Cranes – Design, Practice, and Maintenance phần 9 pptx

260 Cranes – Design, Practice, and Maintenance

Allowed pressures

– Normal duty σvG100 N
mm2

– Heavy duty σvG85 N
mm2

– Very heavy duty σvG70 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. 7.6.1 Details of fatigue-sensitive constructions

Construction and Calculation Methods 261

Fig. 7.6.1 Continued

7.7 The natural frequency

The natural frequency of a crane is a very important subject. It is neces￾sary 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

Fig. 7.7.1 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: WG800 tons

– calculated displacement in horizontal direction under load: DG

6 mmG0,006 m (measured from quay level up to centre of main

girders).

Then CG F

DG 100

0,006G16 666 kN
m

Frequency ωG1C

W1

G116 666

800 G4,56 rad
sec

Natural frequency fG ω

2π

G4,56

2π G0,726 Hz

Vibration time tG 1

f

G 1

0,726G1,38 sec

In order to have a good reasonably stiff crane, the natural frequency

should be in the range of fG0,70 Hz.