Hoist Ropes: Why must they be inspected? A hoist rope is not made to last forever. This is a useful fact, as the kinds of wear that appear on a rope can indicate areas of problems within an installation and provide a professional with vital clues on how to address them before catastrophic damage occurs. If a rope were ever designed to be impervious to system stresses, or even the forces of friction, it could well prove to be destructive to surrounding components that could never hope to be able to achieve such high tolerances. In addition to the impracticality and inefficiency of having a perfect rope, such an item would not be economical for use in any but the most rare kind of installation. Since no perfect rope exists, it is up to the industry professional to periodically review the rope to make sure it is discarded before it becomes operationally unsafe, and to detect any abnormal damage caused by external influences. Hoist Ropes: How often must they be inspected? ASME 17.1-2010/CSAT B44-10 recommends periodic inspections for electric and hydraulic elevators every 6 months, with evaluative tests performed every 12 months. This schedule is also dependent upon environmental factors, the frequency of usage and type of usage, quality of maintenance, age and the condition of the installation. In addition to routine inspections, professionals also recommend that ropes be checked after initial installation, and soon after the first wire breaks have been detected. One should inspect ropes more frequently in cases of abnormal loading. Ropes should be reviewed prior to usage if machinery has been idle for an extended period of time, or if that lifting system has been dismantled, moved to another location, and reused. Additionally, DIN standards mandate that sheaves, rope drums and compensation sheaves be examined whenever a new rope has been installed. For questions concerning details on the frequency of rope inspection, relevant rope discard criteria, or on sheave and drum groove diameters, consult the applicable international standards listed below. Europe : EN, ISO or DIN Great Britain : BS Japan : JIS USA: ASME Wire Defects The following images detail obvious rope defects and offer basic explanations for their cause. Should evidence for any of these conditions be found upon examining a hoist rope, international standards mandate that the rope be discarded immediately. figure 1 Wire Protrusion In this case certain wires or groups of wires rise up as loops on the side of the rope opposite to the sheave groove. Ropes found to be in this condition should be discarded immediately. Wire Protrusion figure 2 Core Protrusion An extremely rare form of deformation in NFC ropes, though it can may occasionally be found in Mixed Core ropes, in which a rope imbalance is indicated by either the protrusion of the core member through the outer rope strands, or the protrusion of an outer strand of the core element through the exterior strands of the rope. Sunken Strand Core Protrusion Sunken Strand/Local Reduction in Rope Diameter figure 3 Local reduction in rope diameter is evidence of core deterioration. This may be due to: internal wear and wire indentation; internal wear due to friction generated by bending between rope strands and wires; fiber core deterioration; fracture of a steel core. If such factors cause rope diameter to decrease by 10% of nominal the rope shall be discarded, even if no broken wires are evident (ISO 4309 3.5.7). Low values of deterioration may not be as apparent under actual examination, particularly if rope stresses are well balanced in the outer strands. Such a condition can lead to a large loss in rope strength. Any suggestion of internal deterioration shall be verified through internal examination procedures or with non-destructive testing. If deterioration is confirmed, then the rope shall be discarded.
figure 4 Strand Protrusion A basket or lantern deformation where an outer rope strand, or a wire strand surrounding the core area, protrudes from the correct laid position. This is normally evidence of a rope imbalance. Strand Protrusion figure 5 Flattened Rope figure 6 Flattened Rope Such damage is evidence of a failure of the rope wires to be able to adjust to changes in length as they are bent. This normally occurs when a rope becomes wedged in the sheave or through poor or less than careful handling technique during the installation process. Flattened rope segments which pass through the sheave will reveal broken wires and quickly deteriorate. This may damage the sheave. The rope should be discarded immediately. Kinks or Tightened Loops A deformation created when a rope loop has been tightened without an allowance being made for rotation about the rope axis. This can result in alteration of rope lay length which leads to excessive rope wear. In severe cases the hoist rope becomes so distorted that it loses all but a small proportion of its overall strength. Any rope with a kink or tightened loop should be immediately discarded. Kink or Tightened Loop Waviness figure 7 Waviness A deformation where the longitudinal axis of the wire rope takes the shape of a helix either in a loaded or unloaded state. While not necessarily indicating a loss in strength, such a deformation can transmit a pulsation throughout the entire rope length and create irregular rope drive. This can lead to accelerated rope wear (as seen in a rise in wire breaks) and excessive rope vibration, which is marked by increased system noise.
figure 8 Birdcage or Basket/Lantern Deformation figure 9 Birdcage or Basket/Lantern Deformation A deformation created by a difference in length between the rope core and the outer layer strand layer. There can be many causes behind such a condition, such as: when running over a tight sheave (i.e., a sheave with a groove radius that is too small for the rope) the entire wire rope is compressed. This reduction in overall rope diameter simultaneously creates an increase in overall rope length. As the outer strands are naturally compressed to a greater extent than the inner core, they also increase in length to a greater degree than the core. A discrepancy between core and outer strand length can lead to the deformation shown. should a rope run over a sheave or drum at too great a fleet angle, the rope will contact the flange of the sheave (or the drum) instead of settling directly into the groove. This forces the rope to roll down into the bottom of the groove, unlaying the outer strands of the rope to a greater degree than the rope core, creating an alteration in length of core and outer strand elements. (Note: See figure 9 for an illustration of this condition). In both cases the sheave and drum will displace the loose outer strands, causing the discrepancy in core and outer strand lengths to appear in one location as a basket or lantern deformation. Large Fleet angle causing rope to roll down into groove. External Wear of Crown Wires figure 10 External Wear Abrasion of the crown wires of outer strands is due to the wires rubbing the grooves of the sheaves and drums under pressure. This rope condition, which is marked by flat surfaces on crown wires, is particularly evident at points of sheave contact when the load is being accelerated or decelerated.
figure 11 Crown breaks originate on the outside of a rope at the contact point between the rope and sheave, or drum. If there is no obvious indication of rope wear the condition is normally cause by bending stresses, or stresses placed upon the rope s wires as it bends under load over a sheave. Close attention must be paid to a rope if two or more valley breaks are found in one lay length. It is a sign that the rope should be considered for discard. figure 12 Valley Wire Breaks Valley breaks originate within the rope itself and are a sign of rope core deterioration. Should evidence of a valley break be found one should inspect the rope segment closely. Two or more valley breaks found in one lay length indicate that a rope should be considered for discard. figure 13 External Corrosion Such a condition can be found in environments where debris and dirt are allowed to accumulate, or those where an excessive amount of moisture is present (which can cause a rope to rust). External corrosion will diminish rope breaking strength by reducing the metallic cross sectional area, and accelerate fatigue by causing surface irregularities which lead to stress cracking. Severe corrosion can cause decreased elasticity in a rope. External Corrosion Rouging figure 14 Rouging Rouging is a fine, red oxide crust that forms on a hoist rope giving it a rusty appearance. The debris exudes from within the rope and mainly deposits itself between the outer rope strands, and then accumulates lightly on the ropes surface (only trace amounts on the crowns of wires), where it spontaneously oxidizes. Though the red color is due to an oxidation of steel wire fragments, it is not evidence of rust which indicates the presence of moisture. Instead, rouging is a sign of abrasion between rope strands and the core, or between adjacent wire strands, and indicates internal rope degradation. Rouging is generally due to the presence of heavy loads and vibrations being placed on a rope and creating pressures that then work lubricant out of the rope core. Note: field lubrication can only clean and retard further rope damage, it cannot bring the rope back to its original state. The hoist rope should be subjected to internal examination should rouging be detected.
figure 15 Detecting Wire Breaks by Bending the rope Detecting Wire Breaks: Why it s not so easy The actual amount of hoist rope that a professional can inspect is comparatively small. The outer wires of the outer strands are not visible along the underside of the rope, and for a zone of nearly 120 of the strand s circumference. This means that only 2/3 of the 50% of any rope s metallic cross sectional area is available for visual inspection (this is equivalent to an amount equaling 1/3 of the rope s entire cross section). Although the amount of area available for inspection for a steel rope remains the same as that for a Mixed or Fiber Core rope, the greater amount of steel used in the construction means that the total proportion of wire available for inspection drops even further by some estimates to as little as 30% of the total metallic cross sectional area. Wire breaks that appear at the crowns of the strands, and obvious breaks that appear in the valleys, are relatively easy to detect. However breaks at the contact points of two adjacent strands or a rope s underside can be difficult to identify. In many cases the only thing that can be done is for a professional to review the rope, secure and remove its loads, and then physically bend it. One can then inspect the rope more closely and identify any breaks. As can be seen in Figure 15, at first glance the rope appears free of wire breaks. However subsequent bending reveals numerous breaks that have developed at the underside of the strands where they contacted the steel core. Long, free, wire ends indicate that the breaks did not occur on the rope s surface. Though certain older industrial publications did advise the use of certain invasive means to assess the state of a rope s steel core or to examine the underside of a rope s outer wires (through the use of a Marlin Spike), we cannot recommend this procedure. Any mechanical intrusion into a rope s construction (no matter how carefully applied) can lead to damage of the rope s design. An Obvious Recommendation Instead we recommend a passive, investigative inspection technique that compares the total number of wire breaks against relevant international standards. The professional then should use that criteria, as well as observations on rope diameter, end connections, and an evaluation of surrounding machinery (such as sheaves and drums) to assess rope condition. Rope Discard Recommendations To obtain the most recent regulations concerning rope replacement we urge you to consult the relevant international standards that specifically affect you. However, we do offer these general guidelines for reference purposes that reflect information based upon ASME 17.6-2010 Table 1.10.1.2-1 Wire Breaks ( Per Lay Length). These guidelines are similar to those listed in on ISO 4344:2004 (E) Table E.1, (Number of Visible Broken Wires). These guidelines do not apply to Governor ropes, 6-Strand Ropes Type of Breaks Wear Wear Conditions Conditions Normal Unfavorable Ropes Show Rouge Distributed Breaks (max) 24 12 12 Unequal Breaks (max) 8 4 4 4 Side-by- Side Breaks 12 6 6 8 and 9-Strand Ropes Type of Breaks Wear Wear Conditions Conditions Normal Unfavorable Ropes Show Rouge Distributed Breaks (max) 32 16 16 Unequal Breaks (max) 10 5 5 4 Side-by- Side Breaks 16 8 8 General Notes: A. Provided tables do not apply to Winding Drum Machines or Governor Ropes. B. Unfavorable Conditions include, but are not limited to, corrosion due to external conditions, excessive wear of individual wires strands, unequal tension, poor sheave grooves. C. If fretting conditons indicated by a red dust or rouge exist, the criteria to be followed are those indicated in Ropes Showing Rouge. No more than one valley break per lay length and valley breaks per lay length are allowed if visible rouge is detected. D. Ropes less than 8 mm (.315 in.) in diameter shall be replaced when rouging is indicated. E. Ropes should be replaced if there is more than one valley break per rope lay, or if valley breaks are indicated at any point where rouging may be found.