Accidents happen is no excuse

Guarding is mandatory because moving machine parts are unforgiving of the unwary.

By Russ Kratowicz, executive editor

PlantServices.com

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. It’s 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 can’t 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.

Cutting wood

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. That’s 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 wheel’s 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.

Mechanical presses

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 can’t be positioned so that its position creates a pinch point between itself and the moving machine parts. It can’t 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 operator’s 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 operator’s 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.