An important feature of any wire rope is the ropes diameter, which is specified by equipment manufacturers and users. This is to ensure the rope selected and supplied, is comparable with the sheaves, winches and operation of the equipment, although other characteristic of the rope (construction, breaking strength, etc) must be considered when selecting a rope.
Within this bulletin we explain how the diameter of a wire rope is referenced and how to measure the diameter of a wire rope.
Nominal diameter (d) is the referenced or given diameter in tables within standards, rope manufacturers/suppliers’ catalogues and on order/certification documentation.
Measured or actual diameter is as the name implies, the diameter of a wire rope at the time the rope diameter is measured.
Off tension diameter is the measured diameter recorded whilst the rope is under no tension.
Under tension diameter is the measured diameter recorded whilst the rope is under a specified tension. e.g., 5% of the ropes minimum breaking force (MBF).
Diameter tolerance is normally specified as a percentage of the nominal diameter, e.g. Nominal diameter 28 mm +2% to +5% (28.56 mm to 29.12 mm).
Note: The measured diameter of a new rope will normally be larger than the nominal diameter specified.
Measuring the diameter of a rope.
Steel wire ropes are constructed by helically twisting individual wires together forming a strand and then strands around a core to form the rope, which creates an uneven exterior profile. It is therefore essential when measuring the diameter of a rope that it is measured correctly. The measured diameter of a rope is the diameter of a circle which encircles the circumference of the wire rope and therefore when measuring a rope, it must be measured across the largest diameter, as shown to the right.
Before measuring the diameter of a rope, ensure that the section of rope to be measured is pulled straight and never attempt to measure a rope whilst it is spooled on a drum or in a coil. To measure the rope a simple calliper gauge can be used, taking care to ensure that the measurement is taken across the largest diameter of the rope (across the crowns of two strands). Since most wire ropes which have been bent in a coil, on a drum or over sheaves within a system have a tendency to be slightly oval, it is therefore good practice to take two measurements on the same section of rope at right angles to each other and take an average of the two measurements to confirm the measured diameter.
If a more accurate measurement is required, this can be obtained by using a calibrated calliper gauge with jaws broad enough to cover at least two adjacent strands, taking
two diameter measurements at approximately right angles to each other and repeat the measurements one metre away along the rope’s length. The average of these four measurements should then be within the tolerances specified in the appropriate standard or specification.
| Permissible difference between any two measurements as a percentage of the nominal rope diameter, for new ropes with strands that exclusively of wire or incorporate solid polymer. |
|
Tolerance on nominal rope diameter, for new ropes with strands that are exclusively of wire or incorporate solid polymer. |
Nominal Rope Diameter
(mm) |
% |
|
Nominal Rope Diameter
(mm) |
Tolerance as a percentage of the nominal rope diameter % |
| From 2 to < 4 |
7 |
|
From 2 to < 4 |
0 to 8 |
| From 4 to < 6 |
6 |
|
From 4 to < 6 |
0 to 7 |
| From 6 to < 8 |
5 |
|
From 6 to < 8 |
0 to 6 |
| 8 and greater |
4 |
|
8 and greater |
0 to 5 |
Note: as specified in EN 12385-4, API 9A, ISO 2408
The difference between any two of the four measurements taken for a new rope, expressed as a percentage of the nominal rope diameter, shall not exceed the values in the table above.
For the majority of Crane/Lifting applications other than Lifts/Elevators & Mining, the various National & International Rope standards specify a diameter tolerance of nominal diameter +0% to +5% for 8 mm diameter ropes and larger. Therefore, ropes when supplied will generally have a measured diameter larger than the nominal diameter.
Local applicable standards should be checked and tolerances other than this should be discussed with Bridon Bekaert at the order stage. In the cases of a dispute and/or absence of specification, it is recommended that a sample of the rope is tensioned up to an equivalent to 5% of the minimum breaking force of the rope is applied and the measuring procedure repeated. Additional guidance is given in EN 12385-1 & ASTM A 1023 on the verification of the measured diameter.
Further guidance is available at
contact@bridon-bekaert.com
Reference: TB.001 Ed.1 Sept 2021