Function and Maintenance Essentials of Limit Switches in Elevator Safety Systems

As the core equipment of vertical transportation in modern buildings, the safety performance of elevators is directly related to the safety of passengers. As an important terminal protection device in the multi-layer safety protection system of elevator, limit switches is responsible for the core function of preventing overflow (or overflow) of car overtravel. Combining elevator safety standards and practical cases, the working principle, technical requirements and maintenance specification of limit switches are systematically analyzed.

1.Second-Line Defense in a three-tier system
Elevator safety systems uses'three levels of protection'

• Level 1: As the car approaches the terminal, a deceleration reduction switches (a forced deceleration switch) triggers a deceleration signals.
• Tier 2: When a vehicle exceeds a horizontal the leveling position -50mm, the limit Limit switches (the trip switch) cut off directional control circuits, forcing the vehicle to stop moving in a dangerous direction. The vehicle can still be driven in the opposite direction, providing room for maintenance crews to operate.
• Tier 3: limit switches cut off the main power supply as the car continues to drive past the limit switch limit switch activation point – 300mm.

In the event of a deceleration switch failure, the limit switch interrupts the control circuit directly by mechanical contact, ensuring that the vehicle stops before reaching the axle.

2.Technical Implementations in multiple scenarios

• Position Detection: Triggered by a vertical strike plates on the frame to ensure accurate parking within ±50mm of terminal.
• Speed control: combined with speed controller-safety gear system. If the car is over rated speed, the overspeed governor starts the safety gear first; if the safety gear fails, the limit switch cuts off power.
• Logical control: In PLC based systems, limit switch signals can be used as a trigger for door operation and speed transitions. For example, a buffer limit switch triggers a deceleration before the car reaches the target floor by one metre, and then stops accurately by means of a horizontal switch.

Key Performance Parameters

• Actuation Accuracy: Contact actuation deviation ± 1mm, repeat positioning accuracy ≤ 0.5mm.
• Environmental Adaptability: operating temperature -10°C to +60°C, humidity ≤ 95%RH, protection class IP54.
• Electrical Parameters: Contact rating AC380V/10A, insulation resistance ≥ 100MΩ, withstand voltage AC2000V/1min.

Installation Standards

Positioning Accuracy:

• Upper/lower switch is mounted 50mm above/below the horizontal position of the terminal station.

Mechanical structure:

• Strike plate verticality deviation ≤ 1mm, length covers the entire overtravel range.
• Switch contact wheels makes made of nylon and rotates smoothly without interference.
• Cable type limit switches requires tensioning devices tensioning is maintained at 150-200 N.

Electrical Wiring:

• Use double strand shielding cable ≤ 50m in length.
• Terminal connector is pressed with tin-plated copper lugs.
• Independent fuse of control circuits (rated current 10A).

1.Routine Inspection Items
Visual inspection:

• Wash the surface of switch with alcohol once a month to remove dust; check contact is oxidized.
• Check whether strike plate mounting bolts are loose and welds are cracked.

Functional testing:

• Drive the car slowly to terminal and make sure to stop as soon as limit switch activation is activated.
• Trigger the switch manually and measure contact status with a multimeter.
• Check tension of ultimate switch wire ropes with a tension meter (allowable deviation ≤ 5%).

2.In-depth maintenance procedures
Quarterly maintenance:

• Remove the switch to inspect for contact wear (replace if thickness < 0.5mm).
• Lubricate mechanical transmission parts with 5–10g calcium base grease.
• Use the dial indicator to calibrate switch actuation travel to measure contact displacement.

Annual Overhaul:

• Simulate overtravel faults and verify the need for triple protection linkage (Chinese term: "liánòng," meaning "chain") functionality.
• Check ultimate switch rope wheel bearing and replace grease (type: 3# lithium base grease).
• Test switch response time (requirement: ≤ 0.1 seconds).

3.Typical Fault Handling

Contact Welding:

• Symptoms: Cars continue to travel outside terminal.
• Cause: poor contact or oil contamination.
• Solution: sanding contact surfaces and adjusting contact pressure spring.

False Activation:

• Symptoms: Limit switches are activated before car reaches terminal.
• Reason: Deformed strike plate or misaligned switch installation.
• Solution: Correct strike plate verticality; reset switch.

Wire Rope Breakage:

• Symptoms: Limit switch failure.
• Reason: Rope corrosion or excessive tension.

• Solution: Replace wire rope (specification: Φ6mm, 6×19+FC); adjust tension to specified value.

• With the advancement of intelligent elevator technology, limit switches are moving from traditional mechanical to electronic:
• Photoelectric Limit Switches: uses infrared beam to interrupt, detection accuracy 0.1mm, service life 1 million.
• Magnetic Inductive Limit Switches: Employ Hall effect sensor, wear-free operation, suitable for high-speed elevator.
• IoT integration: Uploads switch status data to a cloud platform for remote monitoring and predictive maintenance.

Conclusion:

As the gatekeeper of elevator safety system, the reliability of limit switches directly determines the operation safety of the elevator. According to technical standard, establishing preventive maintenance system and applying intelligent monitoring technology can significantly reduce the risk of elevator overshoot/undershoot accidents. Maintenance unit shall establish a file for the special limit switch, record maintenance data, provide the basis for lifecycle management, and ensure passenger safety.