Technology: The TWIN Lift System - Two Cars, One Shaft, One Set of Guide Tracks

Without lifts, high-rise buildings would not be economically viable or functional. Furthermore, in most buildings, the lift shafts form a load-bearing, bracing core and thus an essential part of the overall structure. To meet the needs of tall buildings, whole batteries of lifts are usually required. In the Empire State Building in New York, for example, there are 72; in the Sears Tower in Chicago,104; and in the Jin Mao Building, Shanghai, as many as 130. Lifts occupy a large part of the space of a building, however, and reduce the commercially exploitable proportion of the floor area. The higher and more slender a building is, the more pronounced this will be. What is more, large sections of the lift shafts are unused at any one time. To ensure a more efficient use of this space, therefore, concepts have been developed that allow a lift shaft to accommodate a number of cars above each other. One solution is the double-decker lift, which is found especially in the US and the Far East, but the coupling of two cabins has a number of disadvantages.

The TWIN lift system consists of two traditional friction-driven units housed in a single shaft. The cars are situated vertically above each other and use the same guide tracks. The respective counterweights can run in separate tracks or in a joint track, one above another. With this system, each of the lifts has its own friction driving mechanism and controls. The safety devices – overspeed governor, arresting gear, etc. – are also operated separately for each lift.

The first TWIN lift was installed in the architectural faculty of the University of Stuttgart in 2002. Erected in the 1960s and designed for 800 persons, the structure is 11 storeys high and houses a number of institutes. The building is now used by roughly 2,000 students and staff members daily. When the lift installation was refurbished, a TWIN system was integrated in one of the shafts, thereby extending the capacity from six to seven lifts. The two TWIN-system lifts have a loading capacity of 1,000 kg each (13 persons) and a speed of 2 metres per second (ill. 1). Two gearless synchronous motors were used as driving mechanisms. The overspeed governor and cable suspension were housed in the machine room within the area of the shaft (ill. 3).

This layout is necessary, since the TWIN system can be installed in a number of shafts next to each other. The lifting height is roughly 40 m. The counterweights run next to each other in the original balance track on the rear wall of the shaft (ill. 4). For identification purposes, the components for the lower lift were painted in a different colour from those for the upper lift (ills. 2–4).

In contast to traditional lift plant with collective controls, in a TWIN installation, the user indicates his or her desired destination before entering the car, and the appropriate lift is assigned to the call. That helps to avoid restricting the movement of the other car. In other words, it prevents the lower lift receiving instructions to travel to destinations above the position of the upper car and vice versa. In tall buildings, the TWIN system allows new transport concepts to be tailored to the specific needs of the structure. In new developments, the aim will usually be to minimize the number of lift shafts.

In illustration 1, a traditional lift group, comprising four cars in four shafts (left), is compared with a group with TWIN technology, comprising five cars in three shafts (right). In other words, one shaft could be omitted without reducing the transport capacity. The space thus gained on each floor can be commercially exploited. In the example shown in ill. 1, it would be possible to install three TWIN units as well as a single lift, thereby increasing the number of cars from four to seven.

Following the pilot scheme in the University of Stuttgart, a further TWIN system was installed in the headquarters of the Thyssen-Krupp concern in Düsseldorf, where two shafts were fitted with TWIN plant.