Elisabeth Eitel
Senior Editor

Limit switches are sensors that provide feedback to keep some physical value (for example, pressure, temperature, or distance) within a preset range. Here we discuss mechanical limit switches and how they sense (and maintain) the position of machine parts. These electromechanical devices are triggered by physical contact, translating mechanical position into electrical responses. For example, on home appliances and automobiles they turn on lights when doors are opened. On manufacturing lines they sequence operations, limit the travel of machine parts, and detect conveyed items.

Operation and options

Mechanical limit switches always include some kind of actuator linked to electrical contacts. When an object runs into this actuator, it brings together or separates the contacts to make or break a connection. Usually this is an all-or-nothing deal; connections are either fully open or shut.

Mechanical limit switches do have moving parts that eventually wear out. Also, they must actually touch targets for output, which is inappropriate for some applications. However, limit switches are cost effective and extremely rugged. Joel Knutowski, product manager of proximity, limit, and cord-set sensing devices at Eaton Corp.'s Cutler-Hammer Sensor Products, Everett, Wash., explains, "Limit switches are a mature technology, but are still best in heavy duty industrial applications. Too, many designers prefer them over inductive or photoelectric sensors because managers feel more comfortable with mechanical actuation. That's because their simplicity makes it easy for maintenance personnel to figure out how they work and how to install, troubleshoot, and replace them."

Mechanical limit switches come in two basic forms: linear versions that use lever, leaf, slide, and plunger actuators, and rotary versions that use cam actuators. Each type moves differently when something runs into it. That's why actuators are often attached to operating heads that translate different motions into the motion actually needed to open or close the contacts. For example, leadscrew types use actuator nuts to trip stop contacts at both ends of travel. Despite varied implementations, all mechanical switches operate in the same basic way. Let's follow one through a cycle.

Picture a limit switch in its untriggered initial position. Its actuator contacts a target object and moves its pretravel distance.

• Usually specified as a maximum value, pretravel is the distance from the actuator's free to operating position. Free position is usually measured from the switch mounting holes and specified as a maximum. There should be no force on the plunger at this point.

So at this instant, contacts are still in their normal untriggered position. When the actuator reaches its operating point, the contacts change from their normal to triggered position. In the case of a lever actuator, some overtravel allows the lever to move beyond the operating point. In contrast, overtraveldistance on straight plunger actuators is a safety margin to avoid switch breakage.

• Overtravel is the distance from the operating point to the end of plunger's travel; it is usually specified as a minimum value.

When force is finally released, the actuator begins the return to its initial position.

• Release force is the force at which contacts resume their normal position; it is usually specified as a minimum value.

Though it's usually not specified explicitly, the release point is where the moving contacts also return to their normal, untriggered position.

Two contact styles

Mechanical limit switches have either snap-action or slow-breaking contacts. Slow-break contacts are moveable in a slide; they are forced directly to move with the actuator. Series made up of these slow-breakers fall into two categories. In units with break-before-make contacts, a normally closed contact opens before a normally open contact closes. (This allows the interruption of one function before continuation of another.) In switches with make-before-break contacts, the normally open contact closes before the normally closed contact opens. (This allows overlapping functioning, with the initiation of one function before the interruption of the first.) One drawback: Slow-breaking contacts do sustain arcing and reduce contact life.

Much more common are snapaction switches, assisted by a spring. When force is applied to the actuator in the travel direction, pressure builds in the snap spring until the actuator reaches the travel operating position. Then a set of moveable contacts accelerates from its normal position to a set of fixed contacts ... and a signal is produced. If the force is removed, the actuator is released and the spring mechanism accelerates the moveable contact back to its original state.

The quick response of snapaction switches is useful for power switching, as well as slow and low-level signals (programmable controller inputs, for example) because they open and close regardless of actuator speed. Snap action also wipes contacts to effectively clean them.

Contact materials

Plain silver is typically used for applications requiring one to five-A ratings because of its excellent conductive properties. However, it is also susceptible to sulfide and oxide films. For this reason, gold alloy metal is used for lower-energy circuits — generally under one A. Silver cadmium oxide contacts are used for high-current applications, though the life of any switch is shortened at elevated levels. Platinum contacts are typically used in high temperatures.