GRADES OF WIRE ROPE

Wire rope is made in a number of different grades. Three of the most common are mild plow steel, plow steel, and improved plow steel.

Mild plow steel rope is tough and pliable. It can stand up under repeated strain and stress, and it has a tensile strength of from 200,000 to 220,000 pounds per square inch (psi). Plow steel wire rope is unusually tough and strong. It has a tensile strength (resistance to lengthwise stress) of 220,000 to 240,000 psi. This rope is suitable for hauling, hoisting, and logging. Improved plow steel rope is one of the best grades of rope available, and most, if not all, of the wire rope in your work will probably be made of this material. It is stronger, tougher, and more resistant to wear than either plow steel or mild plow steel. Each square inch of improved plow steel can withstand a strain of 240,000 to 260,000 psi.

Figure 4-6.-Two common types of wire rope.

MEASURING WIRE ROPE

The size of wire rope is designated by its diameter. The true diameter of a wire rope is the diameter of a circle that will just enclose all of its strands. Correct and incorrect methods of measuring wire rope are illustrated in figure 4-7. In particular, note that the correct way is to measure from the top of one strand to the top of the strand directly opposite it. The wrong way is to measure across two strands side by side. Use calipers to take the measurement. If calipers are not available, an adjustable wrench will do.

To ensure an accurate measurement of the diameter of a wire rope, always measure the rope at three places, at least 5 feet apart. Use the average of the three measurements as the diameter of the rope.

SAFE WORKING LOAD

The term "safe working load" (swl), as used in reference to wire rope, means the load that can be applied and still obtain the most efficient service and also prolong the life of the rope. Most manufacturers provide tables that show the safe working load for their rope under various conditions. In the absence of these tables, you must apply a thumb rule formula to obtain the swl. There are rules of thumb that may be used to compute the strength of wire rope. The one recommended by the Naval Facilities Engineering Command (NAVFAC) is swl (in tons)= DZx 8. This particular formula provides an ample safety margin to account for such variables as the number, size, and location of sheaves and drums on which the rope runs. Also included are dynamic stresses, such as the speed of operation and the acceleration and deceleration of the load. All can affect the endurance and breaking strength of the rope.

Let's work an example. In the above formula, D represents the diameter of the rope in inches. Suppose you want to find the swl of a 2-inch rope. Using the formula above, your figures would be: swl = 2'x 8, or 4 x 8 = 32. The answer is 32, meaning that the rope has a swl of 32 tons.

It is very important to remember that any formula for determining swl is only a rule of thumb. In computing the swl of old rope, worn rope, or rope that is otherwise in poor condition, you should reduce the swl as much as 50 percent, depending on the condition of the rope. The manufacturer's data concerning the breaking strength (BS) of wire rope should be used if available. But if you do not have that information, one rule of thumb recommended is BS = C2x 8,000 pounds.

As you recall, wire rope is measured by the diameter (D). To obtain the circumference (C) required in the formula, multiply D by pi (usually shown by the Greek letter 7t), which is approximately 3.1416. Thus, the formula to find the circumference is C = Dir.

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