The load capacity of a lifting accessory, i.e. its working load limit (WLL), is determined at the design stage by structural analysis or testing – not by multiplying the mass of the load by a single safety factor. According to the standard for detachable lifting accessories, SFS-EN 13155:2020+A1:2025, strength is demonstrated either by calculation, with the result verified by a proof load test, or by testing alone. For the user, the load capacity is shown as the WLL value marked on the accessory, which must not be exceeded.
What do load capacity and WLL mean?
WLL (working load limit) is the maximum load that a lifting accessory may carry in normal use. It must be marked on the accessory, and a lifting accessory without this marking must not be used. On older accessories, the same value may appear as SWL (safe working load). Markings are explained in more detail in the article Markings and Color Codes on Lifting Accessories: A Guide to Safe Use.
Load capacity is not the breaking load. The accessory must withstand its working load limit repeatedly throughout its service life without permanent deformation, so there is always a clear margin to failure. The dead weight of the lifting accessory is not included in the WLL value but is stated separately. The capacity of the crane must therefore be sufficient for the combined mass of the load and the lifting accessory.
How is load capacity verified according to EN 13155?
The version SFS-EN 13155:2020+A1:2025, which was harmonised under the Machinery Directive on 13 March 2026, provides two ways to demonstrate that a lifting accessory withstands its rated load.
| Calculation and proof load test | Testing only | |
|---|---|---|
| How strength is demonstrated | By structural analysis in accordance with EN 13001-1 and EN 13001-2 | By proof load testing, without calculation |
| What is examined | The critical points of the structure: material, welds, lifting points, stability and, where necessary, fatigue | The behaviour of the structure under the proof load |
| Proof load | Verifies the calculation: 1.5 × WLL when WLL ≤ 20 t · (1.56 − WLL/350) × WLL when 20 t < WLL < 160 t · 1.1 × WLL when WLL ≥ 160 t | Type: 2 × WLL without permanent deformation and 3 × WLL without releasing the load. Individual item: 2 × WLL |
| Usually suitable for | Individually manufactured accessories and series in which the structure and mass are to be optimised | Accessories for which a high proof load is easy to apply – can result in a lighter structure |
On the calculation route, the calculation alone is not enough; the result is verified by a proof load test. The changes to the standard and proof load testing are explained in more detail in the article proof load test for lifting accessories and the EN 13155 update.
Why is a “4:1 safety factor” not enough for dimensioning?
A common piece of advice online is that the load capacity of a lifting accessory is obtained by dividing the breaking load by a factor of 4. However, fixed factors belong to the product standards of standard components such as chains, wire ropes and lifting slings. The load capacity of a customer-specific steel lifting accessory, such as a lifting beam or a gripper, is not based on a single factor.
The designer chooses the safety factor according to what the strength is compared with, for example the yield strength or the tensile strength of the material. In addition, different types of structural details may have their own safety factors that must be taken into account. A factor of 4 relative to tensile strength is often on the safe side, but it cannot be used as a general rule. The weakest point of the structure determines the load capacity.
In a welded structure, the welded joints are often the critical point for dimensioning. That is why the designer takes the weld quality level into account in the calculation, and the manufacturer uses an approved welding procedure specification (WPS) – read more in the article WPS and welding procedure specifications for lifting accessories.
Which factors affect load capacity in design?
- Mass and centre of gravity of the load: an off-centre centre of gravity increases the load on an individual lifting point and can tilt the load.
- Lifting method and lifting angle: inclined chains and slings increase the forces acting on the structure (see the table below).
- Dynamic loads: starting and stopping the lift and swinging of the load momentarily increase the load.
- Frequency of use: with frequently repeated lifts, fatigue governs the dimensioning before static strength does.
- Environment: temperature, corrosion and, for example, cleanroom requirements affect the choice of material.
When two chains or slings rise to a single point, the force in each leg increases as the angle increases. The figures follow directly from geometry:
| Angle between the legs | Leg force relative to a vertical lift |
|---|---|
| 0° (vertical legs) | 1,00 |
| 60° | 1,15 |
| 90° | 1,41 |
| 120° | 2,00 |
This is why the WLL of a lifting accessory applies only at the angles and with the lifting methods defined in the instructions for use.
Can the load capacity of a lifting accessory in use be increased?
Yes, but only by reassessing the accessory. The designer calculates the structure according to the current standard, the load capacity is verified by a proof load test where necessary, and new CE documentation and markings are prepared for the accessory. A new plate alone is not enough. The procedure is explained in the article CE marking of lifting accessories.
If the load capacity marking of an accessory in use has been lost, the accessory must not be used until the load capacity has been determined and the marking renewed. Inspections of accessories in use are explained in the article inspection of lifting accessories.
How Hefmec works
We design lifting accessories for the customer’s workpiece and lifting operation. We dimension the load capacity by analytical calculation and, where necessary, by FEM analysis, and the same package includes proof load test planning and the technical documentation required for CE marking. We do the same for existing accessories when their load capacity needs to be determined or increased.
Frequently asked questions
Is WLL the same as SWL?
In practice, yes. Both mean the maximum permitted working load. WLL is the term used in current European standards.
Can I calculate the load capacity of a lifting accessory myself?
Yes, if the person doing the calculation is familiar with the applicable standards. The manufacturer of the lifting accessory is responsible for ensuring that the calculation and proof load test have been carried out and documented in the technical documentation.
Does every lifting accessory need a proof load test?
An individually manufactured lifting accessory is always proof load tested. For series-produced accessories, a type test is carried out on at least one item in the production series, and the sampling for individual verification of the other items is defined according to the manufacturing method and series size.
Still have questions?
For questions about the dimensioning and structural analysis of lifting accessories, you can contact Service Manager Aki Jalonen directly at +358 50 492 7579. Read more: structural analysis and proof load testing.
