Các tần số tự nhiên Các tần số tự nhiên của cần cẩu là một chủ đề rất quan trọng. Nó là cần thiết để tính toán con số này để biết liệu cần cẩu là đủ cứng, linh hoạt hoặc thậm chí run rẩy. Đối với các cần cẩu với nhanh chóng chạy xe đẩy tần số tự nhiên, theo hướng du lịch xe đẩy, được kiểm soát. | 260 Cranes-Design Practice and Maintenance Allowed pressures - Normal duty ơv 100N mm2 - Heavy duty ơv 85 N mm2 - Very heavy duty Ơv 70 N mm2 For alternating loads these figures have to be diminished by 30 percent. D-Design details In constructions which are being exposed to fatigue loads a number of design details can often be avoided to prevent fatigue cracks. The following figures give some of these details. Fig. Details of fatigue-sensitive constructions Construction and Calculation Methods 261 Fig. Continued The natural frequency The natural frequency of a crane is a very important subject. It is necessary to calculate this figure in order to know whether a crane is stiff enough flexible or even shaky. For cranes with fast running trolleys the natural frequency in trolley travel direction has to be controlled. 262 Cranes-Design Practice and Maintenance F 100 kN 0 6 mm N 1 - 800 tons Fig. The natural frequency How to control this - Using a computer make a calculation of the displacement in the horizontal direction of the main- or trolley-girders of the crane under a particular horizontal force. Assuming the following - weight of complete upperstructure plus half of the underportal plus trolley and load W 800 tons - calculated displacement in horizontal direction under load D 6 mm 0 006 m measured from quay level up to centre of main girders . Then C 100 16 666 kN m D 0 006 Frequency Ũ . 4 6 666 4 56 rad sec V W1 V 800 Natural frequency f n 0 726 Hz Vibration time t f Q 726 1 38 sec In order to have a good reasonably stiff crane the natural frequency should be in the range of f 0 70 .
