Marking of single-sheave blocks

• Pages: 205-209

H. Marking of single-sheave blocks

General provisions

H. 1.1. This appendix explains the method of marking the safe

working load (SWL) of a single-sheave block in a derrick rig. 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 does, in fact, ignore friction and rope stiffness; these factors

should nevertheless be taken into account when determining the

resultant force on the head fittings of heel blocks, span gear blocks

and other equipment. This is the responsibility of the competent

person who prepares the ship's rigging plan.

Method

H.2.l. A single-sheave block may be rigged at various positions

in the derrick rig, as for example in the span gear, upper and lower

cargo blocks or heel blocks; it 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. the par-

ticular case 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 (see figure 1). The SWL marked on the

block is the dead weight (M tonnes) that can safely be 4fted 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

the block in such a way that the head fitting, axle pin and strop are

capable of safely withstanding the resultant force of 2M tonnes.

Consequently a proof load of twice the designed SWL (i.e. 4M

tonnes) should be applied to the block.

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