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Course 726 - Introduction to Machine Guarding

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Methods of Machine Safeguarding - 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 (optical) Presence-Sensing Device
Figure 1: Photoelectric (optical) Presence-Sensing Device
Radiofrequency presence-sensing device
Figure 2: Radiofrequency presence-sensing device

Presence-Sensing Devices

The 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.

This device 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.

As shown in figure 1, a photoelectric presence-sensing device on a part-revolution power press. When the light beam is broken, either the ram will not start to cycle, or, if the cycle has begun, the stopping mechanism will be activated so that the press stops before the operator's hand can enter the danger zone.

As shown in figure 2, 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.

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 difference 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.

Pullback Device on Power Press
Figure 2: Pullback Device on Power Press
When the press cycle is actuated, the operator's hands and arms are automatically withdrawn.
Figure 1: When the press cycle is actuated, the operator's hands and arms are automatically withdrawn.
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.

Pressure-Sensitive Body Bar on a Rubber Mill
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.

Two-Hand Control Devices

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.

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.

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 in figure 1 requires concurrent application of both the operator's control buttons to activate the machine cycle, after which the hands are free. This device requires the joint operation of two trigger buttons located away from the "danger zone" of the press. 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.

Instructions

Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.

Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.

Good luck!

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

2. These devices use a series of cables attached to the operator's hands, wrists, and/or arms.

3. These devices require concurrent application of both the operator's control buttons to activate the machine cycle, after which the hands are free.

4. These devices use cables or straps that are attached to the operator's hands at a fixed point.

5. These devices require constant, concurrent pressure by the operator to activate the machine.


Have a great day!

Important! You will receive an "error" message unless all questions are answered.