How to Determine Sheave Size
Blocks for fiber lines come in 4-, 5-, 6-, 7-, 8-, 10-, 12- and 14-inch sizes. Sheaves for wire rope start at 4 inches. The sheave size for fiber lines is based on the length of the cheek of the block. The cheeks of a block are the two side plates of the block. Blocks come in three shapes: diamond, oval or round. The length of the cheek for diamond blocks is the length of one side of the diamond shape; for oval and round blocks, it's the distance across the side from left to right.
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Instructions
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Fiber Rope
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1
Measure the diameter of the line. Multiply the the diameter by 3.1416 ("Pi," if you're using a calculator) to determine the circumference of the line.
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2
Multiply the circumference of the line by 3. Three is the constant used for calculating fiber rope sheave size.
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3
Round up to the next largest sheave size, if the calculation has a fractional part. For example, a 3/4-inch nylon line has a circumference of 2.356 inches (0.5 x 3.1416 = 2.356). Multiplied by the constant for fiber rope, the result, 7.068 inches (3 x 2.356 =7.068) is larger than a 7-inch block by 0.68 inch; an 8-inch block is the correct choice.
Wire Rope
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4
Measure the widest diameter of the rope by placing it in a caliper so that only two strands are in contact with the jaws of the caliper.
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5
Multiply the diameter of the wire rope by 20. Twenty is the constant used for calculating wire rope sheave sizes.
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Round up to the next largest sheave size, if the calculation has a fractional part. If he diameter of the wire rope is 2.375 inches, multiplying by the constant for wire rope sheaves gives a fractional result (2.375 x 20 = 47.5 inches) 0.5 inch larger than a 47-inch sheave. A 48-inch sheave is the correct choice
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References
- Photo Credit ship's block and tackle image by David Smith from Fotolia.com
