Practical safeguards.
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 risk, but are also more likely to inhibit the normal operation of the machine. In addition, they may in themselves create hazards and are likely to be difficult and hence expensive to maintain.
Fixed guards
Fixed guards have the advantage of being simple, always in position, difficult to remove and almost maintenance free. Their disadvantage is that they do not always properly prevent access; they are often left off by maintenance staff and can create difficulties for the operation of the machine. A fixed 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 sufficient to withstand the stresses of the process and environmental conditions. If visibility or free air flow (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 fixed guard is the distance fixed 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.
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Adjustable guards
User adjusted guard
These are fixed or movable guards, which are adjustable for a particular operation during which they remain fixed. 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). Adjustable guards may be the only option with cutting tools, which are otherwise very difficult to guard, but they have the disadvantage of requiring frequent readjustment.
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 adjustable guard for a rotating shaft such as a pedestal drill.
Self-adjusting guard on a wood saw. Working area should be well lit and kept free of anything which might cause the operator to slip or trip.
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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 fitted 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.
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 disconnected. 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.
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