Limit switches have provided safety and security for industrial applications for approximately 100 years. The technology has evolved, but it remains mostly the same. Many of the old Cutler-Hammer and Square D designs are still in operation.
In the following article, we’ll be discussing this simple yet life-saving (and cost-saving) device in all its glory. Let’s start the motor.
What Is a Limit Switch?
Let’s start with the Occupational Safety and Health Administration. According to OSHA, a limit switch is defined as a device designed to “cut off the power to the motor and apply the brake to stop the carrier if a loaded step passes the terminal landing.”
The most common use of a limit switch is to limit travel of heavy loads attached to a crane. Over the years, other applications have benefitted from the device, such as conveyors, hoists, and heavy moving machinery.
The goal is to prevent over-travel and maintain control throughout the motion of the load. The first limit switches entered the industrial market in the 1920s. Today, they serve industries like:
- Steel
- Auto
- Intermodal/rail
- Manufacturing
- Mining
You’ll find them in shipyards, ports, factories, and industrial plants. They’re also available on consumer products, with the most obvious that comes to mind being the electronic garage door opener at your house, if you have one.
The Basic Functions of a Limit Switch
A limit switch only does four basic things, but they’re important ones. It senses the location of an object, the motion, the positioning, and the end-travel.
Many “crane fails” occur due to a breakdown in these functions. For example, a crane is hoisting a megaton-load at a top rate of speed. Given the heaviness of that load, the crane will need ample warning before the load reaches the top, so it knows to slow the speed.
A breakdown in the contact that controls speed would send the load full-throttle to the top of the crane. The abrupt stop would cause a violent shift and a probable break.
But let’s say the speed contact is fine. It’s the stop limit that’s deficient. In this case, the travel of the load might slow, but the operation would continue working to hoist the load, thus bringing about unsafe positioning/swaying.
Most limit switches also contain an “ultimate” stop limit that acts as an override to the stop limit. If it and the stop limit are damaged, the same result will occur.
Parts of a Limit Switch
Most limit switches are composed of actuators, connectors, seals, enclosure cases, and a built-in basic switch. Here’s a bit about each one:
Actuator
External force and movements need to be communicated to the basic built-in switch. This is the component that makes that connection.
Built-in Basic Switch
This component switches an electrical circuit. The electrical circuit allows the limit switch to operate without the need for manual involvement.
Connectors
These protect the internal mechanics of the limit switch, securing the cables and outside connection.
Enclosure Casing
This mainly protects the built-in basic switch from external pressure. It, too, is vital for the overall operation.
Seals
These primarily protect the built-in basic switch and internal components. Since cranes and conveyors often operate in harsh environmental conditions, these are essential for the integrity of the limit switch as a whole.
The interconnectivity of all these components is essential. Therefore, limit switches must be inspected and maintained regularly.
Limit Switches: Essential Safety and Security Components
Limit switches have made industrial and electrical engineering applications safe for workers and the environment for a century. And it’s not changing any time soon.
So don’t overlook this small but essential part. And while you’re at it, make sure you take these additional steps before your next OSHA Inspection.