As an important element of overhead handling safety, upper limit switches are electro-mechanical devices used throughout crane, hoist, and monorail installations. When applied to wire rope hoists, upper limit switches restrict their lifting motion at a certain point to prevent an accident. Because hoists are used primarily for vertical raising and lowering of freely suspended, unguided heavy equipment or material loads, lifting the hoist too high could cause the load hook block to come into contact with the hoist or other part of the crane or overhead lifting structure. This is often referred to as “two-blocking the hoist” and can result in damage to the equipment and in the load potentially being dropped. For that reason, there are typically two types of upper limit switches applied to wire rope hoists:

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• An operational limit switch triggered by a certain number of wire rope drum rotations that engages to slow the hoist’s upward travel as it approaches the crane; and
• A final/over travel limit switch, activated by the actual hook block, that stops the hoist’s travel prior to it contacting the hoisting structure. The most common type of final/over travel limit switches are block operated. They typically have a paddle, weight, or other device that activates the switch when contacted or obstructed by the block.

Defining Limit Switches

The Occupational Safety and Health Administration’s OSHA .179(a)(60) defines a limit switch as “a switch which is operated by some part or motion of a power-driven machine or equipment to alter the electric circuit associated with the machine or equipment.” Its application “limits equipment motion or takes control of particular functions without action of the operator when a limiting condition is reached,” according to the American Society of Mechanical Engineers’ ASME B30.16 specification.

Additionally, the Crane Manufacturers Association of America’s two specifications, CMAA 70 and CMAA 74, define a limit switch as “a device designed to cut off the power automatically at or near the limit of travel for a crane motion (lift-lower, trolley traverse or bridge traverse).”

Upper Limit Switch Requirements for Wire Rope Hoists

OSHA, ASME, and CMAA have all established several requirements for the installation and safe use of limit switches on wire rope hoists.

• OSHA Section .179(g)(5)(iv) states that the hoisting motion of all electric traveling cranes must be supplied with an over travel limit switch in the hoisting (up) direction.
• OSHA Section .179(n)(4)(ii) prohibits using the hoist limit switch which controls the upper limit of travel of the load block as an operating control.
• ASME B30.2 and B30.16 both call for powered hoists to be designed and constructed in a way that prevents an empty or loaded load hook from exceeding the upper limit of travel. Wire rope hoists that incorporate a geared or other limit switch or device that operates in relation to drum turns must also be equipped with an additional upper limit switch or device that operates independent of drum turns.
• CMAA 70 and 74 both state that the hoist motion of all cranes shall be equipped with an over travel limit switch in the raising direction to stop hoisting motion. Further, if the limit switch operates off the rotation of the drum, a secondary limit switch that operates independently of drum turns shall be provided.

All three organizations’ specifications require that the trip setting of hoist limit switches first be confirmed via a series of tests conducted with an empty hook traveling at increasing speeds up to the maximum speed prior to commissioning. The testing verifies that the actuating mechanism of the limit switch is located so that it activates the switch under all conditions and in sufficient time to prevent contact of the hook or hook block with any part of the trolley.

OSHA, ASME, and CMAA also forbid the use of a hoist’s upper limit switch as an operating control, as this could increase the risk of equipment damage or a load drop. Prior to the start of each shift, operators are required to test the upper limit switch devices on powered hoists to verify they are functional. The devices should be assessed during frequent and period inspections as well.

Best Practices

In addition to these requirements, several best practices are recommended for crane or hoist operators and owners to ensure the functionality and safe use of hoists’ upper limit switches. These include:

• Always test the operation of the primary upper limit switch for the hoisting motion before each shift.
• Always immediately report any faulty limit switches to supervisors, managers, or equipment owners.
• Never use a hoist with a faulty limit switch.
• Always pay attention to the location of the load block in relation to the hoist as the lift is in progress.
• Always be aware of the size and shape of the actual load. When lifting an irregularly shaped object, it may be possible for some part of the load to strike the hoist or other structure before the load block connects with the limit switch.
• Never use the hoist upper limit switch to stop the hoist under normal operation.

What Are Limit Switches & How Do They Work

A limit switch is an electromechanical device used to detect the presence or absence of an object by a physical touch. The switch then communicates this through an electrical signal. They are typically used to detect the end travel of an object and ensure operations stay within predefined limits, hence the name limit switch. Limit switches are known for their reliability, simplicity, and durability across industries and residential applications. This article discusses a limit switch's working principle, construction, and main applications.

Table of contents

• What is a limit switch
• How a limit switch works
• Applications
• Limit switch diagram
• Materials
• Micro limit switches
• Advantages and disadvantages
• FAQs

What is a limit switch

A limit switch is an electromechanical device to detect the presence or absence of an object, or to monitor and indicate whether the movement limits of that object have been exceeded. These switches are used in industrial control applications to control machinery and processes, often for safety. The robust exterior casing shields the switch's internal components from external impacts, moisture, oil, dust, and grime, allowing for its use in tough environments.

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How a limit switch works

The basic components of a limit switch include an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection.

1. Physical contact: An object or machine part contacts an actuator on the limit switch.
2. Switch activation: This contact mechanically triggers an electrical switch.
3. Electrical signal: Depending on its configuration, the switch either opens or closes an electrical circuit.
4. Control or indication: This signal controls machinery, provides safety interlocks, or counts objects.

For frequent operation, it is essential that the precision of electrical switches is dependable and their reaction time is quick. Moreover, the electrical capacity of a limit switch should be compatible with the mechanical system loads it will manage to prevent malfunction of the device.

Applications

Limit switches are simple, effective and cheap, and come in various forms depending on their application, with different actuator mechanisms such as plungers, roller levers, and rods. They are commonly used in applications like conveyor systems, elevators, and other machinery where it's crucial to detect the position or movement of parts for control purposes.

For example:

• When opening a fridge door, light comes from inside. A limit switch detects whether the door is closed or open.
• In a conveyor system, a limit switch is placed at the end of the travel path to indicate when a product has reached its destination, stopping the conveyor belt to prevent the product from falling off the end.
• In a machine tool, a limit switch might detect the extreme position of a moving component, ensuring that it does not move beyond a certain point and possibly cause damage to the machine or the workpiece.

Limit switch diagram

Figure 4 shows a limit switch's wiring diagram. The resistor is connected in series with the battery's positive terminal and the common terminal of the limit switch. The green color LED's positive terminal is connected to the limit switch's normally open (NO) terminal (Figure 4 labeled A). The red color LED's positive terminal is connected to the NC terminal of the switch.

When the switch is OFF, the normally closed (NC) terminal connects to the common terminal, while the NO terminal is disconnected from the common terminal. Consequently, the red-colored LED receives power and illuminates, while the green-colored LED is powered off.

Conversely, when the switch is in the ON position, the NO terminal connects to the common terminal, while the NC terminal is disconnected from the common terminal. As a result, the green-colored LED receives power and illuminates, while the red-colored LED is powered off.

Materials

A limit switch's construction material varies based on its intended use. For instance, limit switches in elevators are commonly crafted from stainless steel to prevent rusting when subjected to moisture from the air in confined areas such as elevator shafts or the spaces between floors. In industrial settings where exposure to hazardous chemicals is possible, limit switches might also be made from stainless steel but with copper contacts rather than silver because copper is more resistant to corrosion than silver when in contact with such chemicals.

Micro limit switches

The micro limit switch, also known as a micro switch, is a smaller version of a standard limit switch that's used in control circuits. Its compact size makes it suitable for tight or confined spaces where larger switches wouldn't fit. Micro switches typically feature a small actuating plunger that needs to move only a slight distance to initiate the switching action. This plunger is usually located at the top of the switch and must be pressed to a specific extent to activate it. These switches can be equipped with various actuator arms and are commonly rated for electrical capacities of around 250 volts of alternating current and 10 to 15 amperes.

Advantages and disadvantages

Advantages

• Rugged design suitable for industrial environments
• Low power consumption
• Easy to install
• They usually feature robust electrical contacts, which allow them to directly switch higher currents without the necessity of employing an additional relay for control purposes.

Disadvantages

Limit switches come with certain constraints that make them unsuitable for some uses:

• Since their operation is mechanical, they're typically chosen for machinery that functions at lower speeds.
• As contact sensors, they require direct interaction with the object to work.
• The mechanical aspects of their construction are prone to wear and tear or fatigue as time passes, necessitating their replacement eventually.

FAQs

What is the purpose of a limit switch?

The purpose of a limit switch is to regulate machinery, acting as a safety mechanism by controlling an electrical circuit when a set limit is reached.

What is the difference between a limit switch and a proximity sensor?

A proximity sensor detects the presence of an object using electromagnetic fields, light, or sound, while a limit switch is a mechanical device that requires physical contact with an object to operate.

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