GRADES OF WIRE ROPE

Wire rope is made in a number of different grades, three of which 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 200,000 to 220,000 pounds per square inch (psi).

PLOW STEEL wire rope is unusually tough and strong. This steel 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

Figure 6-22.\Two common types of wire rope.

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 stand a strain of 240,000 to 260,000 psi.

MEASURING WIRE ROPE

The size of wire rope is designated by its diameter. The true diameter of a wire rope is considered as being the diameter of the circle that will just enclose all of its strands. Both the correct and incorrect methods of measuring wire rope are shown in figure 6-23. Note, in particular, 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 have to apply a thumb-rule formula to obtain the SWL. There are rules of thumb that may be used

Figure 6-23.\Correct and incorrect methods of measuring wire rope.

to compute the strength of wire rope. The one recommended by the Naval Facilities Engineering Command (NAVFAC) is as follows:

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, and such dynamic stresses as the speed of operation and the acceleration and deceleration of the load, all of which can affect the endurance and breaking strength of the rope.

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 as follows:

SWL = (2)² 4

SWL = 4 4 = 16

The answer is 16, meaning that the rope has an SWL of 16 tons.

It is 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 as follows:

As you know, wire rope is measured by the diameter (D). To obtain the circumference (C) required in the formula, multiply D by pi (p), which is approximately 3.1416. Thus, the formula to find the circumference is C = Dp.