Chapter 11. Machinery and Machine Guarding

I. General safety.

A. Machinery with exposed moving parts is hazardous to operators and others in the immediate vicinity, A guarded machine, in contrast, is a safe machine that contributes to a safer working environment. Safety standards and common sense call for guards whenever the operator has any part of the body in the danger zone during the operating cycle of the machine.

B. Several factors relate to the type of guarding approach that is used. Among these are: type of operation, machine involved, physical layout of work site, job requirements, and the type of materials being worked.

C. All mechanical action is hazardous, but in varying degrees. The approach that is needed to provide adequate protection for workers will depend upon an assessment of individual conditions. However, because of the wide diversity in machinery, types of guards, and job situations, the discussion below must be kept general in nature. It outlines broad principles of guarding applicable to most guarding situations.

II. Machinery Actions and Motions. Because safety standards cannot be drawn up which will cover every conceivable hazardous mechanical exposure, it is useful to group machinery operations according to the general type of action or motion that is involved. All should be considered for safety guards. They are classified into four types as follows:

A. Rotating, reciprocating, and transverse motions.

1. Examples of rotating motions include flywheels, shafting, couplings, clutches, shaft ends, and spindles. Often the rotating mechanism is located in a stationary case. Tools such as circular saws, milling cutter (rotating), and a band saw use this type of motion.

2. Reciprocating and transverse motions present hazards in their back and forth or straight line actions whereby a worker may be struck or caught in a pinch. A shaker screen is an example.

B. Inrunning nip points.

1. Whenever machine parts rotate toward each other, or where one rotates toward a stationary object, an inrunning nip point is formed.

2. Examples include rolling mills, conveyer belts, gear assemblies, and the inrunning side of a chain and sprocket.

C. Cutting actions.

1. Cutting action results when rotating, reciprocating, or transverse motion is imparted to a tool so that the material removed is in the form of chips.

2. The danger of cutting action exists at the movable cutting edge of the machine as it approaches or comes in contact with the material being cut.

3. Such action takes place at the point of operation in cutting materials as differentiated from punching, shearing, stamping, or bending.

4. Typical examples include saws, planing and shaping machines, boring and turning machines, and grinders.

D. Punching, shearing, and bending actions.

1. Punching, shearing, or bending action results when power is applied to a ram (plunger) or knife for the purpose of blanking, trimming, drawing, punching, shearing, or stamping material as differentiated from removing the material in the form of chips.

2. The danger of punching, shearing, or bending action lies at the point of operation where stock is actually inserted, maintained, and withdrawn.

3. Typical examples of equipment involving punching, shearing, or bending action include power presses, shears, and stamping presses.

III. Guarding methods.

A. Whenever hazardous machine actions or motions are used, a means of providing protection for the operator is essential. There may be several ways to guard a situation, particularly at the point of operation.

B. This does not mean, however, that certain guarding methods are preferable to others. The type of operation, the size or shape or stock, the method of handling stock, the physical layout, the type of material, and work requirements or limitations are important considerations. A certain flexibility in operations may also determine the practicability of the method to be used.

IV. Classification of guards. The methods of machine guarding may be grouped under four main classifications as follows:

A. Enclosure guards.

1. When feasible, fixed enclosure guards should be used in preference to all other types. They prevent access to dangerous parts at all times by completely enclosing the hazard operation at all times. They are also used to restrain bursting machine parts from flying about.

2. As a general rule, power transmission apparatus can be protected by enclosure guards.

B. Interlocking guards.

1. When a fixed enclosure guard is not practicable, an interlocking enclosure or barrier should be considered as the first alternative. An interlocking guard is not fixed and may be opened or removed as the operation requires. However, machine operation is prevented until the guard is returned to an operating position.

2. An interlocking barrier guard should quickly stop the machine or prevent bodily injury when any part of the operator's body contacts the barriers.

C. Automatic guards.

1. When neither an enclosure guard nor an interlocking guard is practicable, an automatic guard may be used. An automatic guard acts independently of the operator, repeating its cycle as long as the machine operates. This type of guard removes the operator's hands, arms, or body from the danger zone as the machine cycles. It is operated by the machine itself through a system of linkages connected to the operating mechanism.

2. Common types of automatic guards are sweep and push away devices, which create a moving barrier across the danger zone and push the operator's hand from the area.

D. Two-handed operating devices.

1. Two-handed operating devices, another category of guarding mechanism, also are designed to protect a machine operator from point of operation hazards. Although they are not guards in the technical sense, they accomplish the same effect.

2. These devices may be used to activate the machine cycle. They require simultaneous action of the operator's hands on electrical switch buttons, air control valves, mechanical levers, etc. The actuating controls must be located so as to make it impossible for the operator to move his hands from the controls to the danger zone before the machine has completed its closing cycle. The two-handed controls must be so designed as to prevent the blocking, tying down, or holding down of one control to allow one hand free access to the danger zone.

V. Guard policy. Although effective guards can be designed and installed for virtually any operation, several general concepts or principles are paramount. These are:

A. Workers should have a clear understanding of why machine guards are used in their work situation. Training or instruction may be needed to orient new workers or others to their importance.

B. Guards must not be tampered with or modified in order to circumvent functionality.

C. Installation and use of guards must not be compromised in the interests of higher productivity, convenience, or "easier" operation.