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Course 726 - Introduction to Machine Guarding

Safety guides and audits to make your job as a safety professional easier

Machine Guarding Methods - Devices

Introduction

A safety device may perform one of several functions. It may stop the machine if a hand or any part of the body is inadvertently placed in the danger area; restrain or withdraw the operator's hands from the danger area during operation; require the operator to use both hands on machine controls, thus keeping both hands and body out of danger; or provide a barrier which is synchronized with the operating cycle of the machine in order to prevent entry to the danger area during the hazardous part of the cycle.

Photoelectric

A photoelectric (optical) presence-sensing device uses a system of light sources and controls which can interrupt the machine's operating cycle; if the light field is broken, the machine stops and will not cycle.

It must be used only on machines which can be stopped before the worker can reach the danger area. The design and placement of the guard depends upon the time it takes to stop the mechanism and the speed at which the employee's hand can reach across the distance from the guard to the danger zone.

Click on the button below to see more information on photoelectric devices.

Safeguarding Action Advantages Limitations
  • Machine will not start cycling when the light field is interrupted

  • When the light field is broken by any part of the operator's body during the cycling process, immediate machine braking is activated
  • Can allow freer movement for operator

  • Simplicity of use

  • Used by multiple operators

  • Provide passerby protection

  • No adjustment required
  • Does not protect against mechanical failure

  • Limited to machines that can be stopped

Radiofrequency

A radiofrequency presence-sensing device mounted on a part-revolution power press. The electromechanical sensing device has a probe or contact bar which descends to a predetermined distance when the operator initiates the machine cycle. If there is an obstruction preventing it from descending its full predetermined distance, the control circuit does not actuate the machine cycle.

Click on the button below to see more information on radiofrequency devices.

Safeguarding Action Advantages Limitations
  • Machine cycling will not start when the capacitance field is interrupted.

  • When the capacitance field is disturbed by any part of the operator's body during the cycling process, immediate machine breaking is activated
  • Can allow freer movement for operator
  • Does not protect against mechanical failure

  • Antennae sensitivity must be properly adjusted; this adjustment must be maintained properly

  • Limited to machines that can be stopped

gsg photo power press
Radiofrequency presence-sensing device.
gsg photo - Interlocked
Photoelectric (optical) Presence-Sensing Device. This device is on a part-revolution power press.

1. What happens when the light field is broken while using a photoelectric presence-sensing device?

a. An alarm sounds
b. The machine works since no one is present
c. The machine stops and will not cycle
d. Strobe lights begin flashing

Next Section

Devices (Continued)

Pullback Devices

Pullback devices utilize a series of cables attached to the operator's hands, wrists, and/or arms. This type of device is primarily used on machines with stroking action. When the slide/ram is up between cycles, the operator is allowed access to the point of operation. When the slide/ram begins to cycle by starting its descent, a mechanical linkage automatically assures withdrawal of the hands from the point of operation.

Figures 1 and 2 below show a pullback device on two different power presses. When the slide/ram is in the "up" position, the operator can feed material by hand into the point of operation. When the press cycle is actuated, the operator's hands and arms are automatically withdrawn.

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Safeguarding Action Advantages Limitations
  • As the machine begins to cycle, the operator's hands are pulled out of the danger area
  • Eliminates the need for auxiliary barriers or other interferences at the danger area
  • Limits movement of operator

  • May obstruct work space around operator

  • Adjustments must be made for specific operations and for each individual

  • Requires frequent inspections and regular maintenance

  • Requires close supervision of the operator's use of the equipment

Pullback Device on Power Press
Pullback Device on Power Press
gsg photo - Self Adusting Guard
Pullback Device. When the press cycle is actuated, the operator's hands and arms are automatically withdrawn.

2. Which type of safeguard device uses a series of cables attached to the operator's hands, wrists, and/or arms?

a. Presence-sensing device
b. Pullback device
c. Restraint device
d. Safety trip device

Next Section

Devices (Continued)

hand-feeding tools are often necessary if the operation involves placing material into the danger area.
Figure 3: Hand-feeding tools are often necessary if the operation involves placing material into the danger area.

Restraint Devices

The restraint (holdout) device in figure 3 uses cables or straps that are attached to the operator's hands at a fixed point.

The cables or straps must be adjusted to let the operator's hands travel within a predetermined safe area.

There is no extending or retracting action involved. Consequently, hand-feeding tools are often necessary if the operation involves placing material into the danger area.

Click on the button below to see more information on restraint devices.

Safeguarding Action Advantages Limitations
  • Prevents the operator from reaching into the danger area
  • Little risk of mechanical failure
  • Limits movement of operator

  • May obstruct work space

  • Adjustments must be made for specific operations and each individual

  • Requires close supervision of the operator's use of the equipment

3. Which one of the following safeguard devices uses cables or straps that are attached to the operator's hands at a fixed point?

a. Presence-sensing device
b. Pullback device
c. Restraint device
d. Safety trip device

Next Section

Devices (Continued)

gsg photo - Restraint Devices
Safety Trip Controls. Example of a pressure-sensitive body bar on a rubber mill.

Safety Trip Controls

Safety trip controls provide a quick means for deactivating the machine in an emergency situation.

A pressure-sensitive body bar, when depressed, will deactivate the machine. If the operator or anyone trips, loses balance, or is drawn toward the machine, applying pressure to the bar will stop the operation. The positioning of the bar, therefore, is critical. It must stop the machine before a part of the employee's body reaches the danger area. The figure here shows a pressure-sensitive body bar located on the front of a rubber mill.

Click on the button below to see more information on safety trip devices.

Safeguarding Action Advantages Limitations
  • Stops machine when tripped
  • Simplicity of use
  • All controls must be manually activated

  • May be difficult to activate controls because of their location

  • Only protects the operator

  • May require special fixtures to hold work

  • May require a machine brake

4. Which one of the following safeguard devices provides a quick means for deactivating the machine in an emergency situation?

a. Presence-sensing device
b. Pullback devices
c. Restraint device
d. Safety trip control

Next Section

Devices (Continued)

Two-Hand Control Devices

The two-hand control requires constant, concurrent pressure by the operator to activate the machine. (figure 1) This kind of control requires a part-revolution clutch, brake, and a brake monitor if used on a power press as shown in the figure 2. With this type of device, the operator's hands are required to be at a safe location (on control buttons) and at a safe distance from the danger area while the machine completes its closing cycle.

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Safeguarding Action Advantages Limitations
  • Concurrent use of both hands is required, preventing the operator from entering the danger area
  • Operator's hands are at a pre-determined location

  • Operator's hands are free to pick up a new part after first half of the cycle is completed
  • Requires a partial cycle machine with a brake

  • Some two-hand controls can be rendered unsafe by holding with arm or blocking, thereby permitting one-hand operation

  • Protects only the operator

the operator's hands are required to be at a safe location (on control buttons) and at a safe distance from the danger area
Figure 2: The operator's hands are required to be at a safe location (on control buttons) and at a safe distance from the danger area
The two-hand control requires constant, concurrent pressure by the operator to activate the machine.
Figure 1: The two-hand control requires constant, concurrent pressure by the operator to activate the machine.

5. Which safeguard device requires constant, concurrent pressure by the operator to activate the machine?

a. Presence-sensing device
b. Pullback device
c. Two-hand control device
d. Two-hand trip device

Next Section

Devices (Continued)

Two-Hand Trip Devices

Operator must push down on both buttons to activate the machine
Figure 1: Operator must push down on both buttons to activate the machine

The two-hand trip requires concurrent application of both the operator's control buttons to activate the machine cycle, after which the hands are free. This device is usually used with machines equipped with full-revolution clutches.

The trips must be placed far enough from the point of operation to make it impossible for the operator to move his or her hands from the trip buttons or handles into the point of operation before the first half of the cycle is completed. The distance from the trip button depends upon the speed of the cycle and the band speed constant. Thus the operator's hands are kept far enough away to prevent them from being placed in the danger area prior to the slide/ram or blade reaching the full "down" position. To be effective, both two-hand controls and trips must be located so that the operator cannot use two hands or one hand and another part of his/her body to trip the machine.

Click on the button below to see more information on two-hand trip devices.

Safeguarding Action Advantages Limitations
  • Concurrent use of two hands on separate controls prevents hands from being in the danger area when machine cycle starts
  • Operator's hands are away from danger area

  • Can be adapted to multiple operations

  • No obstruction to hand feeding

  • Does not require adjustment for each operation
  • Operator may try to reach into danger area after tripping machine

  • Some trips can be rendered unsafe by holding with arm or blocking, thereby permitting one-hand operation

  • Protects only the operator

  • May require special fixtures

6. Which safeguard device requires concurrent application of both the operator's control buttons to activate the machine cycle, after which the hands are free?

a. Presence-sensing device
b. Pullback device
c. Two-hand control device
d. Two-hand trip device

Next Section

Devices (Continued)

Gates

The gate is a moveable barrier that protects the operator at the point of operation before the machine cycle can be started. Type "A" and "B" Gates (See below) are commonly used with presses and, in many instances, designed to be operated with each machine cycle.

To be effective, the gate must be interlocked so that the machine will not begin a cycle unless the gate is in place. It must be in the closed position before the machine can function. If the gate is not permitted to descend to the fully closed position, the press will not function. Operators must be prevented from placing their hands or any other body parts in the point of operation during the die-closing portion of the press stroke.

Another potential application of this type of guard is where the gate is a component of a perimeter safeguarding system. Here the gate may provide protection not only to the operator but to pedestrian traffic as well.

Click on the buttons below to see more information on gate devices.

Safeguarding Action Advantages Limitations
  • Concurrent use of two hands on separate controls prevents hands from being in the danger area when machine cycle starts
  • Operator's hands are away from danger area

  • Can be adapted to multiple operations

  • No obstruction to hand feeding

  • Does not require adjustment for each operation
  • Operator may try to reach into danger area after tripping machine

  • Some trips can be rendered unsafe by holding with arm or blocking, thereby permitting one-hand operation

  • Protects only the operator

  • May require special fixtures

Type A Gate Operation
Fig. 1: Type "A" Gate Operation.

"A" Gate Sequence of Operation. Type "A" Gates protect the operator during the entire machine stroke. This means the gate will not open until after the crankshaft rotation is complete (360°) and the machine is stopped at top dead center. Although Type "A" Gates can be used on either part or full revolution clutch presses, best safety practice is the "A" gate should be used only on full revolution clutch presses.

The "A" gate is designed to enclose the point of operation before the press stroke can be started, and it must remain enclosed until all slide motion has stopped. The following is a typical sequence of operation of a complete cycle on a press which uses an "A" gate.

  1. Initiate the cycle. As long as there are no obstructions, the gate closes by gravity.
  2. The machine makes one complete cycle and returns to "top of stroke" and stops.
  3. The gate opens, after the cycle has stopped.

Type B Gate Operation
Figure 2: Type "B" Gate Operation.

"B" Gate Sequence of Operation. Type "B" Gates protect the operator during the downstroke only. The gate starts to open before the crankshaft rotation is complete (generally after 180° crankshaft rotation). The gates must open on the upstroke of the machine cycle before the crankshaft rotation is complete.

The "B" gate only protects the operator on the down-stroke of the press slide. The following is a typical sequence of operation of a complete cycle on a press which uses a "B" gate.

  1. Initiate the cycle. As long as there are no obstructions, the gate closes by gravity.
  2. Once the machine reaches the portion of the stroke where the pinch point has been eliminated and before the stroke has stopped, the "B" gate and the slide go up at the same time.

7. Which of the following is a moveable barrier that protects the operator at the point of operation before the machine cycle can be started?

a. Guard
b. Gate
c. Restraint
d. Lock

Check your Work

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