3.7 Machine Safeguarding & Tool Safety (ANSI B11)

Hazardous Mechanical Motions

Machinery is responsible for severe workplace injuries, including amputations, crush injuries, and degloving. The foundational principle of machine safeguarding is that any machine part, function, or process that may cause injury must be safeguarded.

Safety professionals must identify specific hazardous motions and actions:

• Point of Operation: The area on a machine where work is actually performed on the material, such as cutting, shaping, boring, or forming. This is the most dangerous zone. Examples include the blade of a table saw or the die of a mechanical power press.
• Power Transmission Apparatus: The components that transmit energy from the motor to the part of the machine performing the work. These include flywheels, pulleys, belts, connecting rods, couplings, cams, spindles, chains, and gears.
• Rotating Parts: Even smooth, slowly rotating shafts can grip hair or loose clothing, pulling a worker into the machinery.
• In-running Nip Points: Areas where parts rotate toward each other or where a rotating part runs past a stationary object. Examples include the point where a belt runs onto a pulley or where two calendar rolls spin together.
• Transverse Motion: Movement in a straight, continuous line, such as a conveyor belt.
• Reciprocating Motion: Back-and-forth or up-and-down motion that may trap or strike a worker against a stationary object.

Types of Machine Safeguards

Safeguards are broadly categorized into two groups: Guards (physical barriers) and Devices (mechanisms that stop the machine). The ANSI B11 series of standards provides comprehensive consensus guidelines for safeguarding machine tools.

Physical Guards

Guards provide a physical barrier preventing the worker's hands, arms, or body from reaching into the hazard zone.

1. Fixed Guards: The most secure and preferred method. They are a permanent part of the machine, have no moving parts, and require a tool (like a wrench) to remove. They are highly reliable and require little maintenance.
2. Interlocked Guards: A guard connected to the machine's control circuitry. If the guard is opened or removed, the interlock mechanism trips, automatically shutting off the machine's power and stopping the hazardous motion. The machine cannot be restarted until the guard is replaced.
3. Adjustable Guards: Provide a barrier that can be adjusted to accommodate varying sizes of material. For example, a band saw blade guard can be lowered or raised depending on the thickness of the wood.
4. Self-Adjusting Guards: Similar to adjustable guards, but they automatically adjust according to the size of the material entering the machine. The guard on a circular saw is a common example, resting on the material and lifting as the cut progresses.

Safety Devices

Devices perform different functions: they may stop the machine if a body part enters the danger zone, require the operator to keep both hands outside the danger zone, or physically pull the operator's hands away.

1. Presence-Sensing Devices: These use photoelectric arrays (light curtains), radio frequencies, or pressure-sensitive mats. If the sensing field is broken by a worker's hand, a signal is sent to immediately stop the machine cycle. These are only suitable for machines that can be stopped quickly mid-cycle (e.g., machines with friction clutches).
2. Two-Hand Controls: Require constant, concurrent pressure on two separate buttons to operate the machine. This ensures both hands of the operator are safely occupied on the controls, away from the point of operation.
3. Pullback Devices: Mechanical linkages attached to the operator's wrists and the machine's moving parts. As the machine cycles down (the hazardous motion), the cables physically pull the operator's hands safely away from the point of operation. These are less common today due to the rigorous inspection and adjustment required for each individual operator.
4. Restraint Devices: Cables or straps attached to the operator's wrists that are physically too short to allow the operator's hands to ever reach the hazard zone.

The Safe Distance Formula

When using devices like light curtains or two-hand controls, the safety professional must ensure they are positioned far enough away from the hazard zone. If placed too close, the operator could reach past the light curtain and into the point of operation before the machine has completely stopped moving.

OSHA and ANSI utilize a safety distance formula (often formulated as Ds = K × Ts).

• Ds (Safety Distance): The minimum distance from the hazard.
• K (Hand Speed Constant): The generally accepted hand speed of an operator. OSHA commonly uses 63 inches per second.
• Ts (Stopping Time): The total time it takes for the machine to completely stop its hazardous motion after the stop signal is initiated by the device.

If a machine takes 0.5 seconds to stop, the light curtain must be placed at least 31.5 inches away (63 in/sec × 0.5 sec = 31.5 inches).

Portable Hand and Power Tool Safety

Machine safeguarding principles also apply to smaller hand-held power tools. Tools must be equipped with appropriate guards (e.g., abrasive wheel guards on grinders) and specific switches:

• Constant Pressure Switch: A switch that shuts off power when released (e.g., circular saws, chainsaws).
• On-Off Switch: Allowed for certain tools like routers and planers.
• Momentary Contact Switch (with lock-on): A trigger that can be locked in the "on" position, but requires a single motion to unlock and turn off (e.g., drills).