PUWER requires that access to dangerous parts of machinery should be prevented in a preferred order or hierarchy of control methods. The standard required is a
‘practicable’ one, so that the only acceptable reason for non-compliance is that there is no technical solution. Cost is not a factor. (See Chapter 1 for more details on stand- ards of compliance.)
The levels of protection required are, in order of implementation:
➤ fi xed enclosing guarding
➤ other guards or protection devices, such as inter- locked guards and pressure sensitive mats
➤ protection appliances, such as jigs, holders and push-sticks and
➤ the provision of information, instruction, training and supervision.
Since the mechanical hazard of machinery arises prin- cipally from someone coming into contact or entangle- ment with dangerous components, risk reduction is based on preventing this contact occurring.
This may be by means of:
➤ a physical barrier between the individual and the component (e.g. a fi xed enclosing guard)
➤ a device which only allows access when the com- ponent is in a safe state (e.g. an interlocked guard which prevents the machine starting unless a guard is closed and acts to stop the machine if the guard is opened) or
➤ a device which detects that the individual is entering a risk area and then stops the machine (e.g. certain photoelectric guards and pressure-sensitive mats).
The best method should, ideally, be chosen by the designer as early in the life of the machine as possible.
It is often found that safeguards which are ‘bolted on’
instead of ‘built in’ are not only less effective in reducing
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risk, but are also more likely to inhibit the normal oper - ation of the machine. In addition, they may in them- selves create hazards and are likely to be diffi cult and hence expensive to maintain.
11.13.1 Fixed guards
Fixed guards have the advantage of being simple, always in position, diffi cult to remove and almost main- tenance free. Their disadvantage is that they do not always properly prevent access, they are often left off by maintenance staff and can create diffi culties for the operation of the machine.
A fi xed guard has no moving parts and should, by its design, prevent access to the dangerous parts of the machinery. It must be of robust construction and suffi cient to withstand the stresses of the process and environmental conditions. If visibility or free air fl ow (e.g.
for cooling) are necessary, this must be allowed for in the design and construction of the guard. If the guard can be
opened or removed, this must only be possible with the aid of a tool.
An alternative fi xed guard is the distance fi xed guard, which does not completely enclose a hazard, but which reduces access by virtue of its dimensions and its distance from the hazard. Where perimeter-fence guards are used, the guard must follow the contours of the machinery as far as possible, thus minimizing space between the guard and the machinery. With this type of guard it is important that the safety devices and operating systems prevent the machinery being operated with the guards closed and someone inside the guard, i.e. in the danger area. Figure 11.12 shows a range of fi xed guards for some of the examples shown in Figure 11.10.
11.13.2 Adjustable guards
User adjusted guard
These are fi xed or movable guards, which are adjustable for a particular operation during which they remain fi xed.
Figure 11.12 Range of fi xed guards.
Shaft wit projectios Rack and pinion gears
Axial flow fans Belt conveyor
Counter-rotating rolls Pulley belts
Meshing gears
Chain and sprockets
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Work equipment hazards and control
175 They are particularly used with machine tools where
some access to the dangerous part is required (e.g. drills, circular saws, milling machines) and where the clearance required will vary (e.g. with the size of the cutter in use on a horizontal milling machine or with the size of the timber being sawn on a circular saw bench) (Figure 11.13).
Adjustable guards may be the only option with cutting tools, which are otherwise very diffi cult to guard, but they have the disadvantage of requiring frequent re- adjustment. By the nature of the machines on which they are most frequently used, there will still be some access to the dangerous parts, so these machines must only be used by suitably trained operators. Jigs, push sticks and false tables must be used wherever possible to minimize hazards during the feeding of the work-piece. The
working area should be well lit and kept free of anything which might cause the operator to slip or trip.
Self-adjusting guard
A self-adjusting guard is one which adjusts itself to accommodate, for example, the passage of material.
A good example is the spring-loaded guard fi tted to many portable circular saws.
As with adjustable guards (see above) they only provide a partial solution in that they may well still allow access to the dangerous part of the machinery. They require careful maintenance to ensure they work to the best advantage (Figure 11.14).
11.13.3 Interlocking guard
The advantages of interlocked guards are that they allow safe access to operate and maintain the machine without dismantling the safety devices. Their disadvantage stems from the constant need to ensure that they are operating correctly and designed to be fail safe. Maintenance and inspection procedures must be very strict.
This is a guard which is movable (or which has a movable part) whose movement is connected with the power or control system of the machine.
An interlocking guard must be so connected to the machine controls such that:
➤ until the guard is closed the interlock prevents the machinery from operating by interrupting the power medium
➤ either the guard remains locked closed until the risk of injury from the hazard has passed or opening the guard causes the hazard to be eliminated before access is possible.
A passenger lift or hoist is a good illustration of these principles: the lift will not move unless the doors are closed, and the doors remain closed and locked until the lift is stationary and in such a position that it is safe for the doors to open.
Special care is needed with systems which have stored energy. This might be the momentum of a heavy moving part, stored pressure in a hydraulic or pneumatic system, or even the simple fact of a part being able to move under gravity even though the power is discon- nected. In these situations, dangerous movement may continue or be possible with the guard open, and these factors need to be considered in the overall design.
Braking devices (to arrest movement when the guard is opened) or delay devices (to prevent the guard opening until the machinery is safe) may be needed. All interlocking systems must be designed to minimize the risk of failure- to-danger and should not be easy to defeat (Figure 11.15).
Figure 11.13 Adjustable guard for a rotating shaft, such as a pedestal drill.
Figure 11.14 Self-adjusting guard on a wood saw.
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