Two-leg sling safety coefficient testing verifies that twin-leg lifting slings achieve the minimum required ratio of breaking force to working load limit specified by standards such as EN 1492-1, EN 1492-2, and the ISO textile sling series. This article explains the sample types, test items, laboratory procedures, calculation logic, and acceptance criteria used by accredited third-party laboratories to confirm the safety factor of two-leg sling assemblies. It also clarifies how leg angles, verification loads, and construction details influence the verified coefficient.
Technical Overview
A two-leg sling (twin-leg sling) is a lifting assembly in which two load-bearing legs are connected to a common master link, coupling link, or top fitting, terminating in soft eyes, hooks, or shackles at the lower ends. It is widely used for balanced lifting of beams, pipes, steel structures, machinery, and packaged loads where two suspension points are required. The safety coefficient, also called the safety factor or design factor, is defined as the minimum breaking force of the sling divided by its working load limit (WLL), and it represents the structural reserve built into the product.
For slings made of synthetic fibers, the governing standards include EN 1492-1 for flat woven webbing slings, EN 1492-2 for round slings, and the corresponding ISO and GB national standard series. These standards require a minimum safety coefficient of 7:1, meaning the measured breaking force must reach at least seven times the WLL of the leg in a straight pull. By comparison, steel wire rope slings are typically designed to 5:1 under the EN 13414 series, and chain slings to 4:1 under the EN 818 series. For multi-leg assemblies, the assembly WLL is additionally governed by the number of legs and the angle between each leg and the vertical.
Safety coefficient testing is performed for several purposes: design and type verification of new sling constructions, factory production control, delivery acceptance by purchasers, periodic evaluation of in-service slings, and forensic assessment after suspected overload or damage. Testing is normally carried out by accredited third-party laboratories using calibrated tensile testing equipment, with results expressed as the measured breaking force, the calculated coefficient, and a pass or fail verdict against the applicable limit.
Testing Samples
Two-Leg Flat Woven Webbing Slings
These samples consist of synthetic flat webbing, typically polyester, polyamide, or polypropylene, with reinforced sewn eyes at both ends and a stitched seam joining the legs to the top master link. Samples should be full-size, unused, and representative of serial production, including the actual sewing pattern, thread type, edge protection, and wear sleeves. The webbing material, width, thickness, number of plies, and elongation characteristics are recorded before testing.
Two-Leg Round Sling Assemblies
Round sling samples are endless constructions in which a core of untwisted multifilament yarns is enclosed in a woven protective jacket. Two-leg assemblies combine two round slings with a master link and, where applicable, metal end fittings. Testing focuses on core yarn strength, jacket integrity, label durability, and the interaction between the textile body and the fittings.
Composite and Fitted Configurations
Some two-leg slings incorporate chain legs, wire rope legs, shortening clutches, hooks, or shackles combined with textile sections. In these cases, each load path component must be evaluated, and the weakest element determines the verified coefficient of the assembly. For comparison or forensic purposes, in-service samples showing cuts, abrasion, chemical attack, heat exposure, or illegible markings may also be submitted to quantify residual strength.
Regardless of type, sampling normally requires at least three identical specimens of the same batch and construction, because breaking force results exhibit statistical scatter and standards evaluate both individual values and, where applicable, the mean of the series.
Testing Items
The scope of a two-leg sling safety coefficient evaluation typically covers the following items:
- Ultimate breaking force of the complete leg and of the full assembly
- Calculated safety coefficient, expressed as breaking force divided by WLL, evaluated against the 7:1 requirement for textile slings
- Verification (proof) load test at a specified multiple of the WLL, commonly two times WLL, with no rupture, yarn damage, or seam displacement
- Working load limit confirmation for straight, choke, and basket hitch configurations, and for two-leg mode factors
- Leg length tolerance and length matching between the two legs
- Elongation and permanent set at reference loads
- Stitch seam strength and eye or loop strength for textile slings
- Strength and function of fittings such as master links, hooks, and shortening devices
- Verification of marking and labeling, including WLL table, standard reference, traceability code, and color coding
- Visual and dimensional examination of webbing or jacket condition, edge protection, and label legibility
Testing Methods
Specimen Preparation and Conditioning
Specimens are conditioned in a controlled laboratory atmosphere before testing, and the test is performed at room temperature on a calibrated universal tensile testing machine of sufficient capacity. Full-size legs and complete assemblies are tested in preference to cut webbing strips, because the sewing pattern, eyes, and fittings are integral to the real safety coefficient. Loading pins, shackles, or mandrels of adequate diameter are used at both ends so that the textile is loaded in bearing rather than being cut or pinched, reproducing realistic service conditions.
Verification Load Test
The specimen is loaded gradually to the specified verification force, typically two times the WLL, and held at that force for a defined period. During and after the hold, the sling is examined for yarn rupture, seam slippage, jacket damage, fitting deformation, or any permanent distortion. A specimen that sustains the verification load without damage passes this non-destructive stage, which may also be applied to individual serial products during factory production control.
Destructive Breaking Force Test
The specimen is then loaded in a quasi-static manner, at a controlled rate without shock, until failure occurs or the force falls for the first time. The maximum force reached is recorded as the breaking force, and the mode and location of failure are documented, for example core yarn breakage, eye rupture, seam failure, or fitting failure. The force-elongation curve is recorded throughout the test to capture elastic behavior, elongation at working load, and the failure signature.
Safety Coefficient Calculation and Evaluation
The safety coefficient is calculated as the measured breaking force divided by the WLL of the leg in straight tension. For textile slings, the result must be at least 7.0 for each leg tested. For two-leg assemblies, the rated WLL is derived from the leg WLL multiplied by the mode factor: a factor of 1.4 applies when the angle between each leg and the vertical does not exceed 45 degrees, and a factor of 1.0 applies for angles between 45 and 60 degrees, with 60 degrees being the maximum permitted angle. The effect of hitch type, for example choke hitch, is likewise taken into account through the tabulated mode factors of the applicable standard.
Supplementary Examinations
After mechanical testing, supplementary checks are performed. Leg lengths are measured under a low reference tension and compared with the nominal length and the permitted tolerance. Markings are verified against the standard color code, for example violet for one tonne up through green, yellow, grey, red, brown, and blue at higher capacities for textile slings. Temperature range declarations, for example approximately minus 40 to plus 100 degrees Celsius for polyester and polyamide and up to plus 80 degrees Celsius for polypropylene, are checked for consistency with the material used.
Applications
Certified two-leg slings with verified safety coefficients are essential wherever loads are lifted from two suspension points, and safety coefficient testing supports the full life cycle of the product, from development and certification to procurement, use, and retirement decisions. Typical application sectors include:
- Steel mills, foundries, and metal fabrication for lifting plates, sections, and coils
- Construction and infrastructure for precast elements, rebar bundles, and formwork
- Ports, shipping, and logistics for general cargo handling
- Power generation, including wind energy component handling
- Machinery manufacturing, shipbuilding, and offshore platform maintenance
- Rental fleets and lifting equipment suppliers requiring batch acceptance and traceability
Test reports are used for product certification, conformity marking, tender documentation, quality disputes, insurance and accident investigations, and as technical justification for continued service or condemnation of in-service slings.
FAQ
What safety factor applies to a two-leg textile sling?
Under EN 1492-1 and EN 1492-2, flat woven webbing slings and round slings made of man-made fibers must achieve a minimum breaking force of seven times the WLL, that is, a safety coefficient of 7:1. Wire rope slings are generally designed to 5:1 and chain slings to 4:1 under their respective standards.
Why does the leg angle matter for a two-leg sling?
As the angle between each leg and the vertical increases, the force in each leg increases. Standards therefore permit a mode factor of 1.4 for angles up to 45 degrees and only 1.0 for angles from 45 to 60 degrees, and prohibit angles beyond 60 degrees. Mechanical testing is performed in straight tension, and the angle effect is applied through these standardized factors when rating the assembly.
Is the safety coefficient test destructive?
The full breaking force determination is destructive, because the specimen is loaded to failure. However, a separate verification load test at approximately two times WLL is non-destructive and demonstrates that the sling can sustain its proof force without damage, which is useful for routine production control.
Should the full assembly or a single leg be tested?
Both approaches are used. Testing complete assemblies exercises the master link, junction sewing, and fittings together, while testing individual legs characterizes the textile strength itself. For a complete evaluation, laboratories normally test legs for breaking force and verify the assembly, including its fittings, at the verification load.
How often should in-service two-leg slings be evaluated?
Slings should receive a visual inspection before each use, a documented periodic inspection by a competent person at intervals defined by the employer based on service conditions, and a laboratory assessment after repair, suspected overload, chemical or heat exposure, or any incident affecting structural integrity.
What are common causes of failing the 7:1 coefficient?
Frequent causes include inferior yarn tenacity, incorrect webbing width or ply count, inadequate seam design or sewing density, damaged core yarns or jackets, UV and chemical degradation, and the use of substandard fittings. A detailed failure analysis in the test report identifies whether the weak point lies in the textile body, the seam, the eyes, or the hardware.