Strength & Flexibility in Lifting

Technical Information

Wire Rope Technical Information

Wire Rope Markers

Wire rope manufactured in the United States normally has some type of colored marker to identify the manufacturer. Two types of markers may be used -- strand markers and core markers.

A strand marker can be seen by looking at the wire rope; it is simply a colored lubricant applied externally to one strand during manufacture. Strand markers are not used in mining rope, elevator rope, galvanized rope, compacted rope or any rope that is post-lubricated. Strand markers are used in the manufacture of all standard (round) wire ropes but are not a confirmation of the manufacturer in an of itself.

Core markers are used in most wire rope manufactured in the United States, but cannot be seen unless the wire rope is disassembled. Manufacturers add colored threads of filaments to fiber and steel cores.

For example, every Bethlehem Wire Rope product contains one or both types of markers. WWW uses a purple strand in the manufacture of all standard EIP and EEIP Bethlehem Wire Rope, excluding those ropes cited previously. In addition, every Bethlehem Wire Rope product contains two filaments in the core (either fiber or steel) - one yellow and one purple filament.

Wire Rope Diameter Tolerances

Wire rope is always manufactured larger -- never smaller -- than the nominal diameter when specified in inches. The allowable tolerances are shown in the table. In standard practice, the nominal diameter is the minimum diameter. All tolerances are taken on the plus side when specified in inches. Wire rope is not termed oversize until its diameter exceeds the allowable maximum. For example, a 1" nominal diameter wire rope may vary between 1" and 1.05" in diameter.

Nominal Diameter Under Over
- 1/8" - 0% + 8%
1/8" - 3/16" - 0% + 7%
3/16" - 5/16" - 0% + 6%
5/16" + - 0% + 5%

Wire Rope Strength Design Factors

The rope strength design factor is the ratio of the rated strength of the rope to its operating stress. If a particular rope has a rated strength of 100,000 lbs. and is working under an operating stress of 20,000 lbs., it has a rope strength design factor of 5. It is operating at one-fifth or 20% of its rated strength.

Many codes refer to this factor as the "safety factor" which is a misleading term since this ratio obviously does not include many facets of an operation which must be considered in determining safety.

Wire rope is an expendable item -- a replacement part of a machine or installation. For economic and other reasons, some installations require ropes to operate at high stresses (low rope strength design factors). On some installations where high risk is involved, high rope strength design factors must be maintained. However, operating and safety codes exist for most applications and these codes give specific factors for usage. When a machine is working and large dynamic loadings (shockloadings) are imparted to the rope, the rope strength design factor will be reduced, which may result in overstressing of the rope. Reduced rope strength design factors frequently result in reduced service life of wire rope.

Wire Rope Physical Stretch Properties

The following discussion relates to conventional 6 or 8 strand ropes that have either a fiber or steel core. It is not applicable to rotation-resistant ropes since these constitute a separate case.

Wire rope is an elastic member; it stretches and elongates under load. This stretch is derived from two sources:

Constructional stretch

When a load is applied to wire rope, the helically-laid wires and strands act in a restricting manner, thereby compressing the core and bringing all of the rope elements into closer contact. The result is a slight reduction in diameter and an accompanying lengthening of the rope. Constructional stretch is influenced by:

  • Type of core
  • Rope construction
  • Length of lay
  • Material

Ropes with a WSC or IWRC have less constructional stretch than those with a fiber core. The reason for this is steel cannot compress as much as the fiber core. Usually, constructional stretch will increase at an early stage in the rope's life. However, some fiber core ropes, if lightly loaded (as in the case of elevator ropes), may display a degree of constructional stretch over a considerable portion of their lives. A definite value for determining constructional stretch cannot be assigned since it is influenced by several factors. The Constructional Stretch table gives some idea of the approximate stretch as a percentage of rope under load.

Rope Construction Approximate Stretch
6-Strand Fiber Core 1/2% to 3/4%
6-Strand IWRC 1/4% to 1/2%
8-Strand Fiber Core 3/4% to 1%

Elastic stretch

Elastic stretch results from recoverable deformation of the metal itself. Here again, a quantity cannot be precisely calculated. However, the equation shown below can provide a reasonable approximation for many situations.

In actuality, there may be a third source of stretch -- a result of the rope's rotating on its own axis. Such elongation, which may occur either as a result of using a swivel, or from the effect of a free turning load, is brought about by the unlaying of rope strands. This type of stretch is undesirable and may lead to rope failure.

Changes in length (ft.)  =  ( Change in load (lbs.) x Length (ft.) ) / ( Area (inches^2) x Modulus of Elasticity (psi) )

Block Twisting

Block twisting or cabling is one of the most frequently encountered wire rope problems in the construction industry. When this problem occurs, the wire rope is most often blamed, and other equally important factors in the operation are overlooked.

Personnel experienced with handling wire rope know that conventional wire ropes will twist or unlay slightly when a load is applied. In a reeved hoisting system subjected to loading and unloading, such as a hoist line, this results in block twisting and possibly distortion of the wire rope. Cabling of the block most frequently occurs as the load in the wire rope is released and the falls are in a lowered position. Cabling may be considered as the twisting of the block beyond one-half revolution (180° twisting) of the traveling block. When this condition occurs, the operator shows good judgement in not making additional lifts until the conditions causing the problem are corrected.

The following machine and site conditions should be investigated for possible improvement in block twisting.

  1. Reduce wire rope length. Longer rope lengths cause more twisting than short rope lengths. This applies particularly to the amount of wire rope in the fall.
  2. Reduce the amount of load lifted. Heavily loaded ropes have more torque and twist than lightly loaded ropes. This condition would also apply to the speed of loading or shockloading, since this condition also causes higher wire rope loading.
  3. Eliminate odd-part reeving where the wire rope dead end is on the traveling block. Wire rope torque, from the application of load, is greatest at the rope dead end.
  4. Relocate the rope dead end at the boom in order to increase the separation between the dead end and the other rope parts. This applies a stabilizing load directly to the traveling block. The original equipment manufacturer should be consulted before making this modification.
  5. Increase sheave size. This increases the amount of separation between wire rope parts and may improve the situation by applying stabilizing loads and reducing the amount of rope torque transmitted to the traveling block.
  6. Restrain the twisting block with a tag line.

The use of rotation-resistant wire ropes will not likely be required unless the intended length of rope fall exceeds 100 feet, or the length of the hoist line exceeds 600 feet. In the event these latter conditions exist, the end user should anticipate using a combination of the rotation-resistant wire rope and the foregoing field suggestions.

Effect of Sheave Size on Wire Rope

Wire ropes are manufactured in a great variety of constructions to meet the varying demands of wire rope usage. Where abrasion is an important factor, the rope must be made of a coarse construction containing relatively large outer wires. In other cases, the great amount of bending to which the rope is subjected is more important. Here, a more flexible construction, containing many relatively small wires, is required. In either case, however, if the rope operates over inadequate size sheaves, the severe bending stresses imposed will cause the wires to break from fatigue, even though actual wear is slight. The smaller the diameter of the sheave, the sooner these fatigue breaks will occur and the shorter rope life becomes.

Another undesirable effect of small sheaves is accelerated wear of both rope and sheave groove. The pressure per unit area of rope on sheave groove for a given load is inversely proportional to the size of the sheave. In other words, the smaller the sheave the greater the rope pressure per unit area on the groove. Both sheaves and rope life can obviously be prolonged by using the proper diameter sheave for the size and construction of rope.

Sheave diameter can also influence rope strength. When a wire rope is bent around a sheave, there is a loss of effective strength due to the inability of the individual strands and wires to adjust themselves entirely to their changed position. Tests show that rope strength efficiency decreases to a marked degree as the sheave diameter is reduced with respect to the diameter of the rope.

A definite relationship exists between rope service and sheave size. As a guide to users, wire rope manufacturers have established standards for sheave sizes to be used with various rope constructions.