STRENGTH OF ROPE, ETC.
Reading edition
Rope is measured by its circumference. A four-stranded rope is about one-fifth weaker than a three-stranded one.
Generally blocks should be three times the size of the rope which it is intended to reeve in them.
The hauling part bears twice the strain of the standing part of a fall, the pin of a block is often more worn on one of its sides than on the other and should be turned frequently.
Sheaves and pins of blocks should be carefully examined at short intervals.
BREAKING STRAINS, ETC.
Hawser-Laid Rope Rule
Square the circumference and divide by 3 for the breaking strain; in tons. Divide by 4 for the proof strain; divide by 6 for the working strain.
Worked Example
A rope 4 inches in circumference; required the breaking strain.
4 inches in circumference.
× 4
÷ 3) 16
Ans. 5.3 tons = breaking strain.
4 inches in circumference.
× 4
÷ 4) 16
Ans. 4 tons = proof strain.
4 inches in circumference.
× 4
÷ 6) 16
Ans. 2.7 tons = working strain.
To find what weight a rope will lift when rove as a tackle.
Multiply the weight the rope is capable of suspending by the number of parts at the movable block and subtract ¼ of this for resistance.
To determine the relative strength of chain and rope.
Consider the proportional strength to be 10 to 1, using the diameter of the chain and the circumference of the rope; ½-inch chain may replace 5-inch rope.
Table showing the sized wire rope which may be used as a substitute for hempen rope.
Hemp Rope Inches Wire Rope Inches
3 1½
4 1¾
5 2
6 2½
7 3
8 3½
9 4
10 4½
11 5
STEEL WIRE ROPES
1. The four qualities of steel wire used for wire making are:
Breaking Strain p.s.i.—Extra plough steel, 110 to 120 tons. Mild plough steel, 95 to 100 tons. Best patent steel, 80 to 85 tons. Bessemer steel, 40 to 45 tons.
2. Specifications.—Specification should state: (1) Length of rope. (2) Size of gear. (3) Speed. (4) Load, exclusive of rope. (5) If for wet workings. (6) Gradients. (7) Particulars of curves.
3. Working Load.—The maximum working load at average speed, including weight of rope, should not exceed a tenth of the breaking strain as tabulated below.
4. Sheaves and Barrels.—Great care should be taken that wire ropes are not worked round drums or over pulleys of insufficient circumference, that they do not strike against any hard substance while in motion. They should be about 30 times the circumference of the rope in diameter.
5. Uncoiling.—Much care should be taken in uncoiling wire ropes, to prevent kinking. The coil should not be laid stationary, but should be placed on a turntable or reel and unwound from the outer end.
6. Grease.—To prevent corrosion, all working ropes should receive a regular dressing of wire rope grease thoroughly laid on.
7. Starting.—The greatest strain on a rope being at the moment of starting, every care should be taken to insure perfect steadiness of movement, as jerking is ruinous to ropes.
Weights and Breaking Strengths of Round Wire Ropes.
Diam. Inches. Circumf. Inches. Lbs. per Fathom. Plough Steel Best Patent Steel. Best Bessemer Steel.
Extra. Mild.
Tons. Tons. Tons. Tons.
½ 1 ½ 2 ¼ 8 ½ 7 ½ 6 ¼ 3 ¼
1 ¾ 3 11 ½ 10 8 ½ 4 ¼
⅝ 2 3 ¾ 15 ¼ 13 11 5 ½
2 ¼ 5 19 ¼ 16 ½ 14 7
¾ 2 ⅜ 5 ½ 20 17 ¼ 15 ½ 8
2 ½ 6 ¼ 23 ¾ 20 ¼ 17 ½ 8 ¾
⅞ 2 ¾ 7 ½ 28 ¾ 24 ½ 21 10 ½
3 8 ¾ 34 ¼ 29 ¾ 25 12 ½
1 3 ⅛ 9 ½ 36 ½ 32 27 13 ½
3 ¼ 11 ¼ 40 34 29 14 ¾
3 ½ 12 ¼ 46 ½ 39 ¼ 34 17
3 ¾ 14 53 ½ 45 ¾ 39 19 ½
1 ¼ 4 16 61 54 44 ½ 22 ¼
4 ¼ 18 69 62 50 25
4 ½ 20 77 70 56 28
1 ½ 4 ¾ 22 ½ 86 76 ½ 63 31
5 25 95 85 ½ 70 34 ½
5 ¼ 27 105 96 77 38 ½
1 ¾ 5 ½ 30 115 106 84 42
5 ¾ 33 126 114 92 46
6 36 138 125 100 50
2 6 ¼ 39 155 133 120 60