Factors Affecting the Selection of Wire Rope
The key to choosing the rope best suited for the job lies in an accurate estimation of the important requirements. Correct appraisal of the following will simplify the selection process:
- Strength- resistance to breaking
- Resistance to bending fatigue
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Resistance to vibration fatigue
- Resistance to abrasion
- Resistance to crushing
- Resistance to Strength
It is essentially impossible for any single rope to have top values in all of the above qualities. The rule, in fact, seems to be that a high rating in one almost always means lower ratings in others. The first task is to make a careful analysis of the job requirements, establishing priorities among these requirements and then selecting the rope on a trade off basis. This will provide the best possible balance by sacrificing the least essential advantages in order to obtain maximum benefits in the most important requirements.
The following are brief explanations of six factors previously listed:
1) Strength- resistance to breaking
It has been noted that wire rope is a machine-a fairly complex device that transmits and modifies force and motion. Thus, the very first consideration in choosing a “machine” is to determine the potential workload. Stated in terms of wire rope, this means establishing the actual load that is to be moved. To this known dead weight, there must be added those loads that are caused by abrupt starts (acceleration), sudden stops, shock loads, high speeds and friction of sheave bearings. Another item in this equation is the loss of efficiency that occurs when the rope is bent over sheaves. All of these loads must be summed up in order to determine the true total load that will ultimately be handled.
For an average operation, this figure is generally multiplied by a “design factor” of 5. For increased mobility or design space economy, a design factor of less than 5 is used at times, but if the load is especially valuable or if there is danger to human life, a larger design factor (up to 8 or 9) is used in some instances. A still larger factor is grade and core of wire rope to be considered.
2) Resistance to bending fatigue
To describe this, a close analogy can be made with a paper clip. If it is repeatedly bend back and forth at one point it will eventually break. The reason for this a phenomenon called “metal fatigue”. To some degree, the same thing happens when a wire rope is bent around sheaves, drums and rollers. The sharper, or more acute, the bend, the quicker the fatigue factor does its work. Accelerating the rate of travel also speeds up fatigue; close coupled reverse bending will speed it up at a faster rate.
Fatigue can be greatly reduced if sheaves and drums have, at the very least, the suggested minimum diameter. As for the rope, there is one governing overall rule: the greater the number of wires in each strand, the greater the resistance of the rope to bending fatigue. The subject of metal fatigue is covered in more detail by a larger body of literature. It is not the intent of this publication to discuss, even in broad terms, the theoretical concepts of the phenomenon. It will simply be noted here that the concept of fatigue as a cause of metal “crystallization” is incorrect since all metals are at all time crystalline in structure. The crystalline appearance in many fractures is not indicative of “crystallization”.
3) Resistance to vibration fatigue
Vibration, from whatever source, sends shock waves through the rope. These waves are a form of energy that must be absorbed at some point. This point may appear at various places – the end attachment, the tangent where the rope contact the sheave or at any other place where the waves are damped and the energy absorbed.
In the normal operation of a machine or hoist, wire ropes develop a wave action that can be observed either as a low frequency or as a sharp, high frequency cycle. A good example of this is found in shaft hoists. When the cage is just starting up the rope has a very slow swing within the shaft. But, by the time the cage reaches the top of the shaft, the initially low frequency has become a high frequency vibration.
vibration. The result is eventual breakage of the wires at the attachment of the cage.
Another type of vibrational fatigue is found in operations where there is cyclic loading. Such loadings would be found, for example, in the boom suspension system of draglines. Here, the energy is absorbed at the end fittings of the pendants or at the tangent point where the rope contacts the sheave. In this case, the “vibration” is torsional as well as traverse.
4) Resistance to Abrasion
Abrasion is one of the most common destructive conditions to which wire rope is exposed. It will occur whenever a rope rubs against, or is dragged through, any soil or other material. It happens whenever a rope passes around a sheave or drum. And, it takes place internally within the rope whenever it is loaded or bent. Abrasive action weakens the rope simply by removing metal from both side and outside wires.
When excessive wear is encountered in an operation, the problem usually stems from faulty sheave alignment, incorrect groove diameters, an inappropriate fleet angle or improper drum winding. There may, however, be other causes. If on investigation, none of these common conditions are found to be causative factors, the solution may lie in changing to a more suitable rope construction. In making such a change, it is helpful to remember that larger outer wires and Lang-lay ropes are more abrasion resistance than regular-lay ropes.
5) Resistance to Crushing
Rope can be crushed by its own pressure against a sheave, from improperly sized groves and from multiple layers on a drum.
The sheave diameter and the load determine the pressure of rope against a sheave. The pressure of rope to a drum is influenced in great measure by the support of the groove; smooth drums have a more adverse effect than those that are grooved.
Multiple layer winding is also a cause of wear even when the winding is done in an orderly (thread-winding) manner. Irregular or scramble winding is an even greater cause of damage.
Obviously, in each of these cases, reducing that load will ease the condition. If, however, this is not feasible, offending sheaves may be replaced with sheaves that have larger tread diameters. Unsuitable drums and /or winding conditions should be corrected. Otherwise, the rope will have to be replaced by one with a construction better designed to resist the abuse.
If the original rope has a fiber core, the replacement should have a steel core because a steel core rope will provide greater physical support. Also it is well to remember that regular-lay are better able to resist crushing than Lang-lay ropes.
6) Reserve Strength
The reserve strength to a wire rope is defined as the combined strength of all the wires it contains; expect those in the outside layer of strands.
The “X-Chart” Abrasion Resistance VS Bending Fatigue Resistance
While there is a possibility, there is little likelihood that an application can be found for which there is precisely suitable wire rope-one that can every indicated requirement.
As with all engineering design problems, feasible solutions demand compromise to some degree. At times become necessary to settle for less than optimum resistance to abrasion in order to obtain maximum flexibility, the latter being a more important requirement for the given job. A typical example of this kind of trade-off would be in selecting a highly flexible rope on an overhead crane. Conversely, in a haulage installation, a rope with greater resistance to abrasion would be chosen despite the fact that such ropes are markedly less flexible.
Two compelling facts that govern most decisions as to the selection of a wire rope are: abrasion resistance and resistance to bending fatigue. Striking a proper balance with respect to these two important characteristics demands judgment of a very high order. A graphic presentation of this comparison of qualities between the most widely used rope construction an others is given by means of the “X- Chart”.
Referring to this chart when selecting a rope, the mid-point (at the X) comes closest to an even balance between abrasion resistance and resistance to bending fatigue. Reading up or down along either leg of the X, the inverse relationship becomes more apparent as one quality increases and the other decreases.
The term flexibility is frequently thought of as being synonymous with resistance to bending fatigue. This it not true. Flexibility refers to the capability of flexing or bending. While a high degree of fatigue resistance may sometimes accompany the flexibility characteristic, it does not necessarily follow that this is so. A fiber core rope, for example, is more flexible than IWRC rope. Yet, when the IWRC rope is bent around sheaves at relatively high loads, it will usually perform better than the more flexible fiber core rope. The reason for this lies in the ability of IWRC rope to retain it roundness and freedom on internal movement. Under the same conditions, a fiber core rope will flatten and inhibit free internal adjustment, thereby leading to early failure.
As noted earlier, a design choice is almost invariably the result of compromise. Ultimately, what is sought is an efficient economical solution, hence whatever the compromise, it must help achieve this goal.
Breaking In a New Wire Rope
A new wire rope requires careful installation and close adherence to following all the appropriate procedures previously notes. After the rope has been installed and the ends secured in the correct manner, the mechanisms should be started carefully and then permitted to run through a cycle of operation at very slow speed. During this trial operation, a very close watch should be kept on all parts-sheaves, drums, rollers- to make certain that the rope runs freely and without any possible obstructions as it makes its way through the system. If no problems appear in running the rope, the next step should include several run-through of the normal operational cycle under light load and at reduced speed. This procedure allows the component parts of the new rope to make a gradual adjustment to the actual operating conditions.
Wire Rope and Operations Inspection
It is essential to maintain a well-planned program of periodic inspection. Frequently, there are statutory and/or regulatory agencies whose requirements must be adhered to, but whether or not these exist in a given locale; the wire rope user can be guided by the suggested procedures that follow.
Abrasion, bending, and crushing represent the ABC’s of wire rope abuse and it is the primary goal of good inspection practice to discover such conditions with minimum effort. When any sudden degradation indicates a loss of original rope strength, a decision must be made quickly as to allowing the rope to remain in service. An experienced inspector can only make a decision, such as this. Their determination will be based on:
- Details of the equipment’s operation
- Frequency of inspection
- Maintenance history
- Consequences of failure
- Historical records of similar equipment