Blocks are constructed for use with fiber line or wirerope. Wire rope blocks are heavily constructed and havelarge sheaves with deep grooves. Fiber line blocks aregenerally not as heavily constructed as wire rope blocksand have smaller sheaves with shallow, wide grooves.A large sheave is needed with wire rope to prevent sharpbending. Because fiber line is more flexible and pliable,it does not require sheaves as large as that required forwire rope of the same size.Blocks, fitted with one, two, three, or four sheaves,are often referred to as single, double, triple, andquadruple blocks. Blocks are fitted with a number ofattachments, such as hooks, shackles, eyes, and rings.Figure 13-45 shows two metal framed, heavy-dutyblocks. Block A is designed for manila line, and blockB is for wire rope.Block to Line RatioThe size of a fiber line block is designated by thelength in inches of the shell or cheek. The size of astandard wire rope block is controlled by the diameterof the rope. With nonstandard and special-purpose wirerope blocks, the size is found by measuring the diameterof one of its sheaves in inches.Use care in selecting the proper size line or wire forthe block to be used. If a fiber line is reeved onto a tacklewhose sheaves are below a certain minimum diameter,the line becomes distorted that causes unnecessary wear.A wire rope too large for a sheave tends to be pinchedthat damages the sheave. Also, the wire will be damagedbecause of too short a radius of bend. A wire rope toosmall for a sheave lacks the necessary bearing surface,Figure 13-45.-Heavy-duty blocks.puts the strain on only a few strands, and shortens thelife of the wire.With fiber line, the length of the block used shouldbe about three times the circumference of the line.However, an inch or so either way does not matter toomuch; for example, a 3-inch line maybe reeved onto an8-inch block with no ill effects. As a rule, you are morelikely to know the block size than the sheave diameter.However, the sheave diameter should be about twice thesize of the circumference of the line used.Wire rope manufacturers issue tables that give theproper sheave diameters used with the various types andsizes of wire rope they manufacture. In the absence ofthese, a rough rule of thumb is that the sheave diametershould be about 20 times the diameter of the wire.Remember with wire rope, it is the diameter, rather thancircumference, and this rule refers to the diameter of thesheave, rather than to the size of the block, as with line.Block SafetySafety items when using block and tackle are asfollows:l Always stress safety when hoisting and movingheavy objects around personnel with block and tackle.. Always check the condition of blocks andsheaves before using them on a job to make sure theyare in safe working order. See that the blocks areproperly greased. Also, make sure that the line andsheave are the right size for the job.. Remember that sheaves or drums which havebecome worn, chipped, or corrugated must not be used,because they will injure the line. Always find outwhether you have enough mechanical advantage in theamount of blocks to make the load as easy to handle aspossible.. You must NOT use wire rope in sheaves andblocks designed for fiber line. They are not strongenough for that type of service, and the wire rope willnot properly fit the sheaves grooves. Likewise, sheavesand blocks built for wire rope should NEVER be usedfor fiber line.CHAIN HOISTSChain hoists provide a convenient and efficientmethod for hoisting by hand under particularcircumstances. The chief advantages of chain hoists arethat the load can remain stationary without requiringattention and that the hoist can be operated by one man13-25
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