There are many ways to design and use machine safeguarding. The type of operation, size or shape of stock, method of handling, physical layout of the work area, type of material, and production requirements or limitations will help to determine the appropriate machine safeguarding method for the individual machine.
As a general rule, a power transmission apparatus is best protected by fixed guards that enclose the danger areas. For hazards at the point of operation, where moving parts actually perform work on stock, several kinds of machine safeguarding may be possible. One must always choose the most effective and practical means available.
We can group machine safeguards under five general classifications.
As its name implies, a fixed guard is a permanent part of the machine. It is not dependent upon moving parts to perform its intended function. It may be constructed of sheet metal, screen, wire cloth, bars, plastic, or any other material that is substantial enough to withstand whatever impact it may receive and to endure prolonged use. This guard is usually preferable to all other types because of its relative simplicity and permanence.
Adjustable guards are useful because they allow flexibility in accommodating various sizes of stock.
When this type of guard is opened or removed, the tripping mechanism and/or power automatically shuts off or disengages, and the machine cannot cycle or be started until the guard is back in place. An interlocked guard may use electrical, mechanical, hydraulic, or pneumatic power or any combination of these. Interlocks should not prevent "inching" by remote control if required. Replacing the guard should not automatically restart the machine.
The openings of these barriers are determined by the movement of the stock. As the operator moves the stock into the danger area, the guard is pushed away, providing an opening which is only large enough to admit the stock. After the stock is removed, the guard returns to the rest position. This guard protects the operator by placing a barrier between the danger area and the operator. The guards may be constructed of plastic, metal, or other substantial material. Self-adjusting guards offer different degrees of protection.
A safety device may perform one of several functions. It may stop the machine if a hand or any part of the body is inadvertently placed in the danger area; restrain or withdraw the operator's hands from the danger area during operation; require the operator to use both hands on machine controls, thus keeping both hands and body out of danger; or provide a barrier which is synchronized with the operating cycle of the machine in order to prevent entry to the danger area during the hazardous part of the cycle.
Presence-sensing devices use a system of light or radiofrequency sources and controls which can interrupt the machine's operating cycle; if the light or electric field is broken, the machine stops and will not cycle.
Pullback devices use a series of cables attached to the operator's hands, wrists, and/or arms. This type of device is primarily used on machines with stroking action. When the slide/ram is up between cycles, the operator is allowed access to the point of operation. When the slide/ram begins to cycle by starting its descent, a mechanical linkage automatically assures withdrawal of the hands from the point of operation.
The restraint (holdout) device in the center figure below uses cables or straps that are attached to the operator's hands at a fixed point. The cables or straps must be adjusted to let the operator's hands travel only within a predetermined safe area. There is no extending or retracting action required because the hands are never allowed to extend into the danger area. Consequently, hand-feeding tools are often necessary if the operation involves placing material into the danger area.
Safety trip controls provide a quick means for deactivating the machine in an emergency situation. A pressure-sensitive body bar, when depressed, will deactivate the machine. If the operator or anyone trips, loses balance, or is drawn toward the machine, applying pressure to the bar will stop the operation. The positioning of the bar, therefore, is critical. It must stop the machine before a part of the employee's body reaches the danger area. Unfortunately, it may be easy to defeat the body bar by going under it into the danger zone. The figure here shows a pressure-sensitive body bar located on the front of a rubber mill.
The two-hand control device requires constant, concurrent pressure by the operator to activate the machine. This kind of control requires a part-revolution clutch, brake, and a brake monitor if used on a power press. With this type of device, the operator's hands are required to be at a safe location (on control buttons) and at a safe distance from the danger area while the machine completes its closing cycle.
Two-hand trips are used as a safeguarding device on a full revolution clutch power press only. This device requires the joint operation of two trigger buttons located away from the "danger zone" of the press. Activation of the machine stroke requires only a "trip" of the controls whereas a two-hand control requires continued pressure. The two-hand trip requires the operators hands to be away from the point of operation to activate the machine stroke.
To consider a part of a machine to be machine safeguarded by location, the dangerous moving part of a machine must be positioned so that those areas are not accessible or do not present a hazard to a worker during the normal operation of the machine. This can be accomplished in two ways:
The feeding process can be machine guarded by location if a safe distance can be maintained to protect the worker's hands. The dimensions of the stock being worked on may provide adequate safety.
For instance, if the stock is several feet long and only one end of the stock is being worked on, the operator may be able to hold the opposite end while the work is being performed. An example would be a single-end punching machine. However, depending upon the machine, protection might still be required for other personnel.
The positioning of the operator's control station provides another potential approach to machine safeguarding by location. Operator controls may be located at a safe distance from the machine if there is no reason for the operator to tend it.
Many feeding and ejection methods do not require the operator to place his or her hands in the danger area. In some cases, no operator involvement is necessary after the machine is set up. In other situations, operators can manually feed the stock with the assistance of a feeding mechanism. Properly designed ejection methods do not require any operator involvement after the machine starts to function.
Some feeding and ejection methods may even create hazards themselves. For instance, a robot may eliminate the need for an operator to be near the machine but may create a new hazard itself by the movement of its arm.
Using feeding and ejection methods does not eliminate the need for guards and devices. Guards and devices must be used wherever they are necessary and possible in order to provide protection from exposure to hazards.
Automatic feeds reduce the exposure of the operator during the work process, and sometimes do not require any effort by the operator after the machine is set up and running.
With semi-automatic feeding as in the case of a power press, the operator uses a mechanism to place the piece being processed under the ram at each stroke. The operator does not need to reach into the danger area, and the danger area is completely enclosed.
The figure below shows a semi-automatic ejection mechanism used on a power press. When the plunger is withdrawn from the die area, the ejector leg, which is mechanically coupled to the plunger, kicks the completed work out.
Robots are machines that load and unload stock, assemble parts, transfer objects, or perform other tasks. They are used to accomplish many different functions such as material handling, assembly, arc welding, resistance welding, machine tool load/unload functions, painting/spraying, etc. According to OSHA, since 2000 most occupational accidents involving robots have been fatalities.
All industrial robots are either servo or non-servo controlled.
The use of robotics in the workplace can also pose potential mechanical and human hazards.
The three figures on this page show a type of robot in operation, the danger areas it can create, and an example of the kind of task (feeding a press) it can perform.
While these aids do not give complete protection from machine hazards, they may provide the operator with an extra margin of safety. Sound judgment is needed in their application and usage. Below are several examples of possible applications.
An awareness barrier does not provide physical protection, but serves only to remind a person that he or she is approaching the danger area. Generally, awareness barriers are not considered adequate when continual exposure to the hazard exists.
The figure on the left, below, shows a rope used as an awareness barrier on the rear of a power squaring shear. Although the barrier does not physically prevent a person from entering the danger area, it calls attention to it. For an employee to enter the danger area an overt act must take place, that is, the employee must either reach or step over, under or through the barrier.
Shields, another aid, may be used to provide protection from flying particles, splashing cutting oils, or coolants. The figure on the right, below, shows more potential applications with drills and lathes.
Special hand tools may be used to place or remove stock, particularly from or into the point of operation of a machine. A typical use would be for reaching into the danger area of a press or press brake. The figure on the left, below, shows an assortment of tools for this purpose. Holding tools should not be used instead of other machine guards; they are merely a supplement to the protection that other guards provide.
A push stick or block, such as those in the figure below may be used when feeding stock into a saw blade. When it becomes necessary for hands to be in close proximity to the blade, the push stick or block may provide a few inches of safety and prevent a severe injury. The figure on the right, below, shows how the push stick works by fitting over the fence.
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