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From childhood, many of us have been conditioned to think of a machine as some device with gears, shafts, belts, cams, and assorted whirring parts. Yet, by the rules of physics, an ordinary pry bar is a simple machine, even though it has only one part.
A wire rope is, in reality, a very complicated machine. A typical 6 x 25 rope has 150 wires in its strands, all of which move independently and together in a very complicated pattern around the core as the rope bends. Clearances between wires and strands are balanced when a rope is designed so that proper bearing clearances will exist to permit internal movement and adjustment of wires and strands when the rope has to bend. These clearances will vary as bending occurs, but are of the same range as the clearances found in automobile engine bearings.
Understanding and accepting the “machine idea” gives a rope user a greater respect for rope, and enables him to obtain better performance and longer useful life from rope applications. Anyone who uses a rope can use it more efficiently and effectively when he fully understands the machine concept.
How A Wire Rope Machine Works
The extent to which wires move in a rope when it bends is illustrated by the following example—what actually happens when you wrap a 1-inch rope over a 30-inch sheave.
Between the point where the rope first touches the sheave on one side, and where it leaves the sheave on the other side, the length of rope in contact with the sheave would be 3-1/8 inches shorter than the length of the side away from the sheave—if the rope did not move and adjust internally by wires sliding back and forth.
The mathematics is simple: Just subtract half the circumference of a 30″ circle from half the circumference of a 32″ circle.
Circumference = π x Diameter
C= 3.1416 x 32 = 100.5312
C = 3.1416 x 30 = 94.2490
6.2931 / 2 = 3.14
Thus, circumference of a 32-inch circle is slightly more than 6-1/4″ longer than that of a 30-inch circle. Since a rope only touches half of a sheave at any time, the length differential which a rope must accommodate is 3-1/8″.
By this same reasoning, a 1-inch rope wrapped on a 30-inch hoist, the drum must compensate internally for a 6-1/4″ length differential in each wrap.
This change of dimension is achieved by the sliding and adjusting of the strands in relation to one another, and a similar sliding and adjusting of the individual wires within each strand.
By painting stripes around a wire rope as illustrated here, and actually bending the rope, we can see the movement of strands as the rope bends. Anytime a rope flexes, this movement takes place. The sharper the bend, the more the movement.
Quite obviously, the grade of wire will affect such things as strength, resistance to wear, fatigue resistance, corrosion resistance and so on. Today, the greatest portion of all wire rope is made from two grades of wire—Extra Improved, Plow Steel (EIP) and Double Extra Improved Plow Steel (EEIP). Both are tough, strong, wear-resisting carbon steel, with EEIP providing about 10% greater tensile strength. Sometimes wire is plated or galvanized before strands are formed, where special corrosion or wear characteristics are desired. Most wire is “Bright”—that is, without any surface coating or treatment.
Strands are basic building blocks. A strand consists of a “center” which supports a specified number of wires around it in one or more layers. The strands provide all the tensile strength of a fiber core rope, and 92-1/2% of the strength of an IWRC six strand rope.
Such physical characteristics as fatigue resistance and resistance to abrasion are directly affected by the design of strands. In most strands with two or more layers of wires, inner layers support outer layers in such a manner that all wires may slide and adjust freely when the strand flexes.
As a general rule, a strand made up of a small number of large wires will be more abrasion resistant and less fatigue resistant than a strand of the same size made up of many smaller wires.
Standard Rope Classifications
Most common wire rope constructions are grouped into four standard classifications, based on the number of strands and wires per strand, as shown in this chart. All ropes of the same size and wire grade in each classification have the SAME strength and weight ratings, and usually the same pricing. Ropes within each classification may differ in working characteristics such as abrasion and fatigue resistance.
Basic Strand Constructions
What is sometimes called the “Single Layer Principle” is the basis for this strand construction. Probably the most common example is a single wire center with six wires of the same diameter around it. It is called simply, a 7-wire (1-6) strand.
This construction has two layers of uniform size wire around a center wire, with the inner layer having half the number of wires as the outer layer. Small filler wires, equal in number to the inner layer, are laid in the valleys of the inner layer. Example: 25 Filler Wire (1-6-6f-12) strand
The Seale Principle features two layers of wires around a center wire, with the same number of wires in each layer. All wires in each layer are the same diameter, and the strand is designed so that the large outer wires rest in the valleys between the smaller inner wires. Example: 19 Seale (1-9-9) strand.
The Warrington Principle is a 2-layer construction with uniform-sized wires in the inner layer, and two diameters of wire alternating large and small in the outer layer. The larger outer-layer wires rest in the valleys, and the smaller ones on the crowns, of the inner layer. Example: 19 Warrington [1-6-(6+6)] strand.
When a strand is formed in a SINGLE OPERATION using two or more of the above constructions, it is referred to as a “Combined Pattern.” This example is basically a Seale strand in its first two layers. The third layer utilizes the Warrington Principle, and the outer layer is a typical Seale pattern of same-size wires. It is described: 49 Seale Warrington Seale [1-8-8-(8+8)-16] strand.
In contrast to all the above strand types which are formed in a single operation, a Multiple Operation construction strand is one in which one of the above designs is covered with one or more layers of uniform-sized wires in a different work operation. The second operation is necessary because the outer layers must have a different length of lay or direction of lay. This example is a Warrington strand overlayed with 18 same-size wires. It is described: 37 Warrington 2-Operation [1-6-(6+6)/18] strand.
Each Characteristic Affects Other Characteristics
Every wire rope has its own “personality” which is a reflection of its engineered design. Each rope construction has been established to produce a desired combination of operating characteristics which will best meet the performance requirements of the work, or application, for which that design is intended…and each rope construction is, therefore, a design compromise.
The best illustration of a design compromise–or best combination of desired characteristics—is the interrelationship between resistance to abrasion and fatigue resistance.
Fatigue resistance (a rope’s capability to bend repeatedly under stress) is accomplished by using many wires in the strands. Resistance to metal loss through abrasion is achieved primarily with a rope design which uses fewer and, therefore, larger wires in the outer layer to reduce the effects of surface wear.
Therefore, from a design standpoint, when anything is done to alter either abrasion resistance or fatigue resistance, both of these features will be affected.
Wire rope strength is usually measured in tons of 2000 pounds. In published material wire rope strength is shown as minimum breaking force. Minimum breaking force refers to calculated strength figures that have been accepted by the wire rope industry.
When placed under tension on a test device a new rope should break at a figure equal to, or higher than, the minimum breaking force shown for that rope.
To account for variables which might exist when such tests are made to determine the breaking strength of a new wire rope an “acceptance” strength may be used. The acceptance strength is 2-1/2% lower than the minimum breaking force and ropes must meet or exceed this strength.
The minimum breaking force applies to new, unused rope. A rope should never operate at, or near, the minimum breaking force. During its useful life, a rope loses strength gradually due to natural causes, such as surface wear and metal fatigue.
2. Reserve Strength
The Reserve Strength of a standard rope is a relationship between the strength represented by all the wires in the outer strands and the wires remaining in the outer strands with the outer layer of wires removed. Reserve Strength is calculated using actual metallic areas of the individual wires. Since there is a direct relationship between metallic area and strength, Reserve Strength is usually expressed as a percentage of the rope’s minimum breaking force. Reserve Strength is used as a relative comparison between the internal wire load bearing capabilities of different rope constructions.
Reserve Strength is an important consideration in selection, inspection and evaluation of a rope for applications where the consequences of a rope failure are great. The use of Reserve Strength is premised on the theory that the outer wires of the strands are the first to be subjected to damage or wear. Therefore, the Reserve Strength figures are less significant when the rope is subjected to internal wear, damage, abuse, corrosion or distortion.
The more wires there are in the outer layer of a strand construction, the greater will be the rope’s Reserve Strength. Geometrically, as more wires are required in the outer layer of a strand, they must be smaller in diameter. This results in greater metallic area remaining to be filled by the inner wires. Separate columns are shown for standard Fiber Core and IWRC ropes. For Fiber Core ropes, the Reserve Strength is the approximate percentage of the rope’s metallic area made up by the inner wires of the outer strands.
An IWRC in a rope is considered to contribute 7-1/2% to the rope’s total strength. By definition, the core is not included in the Reserve Strength calculation so a 7-1/2% reduction has been made for ropes with an IWRC.
Rotation Resistant ropes, due to their construction, can experience different modes of wear and failure than standard ropes. Therefore, their Reserve Strength is calculated differently. For Rotation Resistant ropes, the Reserve Strength is based on the percentage of the metallic area represented by the core strand plus the inner wires of the strands of both the outer and inner layers.
3. Resistance to Metal Loss and Deformation
Metal loss refers to the actual wearing away of metal from the outer wires of a rope, and metal deformation is the changing of the shape of outer wires of a rope.
In general, resistance to metal loss by abrasion (usually called “abrasion resistance”) refers to a rope’s ability to withstand metal being worn away along its exterior. This reduces strength of a rope.
The most common form of metal deformation is generally called “peening”—since outside wires of a peened rope appear to have been “hammered” along their exposed surface. Peening usually occurs on drums, caused by rope-to-rope contact during take-up of the rope on the drum. It may also occur on sheaves.
Peening causes metal fatigue, which in turn may cause wire failure. The “hammering”, which causes metal of the wire to flow into a new shape, realigns the grain structure of the metal, thereby affecting its fatigue resistance. The out-of-round shape also impairs wire movement when the rope bends.
4. Crushing Resistance
Crushing is the effect of external pressure on a rope, which damages it by distorting the cross section shape of the rope, its strands or core—or all three.
Crushing resistance therefore is ability to withstand or resist external forces, and is a term generally used to express comparison between ropes.
When a rope is damaged by crushing, the wires, strands and core are prevented from moving and adjusting normally in operation. In a general sense, IWRC ropes are more crush resistant than fiber core ropes. Lang Lay ropes are less crush resistant than Regular Lay ropes…and 6-strand ropes have greater crush resistance than 8-strand ropes.
5. Fatigue Resistance
Fatigue resistance involves metal fatigue of the wires that make up a rope. To have high fatigue resistance, wires must be capable of bending repeatedly under stress—as when a rope passes over a sheave.
Increased fatigue resistance is achieved in a rope design by using a large number of wires. It involves both the basic metallurgy and the diameters of wires.
In general, a rope made of many wires will have greater fatigue resistance than a same-size rope made of fewer larger wires, because smaller wires have greater ability to bend as the rope passes over sheaves or around drums. To overcome the effects of fatigue, ropes must never bend over sheaves or drums with diameter so small as to kink wires or bend them excessively. There are precise recommendations for sheave and drum sizes to properly accommodate all sizes and types of ropes.
Every rope is subject to metal fatigue from bending stress while in operation, and therefore, the rope’s strength gradually diminishes as the rope is used.
Bend-ability relates to the ability of a rope to bend easily if an arc. Four primary factors affect this capability:
- Diameter of wires that make the rope.
- Rope and Strand Construction.
- Metal Composition of wires and finish, such as galvanizing.
- Type of rope core—fiber or IWRC.
Some rope constructions are by nature more bend-able than others. Small ropes are more bend-able than big ones. Fiber core ropes bend more easily than comparable IWRC ropes. As a general rule, ropes made of many wires are more bend-able than same-size ropes made with fewer larger wires.
The word “stability” is most often used to describe handling and working characteristics of a rope. It is not a precise term, since the idea expressed is to some degree a matter of opinion, and is more nearly a “personality” trait than any other rope feature.
For example, a rope is called stable when it spools smoothly on and off a drum…or doesn’t tend to tangle when a multi-part reeving system is relaxed.
Strand and rope construction contribute most to stability. Preformed rope is usually more stable than nonpreformed, and Lang Lay rope tends to be less stable than Regular Lay. A rope made of simple 7-wire strands will usually be more stable than a more complicated construction with many wires per strand.
There is no specific measurement of ropes have stability.
Wire rope is identified not only by its component parts, but also by its construction, i.e., by the way the wires have been laid to form strands, and by the way the strands have been laid around the core.
In Figure 1, “A” and “C” show strands as normally laid into the rope to the right in a fashion similar to the threading in a right-hand bolt. Conversely, the “left lay” rope strands (illustrations “B” and “D”) are laid in the opposite direction.
Again in Figure 1, the first two (“A” and “B”) show regular lay ropes. Following these are the types known as lang lay ropes (“C” and “D”). Note that the wires in regular lay ropes appear to line up with the axis of the rope; in lang lay rope the wires form an angle with the axis of the rope. This difference in appearance is a result of variations in manufacturing techniques: regular lay ropes are made so that the direction of the wire lay in the strand is opposite to the direction of the strand lay in the rope; lang lay ropes are made with both strand lay and rope lay in the same direction. Finally, “E,” called alternate lay, consists of alternating regular and lang lay strands.
Figure 1: A comparison of typical wire rope lay
A. Right Regular Lay
B. Left Regular Lay
C. Right Lang Lay
D. Left Lang Lay
E. Right Alternate Lay
The Right Way to Unreel and Uncoil a Wire Rope
There is always a danger of kinking a wire rope if you improperly unreel or uncoil it. You should mount a reel on jacks or a turntable so that it will revolve as you pull the rope off. Apply sufficient tension by means of a board acting as a brake against the reel flange to keep slack from accumulating. With a coil, stand it on edge and roll it in a straight line away from the free end. You may also place a coil on a revolving stand and pull the rope as you would from a reel on a turntable.
How to Store Wire Rope Properly
We recommend you store your wire rope under a roof or a weatherproof covering so that moisture cannot reach it. Similarly, you must avoid acid fumes or any other corrosive atmosphere – including ocean spray – in order to protect the rope from rust. If you’re storing a reel for a lengthy period, you may want to order your rope with a protective wrap. If not, at least coat the outer layers of rope with a good rope lubricant.
If you ever take a rope out of service and want to store it for future use, you should place it on a reel after you’ve thoroughly cleaned and relubricated it. Give the same storage considerations to your used rope as you would your new rope.
Be sure to keep your wire rope in storage away from steam or hot water pipes, heated air ducts or any other source of heat that can thin out lubricant and cause it to drain out of your rope.
The radius of bend has an effect on the strength of wire rope. In order to take this fact into account in selecting the size sheave to be used with a given diameter wire rope, the following table can be used as a guide:
For Example: Using a 1/2″ dia. wire rope with a 10″ dia. sheave, Ratio “A” = 10 ÷ 1/2″ = 20 and the strength efficiency = 91% as compared to the catalog strength of wire rope.
The repeated bending and straightening of the wire rope causes a cyclic change of stress known as “fatiguing.” The radius of bend has considerable effect on the fatigue life of wire rope and the following can be used as comparison of relative fatigue life as influenced by sheave diameter:
For Example: Using a 12″ dia. sheave with a 3/4″ dia. wire rope, Ratio “B” = 12 ÷ 3/4″ = 16 and the units of fatigue life = 2.1. However, a 22.5″ dia. sheave using a 3/4” wire rope has a Ratio “B”= 225 ÷ 3/4″ = 30 and the units of fatigue life = 10. So, the expected extension of fatigue life when using a 22.5″ dia. instead of a 12″ diameter sheave would be 10 ÷ 2.1 or 4.7 times greater.
To determine the weight of the block or overhaul ball that is required to free fall the block, the following information is needed:
- Size of wire rope
- Number of line parts
- Type of sheave bearing
- Length of crane boom
- Drum Friction (nominally, 100 pounds)
Formula to Determine Block Weight:
Required Block Weight = Multiply Boom Length by Rope Weight Factor “A” and add Drum Friction then Multiply by Overhaul Factor “B”.
For Example: Using 5 parts of 7/8″ Wire Rope, 50 ft. Boom and Roller Bearing Sheaves, Required Weight = [(50 x 1.42) + 100] x 5.38 = 920 lbs.
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