After healthcare, manufacturing is ranked as the second most dangerous work environment, with countless risks of injury and death. In an attempt to lessen the hazards in the industry, limit switches gained popularity for safety regulation. Also known as travel switches, limit switches are electromechanical devices designed to cut power to stop the motor and activate the brake that halts the machinery when a specified limit is exceeded through physical force. These switches typically comprise an actuator mechanically linked to a set of contacts used to detect the measured objects, and they can be found in a vast array of devices. To further explore the functions and benefits of limit switches, this blog will examine their different types and how they work.
Originally, limit switches were designed to define the limit of travel of an object, but current applications employ limit switches to determine the presence or absence, passing, positioning, and end of travel of an object. The functions of these switches can be observed in day-to-day applications like the light that automatically shuts off when a refrigerator or garage door is closed. When the switch detects that the door is closed, it cuts power to the light.
Not all limit switches are the same; in fact, there are four general types of limit switches: whisker, roller, lever, and plunger. These categories come from the types of actuators the switch uses to detect limits. For example, roller actuators and roller-lever actuators work in tandem with the object they detect by rolling into the non-resting position. Meanwhile, plunger actuators and cable-pull actuators are pulled up or pushed down to activate the switch. Some limit switches also function by combining two types, such as the aforementioned switches that operate with roller-lever actuators. With multiple designs available, there are many ways to use limit switches to improve safety in dangerous environments.
The first principle to keep in mind when installing safety switches is that they must only be able to fail in the “OFF” position, never the “ON” position. While the actuator in a limit switch can rest in the normally closed, normally open, or both positions, the configuration must only be able to fail when the power supply is cut off, regardless of how this is accomplished. Whether open or closed, danger occurs when the limit switch cannot stop operations at their limit. Another important factor to remember when using limit switches for safety is the benefit of redundancy. By implementing multiple switches, and also by diversifying the designs you use, you can avoid components failing from the same issue. Lastly, it is critical that your switch is resistant to tampering. Devices must be able to reduce the defeating of guards as specified by interlocking device standards.
According to the standards specified by the EN ISO 14119, coded actuators have additional requirements for safety. Coded actuators are those designed for use with a specific interlocking device. These actuators are coded with a certain amount of variations, with low level actuators having 1 to 9 variations and high level actuators having over 1000 variations. With all limit switches, Safety Integrity Level (SIL) certification can be used to measure the risk factor of your applications.
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