Marking of single-sheave blocks

Pages:476-480
 
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476
Appendix H
Appendix H
Marking of single-sheave blocks
H.1. General provisions
H.1.1. The safe working load (SWL) of a single-sheave block
in a derrick rig should be marked in accordance with the following
indications. For the sake of simplicity, the effect of friction and
rope stiffness (i.e. the effort required to bend the rope round the
sheave) has been ignored. In practice, the assessment of the SWL
of the block, as in paragraph H.2.2, ignores friction and rope stiff-
ness. These factors should nevertheless be taken into account
when determining the resultant force on the head fittings of
the heel blocks, span gear blocks and other equipment. This is
the responsibility of the competent person who prepares the
ship’s rigging plan.
H.2. Method
H.2.1. A single-sheave block may be rigged at various po-
sitions in the derrick rig, for example in the span gear, upper and
lower cargo blocks or heel blocks, and may be used with or without
a becket.
H.2.2. The SWL of a single-sheave block is always assessed in
accordance with one fundamental condition of loading, i.e. where
the block is suspended by its head fitting and the dead weight or
cargo load is attached to a wire rope passing round the sheave in
such a way that the hauling part is parallel to the part to which the
load is attached (figure H1). The SWL marked on the block is the
dead weight (M tonnes) that can be safely lifted by the block when
rigged in this way.
H.2.3. When the block is rigged as in paragraph H.2.2, the
resultant force on the head fitting is twice the SWL marked on
the block, i.e. 2M tonnes. The block manufacturer should design

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