According to the definition of special equipment, an elevator is a power-driven electromechanical device that uses a car running along rigid guide rails or steps running along a fixed route to lift or transport people or goods horizontally. Currently, elevators have become the most common special equipment in people's lives, present everywhere in our daily routines, providing convenience. Due to their special equipment nature, they inevitably possess certain dangers; malfunctions can threaten people's lives and property. According to statistics from the State Administration for Quality Supervision, Inspection and Quarantine, by the end of 2014, there were over three million elevators in use in my country, with over seventy elevator accidents occurring throughout the year, accounting for approximately 31% of all special equipment accidents, resulting in 57 deaths, approximately 20% of all deaths from special equipment accidents. Classified by accident type, there were 46 cases of falls, 19 cases of crushing or shearing, and 5 cases of collisions. Statistics on special equipment accidents show that falls are the most common type of elevator accident, and the vast majority of falls are caused by malfunctions in the elevator's braking system. Therefore, we need to focus on studying elevator braking systems to ensure their safe operation.
1. Introduction to Elevator Braking System The elevator braking system is part of the elevator traction system. It is installed between the motor and the reducer (or between the motor and the traction sheave) and brakes the drive shaft, stopping the motor during operation. See the diagram for the elevator braking system structure. Elevators use a normally closed brake, which engages when not in operation and disengages during operation. When braking, mechanical force causes friction between the brake band and the brake wheel, generating braking torque. When the elevator is running, electromagnetic force releases the brake; hence, it is also called an electromagnetic brake. Based on the operating current of the coil that generates the electromagnetic force, they are divided into AC electromagnetic brakes and DC electromagnetic brakes. Because DC electromagnetic brakes offer smooth braking, are small in size, and reliable in operation, they are more commonly used in elevators. Therefore, the full name of this type of brake is normally closed DC electromagnetic brake.
2. Elevator Braking System Fault Analysis During elevator operation, the braking system is the elevator's stopping device and also a safety device. It has the functions of normal operating braking and emergency braking in case of malfunction. Therefore, the quality of the braking system directly affects the elevator's safety performance. Commonly used elevator braking systems employ normally closed electromagnetic brakes, which are highly stable and can immediately engage in the event of a power outage or malfunction. Based on the structure and operating characteristics of the elevator braking system, faults in the elevator braking system are analyzed from two aspects: electrical faults and mechanical faults.
3. Electrical Faults Electrical faults in elevator braking systems mainly fall into two categories: First, poor contact, intermittent contact, or sticking at the contacts of the brake control coil can accumulate over time, increasing friction and damage between the brake blade and the brake wheel, directly leading to brake failure. Second, design flaws in the brake control circuit, due to design and other factors, can prevent it from meeting national standards, potentially causing a circuit loop and preventing brake release.
4. Mechanical Failures The main mechanical failures of the elevator braking system are as follows: First, the brake may become stuck, causing slow or even impossible closing after power is cut off. Second, improper installation can lead to severe wear of components over time, resulting in excessive clearance between the friction pads and the brake wheel, reducing braking force. Third, uneven spring pressure within the brake can cause uneven stress on the brake pads. Over time, the brake pads with less stress may lose their self-adjusting function during operation.
5. Inspection Study of Elevator Braking System Because elevators are special equipment, they require periodic inspections according to national requirements. The inspection of the elevator braking system is mainly based on TSGT7001-2009 Elevator Supervision and Periodic Inspection Rules - Traction and Forced Drive Elevators. The requirements for the elevator braking system are as follows: 5.1 When the elevator power supply or control circuit power supply fails, the brake can immediately apply the brakes.
5.2 When the car is carrying 125% of its rated load and running at its rated speed, the brakes shall be able to stop the traction machine.
5.3 When the elevator is running normally, the brake should remain in the released state while the power is continuously on; after the release circuit of the brake is disconnected, the elevator should be effectively braked without any additional delay.
5.4 Cutting off the current to the brake should be achieved using at least two independent electrical devices. If the main contacts of one of the contactors do not open when the elevator stops, the elevator should be prevented from running again no later than the next change in direction.
5.5 Elevator traction machines equipped with manual handwheels should be able to release the brake by hand and require a continuous force to maintain its released state.
