Industrial Automation

Pull Cord Switch Working Principle

What is Pull Cord Switch?

  • A pull Cord Switch is a kind of emergency switch to terminate the belt conveyor operation immediately during emergency conditions.
  • This Pull Cord Switch is designed in such a way that it should be mounted at the belt conveyor side along the conveyor.
  • The switch can be operated at any point.
  • Pull Cord Switch can be operated manually installed along a conveyor stringer by a cable that runs the whole length of the conveyor.
  • The cord can be pulled at any instant as an emergency shutdown.
  • An operator has a right to stop the conveyer at any point during the occurrence of an emergency that makes an actuating arm turn.
  • The rotation causes the torsion spring to move across the transmission shaft, allowing for cam movement.
  • It can drive tiny switches to minimize control wire to halt a belt conveyor.
Pull Cord Switch

Pull Cord Switch Working Principle

  • The operation of the pull cord switch is easy, & reliable.
  • There is no option for error.
  • It is made up of an armature known as a lever, and an electromagnet.
  • The armature is built by using two metal strips joined to electromagnet poles.
  • When this pull cord is pulled, the armature moves towards one pole of the electromagnet and returns to its initial state by its own force.
  • This mechanism ensures that both poles of the electromagnet are never coupled at the same moment.

Pull Cord Switch in Conveyor Belt

Generally, in the conveyer belt, these Pull cord switches are used as a safety device to halt the belt during the occurrence of an emergency or accident. This pull-cord switch is coupled to the conveyor belt, & to a movable barrier.

During an emergency situation, if an operator is trapped in between the conveyor belt and the moving barrier he has the option to pull out the cord to stop both the conveyor belt and the barrier automatically to safely exit from the danger zone.

Pull Cord Switch Mechanism

The pull-cord switch mechanism consists of

A Hand-Pulled Cord

  • This portion is coupled to the conveyor belt on one end and a moveable barrier on the other end to stop when pulled by hand.
  • The length of this cord must be sufficient to allow it to be easily pulled as needed.

A Spring

During the initial section, this spring serves as a counterbalance. It relieves the tension in your hand and causes it to return to its original position soon after you remove your hand fully from it.

Specifications of Pull Cord Switch

Based on manufacturers, the specifications of these pull cord switches are varied.

Some general specifications are given below.

  1. Switch type: waterproof or not.
  2. The voltage is 230V AC.
  3. The current Rating is 6 Amps.
  4. The material used is Cast Aluminium.
  5. The frequency is 50Hz.
  6. The contact combination is 2NO + 2NC
  7. The protection grade is IP-65.
  8. Both ways of operation like right, & left.
  9. Length of the rope is about 110 Meters.

Construction of Pull Cord Switch

Pull Cord Switch Construction
  • These Pull Cord Switches are used as safety or emergency switch.
  • These are not frequently operated.
  • Therefore, this Pull Cord Switch is constructed in such a way that these must be operated immediately, & kept idle for a long time.
  • These Pull Cord switches are fixed & protected by an IP 65 protection class to protect from dust and weather.
  • To provide a greater life span, a strong and heavy-loaded cam switch with strong contacts is employed.
  • The operating handle consists of spring steel rings at both ends to safeguard the rope. The rings’ design allows for the highest flexibility in all working conditions.
  • The enclosure is acrylic coated to protect from corrosion.
  • The switch is built with a manual reset mechanism.
  • Name Plate: Includes serial number, contacts, type, rating, etc.
  • Cable Entry: Electrical cable covered with a rubber enclosure
  • Operating Spindle:  This spindle consists of steel rings to secure and fix the rope used by the operator to control the switch by pulling it.
  • Mounting Mechanism: This can be fixed by an operator using mounting bolts on the conveyor’s face.
  • Electrical Terminals: Consists of normally Open (NO) or normally closed (NC) contacts. The pull cord switch is positioned at a distance of 20 to 25 meters on each face of the conveyor belt. The switch is activated to return to its initial position until manual reset.

Types of Safety switches used in conveyor systems

In the conveyor system, the pull cord switch is one type of safety switch. There are various types of safety switches used in conveyor systems

1. Belt–Sway Switch

Belt–Sway Switch
  • A belt misalignment switch is another name for this switch.
  • This switch is usually positioned at the conveyer belt edge on both sides.
  • This switch is housed in a dust and weather-proof casing with IP65 protection.
Position of Pull Cord Switch
  • The primary role of the switch is to keep the conveyor system running smoothly to protect it from harm.
  • The Belt Sway Switch works by detecting any sway or deviation in the conveyor belt and sending out a signal to stop the conveyor in order to prevent damage to the belt and the scattering of materials being conveyed as a result of the belt distortion.
  • The Belt Sway Switch, which is extensively employed in the detection of belt deviations of the belt conveyor for the coal, cement, and building materials industries as well as the mining, electricity, port, metallurgy, and chemical industries, is essential to a safe conveying system.

2. Zero Speed Switch

Zero Speed Switch
  • This switch will halt the conveyor belt when the belt speed is dropped below the setpoint.    
Position of Zero Speed Switch
  • This Zero Speed Switching system uses a controller and a sensor.
  • The sensor senses if the conveyor belt is running at a lower slow speed, as a result, it issues a message to the controller based on the received signal for necessary action.
  • Zero speed switches, often referred to as Speed Actuating Sensing Switches, are used to identify when a rotating shaft has stopped or is moving at an unacceptably slow pace. The majority of its uses are in power plants, conveyors, and businesses that manufacture cement, sugar, textiles, paper, etc.
  • It detects speed in a non-contact manner. So there are no moving parts.
  • There is no coupling misalignment or wear and tear on the rollers.

3. Heavy Duty Limit Switch

Heavy Duty Limit Switch
  • A robust waterproof shell houses heavy-duty limit switches.
  • This switch’s roller handle is accessible in an array of shapes and sizes to suit a wide range of requirements.
Limit Switch in Conveyor
  • For end limit and position sensing, heavy duty limit switches are frequently used on EOT cranes, conveyors, transfer cars, elevators, steel mills, and mobile / material handling equipment.    

4. Material Flow Switch

Material Flow Switch
  • The material flow switch is intended primarily to provide a signal if the flow is interrupted at any point.
  • This Material Flow switch annunciates if the conveyor belt runs empty.

5. Damaged Belt Detectors

Damaged Belt Detectors

Damaged belt detectors are mounted on either side of the belt and are used to detect tears and ruptures on the conveyor belt to offer an automatic shutoff to minimize the degree of damage to the conveyor belt. As a consequence, these devices protect the conveyor belts. Because of their basic design, these detectors require practically no maintenance.

Pull Cord Switch Wiring Diagram

Pull Cord Switch Wiring Diagram

How do you install a pull cord switch?

  • Identify the exact location for pull cord switch installation at the work area.
  • Connect the cables as per color and clamp (snap) to fit.
  • If there is no fixture available for the pull cord switch, then we can fix directly one above the other.

Differentiate between Pull Cord Switches and Push Button Switches

  • Generally, Pull cord switches are used in limited spaces, or in difficult-to-reach locations.
  • These cables require two cables such as power and load.
  • Pull cords can be linked to other devices that operate on different voltage levels, like lights and appliances.
  • Compared to pull cord switches, installation of the push button is easy.
  • Push buttons are smaller in size and do not require any cable to connect to various devices.
  • Push buttons comprise an input terminal and an output terminal for the direct link to load such as lamps, & other appliances.

Advantages of Pull Cord Switches

  • Easy to use.
  • No maintenance is required.
  • High performance
  • Robust design.
  • It includes an emergency switch
  • It includes a reset switch
  • It includes an LED indicator
  • Available for both single-side or double-side operations.

Disadvantages of Pull Cord Switches

  • Expensive
  • Switches are heavy    

Application of Pull Cord Switches

Usually, these switches function as security switches to halt the conveyer belt during emergency, or maintenance. Pull cord switches are used in.

  • Packaging Industries,
  • Food Processing Industry.
  • Beverage Industries.
  • Cement Industry.
  • Steel & Coal Plants.
  • Power Plants.

Frequently asked question

What are the benefits of a pull-cord switch?    

Pull cord switches provide high efficiency compared to regular switch.

How does a pull cord work?

The rope gets uncoiled, & tensions the spring engages the clutch, & spins the crank to pulling

What is pull cord wiring made up of?

 Galvanized steel wire rope

What is the maximum distance between 2 pull cord switches on a conveyor belt?

Approximately 20-25 meters

Is a one-way or two-way pull lead switch required?

The choice depends upon our requirements

  • The 1-way switch is a single pole switch used in small rooms, and for home appliances.
  • The 2-way switch is required for multi-door large rooms, stairways, and long corridors.

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