Machine Design

Beware of unsafe guards

One of the basic concepts of machine guarding is that a guard must not become a hazard or contribute to hazards. Thorough hazard analyses easily identify hazardous guards. Ideally, preliminary hazard analyses at the equipment manufacturer will identify and resolve guarding problems before they affect users.

I’ve previously discussed the problem of guards that can be left off without disabling machine operations. This often happens on conveyors of all lengths and capacities, especially on those with tracking problems. It would be easy to add an interlock to the conveyor design that would stop the machine from operating if the guard was missing.

Better communication about guards also improves safety. For instance, if maintenance personnel leave a guard off a machine after they’ve finished with it, a new operator starting the machine may not realize the guard is missing. Two simple ways to combat this are to paint the areas under bolted-on guards in eye-catching, contrasting colors and to post warning labels on the guard and on the machine in the spot where the guard should go. The warnings should inform workers not to operate the machine without the guard in place.

Guards that can themselves turn dangerous are less easily handled. One hazardous but commonly used safety measure is the plastic guarding the blade on many miter saws. I’ve investigated three identical accidents where these guards got bumped out of alignment and into the blade path. When the rotating blades hit the guards, the plastic broke up and forcibly threw dangerously sharp plastic shards into the operators’ fingers and hands. Nothing in the saws’ labeling or manufacturer literature alerted users to this danger.

Another type of unsafe guard is one that fails to stop material from being thrown out of a machine with enough energy to cause injury. One example I investigated involved a punch press that punched 1-in.-diameter, 1/8-in.-thick discs from oiled steel in high volumes. A stack of discs tended to build up in a horizontal collector until the next punch knocked over the stack and sent discs flying out of the machine with substantial force.

After a nearby worker was badly bruised by flying discs, the company operating the machine welded a barrier guard on the side of the disc collector where the discs had flown out. A second incident led them to guard a second side of the collector. The third occurrence was fatal; a flying disc hit the operator in the chest at short range.

If engineers at the machine manufacturer had conducted a proper hazard analysis they would have chosen a better initial design. For instance, sloping the bottom of the collector would have prevented discs from stacking up square to the punch and deflected the force with which they went flying. Such an approach would not have required guarding at all.

And if the employer determined guarding was needed, a thorough hazard analysis would have dictated complete guarding of all four sides.

Lanny Berke is a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at [email protected].

© 2011 Penton Media, Inc.

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