If we assume an average kilo calorie (K cal) output of 200 per hour for sustained work, we can calculate the number of lifts a worker could make under the ideal conditions specified by Frederick.
E
Pushing and pulling
(fxaxwxc) 1000
The ability of force exertion in pushing tasks is covered comprehensively by Kromer (1971)in a publication which also covers the coefficients of friction between various floor and shoe materials. In general, his results agree with those of Snook et al (1969) who found that forces acceptable to 90% of industrial male workers in pulling and pushing tasks over short distances were: Push Initial force
=
25.3 kg (58 lb)
Sustained force 13.1 kg (29.1 lb)
Pull
where E = total energy expenditure per hour in K cal (not more than 200 K cal per hour for an average man); f = number of lifts per hour; a = lifting heights in feet; w = weight of load in pounds, and c = consumption of energy in gram calories per foot pound taken from the graph. Using this formula and substituting values of 200 K cat for E, 4 gram calories per foot lb for/, 2 ft for a, and 40 lb for w, we get:200 = ( f x 2 x 4 0 x 4 )
24-0 kg (53 lb)
1000
14.5 kg (32 lb)
David and Troup (1964) have shown that increases in pressure in the abdominal and thoracic cavities are greater when pushing than when pulling an object. From a safety point of view, when a truck has to move up an incline it is preferable to pull the load up, and when it has to move down an incline it is better to lower the truck ahead of the worker. Simply put, the instruction is that the load should be kept lower than the worker at all times.
Pace of moving loads Although the best patterns for the lifting and moving of loads still need to be established, there is much useful information available. It has been shown that taking weights up from the floor, which involves lowering and raising the body weight, increases the energy expenditure by 50% when compared to lifting a weight from 0-5 m (19½ in) and 1 m (39 in). This information should be taken into account in work place design, because it also eliminates a dangerous step in the lifting of heavy weights, ie, the first 152 mm 228 mm (6 - 9 in) of the lift from the floor. The movement of a load from one location to another is usually more economical physiologically if the load is divided into units of appropriate size and weight within the capability and acceptance range of the operators. Wei~lts of 20 kg (45 lb) are suggested. Kellerman and van Wely ( 1961 ) have shown that a weight of about 17.5 kg (37 lb) is optimum for transporting over distances of 12 m (13 yd), especially for untrained workers. For more experienced workers, a weight of 20 -25 kg (44-55 lb) has been shown to give maximum efficiency, ie, nrinimum energy expenditure per kilogram metre of work performed. Rhythm in the work of lifting and moving light loads increases the mechanical efficiency.
f =
(200x 1000) 320
f = 625 lifts per hour. ~10
~ L i f t range ~Oto2Oi~-~'~ 9 ~ ~IIE'~ o ~ ,- / ~ & 8~lft ronge.9 20t040[~ ~
u>.
4 0 to 60 , n | ,a~20 30
~3-uJ
~'~ Point of maximum ~ Idtmg effioency o Points of maximum efficiency for each lift range
/
/
.__
, _ , ,40 50 Weight, pounds
h 60
F i g 3 The manual lifting data plotted indicate that the maximum lifting efficiency is achieved when a 18 kg (40 Ib) load is lifted in the height range from 1 - 1-5 m (40 to 60 in). All curves 'bottom out', indicating the most efficient weight of lifts in any of the test height ranges. From the chart, hourly tonnage maximums for manual lifting can be calculated using a limit of 200 Kcal of energy consumption per hour. Physiological limits may be used to check industrial engineering studies. The figures illustrate relative heights and not necessarily correct lifting postures. Applied Ergonomics December 1972
193
This is about 10 lifts per minute which ensures maximum productivity. Subjectively this seems a high rate of work and above the range suggested by other authors. A slight reduction in the rate would seem to be more acceptable. For lifts of the same height, a work rate of 333 ft Ib/min has been found acceptable to 90% of industrial male workers.
Davis, P.R. 1959 Lancet 2 155-157. The causation of hernia by weight lifting. Davis, P.R., and Troup, J.D.G. 1964 Ergonomics 7.4,465-474. Pressure in the trunk cavities when pulling, pushing and lifting.
Fife, 1. and Maehin, E.
Discussion
1972 Redgrave's Factories Acts, 925, Butterworth.
It is acknowledged that in many industries workers lift, carry and move objects beyond the limits considered in this paper. It is hoped that with this information available, employers will now have a better idea of what can reasonably be expected of workers as far as tasks involving the lifting and moving of loads is concerned.
Frederick, W.S. 1959 Modern Materials Handling 14.3, 74-76. Human
Answers to the questions posed in the introduction are, of course, difficult to give. No answers applicable to all workers under all circumstances can be given. However, reasonable answers, bearing in mind the multitude of factors which can affect them, would seem to be:1. A weight of about 55 kg (120 lb) seems to be the maximum a worker should be expected to lift - even this should apply only to healthy, experienced males.
energy in manual lifting. Glover, J.R., and Davies, B.T. 1961 JforlndustNurses, 13. 6. Manual handling and lifting. Grandjean, E. 1969 Fitting the task to the man, Taylor and Francis. 15. ILO 1962 Manual Lifting and carrying. CIS Information Sheet No 3, 10 and 13, International Labour Office, Geneva. ILO 1964 Maximum permissible weight to be carried by one worker. Occupational Safety & Health Series No 5, International Labour Office, Geneva.
2. Generally, women can exert less force than men. Adult women have only about 65% of the strength of adult men. The number of lifts a woman can make can be calculated or extrapolated from Fig 3, bearing in mind this figure of 65%. Women should not lift heavy weights above shoulder level.
Kellerman, F.T. and van Wely, P.A. 1961 Ergonomics 4. 3,219. The optimum size and shape
3. All industries where workers are expected to lift heavy weights occasionally, or light weights frequently, should arrange classes of instruction in the correct techniques of lifting and moving loads.
1971 Horizontal static forces exerted by men standing in common working positions on surfaces of various tractions. Aerospace Medical Research Laboratory Report No. AMRL-TR70-114. Wright-Patterson Air Force Base, Ohio.
References
of containers for use by the flower bulb industry.
Kromer, ICM.E.
Poulsen, E., and J~gensen, K. 1971 Applied Ergonomics, 2. 3, 133-137. Back muscle strength, lifting and stooped working postures.
Anon. 1966 Lifting in industry. Pamphlet prepared by the Posture Sub-committee and published by the Chartered Society of Physiotherapy, London.
Snook, S., and lrvine, C.H. 1966 Am Indust Hygiene Ass Jnl, 27, May June, 229, The evaluation of physical tasks in industry.
Anon.
Snook, S., lrvine, C.H., and Bass, S.F.
1970 "A new way to lift". Film circulated by the National Safety Council, Chicago, USA.
1969 Maximum weights and work loads acceptable to male industrial workers while performing lifting, lowering, pushing, pulling, carrying, and walking tasks. Paper presented to American Industrial Conference in Denver, Colarado, 13 May.
Chief Inspector of Factories 1970 Annual Report, Comnd 4758, 99-105, HM SO.
Damon, A., Stoudt, H.W., and McFarland, R.A.
Dashiel, J.F.
U.S. Dept. of Labor 1965 'Teach them to lift'. Bulletin 110. Mechanical and Physical Hazards Series. Bureau of Labor Standards, Washington, DC.
1935 Experimental studies of the influence of social situations on the behaviour of individual adults. Handbook of Social Psychology. Vol 2. Russell & Russell, New York.
Whitney, R.J. 1958 Ergonomics, I. 2. lO1-128. The strength of the lifting action in man.
1966 The human body in equipment design. 321,322. Harvard University Press.
194
AppliedErgonomics December 1972
Moving loads manually B.T. Davies. Senior Lecturer, Department of Engineering Production, University of Birmingham.
Many aspects that must be considered in the manual handling of loads are mentioned. These include legal aspects, recommended loads, techniques of lifting and carrying, dress, pulling and pushing, and pace of work. A new film on lifting and handling is noted and a plea is made for industry to consider training one or two workers as instructors in good techniques of physical moving of loads. An idea frequently expounded is that hard physical work is rarely encountered in industrialised countries and that it has been replaced by mechanical power. While, no doubt, many such examples can be quoted, enquiries continue to be received from industry on such questions as 'how much can we expect a worker to lift and carry?', 'how often can we expect a woman operator to lift a quoted weight?', 'how can we reduce the number of back injuries in this organisation?'.
There are Statutory Regulations dealing with the lifting of heavy loads in the Woollen and Worsted Textiles, Jute & Pottery Industries, as follows:
The question of the physical moving of loads is likely to remain with us for many years and, if the report of the Chief Inspector of Factories (1970) is any guide, the number of injuries attributed to this act will remain around a steady 25% or more of all industrial accidents. Figures published in this report show that of a total of 304 595 reported accidents, 83 187 were categorised under the 'handling of goods'. If we include 'falls on the same level' (24 051)and, without making the connection too tenuous, add 'stepping on or striking against objects or persons' (27 241 ) and 'struck by falling objects' (21 810), we have a total of 156 289. If the number of days lost by accidents due to the faulty lifting and handling of material could be eliminated, a marked contribution to the wellbeing of the country would be made and a great deal of pain and misery removed.
Man
68 kg(150 lb)
54.4 kg ( 120 lb)
Woman over 18 years
29.5 kg(65 lb)
22.6 kg (50 lb)
Female under 18 years
22-6 kg(50 lb)
18.1 kg (40 Ib)
Male under 16 years
22-6 kg(50 Ib)
t8.1 kg (40 lb)
The maximum weight a person can be expected to lift has never been satisfactorily resolved, although much attention has been devoted to this point. The maximum permissible load would appear to be of interest only in exceptional cases; the most physiologically economic or subjectively acceptable load seems to be of more realistic value to industry. Before dealing with these it is perhaps wise to look at the legal aspects of the physical moving of loads. The Factories Act of 1961 states: 'No one must be employed to lift, carry or move any load so heavy as to be likely to cause injury'. Legal interpretation of the Factories Act includes, as quoted by Fife and Machin (1972), the following: 1. "Employed to lift" - a person is not employed to lift if he has been told to ask for help which is readily available. 2. "Likely to cause injury" - Court of Appeal held that a weight of 66 kg (145 Ib) was not likely to cause injury to a man of experience.
190
AppliedErgonomics December
1972
1. The Woollen & Worsted Textiles (Lifting of Heavy Weights) Regulations, 1926. Persons employed
Compact or
Maximum weight not
rigid
compact or rigid
2. Pottery (Health & Welfare) Special Regulations, 1958. A woman or young person must not carry more than 18-1 kg (40 lb) more than 45.7 metres (50 yd) on a level surface. Women shall not carry a sagger weighing more than 13.6 kg (30 lb) or in conjunction with another person more than 22-6 kg (50 lb) or raise a sagger to a height of more than 1.3 m (4ft 6in). 3. The Jute (Safety, Health & Welfare) Regulations, 1948. The Jute Regulations are the same as the Woollen & Worsted Textiles Regulations, except that no maximum weights are mentioned for men, and that young persons under 16 years should lift only 18-1 kg (40 Ib) rigid and 15.8 kg(35 Ib) not rigid. 4. Agriculture (Lifting of heavy weights} Regulations, 1959. (SI 1959 No 2120). The maximum weight of any load consisting of a sack or bag with contents which may be lifted by one worker unaided is 81.5 kg (180 lb) These then are the legal aspects of the lifting and carrying of weights in this country. Fairly comprehensive coverage of the maximum permissible loads in other countries is given in the ILO publication (1964).
We can now look at some figures of suggested weights that workers can be expected to lift. For many years it has been thought that a weight is 'heavy' when it reaches 35% of body weight. If for an 'average' man body weight is considered to be 68 kg (150 lb) and for women 61-21 kg (135 lb), the weight considered heavy is, for men 24 kg (51 lb), and for women 21.3 kg(47 lb). The relevant figures for the 5th and 95th percentiles are:
5th percentile
95th percentile
19-8 kg (44 lb)
27-8 kg (61 lb)
Women 15.7 kg (35 Ib)
27.0 kg (60 lb)
Men
The CIS Information Sheet No 3 (ILO, 1962) suggests that the following weights can be lifted without imposing a critical strain on the lumbar intervertebral discs: men 400 kg (887 lb) With a straight back women 240 kg (555 lb) This is obviously a theoretical value and omits to take into account other important factors. However, when discussing reasonable weight limits for occasional lifting by any method, and taking account of the relationship between efficiency and age, it shows:
Men
Age (years) 14-16 1 6 - 18 18-20 20-35 35-50 over 50
Women
15 kg(33 lb) 19" 42" 2 3 " 51 " 25" 55" 21" 45" 16" 36"
l0 kg (22 lb) 12" 2 6 " 14" 31" 15" 3 3 " 13" 29" 10" 22"
Fig 1 Unstable starting position of lift
It suggests that where lifting is frequent, these figures should be reduced by 25% to take account of the fatigue involved. Compare the large difference between the above figures and those of Poulsen & J6rgensen (1971), who refer to normal, healthy individuals having a normal correlation between back strength and strength of other relevant muscle groups:
From floor to table height Women 25 yrs old.
Height of Lift 150 cm
175 cm
Max range
38kg(851b)
Permissible single lift
27 kg(60 lb) 44 kg (98 lb)
Permissible repeated lift
19kg(421b) 31kg(691b)
63kg(1381b)
Men 25 yrs old.
Max range
80 kg (175 lb) 107 kg (235 lb)
Permissible single lift
56 kg (123 lb) 75 kg (164 lb)
Permissible repeated lift
40kg(891b)
53 k g ( l l 8 1 b )
'Single' here means once or twice per hour and 'repeated' means up to about six times per minute over a short period of a working day. These figures can be further compared with those of Damon et al, (1966), who state that the maximum weight
Fig 2 A good stable starting position for a lift
Applied Ergonomics December 1972
191
that can be lifted without strain by 99% of the young, healthy male population is as follows: To knee level
28.1 kg (62 lb)
To waist level
17-2 kg (38 lb)
To shoulder or eye level
12-2 kg (27 lb)
However, in another section they state that with the load near the floor, the lifting force may be only 75 to 88% of that at the best height. Snook, Irvine, and Bass (1969) investigating maximum acceptable weights while lifting, quote for males: Floor to knuckle
16"7 kg (37.3 lb)
Knuckle to shoulder
15"1 kg (34.4 lb)
Shoulder to arm reach
13"0 kg (29-4 lb)
The marked variation in acceptable or permissible weight to be lifted can thus be seen. It will be appreciated that these results are often obtained from particular experiments under certain specified conditions; extrapolation to general industrial situations must be made with great care. Other aspects of lifting and carrying should be considered. Snook and Irvine (1966) point out that the 20 to 50 year old age group forms 68% of the total labour force and represents 75% of the high cost manual handling accidents. Therefore, attention to the abilities and even training of this group is of major importance.
Dress The pamphlet 'lifting in Industry' (Anon, 1966) points out the importance of the correct clothing for tasks involving lifting and carrying. 'Clothing should be such that it allows easy movement of arms and legs and will not catch in machinery or on a load. Safety shoes or boots, properly laced, with a non-skid sole should be worn. For certain tasks special protective clothing should be worn, ie, industrial aprons, goggles, masks, shoulder pads, and where provided they should always be worn'. Although such advice may sound superfluous, it is a common occurrence for people inappropriately dressed to attempt to avoid soiling their clothes by lifting or carrying objects by holding them away from the body. The simple mechanics of the increasing leverage is often such that the task becomes increasingly difficult and dangerous. Whitney (1958) has shown that the important factor in determining the ability to lift is the distance of the feet from the grasping axis.
Techniques of lifting and carrying Glover and Davies (1961) describe as an objective ot'a manual handling and lifting programme which they introduced into a 6000 employee factory, 'the raising of the status of lifting in men's minds from that of a menial drudge to one of almost a semi-skill, which if thought about can be made easier and if neglected can cause pain and danger'. Only slowly is industry realising that workers often need instruction in the correct methods of lifting and handling. Some forward-looking firms are now taking advantage of a scheme introduced by the author whereby selected workers
192
AppliedErgonomics December
1972
are sent on a week's course where they are trained as instructors in the many techniques of correct lifting and handling. Publicity is often given to a technique of lifting which involves full flexion of the hips and knees with the feet either together or parallel and slightly apart (Grandjean 1969; U.S. Dept. of Labour 1965). This is an unsatisfactory starting position for most lifting tasks. It imposes some strain on the joints and is an extremely unstable position from which to lift. It is a starting position for a lift which cannot be considered safe. The correct techniques are well documented m the the pamphlet "Lifting in Industry" and in a film "A new way to lift" (Anon, 1970), but the main points are repeated here: I. Position of the feet. Good lifting posture starts with the feet. The feet must be far enough apart to give a balanced distribution of the weight: as a general rule the leading foot should point in the direction of the movement. 2. Hips, knees and back. The knees and hips should be bent, and the back kept as straight as possible, with the chin tucked in. 3. Arms. The arms should be held as near to the body as possible. Holding the load as close to the body as possible allows friction between load and clothing to help sustain the load. 4. Grasps. While these vary for different tasks, generally the broader the gripping surface the better. Wherever possible the whole of the hand should be used, not just the fingers. 5. Pace. Lifts should be made smoothly, no jerks or snatches should occur. Two further points must be emphasised when considering techniques of lifting. It has been shown thal a bend and twist, (ie, flexion and rotation of the spine), during a lift, is a common cause of spinal injury and this should be avoided. Davis (1959) has shown that the common 'bend-over' lift predisposes to hernia.
Teamwork in lifting When more than one person is revolved in a lifting task, it is preferable that they should all be of roughly the same height, that they should all lift in accordance with the principles of good lifting, and that the lift should begin on an agreed signal from the senior worker. The final resting place and the route to the resting place of the object should be clearly understood. Dashier (1935) quotes the occurrence of a "Coordination loss' when more than one person is involved in a pulling task, and the same reasoning would seem to apply in lifting. It is unlikely that each person in the team would exert his maximum force at exactly the same time. As a result, the total that can be pulled or lifted is not the sum of each person's capacity. The total reduces quite markedly as the number participating increases. "One person could on average pull 63 kg (140 tb), two persons managed, not 126 kg(280 lb) but t 18 kg (261 lb), a trio pulled 160 kg (345 lb) not 189 kg (420 lb) and eight people pulled 248 kg (558 lb) not 504 kg(1 120 lb)". Although space is often at a premium in mdustly, enough space and clean floors are essential for safe lifting
and handling. What 'enough space' may be for a particular task is something for ergonomists to define. It seems likely that many accidents in the categories mentioned earlier, ie, 'falls on the same level', 'stepping on objects' etc, may be due to lack of clear working space.
Yokes and harnesses These are often advocated to facilitate the moving of loads and have been shown to reduce energy expenditure in many tasks. From a safety point of view, however, they are not to be recommended. If for any reason the load gets out of control, it is often difficult for the worker to disengage promptly from the load.
A comprehensive investigation carried out by Frederick (1959) has shown that the best area for manual lifting is between 1 and 1.5 m (40 and 60 in) from floor level and that the best weight unit for lifting within the recommended area is about 18 kg (40 lb). This article is extensively quoted here because it is felt that with the work of Snook et al (1969) this is the most practical and useful article appearing in the literature on the physical handling of loads. The compounded results of Frederick's experiment are shown in Fig 3. If we assume an average kilo calorie (K cal) output of 200 per hour for sustained work, we can calculate the number of lifts a worker could make under the ideal conditions specified by Frederick.
E
Pushing and pulling
(fxaxwxc) 1000
The ability of force exertion in pushing tasks is covered comprehensively by Kromer (1971)in a publication which also covers the coefficients of friction between various floor and shoe materials. In general, his results agree with those of Snook et al (1969) who found that forces acceptable to 90% of industrial male workers in pulling and pushing tasks over short distances were: Push Initial force
=
25.3 kg (58 lb)
Sustained force 13.1 kg (29.1 lb)
Pull
where E = total energy expenditure per hour in K cal (not more than 200 K cal per hour for an average man); f = number of lifts per hour; a = lifting heights in feet; w = weight of load in pounds, and c = consumption of energy in gram calories per foot pound taken from the graph. Using this formula and substituting values of 200 K cat for E, 4 gram calories per foot lb for/, 2 ft for a, and 40 lb for w, we get:200 = ( f x 2 x 4 0 x 4 )
24-0 kg (53 lb)
1000
14.5 kg (32 lb)
David and Troup (1964) have shown that increases in pressure in the abdominal and thoracic cavities are greater when pushing than when pulling an object. From a safety point of view, when a truck has to move up an incline it is preferable to pull the load up, and when it has to move down an incline it is better to lower the truck ahead of the worker. Simply put, the instruction is that the load should be kept lower than the worker at all times.
Pace of moving loads Although the best patterns for the lifting and moving of loads still need to be established, there is much useful information available. It has been shown that taking weights up from the floor, which involves lowering and raising the body weight, increases the energy expenditure by 50% when compared to lifting a weight from 0-5 m (19½ in) and 1 m (39 in). This information should be taken into account in work place design, because it also eliminates a dangerous step in the lifting of heavy weights, ie, the first 152 mm 228 mm (6 - 9 in) of the lift from the floor. The movement of a load from one location to another is usually more economical physiologically if the load is divided into units of appropriate size and weight within the capability and acceptance range of the operators. Wei~lts of 20 kg (45 lb) are suggested. Kellerman and van Wely ( 1961 ) have shown that a weight of about 17.5 kg (37 lb) is optimum for transporting over distances of 12 m (13 yd), especially for untrained workers. For more experienced workers, a weight of 20 -25 kg (44-55 lb) has been shown to give maximum efficiency, ie, nrinimum energy expenditure per kilogram metre of work performed. Rhythm in the work of lifting and moving light loads increases the mechanical efficiency.
f =
(200x 1000) 320
f = 625 lifts per hour. ~10
~ L i f t range ~Oto2Oi~-~'~ 9 ~ ~IIE'~ o ~ ,- / ~ & 8~lft ronge.9 20t040[~ ~
u>.
4 0 to 60 , n | ,a~20 30
~3-uJ
~'~ Point of maximum ~ Idtmg effioency o Points of maximum efficiency for each lift range
/
/
.__
, _ , ,40 50 Weight, pounds
h 60
F i g 3 The manual lifting data plotted indicate that the maximum lifting efficiency is achieved when a 18 kg (40 Ib) load is lifted in the height range from 1 - 1-5 m (40 to 60 in). All curves 'bottom out', indicating the most efficient weight of lifts in any of the test height ranges. From the chart, hourly tonnage maximums for manual lifting can be calculated using a limit of 200 Kcal of energy consumption per hour. Physiological limits may be used to check industrial engineering studies. The figures illustrate relative heights and not necessarily correct lifting postures. Applied Ergonomics December 1972
193
This is about 10 lifts per minute which ensures maximum productivity. Subjectively this seems a high rate of work and above the range suggested by other authors. A slight reduction in the rate would seem to be more acceptable. For lifts of the same height, a work rate of 333 ft Ib/min has been found acceptable to 90% of industrial male workers.
Davis, P.R. 1959 Lancet 2 155-157. The causation of hernia by weight lifting. Davis, P.R., and Troup, J.D.G. 1964 Ergonomics 7.4,465-474. Pressure in the trunk cavities when pulling, pushing and lifting.
Fife, 1. and Maehin, E.
Discussion
1972 Redgrave's Factories Acts, 925, Butterworth.
It is acknowledged that in many industries workers lift, carry and move objects beyond the limits considered in this paper. It is hoped that with this information available, employers will now have a better idea of what can reasonably be expected of workers as far as tasks involving the lifting and moving of loads is concerned.
Frederick, W.S. 1959 Modern Materials Handling 14.3, 74-76. Human
Answers to the questions posed in the introduction are, of course, difficult to give. No answers applicable to all workers under all circumstances can be given. However, reasonable answers, bearing in mind the multitude of factors which can affect them, would seem to be:1. A weight of about 55 kg (120 lb) seems to be the maximum a worker should be expected to lift - even this should apply only to healthy, experienced males.
energy in manual lifting. Glover, J.R., and Davies, B.T. 1961 JforlndustNurses, 13. 6. Manual handling and lifting. Grandjean, E. 1969 Fitting the task to the man, Taylor and Francis. 15. ILO 1962 Manual Lifting and carrying. CIS Information Sheet No 3, 10 and 13, International Labour Office, Geneva. ILO 1964 Maximum permissible weight to be carried by one worker. Occupational Safety & Health Series No 5, International Labour Office, Geneva.
2. Generally, women can exert less force than men. Adult women have only about 65% of the strength of adult men. The number of lifts a woman can make can be calculated or extrapolated from Fig 3, bearing in mind this figure of 65%. Women should not lift heavy weights above shoulder level.
Kellerman, F.T. and van Wely, P.A. 1961 Ergonomics 4. 3,219. The optimum size and shape
3. All industries where workers are expected to lift heavy weights occasionally, or light weights frequently, should arrange classes of instruction in the correct techniques of lifting and moving loads.
1971 Horizontal static forces exerted by men standing in common working positions on surfaces of various tractions. Aerospace Medical Research Laboratory Report No. AMRL-TR70-114. Wright-Patterson Air Force Base, Ohio.
References
of containers for use by the flower bulb industry.
Kromer, ICM.E.
Poulsen, E., and J~gensen, K. 1971 Applied Ergonomics, 2. 3, 133-137. Back muscle strength, lifting and stooped working postures.
Anon. 1966 Lifting in industry. Pamphlet prepared by the Posture Sub-committee and published by the Chartered Society of Physiotherapy, London.
Snook, S., and lrvine, C.H. 1966 Am Indust Hygiene Ass Jnl, 27, May June, 229, The evaluation of physical tasks in industry.
Anon.
Snook, S., lrvine, C.H., and Bass, S.F.
1970 "A new way to lift". Film circulated by the National Safety Council, Chicago, USA.
1969 Maximum weights and work loads acceptable to male industrial workers while performing lifting, lowering, pushing, pulling, carrying, and walking tasks. Paper presented to American Industrial Conference in Denver, Colarado, 13 May.
Chief Inspector of Factories 1970 Annual Report, Comnd 4758, 99-105, HM SO.
Damon, A., Stoudt, H.W., and McFarland, R.A.
Dashiel, J.F.
U.S. Dept. of Labor 1965 'Teach them to lift'. Bulletin 110. Mechanical and Physical Hazards Series. Bureau of Labor Standards, Washington, DC.
1935 Experimental studies of the influence of social situations on the behaviour of individual adults. Handbook of Social Psychology. Vol 2. Russell & Russell, New York.
Whitney, R.J. 1958 Ergonomics, I. 2. lO1-128. The strength of the lifting action in man.
1966 The human body in equipment design. 321,322. Harvard University Press.
194
AppliedErgonomics December 1972