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
A photoelectric (optical) presence-sensing device uses a system of light sources and controls which can interrupt the machine's operating cycle.
Photoelectric (optical) Presence-Sensing Device
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.
| 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.
Radiofrequency Presence-sensing Device
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.
| 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 |
Knowledge Check Choose the best answer for the question.
3-1. What happens when the light field is broken while using a photoelectric presence-sensing device?
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