Chapter 2 - Methods of Machine Safeguarding
Methods of Machine Safeguarding There are many ways to safeguard machines. The type of operation, the size or shape of stock, the method of handling, the physical layout of the work area, the type of material, and production requirements or limitations will help to determine the appropriate safeguarding method for the individual machine. As a general rule, 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 safeguarding may be possible. One must always choose the most effective and practical means available. We can group safeguards under five general classifications.
- Presence Sensing
- Photoelectrical (optical)
- Radiofrequency (capacitance)
- Safety Controls
- Safety trip control
- Pressure-sensitive body bar
- Safety tripod
- Safety tripwire cable
- Two-hand control
- Two-hand trip
- Potential Feeding and Ejection Methods to Improve Safety for the Operator
- Automatic feed
- Semi-automatic feed
- Automatic ejection
- Semi-automatic ejection
- Miscellaneous Aids
- Awareness barriers
- Miscellaneous protective shields
- Hand-feeding tools and holding fixtures
Guards are barriers which prevent access to danger areas. There are four general types of guards:
Fixed: 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.
Examples of fixed guards...
In Figure 11, a fixed guard on a power press completely encloses the point of operation. The stock is fed through the side of the guard into the die area, with the scrap stock exiting on the opposite side.
Figure 11. Fixed guard on power press.
Figure 12 shows a fixed guard that protects the operator from a mechanism that folds cartons. This guard would not normally be removed except to perform maintenance on the machine. Figure 13 shows a fixed enclosure guard shielding the belt and pulley of a power transmission unit. An inspection panel is provided on top in order to minimize the need for removing the guard. To remain effective, the inspection panel cannot be removed while the mechanism is in operation. In Figure 14, fixed enclosure guards are shown on a bandsaw. These guards protect the operator from the turning wheels and moving saw blade. Normally, the only time for the guards to be opened or removed would be for a blade change or maintenance. It is very important that they be securely fastened while the saw is in use.
Figure 12. Fixed guard on egg carton folding machine.
Figure 13. Fixed guard enclosing belt and pulleys.
Figure 14. Fixed guards on a band saw.
A fixed guard is shown on a veneer clipper in Figure 15. This guard acts as a barrier, protecting fingers from exposure to the blade. Note the side view of the curved portion of the guard.
Figure 15. Fixed guards on veneer clipper.
Figure 16 shows both a fixed blade guard and a throat and gap guard on a power squaring shear. These guards should be removed only for maintenance or blade changes.
Figure 16. Fixed guard on a power squaring shear.
In Figure 17, a transparent, fixed barrier guard is being used on a press brake to protect the operator from the unused portions of the die. This guard is easy to install or remove.
Figure 17. Fixed guard providing protection from unused portion of die on a press brake.
Interlocked: 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. To be effective, all movable guards should be interlocked to prevent occupational hazards. (See also Figure 13.)
Figure 18 shows an interlocked barrier guard mounted on an automatic bread bagging machine. When the guard is removed, the machine will not function.
Figure 18. Interlocked guard on automatic bread bagging machine
In Figure 19, the beater mechanism of a picker machine (used in the textile industry) is covered by an interlocked barrier guard. This guard cannot be raised while the machine is running, nor can the machine be restarted with the guard in the raised position.
Figure 19. Interlocked guard on picker machine
In Figure 20, an interlocked guard covers the rotating cylinder of the dividing head of a roll make-up machine used for making hamburger and hot-dog rolls.
Figure 20. Interlocked guard on roll make-up machine
Adjustable: Adjustable guards are useful because they allow flexibility in accommodating various sizes of stock.
Figure 21 shows a bandsaw with an adjustable guard to protect the operator from the unused portion of the blade. This guard can be adjusted according to the size of stock.
Figure 21. Adjustable guard on horizontal bandsaw.
In Figure 22, the bars adjust to accommodate the size and shape of the stock. Figures 23 and 24 show guards that can be adjusted according to the thickness of the stock.
Figure 22. Adjustable guard on power press.
Figure 23. Adjustable guard on router.
Figure 24. Adjustable guard on shaper.
In Figure 25, the guard adjusts to provide a barrier between the operator and the blade.
Figure 25. Adjustable guard on table saw.
Figure 26 shows an adjustable enclosure guard on a bandsaw.
Figure 26. Adjustable guard on bandsaw.
Self-Adjusting: 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.
Examples of self-adjusting guards...
Figure 27 shows a radial arm saw with a self-adjusting guard. As the blade is pulled across the stock, the guard moves up, staying in contact with the stock.
Figure 27. Self-adjusting guard on radial arm saw.
Figure 28 shows a twin-action, transparent, self-adjusting guard. The first guard rises as the stock enters, then returns to its rest position as the stock moves ahead to raise the second guard.
Figure 28. Self-adjusting guard on table saw.
A self-adjusting guard is shown in Figure 29. As the blade moves through the stock, the guard rises up to the stock surface.
Figure 29. Self-adjusting guard on circular saw.
Figure 30 shows a self-adjusting enclosure guard mounted on a jointer. This guard is moved from the cutting head by the stock. After the stock is removed, the guard will return, under spring tension, to the rest position.
Figure 30. Self-adjusting guard on a jointer
Another type of self-adjusting guard mounted on a jointer is illustrated in Figure 31. The guard moves two ways. An edging operation causes the guard to move horizontally. If the stock is wide enough during a surfacing operation, the stock may be fed under the guard, causing it to move vertically.
Figure 31. Self-adjusting guard on a jointer
|Method||Safeguarding Action||Advantages||May interfere|
|Fixed||Provides a barrier||Can be constructed to suit many specific applications
In-plant construction is often possible
Can provide maximum protection
Usually requires minimum maintenance
Can be suitable to high production, repetitive operations
|May interfere with visibility
Can be limited to specific operations
Machine adjustment and repair often require its removal, thereby necessitating other means of protection for maintenance personnel
|Interlocked||Shuts off or disengages power and prevents starting of machine when guard is open; should require the machine to be stopped before the worker can reach into the danger area||Can provide maximum protection
Allows access to machine for removing jams without time consuming removal of fixed guards
|Requires careful adjustment and maintenance
May be easy to disengage jams
|Adjustable||Provides a barrier that may be adjusted to facilitate a variety of production operations||Can be constructed to suit many specific applications
Can be adjusted to admit varying sizes of stock
|Hands may enter danger area -- protection may not be complete at all times
May require frequent maintenance and/or adjustment
The guard may be made ineffective by the operator
May interfere with visibility
|Self-adjusting||Provides a barrier that moves according to the size of the stock entering the danger area||Off-the-shelf guards are often commercially available||Does not always provide maximum protection
May interfere with visibility
May require frequent maintenance and adjustment
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.
The photoelectric (optical) presence-sensing device uses a system of light sources and controls which can interrupt the machine's operating cycle. If the light field is broken, the machine stops and will not cycle. This device must be used only on machines which can be stopped before the worker can reach the danger area. The design and placement of the guard depends upon the time it takes to stop the mechanism and the speed at which the employee's hand can reach across the distance from the guard to the danger zone.
Figure 32 shows a photoelectric presence-sending device on a part-revolution power press. When the light beam is broken, either the ram will not start to cycle, or, if the cycle has begun, the stopping mechanism will be activated so that the press stops before the operator's hand can enter the danger zone.
Figure 32. Photoelectric presence-sensing device on power press.
A photoelectric presence-sending device used with a press brake is illustrated in Figure 33. The device may be swung up or down to accommodate different production requirements.
Figure 33. Photoelectric presence-sensing device on press brake
The radiofrequency (capacitance) presence-sending device uses a radio beam that is part of the machine control circuit. When the capacitance field is broken, the machine will stop or will not activate. Like the photoelectric device, this device shall only be used on machines which can be stopped before the worker can reach the danger area. This requires the machine to have a friction clutch or other reliable means for stopping.
Figure 34 shows a radiofrequency presence-sensing device mounted on a part-revolution power press.
Figure 34. Radiofrequency presence-sensing device on a power press
The electromechanical sensing device has a probe or contact bar which descends to a predetermined distance when the operator initiates the machine cycle. If there is an obstruction preventing it from descending its full predetermined distance, the control circuit does not actuate the machine cycle.
Figure 35 shows an electromechanical sensing device on an eyeletter. The sensing probe in contact with the operator's finger is also shown.
Figure 35. Electromechanical sensing device on an eyeletter.
Pullback devices utilize 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.
Figure 36 shows a pullback device on a straight-side power press. When the slide/ram is in the "up" position, the operator can feed material by hand into the point of operation. When the press cycle is actuated, the operator's hands and arms are automatically withdrawn. Figure 37 shows a pullback device on a smaller press.
Figure 36. Pullback device on a power press.
Figure 37. Pullback device on a power press.
A pullback device on a press brake is illustrated in Figure 38.
Figure 38. Pullback device on press brake
The restraint (holdout) device in Figure 39 utilizes 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 within a predetermined safe area. There is no extending or retracting action involved. Consequently, hand-feeding tools are often necessary if the operation involves placing material into the danger area.
Figure 39. Restraint device on power press
Page 2 of Chapter 2