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Seamanship Techniques 2 Episode 4

Tham khảo tài liệu 'seamanship techniques 2 episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Wirework and Rigging 103 Gyn Tackle This purchase Figure 4.29 comprises a double and treble block with a cordage or wire fall rove between them. The standing part is secured to the double block. The tackle produces a power gain of 6 or 5 depending on whether it is used to advantage or disadvantage. Three-fold Purchase A heavy duty tackle comprising two triple sheave blocks with a rope or wire fall rove between both blocks this purchase is used extensively in heavy lift work for both topping lift and lifting purchase see Figure 4.30 . There are two methods of reeving the three-fold purchase one with the sheaves of both blocks in the same plane and the second more popular method with the plane of the sheaves in each block at rightangles to each other. The advantage of the latter is that when the lift is made the lower block hangs vertically without toppling over to one side. Chain Blocks There are several types of chain block in general use the most common being a spur geared blocks b lever and ratchet and c wormwheel operation. They are often referred to as chain hoists having a mechanical advantage of between 5 and 250. The lifting capability will be variable but their use for up to 40 tonnes is not uncommon practice. Although they are usually found operating from inside the machinery spaces of vessels for numerous duties they may be employed on deck. Figure 4.27 Luff tackle. Figure 4.28 Double luff tackle. Figure 4.29 Gyn tackle. Figure 4.30 Three-fold purchase. 104 Seamanship Techniques With spur geared blocks a manual drive chain turns a through spindle via geared cog wheels. A ratchet and pawl system is also incorporated so that the load may be held suspended from the load chain. This load chain is held by a sprocket arrangement which is being driven by the operation of the through spindle. The lever and ratchet types generally used for lighter work are smaller and permit optional positioning wherever it is required. They are usually equipped with a reversible