Abstract
Operation of the safety gears is the last means of protecting passengers in a high-speed or even freely falling lift car. EN81-20 stipulates that the average retardation when the safety gears are engaged should lie between 0.2 g n and 1.0 g n while g n is the gravitational acceleration. Decades ago, in Hong Kong, to conduct a formal test on the safety gears, the car was fully loaded and the machine brake was manually released to allow the car fall freely until the overspeed governor triggered the engagement of the safety gears. This test was rather vigorous to both the gear jaws and the guiderails. Recently, tests have been conducted with 125% in-car load while the car travels under an inspection (or levelling) speed and the governor is manually triggered. Such a revised test, though much less vigorous, may not be able to verify the average retardation as required. Even a test with full load and free-falling may not reveal the whole truth by merely measuring the resulting contact scratches on the guiderails because a majority of the retardation could have been assisted by the counterweight via the suspension ropes. A method that involves an empty car traveling downward at 1 m/s with the governor manually triggered was first developed in Germany. This method has been improved to form a comprehensive procedure as detailed in this article so that a no-load rated speed, or slightly reduced speed, test is conducted, which is almost non-invasive or much less vigorous. By calculation, the average retardation when all suspension ropes are broken can be estimated. Cross-checking in our experiments revealed the validity of this procedure. The procedure also involves a test to verify the validity of parameters used to perform the estimation and takes care of some modern overspeed governors which cannot be manually triggered. It is hoped that this procedure could encourage maintenance professionals carry out safety gears tests more often to ensure a high safety standard.
Practical application
This comprehensive procedure could encourage more frequent tests on the car safety gears, which is almost non-invasive for manually triggerable overspeed governors or less vigorous for non-triggerable ones. An empty car traveling downward at rated speed, or at slightly reduced speed, is tested by manually triggering the governor while a partially loaded free-falling car is tested if the governor is not manually triggerable. Results can be used to estimate the average retardation of the lift car when all suspension ropes are broken. A pre-test is included to validate critical parameters necessary for the estimation. Experiments conducted so far were on the safety gears of low- to medium-speed lifts. For high-speed lifts beyond 4 m/s, that may be considered in the next stage of research. Although the title of this article includes the city where the procedure was developed, there is no reason why this testing procedure could not be extensively applicable to any traction lift systems around the world at such speed levels.
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