Ii the cable is to lead horizontally or incline downward, pass it over a log at the outlet to the inclined trench. If the cable is to lead upward, the log is not necessary, but the deadman must be buried deeper. Stakes driven at an angle over the log prevent it from rolling out.

■ 30. Blocks and Tackles (figs. 56 and 57).—The parts or a block are the shell or frame, the sheave or wheel upon which the rope runs, and the pin upon which the wheel turns In the shell. Blocks are designated by the length ol the shell in Inches and by the number of the sheaves. Those with one, two. three, or four sheaves are called single, double, triple, and quadruple. The smallest size of block (length in inches) that will take a given rope is nine times the rope diameter. Self-lubricating blocks should be used where obtainable. a. Definitions.—(1) Snatch bfocfc.—A snatch block is a

50 28-30

50 28-30

single block with the shell open at one side to admit a rope without passing the end through.

<2) Running block.—A running block is attached to the object to be moved. In compound tackle, however, a running block may be suspended by ropes.

(3) Standing block.—A standing block is fixed to some permanent object.

(4) Simple tackle.—A simple tackle consists of one or more blocks rove with a single rope.

b. Uses.—Blocks are used to change the direction of pull and to give mechanical advantage. A man of average weight will phiII about 60 pounds horizontally.

c. Mechanical advantage.—The mechanical advantage cf

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