Civil Engineering Reference
In-Depth Information
The weight force of the hook block must be so great that it moves down if the wire
rope is loosened by the drum. The necessary minimum weight force of the hook
block will be derived for the rope drive according to Fig. 3.87 . It will be pre-
supposed that the stationary sheaves are positioned at about the same height and
that the hanging sheaves of the hook block in the highest position will not be any
great distance from the stationary sheaves. That means that the weight force of the
wire rope pieces in between is small and can be neglected. It will be further
presupposed that all the sheaves have the same diameter.
When the hook block is lowered, the greatest rope tensile force S 1 is on the
fixed point of the rope. When running over a sheave, the rope tensile force will be
reduced by a small force difference DS i . The rope tensile forces in the tackle block
are then
S 1
S 2 ¼ S 1 DS 1
S 3 ¼ S 1 DS 1 DS 2 :
S z ¼ S 1 X
ð 3 : 112 Þ
z
DS i :
i¼1
where
z
is the number of bearing falls of wire ropes, and
DS i
is the loss of rope tensile force between the fall of wire rope i and i + 1
The loss of tensile force DS depends partly on the rope tensile force and partly
not. The rope tensile forces are very low when the empty hook block is lowered.
Therefore, the loss of tensile force which is dependent on the rope tensile force is
very small and the loss of rope tensile force can be used as constant
DS i ¼ DS :
With this, ( 3.112 ) will be simplified to
S z ¼ S 1 ð z 1 Þ DS :
ð 3 : 113 Þ
The weight force Q H = m H *g of the hook block will be borne by the rope
forces
Q H ¼ S 1 þ S 2 þ S 3 þþ S z ¼ X
z
S i :
ð 3 : 114 Þ
i¼1
or
Q H ¼ S 1 þ S 1 DS þ S 1 2DS þ S 1 3DS þþ S 1 ð z 1 Þ DS :
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