Any wire rope that has broken wires, deformed strands, variations in diameter or any change from its normal appearance must be considered for replacement. It is always better to replace a rope when there is any doubt concerning its condition or its ability to perform the required task. The cost or wire replacement is quite insignificant when considered in terms of human injuries the cost of down time, or the cost of replacing broken structures.
Wire rope inspection includes examination of basic items such as:
· Rope diameter Peening
· Rope lay Scrubbing
· External wear Corrosion
· Internal wear Broken wires
Some wire rope sections can break up without any visible warning. Sections where this occurs are usually found at end termination and at points where the rope enters or leaves the sheaves of boom hoist, suspension systems or other semi-operational systems. Because of the “working” that takes place at those sections, neither appreciable external wear nor crown breaks will appear. Under such conditions the core fails thereby allowing adjacent strand nicking. When this happens, valley breaks are likely to appear and the rope should be removed.
If preventative maintenance is performed diligently, rope life can be prolonged. Cutting off an appropriate length of rope at the end termination before the core degrades and valley breaks appear, minimizes degradation at these sections.
Electromagnetic Testing of Wire Rope
Electromagnetic non-destructive testing (NDT) of wire rope can be an extremely tool for the evaluation of the condition of an operating rope. NDT is frequently used on wire ropes used in underground mines, material and passenger tramways and aerial lifts. The NDT devices in use today are capable of detecting localized faults such as wire breaks and damage and loss of metallic area (LMA). A strip chart is produced by the device, which provides a visual display and location of anomalies from a baseline reading. The rope can then be visually inspected for confirmation and assessment of the actual condition.
It is recommended that any new length of wire rope, which will be assessed by NDY equipment, be inspected as soon as it is installed. This inspection will then be the baseline for future NDT inspection from which LMA and other anomalies can be assessed.
NDT inspection should not be, and is not intended to be, the only means for inspection. In all cases a following visual inspection is mandatory.
There are Safety Standards presently in use that requires the use of NDT for certain applications and operating conditions. These standards require the mandatory removal of wire rope from service with more than a 10% LMA. It is important that the 10% loss is from a baseline reading on a new or unused section of the rope and not from a used or worn section.
None of the NDT devices in use can measure loss of strength. Any estimate of loss of strength should be based on the actual rope breaking strength and not on the nominal strength. It is very difficult and often misleading to estimate loss of strength solely on LMA.
Any undetected fault on a sheave, roller or drum, be it of relatively major or minor significance, can cause a rope to wear out many times faster than the wear resulting from normal operations. As a positive means of minimizing abuses and other than normal wear, the procedures here set forth should be adhered to. Every observation and measurement should be carefully recorded and kept in some suitable and accessible file.
- Give close examination to the method by which the rope is attached both to the drum and to the load. Make certain that the proper type of attachment is applied correctly and that any safety devices in use are in satisfactory working order.
- Carefully check the groove and working surface of every sheave, roller and drum to determine whether each (groove and surface) is as near to the correct diameter and contour as circumstances will permit and whether all surface that are in contact with the rope are smooth and free of corrugations or other abrasive defects.
- Check sheaves and rollers to determine whether each turns freely and whether they are properly aligned with the travel of the rope. All bearing must be in good operating condition and furnish adequate support to the sheaves and rollers. Sheaves that are permitted to wobble will create additional forces that accelerate the degradation of the rope.
- If starter, filler and riser strips on drums are used, check their condition and location. Should these be worn, improperly located or badly designed, they will cause poor winding, dog legs and other rope damages.
- Wherever possible, follow the path that the rope will follow through a complete operating cycle. Be on the lookout for spots on the equipment that have been worn bright or cut into by the rope as it moves through the system. Ordinarily, excessive abrasive wear on the rope can be eliminated at these points by means of some type of protector or roller.
During fabrication, ropes receive lubrication, the kind and amount depending on the rope’s size, type and use, if known. This is process treatment will provide the finished rope with ample protection for a reasonable time if it is stored under proper conditions. But, when the rope is put into service, the initial lubrication will normally be less than needed for the full useful life of the rope. Because of this, periodic applications of a suitable rope lubricant are necessary.
Following are important characteristics of a good wire rope lubricant:
- It should be free from acids and alkalis
- It should have sufficient adhesive strength to remain on the rope
- It should be of a viscosity capable of penetrating the interstices between wires and strands
- It should not be soluble in the medium surrounding it under the actual operating conditions
- It should have a high film strength
- It should not resist oxidation
Before applying lubrication, accumulations of dirt or other abrasive material should be removed from the rope. Cleaning is accomplished with a stiff wire brush dipped in solvent, compressed air or live steam. Immediately after it is cleaned, the rope should be lubricated. When it is normal for the rope to operate in dirt, rock or other abrasive material, the lubricant should be selected with great care to make certain that it will penetrate and, at the same time, will not pick up any of the material through which the rope must be dragged.
As a general rule, the most efficient and most economical means to do field lubrication/protection is by using some method or system that continuously applies the lubricant while the rope is in operation. Many techniques are used; these include the continuous bath, dripping, and pouring, swabbing, painting or where circumstances dictate, automatic systems can be used to apply lubricants either by a drip pressure spray method.
Elastic Properties of Wire Rope
The following discussion relates to conventional 6 to 8 strand ropes that have either fiber or steel cores; it is not applicable to rotation resistant ropes since these constitute a separate case.
Wire rope is an elastic member; it stretches or elongates under load. This stretch derives from two sources:
In actuality, there may be a third source of stretch – a result of the rope rotating on its own axis. The unlaying of the rope strands brings about such elongation, which may be occur either as a result of using a swivel or from the effect of a free turning load. Because the third source is a subject that is beyond the scope of this publication, discussion will be directed to constructional and elastic stretch.
When a load is applied to wire rope, the helically laid wires and strands act in a constricting manner thereby compressing the core and bringing all the rope elements into closer contact. The result is a slight reduction n diameter and an accompanying lengthening of the rope.
Constructional stretch is influenced by the following factors:
- Type of core (fiber or steel)
- Rope construction (6×7, 6×25 FW, 6×41 WS, 8x 19S, etc.)
- Length of lay
Ropes with wire strand core (WSC) or independent wire rope core, (IWRC) have less constructional stretch than those with fiber core (FC). The reason for this is the fact that the steel cannot compress as much as the fiber core.
Usually, constructional stretch will cease at an early stage in the rope’s lift. However, some fiber core ropes, if lightly loaded (as in the case of elevator ropes), may deplay a degree of constructional stretch over a considerable portion of their life.
A definite value for determining constructional stretch cannot be assigned since it is influenced by several factors. The following table gives some idea of approximate stretch as a percentage of the rope length under load.
6 Strand FC
1/2% – 3/4%
6 Strand IWRC
1/4% – 1/2%
8 Strand FC
3/4% – 1%
- Varies with the magnitude of the loading.