3.1 General
3.1.1 A typical cargo block is shown diagrammatically in Fig. 6.3.1 with the component items labelled for reference.
3.1.2 The ultimate strength of the block as an assembled unit is in no case to be less than five times the resultant load for which the block is designed.
3.1.3 The safe working load of each block is to be appropriate to its particular position in the rig and is to be not less than the resultant load determined in accordance with the appropriate Chapter of this Code. Blocks are not to be used in positions other than those for which they were approved without first confirming that their safe working load is at least that required for the proposed location.
3.1.4 The required safe working load of the block is to be determined by reference to the resultant load, R, imposed on the block at its particular position in the rig.
3.1.5 The safe working load of a single sheave block is assessed on one particular condition of loading, namely where the block is suspended by its head fitting and the cargo load attached to a wire passing round the sheave such that the hauling part is parallel to the part to which the load is attached, see Fig. 6.3.2. The SWL marked on the block is the weight, W tonnes that can safely be lifted by the block, when rigged in this way. The resultant load, R, on the head fitting (neglecting friction) is, however, twice the SWL marked on the block, i.e., 2W tonnes. The block and head fitting must, therefore, be designed to take a resultant force of 2W tonnes and the proof load applied to the head fitting must be based on this resultant force. That is, the proof load will be 4W tonnes.
3.1.6 When the same block is rigged as a lower cargo block (the load being attached to the head fitting), the SWL marked on the block is unchanged, but the resultant force on the head fitting is only W tonnes. As the block has been designed to withstand a resultant load on the head fitting of 2W tonnes it follows that the block is safe to support a cargo load of 2W tonnes.
3.1.7 For single sheave blocks with beckets the SWL marked on the block is to be not less than one-half the resultant load on the head fitting.
3.1.8 Fig. 6.3.2 gives examples of the use of single sheave blocks and the method of obtaining their SWL's. It should be noted that in all cases with single sheave blocks, the shackle or link securing the block is to be marked with an SWL equal to twice the SWL marked on the block.
3.1.9 The safe working load marked on any multiple sheave block is to correspond to the maximum resultant load on the head fitting of that block.
Sections 2 & 3
b
t a
d Round eye
Oval eye d
d
a r2
t1
t1 t1 t1
e1 e2 t2 d
r2
e2 t2
Fig. 6.2.5
Fixed eyeplates at the derrick boom head
Fig. 6.2.6 Fixed eyeplate at ship's structure
(c) Any block in the rig of a lifting appliance having SWL greater than 20 t.
3.3.2 Cast nylon sheaves may also be used for general cargo handling applications when the manufacturer can indicate satisfactory service experience. Attention is drawn, however, to the fact that whilst tests have indicated longer service life for ropes used with cast nylon sheaves, the ropes do not exhibit the normal warning signs of broken wires but may break without external warning by internal rope fatigue. In consequence it is recommended that one steel sheave is included in the reeving arrangement.
3.3.3 The diameter of the sheave is to be measured to the base of the rope groove and is to be not less than as given in Table 6.3.4.
3.3.4 The depth of the groove in the sheave is to be not less than three quarters of the rope diameter. A depth equal to the rope diameter is recommended. The contour at the bottom of the groove is to be circular over an angle of 128°
and its radius is to be not less than as given in Table 6.3.5.
3.2 Design loads and stresses
3.2.1 The percentage of the resultant load on the head fitting which is transmitted by a sheave is to be taken as not less than the value given in Table 6.3.1.
3.2.2 The percentage of the resultant load on the head fitting which is transmitted to the side straps and partition plates of the sheave is to be taken as not less than the value given in Table 6.3.2.
3.2.3 The load on a becket, where fitted, is to be taken as the maximum load to which it may be subjected in service.
3.2.4 The stresses in the component parts of the block are to be determined from the loads transmitted from the sheaves and straps and are not to exceed the values given in Table 6.3.3.
3.3 Materials and construction
3.3.1 Sheaves may be forged or fabricated from steel plate. In general castings in steel or spheroidal graphite iron may be accepted but grey cast iron or malleable cast iron is not to be used for sheaves in the following circumstances unless specially agreed:
(a) Single sheave block having SWL greater than 10 t.
(b) Multiple sheave block having SWL greater than 20 t.
Head fitting Crosshead
Collar
Grease lubrication
Bottom through pin
Partition plate
Side straps (when fitted)
Rope guards
Side plates
Axle pin
Side straps
Sheave
Becket
Section 3
Fig. 6.3.1 Typical cargo block
Load = W
3.3.5 Side and partition plates and straps are to be castings or fabricated from steel plate. Malleable cast iron may be used when permitted for sheaves, see 3.3.1. The plates are to project beyond the sheaves to provide ample protection for the rope. Means are to be provided to prevent the rope from jamming between the sheave and the side or partition plates by minimizing the clearance or by fitting suitable guards.
3.3.6 Snatch blocks are to be well designed and arrange-ments are to be provided to ensure that the block remains closed at all times when it is in use.
3.3.7 Crossheads and beckets may be cast, forged or machined from plate.
3.3.8 Axle pins are to be positively secured against rotation and lateral movement. The surface finish of the pin is to be suitable for the type of bearing to be used.
3.3.9 Provision is to be made for lubricating all bearings and swivel head fittings without dismantling the block and for withdrawing the axle pin for inspection.
3.4 Blocks for fibre ropes
3.4.1 Blocks intended for use with fibre ropes are not to be fitted with more than three sheaves and a becket or with four sheaves and no becket.
3.4.2 The diameter of the sheave measured to the base of the rope groove is to be not less than five times the nominal diameter of the rope. The depth of the groove is to be not less than one third the diameter of the rope. The contour at the bottom of the groove is to be of a radius in accordance with Table 6.3.5.
3.4.3 Proposals to use materials other than steel or iron castings for the sheaves and body of the block will be considered. Bearing pressures and stresses are to be appropriate to the materials used.
Section 3
NOTE
The values given in the above Figures are for convenience only as no account has been taken of friction.
Fig. 6.3.2
Safe working load of single sheave blocks
Bushed or plain bearings Roller or ball bearings Type of block Number of sheaves
Without becket With becket Without becket With becket
Double 2 52 43 51 42
Treble 3 37 32 35 30
Fourfold 4 29 26 27 24
Fivefold 5 24 22 22 20
Sixfold 6 21 20 19 18
Sevenfold 7 19 18 17 16
Eightfold 8 17 16 15 14
NOTES
Friction allowance taken as 5 per cent for bushed or plain bearings and 2 per cent for roller or ball bearings.
Section 3
Table 6.3.1 Percentage load transmitted by a sheave
Type of block Number of Number of supports Bushed or plain bearings Roller or ball bearings
sheaves Inner Outer Partition Side strap Partition Side strap
Double 2 1 2 63 20 63 19
Treble 3 2 2 40 15 38 14
Fourfold 4 3 2 32 12 30 11
Fivefold 5 4 2 26 10 24 9
Sixfold 6 5 2 23 9 21 8
Sevenfold 7 6 2 21 8 18 7
Eightfold 8 7 2 19 7 16 6
NOTES
1. Friction allowance taken as 5 per cent for bushed or plain bearings and 2 per cent for roller or ball bearings.
2. Where a becket is fitted the partitions and straps are to be designed to take account of the loads imposed on the block.
Table 6.3.2 Percentage load on side plates or supporting straps