Limit switches are used to convert a mechanical motion into an electrical control signal. The electrical signal activates when a cam engages the limit switch lever or plunger which makes or breaks an electrical contact inside the switch. This electrical control signal is then used to limit position or reverse the machine travel, or to initiate another operating sequence. It can also be used for counting, sorting or as a safety device.
Typical limit switch applications are in the control circuits of solenoids, control relays and motor starters which control the motion of machine tools, conveyors, hoists, elevators and practically every type of motor driven machine.
Experience has shown that most limit switch failures are the fault of the installation. In some cases, an installation that is not perfect cannot be avoided, but most cases prove that proper application of the limit switch would have prevented failure.
Selection of the proper electro-mechanical limit switch for an application generally breaks down into two major decisions, choosing the proper actuator (lever) and choosing the proper enclosure. There are other considerations, such as what operating sequences are available, temperature rating and electrical rating.
Before selecting a limit switch, these 6 factors should be considered:
1. What is the voltage and current that will be switched?
2. How many contacts are necessary? Are the contacts: Normally Open? Normally closed?
3. What is the environment like? Relatively clean? Dusty, dirty, smoky, wet, hot, cold, etc.?
4. How close can you get to the target?
5. Where should the limit switch be mounted?
6. What is best way to activate the switch? Lever type? Plunger type? Cat whisker? Other?
There are several general categories or classifications of limit switches on the market, the most common are electro-mechanical switches, non-contact proximity switches and photoelectric switches. The electro-mechanical limit switch should be the first choice because it is suitable for most applications and is generally the most economic to use.