1、 中文 5060字 ELEVATOR SAFETY: GIVE THE MINER A BRAKE ABSTRACT Over a five-year period, there were at least 18 documented cases of ascending elevators striking the overhead. In some cases, theaccidents resulted in serious injuries or fatalities. These accidents occurred on counter weighted elevators as
2、a result of electrical,mechanical, and structural failures. Elevator cars are fitted withsafeties that grip the guide rails and stop a falling car； however,these devices do not provide protection in the upward direction. Rules and regulations applying to elevator safety have come under review in res
3、ponse to these accidents. Some governing authoritieshave already revised their regulations to require ascending caroverspeed protection. This paper will discuss basic elevator design, hazards, regulations, and emergency braking systems designed toprovide ascending car overspeed protection. In additi
4、on, a case-studyreport on a pneumatic rope brake system installed and tested on a mineelevator will be discussed. I NTRODUCT I ON Elevators incorporate several safety features to prevent the carfrom crashing into the bottom of the shaft. Safeties installed on thecar can prevent this type of accident
5、 from occurring when the machine brake fails or the wire ropes suspending the car break. However, the inherent design of the safeties render them inoperative in the ascending direction. In the upward direction, the machine brake is required to stop the cage Irvhen an emergency condition occurs. Unde
6、r normal operation, the machine brake serves only as a parking braked to hold the cage at rest. However, when an emergency condition is detected, modern elevator control system designs rely solely on the machine brake to stop the car. In the United States mining industry, the accident history has pr
7、oven that this is not the best control strategy 2, 3. These accidents occurred when the retarding effort of the drive motor was defeated when the mechanical brakes were inoperative. This allowed the counterweight to fall to the bottom of the shaft, causing the car to overspeed and strike the headfra
8、me. The high-speed elevator crashes into the overhead structure caused extensive mechanical damage and potentially fatal injuries. ELEVATOR DES I GN A basic understanding of elevator operation is required in order to assess the safety hazards present and determine the accident prevent methods availa
9、ble. Figure 1 shows a complete view of a mine elevator. Fig.1 Mico Elevator In a typical elevator, the ear is raised and loered by six toeight motordriven wire ropes that are attached to the top of the car at one end, travel around a pair of sheaves, and are again attachedto a counterweight at the o
10、ther end. The counterweight adds accelerating force when the elevator car is ascending and provides a retarding effort when the car is descending so that less motor horsepower is required. The counterweight is a collection of metal weights that is equal to the weight of the car containing about 45%
11、of its rated load. A set of chains are looped from the bottom of the counterweight to the underside of the car to help maintain balance by offsetting the weight of the suspension ropes Guide rails that run the length of the shaft keep the car andcounterweight from swaying or twisting during their tr
12、avel. Rollersare attached to the car and the counterweight to provide smooth travel along the guide rails. The traction to raise and lower the car comes from the friction of the wire ropes against the grooved sheaves. The main sheave is driven by an electric motor. SUSPENSION riCPCS Motor-generator
13、(M-G) sets typically pro-vide to dc power for the drive motor. Newer systems use a static drive control. The elevator controls vary the motor s speed based on a set of feedback signals that indicate the car s position in the shaftway. As the car approaches its destination, a switch near the landing
14、signals the controls to stop the car at floor level. Additional shaftway limit switches are installed to monitor overtravel conditions. The worst fear of litany passengers is that the elevator will go out of control and fall through space until it smashes into the bottom of the shaft. There are seve
15、ral safety features in modern elevators to prevent this from occurring. The first is the high-strength wire ropes themselves. Each 0. 625-in-diameter extra-high-strength wire rope can support 32, 000 lb, or about twice the average weight of a mine elevator filled with 20 passengers. For safety s sak
16、e and to reduce wear, each car has six to eight of these cables. In addition, elevators have buffers installed at the shaft bottom that can stop the car without killing its passengers if they are struck at the normal speed of the elevator As previously discussed, modern elevators have several speed
17、control features. If they do not work, the controls will disconnect the motor and apply the machine brake. Finally, the elevator itself is equipped with safeties mounted underneath the car. If the car surpasses the rated speed by 15 to 25%, the governor will trip, and the safeties will grip the guid
18、e rails and stop the car. This was the invention that made elevator transportation acceptable for the general public. SAFETY HAZARDS A historical perspective of elevator development can account for today s problems with elevator safety rules and regulations 4. In the beginning of modern elevator his
19、tory, it was realized that although there were several factors of safety in the suspension rope design, the quality of construction and periodic inspection could not be assured. Therefore, the elevator car was equipped with reliable stand by safeties that would stop the car safely if the suspension ropes failed. In 1853, Elisha. Otis, a New York mechanic, designed and demonstrated an instantaneous safety capable of safely stopping a free falling car. This addressed the hazard shown in figure 2.
