Slings and grommets : Details#

Build and terminology#

Steel#

Steel-wire rope#

  • Wire is a metal thread.

  • Several wires are combined into a strand.

  • Strands are bound together around a core to make a rope.

If this is the end-product, then it typically referred to in combination with its construction and/or material. For example a “wire rope”, “steel-wire rope”.

  • A typical product is the 6x36 IWRC (Independent Wire Rope Core )

image-20230530125058469

Cable-laid rope#

The same steps are repeated to create a cable-laid sling or grommet.

The ropes are now used as strands and are referred to as unit-ropes and the end product is cable-laid rope.

In general terms, cable is used to refer to a strong rope.

Pictures#

image-20221202160108698

image-20221202155531763

6x36 Wire rope with steel core (Independent Wire Rope Core)

Cable laid wire rope (sling/grommet)

Synthetic Fiber#

Synthetic ropes are ropes assembled from synthetic fibers (as opposed to natural fibers).

Examples are nylon, polyester, HMPE (high modulus polyethylene). Dyneema® and Spectra® are brand names and are specific types of HMPE.

Grommet specific terms#

A special mention of grommet, because it is easy to confuse properties like length and MBL.

The typical configuration of a grommet and terminology is shown below:

image-20230530125414822

The MBL of a grommet is referred to a twice that of the parts (IMCA defines the breaking load as 12 times that of the unit ropes and a correction for spinning losses). This is the MBL of a grommet “when used in standard configuration”. This means it is 2 times the MBL of rope that is it made off, neglecting friction.

Bending losses and losses due to friction are not considered in this definition. DAVE does account for those when determining the maximum allowed (bending losses) and occuring (friction) tension in the rope.

References:

  • IMCA defines the MBL (CRBL) as 12x that of the unit-ropes. This corresponds to 2x the CRBL of the parts (each have 6 outer unit ropes). For the WLL, IMCA considers the bending factor which depends on the application.

  • BS/EN defines the MBL / SWL as 2x that of a the parts

Physical properties estimation#

To perform a rigging calculation the physical properties (diameter, length, MBL, weight and stiffness) of rigging have to be known.

Purpose is to derive realistic default properties of a cable from its make and known properties.

MBL, diameter, weight and stiffness#

Ideally the physical properties of a sling or grommet are obtained from the manufacturer and verified using tests.

In absence of this, for example because slings still have to be manufactured, DAVE can provide an estimation of of the physical properties based on manufacturer information, the standard, the EN13414 standard and research.

The following table lists the build-in relations between the physical properties

Independent Wire Rope Core
(6x36 Wire Rope)

Cable laid wire

HPME [dyneema]

F = 0.68 RHO = 7850 E = 128 * 1e6 k_MBL = 0.064, SF=5

F = 0.68 * 7 / 9
RHO = 7850
E = 1281e60.6

\(d\) : diameter, [mm]

\( \sqrt (MBL / 0.064)\)
Note 1

from MBL via table
Note 2

MBL [t]

\(0.064 \cdot d^2\)\(128 \cdot 10^6 \cdot A_s\)
note 4

from d via table
Note 2

\(A_s\) Area [m2]

\( {0.68 \cdot \pi \over 4} ({d_s \over 1000})^2\)

\( {7 \over 9} {0.68 \cdot \pi \over 4} ({d_s \over 1000})^2\)
note 3

weight [kg/m]

\(7850 \cdot A_s\)

\(7850 \cdot A_s\)

EA [kN]

\(128 \cdot 10^6 \cdot A_s\)
note 4

\(80\cdot10^6\cdot0.785 \cdot d^2\)
or
\(0.6 \cdot 128 \cdot 10^6 \cdot A_s\)
note 5

Notes:

  1. There are various sources for factor 0.064 used for IWRC

  2. The diameter/SWL relation as informatively given in annex G of BS EN 13414-3:2003+A1 :2008 / EN 13414-3:2003+A1 :2008 (E) ; combined with diameter based the safety-factor (\(SF = 6.33 - 0.022 \cdot d\)) as given in the same code this yields a relation between diameter and MBL.

  3. The factor 7/9 follows from 7 subropes and a diameter of 3 subropes

  4. Supplier data and PAPER

  5. The additional factor 0.6 accounts for the reduction in stiffness considering full-slip conditions (\(E{full-slip}/E_{steel}\)) of the subropes. This theory is applicable for large loads and results in a lower stiffness than for low loads [ref: PAPER].

    From IWRC to cable laid wire is essentially the same step as from steel rods to IWRC. It makes sense to apply the \( E_{full-slip} / E_{steel}\) conversion again. This factor is between 0.48 and 0.71, 0.6 was used as a reasonably conservative value.

    Extract from paper

    DNV recommends values with the same order of magnitude depending on the configuration:

    • DNV 16.2.6.13 : E = 25kN/mm2 and A = 0.785xd2 in combination with a 1.25 SKL for 4-sling lifts using matched pairs of wire single laid slings

    • DNV 16.2.6.13 : E = 80kN/mm2 and A = 0.785xd2 for indeterminate 4-sling lifts using four single laid slings of un-equal length

References:

Lengths#

Length of slings and grommets is measured by inserting pins in both ends and pulling the item taut (under a low tension).

The diameters of the pins are prescribed in the IMCA and EN13414 standard and are copied below for convenience:

Cable diameter [mm]

Pin diameter [mm]

60…150 (EN)
100…150 (IMCA)

300

151…250

500

251…375

750

376…500

1000

The nominal length (L2) is then defined as the length measured between the insides of the eyes:

image-20221206150111812

Note that these pin sizes are a recommendation only. Supplier/manufacturer and purchaser may decide to deviate from these values.

Note that sometimes the “Ultimate length” is used. This is hypothetical length of the sling when measured using pins with a diameter of zero. See https://davedocs.online/DAVE-notebooks/sling_length.html

Circumference length of grommets#

According EN-13414 the length of a grommet may also be defined as “The length of a grommet shall be the length of its circumference, measured along its centreline”.

DAVE#

In DAVE it is recommended to use the SlingGrommet or RiggingString nodes to model slings or grommets. When using SlingGrommet or RiggingString nodes the user can define the length of a sling or grommet using any of the above definitions.

image-20230423165559059

If you can not use the rigging module then the basic components can also be used to model slings and grommets. In that case:

  • When using the basic Sling node, the length if is the ultimate length.

  • When using a basic Cable node to model a grommet then the length is the length of its circumference, measured along its centerline.

Note

When performing length variations, for example as required for DNV, DAVE applies the variation on the ultimate length for slings or circumference length for grommets.