at Lift Landing
By TK NG
For lift arriving at a landing, automatic re-levelling of the lift car is required if difference in the levels of the car door sill and landing sill exceeds a certain limit as required by local code. The re-levelling is effected before the car door is opened to avoid boarding/alighting passengers being tripped over due to excessive level difference.
However, when a fully loaded lift car reaches its terminal floor, such as the ground floor of a high rise building, the difference in passenger load between the moments when the first and the last passenger leaves the lift car may result in substantial change in the vertical position of the lift car. The same would happen when an empty car arrives to pick up passengers. These phenomena are due to the elongation/contraction of the lift suspension ropes under different load conditions. The extent of level change depends on the quantity/size of the suspension ropes used and their Young’s Modulus.
From the above, ∆L = L0 x F / (E x A). Hence, change in vertical position of the lift car (change in ∆L) due to load change (change in F) would be less at top floor given the shorter lengths of suspension ropes (L0) above the lift car.
Re-levelling of the lift car during passenger loading/unloading is not a desirable option as the changing level difference between car floor and landing may aggravate the risk of tripping over. A better way is to limit the level difference after automatic re-levelling and the subsequent car level change between full load and no load. The total level difference should not be more than that allowed by local code to avoid the need of further re-levelling (see Fig 1).
To achieve the aforesaid, not only a “reasonably accurate” re-levelling mechanism is required, a suitable car suspension system is also indispensable. Since the chosen suspension rope size, quantity and their Young’s Modulus, etc. that determine the extent of change in length under different load conditions would have bearing on the design of other parts of the lift system such as the traction sheave, motor, etc., an appropriate lift design must therefore be chosen with great care for individual lift installation.