A few days ago, I saw an article titled "Analysis of an Accident of Elevator Car Rushing Top Caused by a Broken Tooth of a Traction Machine Worm Gear" written by Shi Keyi, Wu Boqi, Zheng Shihong and Yang Sen published on the WeChat official account of "China Elevator" (click to read it). It can be seen from this article that in July 2017, a traction driven cargo elevator suddenly had a car bumping top accident when it was running from the third floor to the fourth floor, resulting in a serious injury to one person and serious damage to the elevator. The main cause of the accident was the fatigue fracture of the worm gear, which served as the reduction mechanism, resulting in the failure of the worm gear and worm gear transmission, and the inability to stop due to the upward overspeed of the elevator car.
The elevator in question was manufactured in December 2007, with a rated speed of 0.50m/s, a rated load capacity of 2000kg, 5 floors/5 stations, a lifting height of 14.3m, a traction machine model of YJ210, a traction ratio of 1:1, a bidirectional speed limiter, and a mechanical trigger safety clamp (for the sake of analysis and understanding, the schematic diagram of the accident elevator structure in Figure 1 of the article is borrowed).
The article provides a detailed analysis of the process and causes of the accident, defects in the worm gear material, and deficiencies in elevator inspection, and proposes improvement measures for elevator inspection. I feel that the article is well written, and I have learned a lot of knowledge and information from it. At the same time, I feel that some content is missing, so I will add some analysis here.
①、 About traction machine design
1. The article mentions: "If the worm gear and worm gear fail, the elevator will be in a state of loss of control, and serious accidents such as top or bottom collision will occur." Why is this so?
From Figure 1, it can be seen that the brake (2) is installed between the electric motor (1) and the reducer (3), which prevents the braking force from directly acting on the traction wheel. This design should clearly not comply with the mandatory GB standards in China. According to the requirements of Article 12.4.2.2 of GB7588-2003 standard: "The braked component should be mechanically and rigidly connected to the traction wheel, drum, or sprocket." (Corresponding to Article 12.4.2.2 of EN81-1:1998 standard: "The component on which the brake operators should be coupled to the traction wheel, drum, or sprocket by direct and positive mechanical means.") That is to say, the braking force of the brake (2) should directly act on the traction wheel, or at least be effectively connected to the traction wheel using a "fixed component" connection type that is "rigid" (such as keys/shafts, etc.) The connection type of "moving parts" such as turbine and worm should not be used.
Why can the design type of traction machine for the involved elevator configuration pass the review and type approval of regulatory authorities? Is there a regulatory loophole? I hope this is just an isolated case or just a manufacturer's action, and not universal. Because the probability of turbine tooth breakage and other failures in this design structure is relatively high, for example, the article mentions that "after investigating the same model and contemporaneous elevators of this manufacturer after the accident, it was found that many elevators also have worm gear tooth surface pitting phenomenon. The traction ratio of elevators with a 1:1 ratio is relatively severe, while elevators with a 2:1 ratio are slightly better. Elevators with the same material of traction machine worm gear from other manufacturers also have severe tooth surface pitting phenomenon." The article suggests that it may be due to the use of "high aluminum zinc based alloy materials," but there is actually a problem that may not have been noticed that the reducer (turbine worm) in the design type shown in Figure 1 has been installed in the elevator for a long time. In a loaded state, if the brake directly acts rigidly on the traction wheel, the reducer will not be loaded when the elevator stops running, The operating conditions of the gearbox are completely different! I don't know if this factor was considered in the design of the reducer? Also, have you considered what consequences would occur if the worm gear and worm gear pair were to fail due to tooth breakage or other factors? As the article states, "The elevator will be in a state of loss of control, and serious accidents such as hitting the top or bottom may occur." Do we need to rely on safety gear or upward protectors (which may not have been installed in old elevators yet) at this time? This clearly goes against the original intention of GB standards requiring dual sets of brakes.
2. The article also mentioned: "The worm gear made of high aluminum zinc based alloy material is greatly affected by temperature, and the lubricating oil in the traction machine gearbox was not replaced for a long time during the accident, which has lost its lubrication and cooling effect. The oil temperature of the accident elevator rapidly increased during operation, causing a significant decrease in the mechanical properties of the high aluminum zinc based alloy worm gear, resulting in failure such as pitting wear and cracks." I am not sure if the gearbox is equipped with a temperature monitoring alarm device and a lubricating oil level monitoring device? I think it is necessary to set it up.
② Selection and control of elevator safety components
The article mentions: "There is a configuration defect in the elevator. The actual traction ratio of the elevator is 1:1, but in the archived supervision and inspection data, the construction self inspection report provided by the construction unit is filled in 2:1, which does not match the actual situation. For the YJ210 traction machine model, after consulting its technical parameters, it was found that the rated load capacity corresponding to the traction ratio of 1:1 is 1000kg, and the rated load capacity corresponding to the traction ratio of 2:1 is 2000kg. The rated load capacity of the elevator is 2000kg, so the traction ratio should be designed as 2:1."
Why did this situation occur? Firstly, there was an error in the selection of traction machines by the elevator manufacturer during the design phase (and logically, it should not be just an isolated case but a batch), and how did the elevator pass the type approval/testing by the testing agency? After being used in large quantities for various projects in various regions and requiring installation self inspection, supervision and inspection, has this problem not been found? Isn't there a requirement for the review of "technical information" in TSG T7001-2009 "Elevator Supervision and Regular Inspection Rules - Traction and Forced Drive Elevators"? Why is the construction self inspection report filled in 2:1? It's hard to imagine! This situation should not occur in marine elevators because the supervision method of marine elevators is indeed different from that of land elevators. Interested friends can take a look at my previous article "Analysis of Differences and Similarities in Supervision and Inspection Modes of Land Elevators and Marine Electric Elevators (Complete)" (click to read).
③ Regarding elevator up speed protection
The elevator uses a rope gripper as an upward overspeed protection device, which is controlled by a bidirectional speed limiter. Why did the rope gripper of the accident elevator not act reliably and provide protection?
The article mentions: "The small steel wire rope used to trigger the rope gripper on the upward side of the speed limiter is in a loose state and cannot reliably trigger the action of the rope gripper. However, in the recent elevator regular inspection, the inspector did not propose any inspection and rectification suggestions for" the rope gripper cannot move "." Do GB standards, supervisory inspections, and regular inspections have design and inspection/testing requirements for upstream overspeed protection devices? The answer should be yes.
Article 9.10 of GB7588-2003 standard has clear requirements for "elevator car up speed protection device", and Article 9.10.11 also specifies that "elevator car up speed protection device is a safety component and should be verified according to the requirements of F7."
TSG T7001-2009 Appendix A Supervision and Regular Inspection Content, Requirements and Methods Table for Traction and Forced Drive Elevators, Sections 2.12 and 8.2 all have testing and verification requirements. It is unknown whether the elevator in question has been inspected and tested according to this rule during supervision and regular inspection?
According to TSG T5001-2009 "Elevator Usage Management and Maintenance Rules" in Table A-4, the seventh item of the annual maintenance items (content) and requirements requires "the operation test of the upstream overspeed protection device works normally"; The TSG T5002-2017 Elevator Maintenance Rules have the same requirements for upward overspeed protection as those in 2009. Has the maintenance not verified that the upward overspeed protection device's action test is working properly?
The article also mentioned: "Due to the relaxation of the steel wire rope on the counterweight side, the elevator car freely fell, causing the safety clamp of the elevator car to act and the elevator car to be stopped on the guide rail." However, at this time, "the swing block of the speed limiter and the electrical safety device are in an upward motion state." Why does the elevator car fall after hitting the top and the safety clamp act to stop the elevator car within a distance of 300mm? What is the principle that triggers the action of the elevator car safety clamp? I haven't mentioned that the speed limiter is in a downward motion state.
④ Regarding the maintenance of the involved elevator on the gearbox
The article mentions: "After the installation of the elevator, the maintenance unit did not regularly replace the lubricating oil of the gearbox according to the requirements of the" Elevator Maintenance and Use Manual "provided by the elevator manufacturer."
According to TSG T5001-2009 Table A-2 Quarterly Maintenance Items (Content) and Requirements Article 1: "The amount of lubricating oil for the gearbox is appropriate, and there is no leakage except for the extended end of the worm gear." Table A-4 Annual Maintenance Items (Content) and Requirements Article 1: "The lubricating oil for the gearbox should be replaced at an appropriate time according to the requirements of the manufacturing unit to ensure that the oil quality meets the requirements.". The requirements for the gearbox in TSG T5002-2017 are consistent with those in 2009. It seems that the maintenance of the elevator involved this time was not done properly!
