Every manufacturing plant depends on mechanical power transmission systems. Nearly every piece of equipment powered by an electric motor also has some form of power transmission that exploits the conversion of electricity to powerful movements. It is that strength that makes a prime mover something that should be respected for its ability to injure, maim and kill.
Unfortunately, safety issues did not arise spontaneously to co-evolve in lock step with other advances spawned by the Industrial Revolution. Its a fact: The modern workplace harbors dangers, some fatal. For example, according to the National Safety Council, more than 5,000 on-the-job deaths and 3.8 million disabling injuries occurred in the U.S. in 1998. In the manufacturing arena alone that same year, there were 660 deaths and 650,000 disabling injuries. Without a doubt, some of those injuries are attributable to power transmission and rotating equipment.
General philosophy of guarding
When designing a guard for a machine, the basic objective is to prevent any possibility of body parts coming into contact with moving machine parts, nip points, flying metal or wood chips and sparks. On one level, where to install a guard is a matter of common sense. Guards should be installed at locations on machinery where a reasonable person would not voluntarily place a hand while their eyes are closed. The physical barrier that prevents the unreasonable or unwary from doing the same thing should be firmly attached to the equipment in question.
Machines have a "point of operation" where work is actually being done. In some cases, special jigs, fixtures or tools are used to move material to and from the point of operation--but those devices, by themselves, do not constitute an adequate machine guard. Some machine guards cant be permanently installed because they must be moved out of the way to access the point of operation. Guards in this category should be interlocked with the equipment so that the drive mechanism is disabled when the guard is removed.
The blade on a hand-fed circular saw needs to be guarded by a hood that rides up and over the material being cut. The idea is to make the blade invisible to the operator. In addition to preventing contact with the whirling teeth, the hood is intended to protect the worker from flying splinters and broken saw teeth. This concept applies to ripsaws and crosscut saws equally.
Bandsaws need to have the entire blade covered--except for the working portion that is in contact with the workpiece. Even the band wheels must be covered because they represent a nip point. The revolving heads on hand-fed jointers must be enclosed entirely, except for a slit that allows the work to be processed. Also, the workpiece in a wood lathe should be covered to prevent it from being thrown should it come loose from one of the centers.
Keeping your nose to the grindstone
Grinding wheels are brittle and can fracture in use. Thats why they must be equipped with guards. There are some exceptions, however. Internal grinding, for which the workpiece provides protection, and portable machines with wheels less than 2 in. in diameter, require no guards. Natural sandstone grinding wheels do not need guards around them, nor do grinding wheels with an abrasive surface on a metal, wooden, cloth or paper disc.
The housing on a floor-mounted grinding wheel should cover at least 3/4 of the wheel, with the top front edge of the housing starting not more than 65 degrees above the wheels center.
Mills and calenders
These machines are large, so enclosing them entirely in guards is impractical. Instead, specific points around these machines need guards. Unless it is physically impossible to reach over, under, around or through to place a body part in peril of becoming entangled in the rolls, trip controls will be required to initiate a shutdown. These controls can be pressure-sensitive body bars, trip rods or wires that function when the operator leans on them.
One must be particularly careful when using a press. The operator positions a workpiece in the die and then either steps on a pedal or pushes a pair of widely-spaced buttons to initiate the stroke. While the press control system must have an antirepeat feature that prevents double cycling, inadvertently activating the stroking mechanism at the wrong time is always a possibility. Hence, there is a need for guards at the point of operation.
The standards for these guards are logical. The guards must prevent fingers from getting near the point of operation, yet permit visibility through holes or spaces, the size of which is a function of the distance from the guard to the point of operation. A guard cant be positioned so that its position creates a pinch point between itself and the moving machine parts. It cant be easily removed by the operator.
A gate must enclose the point of operation before a press stroke can start and remain closed until machine motion has ceased. Pull-out devices are another option. These are attached to the workers hands and forcibly withdraws them before the die closes. Devices that merely sweep the operators hands from danger zone are not acceptable.
Presence-sensing device initiation
A presence-sensing device creates a sensing field or area that deactivates the clutch and activates the brake when any part of the operators body or a hand tool is within the field or area. This safety feature cannot be used on a full-revolution clutch--one that, when tripped, cannot be disengaged until the crankshaft has completed a full revolution and stroke. It can be used on a part-revolution clutch--one that can be disengaged at any point before the crankshaft has completed a full revolution and stroke.
The hardware for this method of protecting operators is more complex than what is needed for a static, mechanical guard. Because the operator must have a certain degree of trust in this safety system, the OSHA regulations that cover presence-sensing devices also are more complex. One point worthy of note is that a photoelectric light curtain is the only sensing device that meets the regulations.
Proposing an alternative device sets in motion the need for a series of demonstration tests and analyses that show the proposed alternative is no less reliable, safe or durable than the light curtain. The manufacturer of the proposed device and the user must certify that the requirements have been met.
Also, the certification must be validated by an independent third-party validation organization. Finally, presence-sensing devices cannot be used where state-of-the-art technology does not allow them to be used safely.
Power train guards
Flywheels 7 ft. or less from the floor must be covered and surrounded with guard rails. The guard must have sufficient strength to support the flywheels weight, should its mounting fail.
Rotating shafts 7 ft. or less from the floor should be covered completely. This also applies to shafts mounted below bench machines. The shaft ends should not project beyond the guard for more than one-half of the shaft diameter.
Pulleys 7 ft. or less from the floor also need to be guarded. Of course, cracked or broken pulleys should be replaced because they can continue to self-destruct during use.
Belts used for power transmission, if located 7 ft. or less from the floor, must be enclosed. Even if the belt is above the 7 ft. limit, it may require guards. If its routed over areas where people can be present, if it travels at more than 1,800 ft. per minute, if the pulleys are positioned more than 10 ft. apart, if belt width is 8 in. or more--each of these possibilities mandates guards for belts high overhead.
For gear reducers, a totally-enclosed gearbox provides adequate guarding. A fabricated guard that is at least 7 ft. high must enclose larger gears that have no gearbox. Hand-operated gears for adjusting machinery do not require guards.
Unless higher than 7 ft. above the floor, powered sprockets and chains must have guards. Manually-operated sprockets are exempt. Shaft couplings cannot have projecting setscrews or other protuberances.
The approved standard materials for fabricating guards are expanded metal, perforated or solid sheet metal, wire mesh on an angle iron frame and iron pipe, all of which must be rigidly fastened to the floor or a machine. Guards must be braced to fixed parts of the machinery or the building every 3 ft. or less of their height.
Sometimes machine guards can be fabricated of wood. If conditions in the manufacturing plant rapidly corrode or destroy metal guards, if extreme heat or cold make metal guards impractical or if the guards are used on a construction site, wood is an acceptable alternative.
For further reading and details
The Occupational Safety and Health Administration has a vested interest in the safety of all workers in this country. To help disseminate the regulations it promulgates, OSHA publishes its relevant work product at http://www.osha-slc.gov/OshStd_toc/OSHA_Std_toc.html and at http://www.osha-slc.gov/Publications/Mach_SafeGuard.