![]() The tables show relative expected rope service life in relationship to D/d ratios and Design Factors. |
Relative Service LifeThe ability of wire rope to withstand repeated bending work over sheaves and onto drums is also called the ‘fatigue resistance’. This terms does NOT describe the ability to withstand mechanical damages nor the crush resistance of the rope. The fatigue resistance of a rope is not time but cycle dependent. Bending fatigue is the ability to withstand repeated bending over sheaves and drums and such ability is depending on factors such as drum diameter, groove dimensions, rope tension, line speed, rope construction, fluctuation between highest and lowest loads, etc. For further information on this topic, please contact us for technical assistance and advice. |
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Loss of Strength over Sheaves and PinsRope breaking strength is determined in a standard test wherein fittings are attached to the ends of the rope and the rope is pulled in a straight line. If however, the rope passes over a curved surface (such as a sheave or pin) its strength is decreased. The amount of such reduction will depend on the severity of the bend as expressed by the D/d ratio. For example, a rope bent around a pin of its own diameter will have only 50% of the strength attributed to it in the standard test. This is called ‘50% efficiency’. Even at D/d ratios of 40, there may be a loss of up to 5%. At smaller D/d ratios, the loss in strength increases quite rapidly. The angle of bend needs not to be 180°, 90°, or even 45°; relatively small bends can cause considerable loss. The table shown derived from standard test data as published by the ‘Wire Rope Technical Board’, is based on static loads only, and is a weighted average of 458 tests over pins and thimbles on 6×19 and 6×37 class ropes. |
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Why multi-strand Wire Rope?The number of outer strands determine the contact area between the rope and the sheave groove. If this area is increased the points of contact are multiplied and contact pressures are reduced. At the same time lateral notching stresses between strands and wires are reduced, resulting in increased fatigue life. Extensive tests programs at the University of Stuttgart, Germany, have proven conclusively that the bending fatigue life of wire rope improves with an increasing number of outer strands. Based on that research we have developed Python® High Performance Wire Rope with 8-, 9-, and 10 outer strands. |
Archive for the ‘Technical Information’ Category
Sheave- and Drum Dimensions
Sheave Dimensions
We recommend only sheaves made of steel or cast steel. The following are recommended values:
Groove radius (r):
Minimum 0.53 to 0.535 x d
Maximum 0.55 x d
Recommended: ≈ 1% over the actual wire rope diameter
Groove depth (h):
Recommended: ≈ 1.5 x d or d x √2
Sheave throat angle:
For normal applications use 35° to 45° opening. Applications with deflection angles of more than 1.5° use 60° opening. Opening angles below 35° must be avoided.
Sheave hardness:
As the hardness of the single wires in a rope can be approximated to be 50-55 RC, the hardness on the sheave surface must be min. 35 RC, better is 40-45 RC.
D/d ratios:
Depending on the actual equipment and usage of wire rope in hoisting- or pulling applications the following D/d ratios are suggested:
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Drum Dimensions
Generally, we recommend grooved drums only. The rope is spooled properly and positively. Depending on the drum/rope diameter relationship helix-grooved drums can be used for up to 3 layers without excessive rope wear. For applications with more than 3 layers (e.g. Mobile cranes) we recommend ‘Lebus’ grooving.
It has to be remembered, however, that rope service life on multiple layer drum systems will always only be a fraction of that compared with single layer helix-grooved drums.
Important: for standard applications the drum grooving should be left hand to accomodate standard right hand lay ropes!
Groove radius (r):
Minimum 0.53 to 0.535 x d
Maximum 0.55 x d
Pitch diameter (p):
The pitch dia. has to be chosen in accordance with the groove radius. Under no circumstance should the pitch dia. be smaller than:
• Minimum 2.065 x groove radius
• Maximum 2.18 x groove radius
If these values are applied to single layer grooved drums the maximum permissible rope-deflection angle for regular wire rope constructions is 4°. For non rotating /rotation-resistant ropes the maximum permissible deflection angle is 1.5° only.
Groove depth (h):
Minimum ≥ 0.374 x d for helically grooved drums
Rotation Resistant and Non-Rotating Wire Rope
HiPacᵀᴹ and ForcePacᵀᴹ Compaction
PYTHON® wire ropes are manufactured using either the HiPacᵀᴹ Compaction- or the ForcePacᵀᴹ Compaction process. Here are the differences:
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HiPacᵀᴹ CompactionMost popular Python® compaction type. This process is applied to the strands and NOT to the rope. The ready made strands are run through a compaction process which compress and shape the individual outer wires to have a flat outer surface. The advantages are: |
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ForcePacᵀᴹ CompactionThis process is usually applied to Python® DoPar type wire ropes or where the rope core is plastic coated. This process is applied after the rope has been manufactured and compresses the entire rope circumference. Individual surface wires are shaped flat as well as closing strand gaps. The advantages are: |
Tensile Strength and Fill Factor
The breaking strength of wire rope can be increased in two ways: either by increasing the wire material TENSILE STRENGTH or by increasing the rope’s FILL FACTOR.
Tensile StrengthThe wires of wire rope are made of high-carbon steel. These carbon steel wires come in various grades. Wire ropes are usually made of Extra Improved Plow Steel (EIPS) or Extra Extra Improved Plow Steel (EEIPS) which roughly equivalents to a wire tensile strength of 1960N/mm² and 2160N/mm². As one can see from the tables, the difference in the rope’s breaking strengths by increasing the material tensile strength is only about 10%. Fill FactorIn order to further increase the breaking strength of wire rope one has to increase the rope’s fill factor. The fill factor measures the metallic cross section of a rope and compares this with the circumscribed area given by the rope diameter. Traditional rope constructions ‘fill’ the rope diameter only up to about 58% with steel. Python® and Compac® wire rope ‘fill’ the rope diameter up to 80% with steel. That is an metallic increase of about 38% which results in a similar increase in rope strength. Two methods can be employed: selecting a different rope CONSTRUCTION or COMPACTING/DIE DRAWING the rope/strands. Many high strength rope constructions use both methods at the same time. |
![]() Fill or Density Factor of different rope constructions
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Cut and Slip Procedure, Lubrication
to a rope may occur first. |
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to lubricate the rope inside and out to maintain optimum rope fatigue properties. |
Cut and Slip Procedure
On multiple layer drums, wire rope will wear out at the crossover points from one wrap to the next. At these crossover points, the rope is subjected to severe abrasion and crushing as it is pushed over the rope ‘grooves’ and rides across the crown of the layer beneath. The scrubbing of the rope, as this is happening, can easily be heard.
In order to extend the rope’s working life, shortening of the rope at the drum anchoring point of approx. 1/3 of the drum circumference, moves the crossover point to a different section of the rope. Now, a rope section previously not subjected to scrubbing and crushing will take the workload.
Lubrication
During fabrication, ropes are lubricated; the kind and amount depending on the rope’s size, type and use, if known. This in-process treatment will provide the finished rope with ample protection for a reasonable time if it is stored under proper conditions, and in the early stages of the rope’s working life. It must be supplemented, however, at regular intervals.
Re-lubrication of a wire rope is not always a simple task. Apart from lubricant being a messy matter in itself, old lubricant, dirt and other particles may cover the outside of a rope to a point were any newly applied lubricant will not be allowed to penetrate the inside of a rope. In these cases it becomes necessary to either thoroughly clean the rope, or to use a high pressure lubrication device which forces new lubricant into the rope.
If the wire rope surface is clean, re-lubrication can also be made with spray cans of specially formulated lubricant which penetrates the inside of a rope.
The re-lubrication procedure and program is very much dependent on the length and size of a rope and on the equipment the rope is installed on. In any case, if a planned program of regular lubrication is not carried out, the rope will deteriorate more rapidly.
Remember that tests have shown that non-lubricated ropes will generate only about 1/3 of the bending cycles than ropes which are well lubicated. Python® ropes with a plastic coated core have the advantage that the inner rope is ‘permanently lubricated’; the lubrication is ‘sealed in’.
Inspection of Sheaves and Drums, Dimensions of Groove Radius
Inspection of Sheaves and DrumsProper maintenance of the equipment on which the ropes operate has an important bearing on rope life. Worn grooves, poor alignment of sheaves and worn parts resulting in shock loads and excessive vibration will have a deteriorating effect. Sheaves should be checked periodically for wear in the grooves which may cause pinching, abrasion, and bird-caging of the rope. If the groove shows signs of rope imprints the sheave must be replaced or re-machined and re-hardened. The same should be done on drums showing similar effect. Poor alignment of sheaves will result in rope wear and wear on the sheave flange. This should be corrected immediately. Excessive wear in the sheave bearings can cause rope fatigue from vibration. Large fleet angles will cause severe abrasion of the rope as it winds onto the drum. Furthermore, the rope will roll into the sheave groove introducing torque and twist which may cause high stranding and bird-cages. |
Dimension of the Groove RadiusThe very first item to be checked when examining sheaves and drums, is the condition of the grooves. To check size, contour and amount of wear, a groove gauge is used. Two types of groove gauges are in general use and it is important to note which of these is being used. The two differ in their percentage over the Nominal Rope Diameter. For new or remachined grooves, and for inspection of fitness for new ropes, the groove gauge should be 1% over the maximum allowable Plus Tolerance of the new rope; alternately, the sheave groove must measure 1% over the Actual Rope Diameter intended to be installed. Many groove gauges on the market are so called ‘No-Go’ gauges and are made with Nominal plus 1/2 of permissible rope Plus Tolerance. If you use these gauges be sure that the existing rope is SMALLER than this gauge. A rope operating in an even slightly undersized groove, deteriorates faster and may develop bird-cages. |
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Corrosion, Rope Removal and Cause
Mechanical Damages
Rope Stretch, Core Wire breaks
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