Licensed Professional Engineers

Forensic Clues #9


Issue 9: Vol. 1 July/August 2004

Forensic Clues #9: Guarding

While operating an excavator, a man was moving a tree with the track hoe bucket. As the operator pulled on the tree by the root ball, the tree broke free, rotated around the root ball, and crashed through the glass panel of the cab. The operator received severe injuries when the falling tree tripped the excavator movement controls, crushing the operator.

Figure 1. Tree Crashes Into Unguarded Cab

This worker was injured as a result of improper guarding. He was fortunate enough to survive the ordeal. Many are not so lucky.

Why Guard?

Guarding is necessary in many situations where a hazard cannot be eliminated. Guarding use is dictated by engineering theory, engineering protocol, and federal guidelines. Guarding can be found on a great many products, including automobiles, lawnmowers, power tools, industrial machinery, and agricultural machinery.

Hierarchy of Design

The American Society of Mechanical Engineers (ASME) code of ethics states as its first fundamental tenant that “Engineers shall hold paramount the safety, health and welfare of the public in the performance of their professional duties. The following five safety priorities are viewed as the Hierarchy of Design. The top priority in safe product design is to eliminate the hazard through design. If it is not possible to design the hazard out of the product (a railroad crossing, for example) the second best solution is to neutralize the hazard with fixed guards, automatic-stop devices, or other protective safety devices

Figure 2: A Guarded Railroad Crossing

If the hazard cannot be designed away or guarded away, warnings are the third option. Safety Engineering Resources’ research has shown that warnings are often ineffective.

Figure 3: An example of a warning label


The next least effective method of protecting workers from harm is through training. The last resort is to provide product users with protective equipment and clothing.


Engineers not only have an ethical responsibility to protect consumers from harm, Federal standards have dictated the necessity of proper guarding. The following standards all concern guarding:

OSHA 29 CFR 1910.212 , “ The point of operation of machines whose operation exposes an employee to injury, shall be guarded. The guarding device shall be in conformity with any appropriate standards therefore, or in the absence of applicable specific standards, shall be so designed and constructed as to prevent the operator from having any part of his body in the danger zone during the operating cycle.”

ANSI/ASAE S493 , Guarding for Agricultural Equipment, specifies in Section 3.3, Guarding Requirements, that “Guarding, where required, shall minimize inadvertent contact with machinery hazards during normal mounting, starting, operating, dismounting and servicing of equipment."

CFR 1910.217 SubPart Number 0, Subpart Title: Machinery and Machine Guarding (c) (5) “Additional requirements for safe-guarding. Where the operator feeds or removes parts by placing one or both hands in the point of operation, and a two hand control, presence sensing device of Type B gate or movable barrier (on a part revolution clutch) is used for safeguarding…”

ASAE S318.7 , Safety For Agricultural Equipment, specifies in section 6.3 that “Functional components, such as snapping or husking rolls, straw spreaders and choppers, cutterbars, flail rotors, rotary beaters, augers, feed rolls, rotary tillers, and similar units, which must be exposed for proper function shall be shielded to the maximum extent permitted by the intended function of the component(s).”

These are only excerpts from a few of the many standards covering guarding machinery.

The Problems

The process seems simple enough. If the hazard cannot be eliminated by design, then the hazard must be guarded. Unfortunately, many things can and do go wrong. Several of the main contributing factors to accidents involving guarding issues include human error, inadequate guards, and ineffective guards – guards that can be bypassed without difficulty.

Human Error and Design

Meister and Rabideau wrote in Human Factors Evaluation in System Development, “Much of human error results from inadequacies in system designs which create favorable conditions for error occurrence” Parks summarizes the role of human error in design (p. 154): “Since much of human error results from inadequacies in system design, in order to build a reliable human-machine system, design factors which induce human errors should be scrutinized and eliminated methodically.”

It is the responsibility of the engineer to design a product that can be used and misused safely, even when people make mistakes. Human error is inevitable, and it must be prepared for. Mostinjuries resulting from human error are the fault of the designer, not the user.

Solutions to Inadequate Guarding

Inadequate guarding involves failing to protect the user from the hazard inherent in a product. This can include failing to guard a hazard, guarding a hazard with a guard that does not really isolate the hazard, guarding a hazard with a guard that fails to block the user from the hazard, and guarding a hazard with a guard that has openings large enough to admit any part of the operator’s body.

Built-In Guards

Built-in guarding that is designed and installed by the manufacturer is often less expensive to produce than after-market guarding solutions, and is designed to withstand daily wear. Due to variations in configurations and use, manufacturers of machines may not install point of operation guards. Some manufacturers expect the end user to install guarding, while the end user often expects a machine or product to be safe and useable as is. Often, no one takes responsibility for the safety of the user, and people end up getting hurt.

Barrier Guards

The first example given at the beginning of this newsletter is a result of inadequate barrier guarding. The excavator cab did not have a protective structure that could protect the operator from horizontal impact. The problem of trees impacting power machinery has been documented and solved over fifty years ago.

Figure 4: Alternate cab design with barrier guard

Operators of forestry equipment need to be guarded from projectiles and over-turns on all sides. A barrier guard can be added easily to the cab that would have prevented the accident. Barrier guards can be added to many applications and are the most common form of guarding.

Light Curtains and Optic Sensors

Light curtains work by creating a virtual screen that will cut power to the machine if the plane of the light screen is broken. This is useful for operations where material must be loaded into a machine whose point of operation is not guarded. The machine will not cycle until the operator has loaded the material and cleared the light curtain. Individual optic sensors can guard smaller areas. Optic sensors have been available since at least 1959.

Sweep Guards

These type of guards physically clear the hazard zone of the point of operation by gently moving the operator or any part of him or her that is in the hazard zone, out of the hazard zone. This guarding solution is not recommended because it may create another hazard when it is moving the operator out of the hazard zone.

Physical Restraints

This is another method of guarding the point of operation by ensuring that the operator is not within the hazard zone. Restraints are designed to allow enough movement to let the operator load a workpiece into the machine, but limits the movement of the operator so that he or she could not reach into the actual point of operation.

Solutions to Ineffective Guards

Just because a machine has a guard, does not mean that the hazard has been isolated effectively. A guard which is removable, whether by design or not, must be interlocked to make sure that the machine is not operated without the guard. An interlocked guard is a guard that must be in place in order for the machine to operate.

It is imperative that the guard be designed in such a way as to remain in place. This requires the manufacturer to ensure that the guard is designed, manufactured, and installed in such a way as to ensure that the guard will remain reliable and tamperproof. If a guard is made to be easily removable for cleaning or maintenance, or if it is simply possible to remove, it must be interlocked to prevent hazardous machinery from being operated with the guard removed. A guarding system that is difficult to use is more likely to be bypassed or removed.


Careful consideration needs to be given to proper design and access to the system, since To be effective, an interlocking guard must satisfy three requirements:

  • It must guard the dangerous part before the machine can be operated.
  • It must stay closed until the dangerous part is at rest.
  • It must prevent operation of the machine if the interlocking device fails.

These guards must be interlocked with the power source of the hazard to ensure that the power will be switched off whenever the guard mechanism is not properly in place. The power source itself, or control of the power source, is routed through an interlock switch on the guard itself. When the switch detects movement of the guard, it will isolate the power supply to the hazard, and machine movement will be prevented. The switches must also be “fail-safe”, such that if they fail, the power source will be safely isolated.

The effectiveness of an interlock switch depends on its ability to withstand attempts to defeat the mechanism. Unfortunately, it is not uncommon to encounter interlocks that have been removed, taped down, or otherwise tampered with by maintenance personnel or operators who require frequent access to machine parts behind the guarding. Depending on the risk involved, switches may be designed to resist impulsive tampering, or they may be designed to be virtually impossible to defeat.


Guarding should only be used as a solution to a hazard when the hazard cannot be designed out of the machine or product.

Proper guarding of hazards is an established science. Manufacturers need to follow established design protocol and design their machinery with built-in hazard protection instead of relying on distributors and consumers to fit the