MOVING PARTS OF MACHINES
Tomas Backstrom, Marianne Doos
This article discusses situations and chains of events leading to accidents attributable to contact with the moving part of machines. People who operate and maintain machinery run the risk of being involved in serious accidents. US statistics suggest that 18,000 amputations and over 800 fatalities in the United States each year are assignable to such causes. According to the US National Institute for Occupational Safety and Health (NIOSH), the “caught in, under, or between” category of injuries in their classification ranked highest among the most important kinds of occupational injuries in 1979. Such injuries generally involved machines (Etherton and Myers 1990). “Contact with moving machine part” has been reported as the principal injury event in just over 10% of occupational accidents ever since this category was introduced into Swedish occupational-injury statistics in 1979.
Most machines have moving parts that can cause injury. Such moving parts may be found at the point of operation where work is performed on the material, such as where cutting, shaping, boring or deforming takes place. They may be found in the apparatus which transmits energy to the parts of the machine carrying out the work, such as flywheels, pulleys, connecting rods, couplers, cams, spindles, chains, cranks and gears. They may be found in other moving parts of the machine such as wheels on mobile equipment, gear motors, pumps, compressors and so forth. Hazardous machine movements can also be found among other sorts of machinery, especially in the auxiliary pieces of equipment which handle and transport such loads as work pieces, materials, waste or tools.
All parts of a machine that move in the course of the performance of work may contribute to accidents causing injury and damages. Both rotating and linear machine movements, as well as their sources of power, can be dangerous:
Rotating motion. Even smooth rotating shafts can grip an item of clothing and, for example, draw a person’s arm into a hazardous position. The danger in a rotating shaft increases if it has projecting parts or uneven or sharp surfaces, such as adjusting screws, bolts, slits, notches or cutting edges. Rotating machine parts give rise to “nip points” in three different ways:
1. There are the points between two rotating parts that rotate in opposite directions and have parallel axes, such as gears or cog-wheels, carriage rollers or mangles.
2. There are the points of contact between rotating parts and parts in linear movement, such as found between a power-transmission belt and its pulley, a chain and a sprocket, or a rack and pinion.
3. Rotating machine movements can give rise to the risk of cuts and crushing injuries when they take place in close proximity to stationary objects-this sort of condition exists between a worm conveyor and its housing, between the spokes of a wheel and the machine bed, or between a grinding wheel and a tool jig.
Linear movements. Vertical, horizontal and reciprocating motion can cause injury in several ways: a person may receive a shove or blow from a machine part, and may be caught between the machine part and some other object, or may be cut by a sharp edge, or sustain a nip injury by being trapped between the moving part and another object (figure 58.8).
Figure 58.8 Examples of mechanical movements that can injure a person
Power sources. Frequently, external sources of power are employed to run a machine which may involve considerable quantities of energy. These include electric, steam, hydraulic, pneumatic and mechanical power systems, all of which, if released or uncontrolled, can give rise to serious injuries or damage. A study of accidents that occurred over one year (1987 to 1988) among farmers in nine villages in northern India showed that fodder-cutting machines, all otherwise of the same design, are more dangerous when powered by a motor or tractor. The relative frequency of accidents involving more than a minor injury (per machine) was 5.1 per thousand for manual cutters and 8.6 per thousand for powered cutters (Mohan and Patel 1992).
Injuries Associated with Machine Movements
Since the forces associated with machine movements are often quite large, it can be presumed that the injuries to which they give rise will be serious. This presumption is confirmed by several sources. “Contact with moving machinery or material being machined” accounted for only 5% of all occupational accidents but for as much as 10% of fatal and major accidents (fractures, amputations and so on) according to British statistics (HSE 1989). Studies of two vehicle-manufacturing workplaces in Sweden point in the same direction. Accidents caused by machine movements gave rise to twice the number of days of sick leave, as measured by median values, compared to non-machine-related accidents. Machine-related accidents also differed from other accidents with regard to part of the body injured: The results indicated that 80% of the injuries sustained in “machine” accidents were to the hands and fingers, while the corresponding proportion for “other” accidents was 40% (Backstrom and Doos 1995).
The risk situation at automated installations has turned out to be both different (in terms of type of accident, sequence of events and degree of injury severity) and more complicated (both in technical terms and with regard to the need for specialized skills) than at installations where conventional machinery is used. The term automated is herein meant to refer to equipment which, without the direct intervention of a human being, can either initiate a machine movement or change its direction or function. Such equipment requires sensor devices (e.g., position sensors or microswitches) and/or some form of sequential controls (e.g., a computer program) to direct and monitor their activities. Over recent decades, a programmable logic controller (PLC) has been increasingly employed as the control unit in production systems. Small computers are now the most common means used for controlling production equipment in the industrialized world, while other means of control, such as electro-mechanical units, are becoming less and less common. In the Swedish manufacturing industry, the use of numerically controlled (NC) machines increased by 11 to 12% per year over the 1980s (Hцrte and Lindberg 1989). In modern industrial production, being injured by “moving parts of machines” is increasingly becoming equivalent to being injured by “computer-controlled machine movements”.
Automated installations are found in more and more sectors of industry, and they have an increasing number of functions. Stores management, materials handling, processing, assembly and packaging are all being automated. Series production has come to resemble process production. If the feeding, machining and ejection of work pieces are mechanized, the operator no longer needs to be in the risk zone during the course of regular, undisturbed production. Research studies of automated manufacturing have shown that accidents occur primarily in the handling of disturbances affecting production. However, people can also get in the way of machine movements in performing other tasks, such as cleaning, adjusting, resetting, controlling and repairing.
When production is automated and the process is no longer under the direct control of the human being, the risk of unexpected machine movements increases. Most operators who work with groups or lines of inter-linked machines have experienced such unexpected machine movements. Many automation accidents occur as a result of just such movements. An automation accident is an accident in which the automatic equipment controlled (or should have controlled) the energy giving rise to the injury. This means that the force which injures the person comes from the machine itself (e.g., the energy of a machine movement). In a study of 177 automation accidents in Sweden, it was found that injury was caused by the “unexpected start” of a part of a machine in 84% of cases (Backstrцm and Harms-Ringdahl 1984). A typical example of an injury caused by a computer-controlled machine movement is shown in figure 58.9.
Figure 58.9 A typical example of an injury caused by a computer-controlled machine movement
One of the studies referred to above (Backstrom and Doos 1995) showed that automatically controlled machine movements were causally linked to longer periods of sick leave than injuries due to other kinds of machine movements, the median value being four times higher at one of the workplaces. The injury pattern of automation accidents was similar to that for other machine accidents (mainly involving hands and fingers), but the tendency was for the former kind of injuries to be more serious (amputations, crushes and fractures).
Computer control, like manual, has weaknesses from the perspective of reliability. There is no guarantee that a computer program will operate without error. The electronics, with their low signal levels, may be sensitive to interference if not properly protected, and the consequences of resultant failures are not always possible to predict. Furthermore, programming changes are often left undocumented. One method used to compensate for this weakness is, for example, by operating “double” systems in which there are two independent chains of functional components and a method for monitoring such that both chains display the same value. If the systems display different values, this indicates a failure in one of them. But there is a possibility that both chains of components may suffer from the same fault and that they both can be put out of order by the same disturbance, thereby giving a false positive reading (as both systems agree). However, in only a few of the cases investigated has it been possible to trace an accident to computer failure (see below), despite the fact that it is common for a single computer to control all the functions of an installation (even the stopping of a machine as a result of the activation of a safety device). As an alternate, consideration may be given to providing a tried-and-tested system with electro-mechanical components for safety functions.
In general, it can be said that a single accident has many causes, including technical, individual, environmental and organizational ones. For preventive purposes, an accident is best looked at not as an isolated event, but as a sequence of events or a process (Backstrцm 1996). In the case of automation accidents, it has been shown that technical problems are frequently part of such a sequence and occur either at one of the early stages of the process or close to the injury event of the accident. Studies in which technical problems involved in automation accidents have been examined suggest that these lie behind 75 to 85% of the accidents. At the same time, in any specific case, there are usually other causes, such as those of an organizational nature. Only in one-tenth of cases has it been found that the direct source of the energy giving rise to an injury could be attributed to technical failure-for example, a machine movement taking place despite the machine’s being in the stop position. Similar figures have been reported in other studies. Usually, a technical problem led to trouble with the equipment, so that the operator had to switch tasks (e.g., to re-position a part that was in a crooked position). The accident then occurred during the implementation of the task, prompted by the technical failure. A quarter of the automation accidents were preceded by a disturbance in the materials flow such as a part becoming stuck or getting into a crooked or otherwise faulty position (see figure 58.10).
Figure 58.10 Types of technical problems involved in automation accidents (number of accidents =127)
In a study of 127 accidents involving automation, 28 of these accidents, described in figure 58.10, were further investigated to determine the types of technical problems which were involved as causal factors (Backstrцm and Dццs, in press). The problems specified in the accident investigations were most frequently caused by jammed, defective or worn-out components. In two cases, a problem was caused by a computer-program error, and in one by electromagnetic interference. In more than half of the cases (17 out of 28), faults had been present for some time but not remedied. Only in 5 of the 28 cases where a technical failure or deviation was referred to, had the defect not manifested itself previously. Some faults had been repaired only to reappear later. Certain defects had been present right from the time of installation, while others resulted from wear and the impact of the environment.
The proportion of automation accidents occurring in the course of the correction of a disturbance to production comes to between one-third and two-thirds of all cases, according to most studies. In other words, there is general agreement that handling production disturbances is a hazardous occupational task. The variation in the extent to which such accidents occur has many explanations, among them those related to the type of production and to how occupational tasks are classified. In some studies of disturbances, only problems and machine stops in the course of regular production have been considered; in others, a wider range of problems have been treated-for example, those involved in the setting up of work.
A very important measure in the prevention of automation accidents is to prepare procedures for removing the causes of production disturbances so that they are not repeated. In a specialized study of production disturbances at time of accident (Dццs and Backstrцm 1994), it was found that the most common task to which disturbances gave rise was the freeing or the correcting of the position of a work piece that had become stuck or wrongly placed. This type of problem initiated one of two rather similar sequences of events: (1) the part was freed and came into its correct position, the machine received an automatic signal to start, and the person was injured by the machine movement initiated, (2) there was not time for the part to be freed or repositioned before the person was injured by a machine movement that came unexpectedly, more quickly or was of greater force than the operator expected. Other disturbance-handling involved prompting a sensor impulse, freeing a jammed machine part, carrying out simple kinds of fault tracing, and arranging for restart (see figure 58.11).
Figure 58.11 Type of disturbance handling at time of accident (number of accidents =76)
The categories of personnel which tend to be injured in automation accidents depend on how work is organized-that is, on which occupational group performs the hazardous tasks. In practice, this is a matter of which person at the workplace is assigned to deal with problems and disturbances on a routine basis. In modern Swedish industry, active interventions are usually demanded from the persons operating the machine. This is why, in the previously mentioned vehicle-manufacturing workplace study in Sweden (Backstrцm and Dццs, accepted for publication), it was found that 82% of the people who sustained injuries from automated machines were production workers or operators. Operators also had a higher relative accident frequency (15 automation accidents per 1,000 operators per year) than maintenance workers (6 per 1,000). The findings of studies which indicate that maintenance workers are more affected are at least partly to be explained by the fact that operators are not allowed to enter machining areas in some companies. In organizations with a different type of task distribution, other categories of personnel-setters, for example-may be given the task of solving any production problems that arise.
The most common corrective measure taken in this connection in order to raise the level of personal safety is to protect the person from hazardous machine movements by using some kind of safety device, such as machine guarding. The main principle here is that of “passive” safety-that is, the provision of protection that does not require action on the part of the worker. It is, however, impossible to judge the effectiveness of protective devices without very good acquaintance with the actual work requirements at the machine in question, a form of knowledge which is normally possessed only by machine operators themselves.
There are many factors that can put even what is apparently good machine protection out of action. In order to perform their work, operators may need to disengage or circumvent a safety device. In one study (Dццs and Backstrцm 1993), it was found that such disengagement or circumvention had taken place in 12 out of 75 of the automation accidents covered. It is often a matter of the operator’s being ambitious, and no longer willing to accept either production problems or the delay to the production process involved in correcting disturbances in accordance with instructions. One way of avoiding this problem is to make the protective device imperceptible, so that it does not affect the pace of production, product quality or task performance. But this is not always possible; and where there are repeated disturbances to production, even minor inconveniences can prompt people not to utilize safety devices. Again, routines should be made available to remove the causes of production disturbances so that these are not repeated. A lack of a means of confirming that safety devices really function according to specifications is a further significant risk factor. Faulty connections, start signals that remain in the system and later give rise to unexpected starts, build-up in air pressure, and sensors that have come loose may all cause failure of protective equipment.
As has been shown, technical solutions to problems may give rise to new problems. Although injuries are caused by machine movements, which are essentially technical by nature, this does not automatically mean that the potential for their eradication lies in purely technical factors. Technical systems will continue to malfunction, and people will fail to handle the situations to which these malfunctions give rise. The risks will continue to exist, and can be held in check only by a wide variety of means. Legislation and control, organizational measures at individual companies (in the form of training, safety rounds, risk analysis and the reporting of disturbances and near accidents), and an emphasis on steady, ongoing improvements are all needed as complements to purely technical development.