A measuring system deals with the question as to the effects different wind loads have on cable cars. Two researchers from the Vienna University of Technology (TU) have thrown light on vibration-control problems, an area little researched up to now and were able to account for the occurrence of lateral vibrations from various wind conditions right through to meteorological calm.
A measuring system which also measures wind direction and wind velocity on a two or three-dimensional basis is positioned on the cab of a cable car or on the chair of a chair lift. The behaviour of the vehicle on impact of the wind can thereby be measured in a wholly self-supporting manner. A dip in a transverse and longitudinal direction when vibrating, the driving speed and the position of the vehicle en route are of particular importance in this regard.
Observations by operating personnel of certain cable car systems have shown that a lateral vibration of the pendulum with large tilt angles can occur not only when the crosswind is blowing gustily but also when the wind is calm. In the meantime this has been quantified using measurement data.
"On the basis of this measurement data, we confirmed that the cabs undergo vibratory excitation due to continuously circulating bicable tramways at certain velocities. In the case of certain continuously circulating bicable tramways, this phenomenon is manifested at a lower driving speed range of approximately 2.0 to 3.0 metres per second. Normal driving speeds lie between 4 and 6 m/s“, says Klaus Hoffmann, head of the Institute for Engineering Design and Logistics Engineering at the TU Vienna.
The measurements confirmed the suspicion that the afore-mentioned oscillations are due to periodical vortex shedding in the cab originating with the airsteam. Particularly strong vibrations occur when the vortex shedding frequency is identical to the natural frequency of the cab. Basically, the phenomenon can only be avoided if longer journeys are carried out at a driving speed range between 2 and 3 m/s.
"Thanks to the extensive European set of standards in accordance with which the cable cars are manufactured, there is a very high internationally comparable safety level today. Each cable car must undergo certain safety checks before it may be used by the operating company. Neveretheless, questions concerning the windproof capacity of cable cars in instruction manuals are often kept vague. Thus, the instruction manuals for monocable aerial ropeways, chair lifts or cabin tramways state the wind velocity at which the tramway can continue to be operated.
At a constant crosswind of 50 to 65 km/h the availability limit of monocable aerial ropeways has normally been attained“, says Robert Liehl, a doctoral candidate at the institute. Apart from wind sensors on the supports whose data is only partly representative for wind conditions along the entire cable car, the entire cable line must also be monitored in these situations by the operating personnel using a pair of binoculars. Hoffmann: „Even for lower average wind velocities, sudden gusts of wind can lead to critical situations. Our measurement data should supply additional information to be able to provide more accurate details of the vibration behaviour of cable car vehicles during different operating conditions.“
During this research project the measuring system was patented in 2002 and has since then been used to carry out field measurements of wind loads and pendulum vibrations for different cable cars and chair lifts. Thanks to the support of the two world market leaders in cable car construction - Doppelmayr and Leitner – this measurement system can continue to be developed further.
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