SWL, WLL, Rated Capacity… Which is Correct?

SWL, WLL, Rated Capacity… Which is Correct?

Safe Working Load (SWL) is a commonly understood term among Engineers and laypersons alike. However, conventional wisdom tells us that the term SWL has been discontinued and that we must use an alternative. There are many similar terms that have been introduced to usurp SWL; however, at first glance, these terms don’t appear to be applied consistently. To introduce further confusion, we still regularly see SWL on lifting equipment markings. So, which is the correct term to use?

  1. Introduction
  2. SWL, WLL, Rated Capacity, Rated Load, what else?
  3. Detailed List of Terms Currently in Use
  4. So, is SWL Out of Date?
  5. Where are Some Sources of Confusion?
  6. Do you have any Examples of Litigation Referring to the Safe in SWL?

References

  1. EN 13155:2003+A2:2009 – Cranes – Safety – Non-fixed load lifting attachments
  2. ASME BTH-1-2017 – Design of Below-the-Hook Lifting Devices
  3. DIRECTIVE 2006/42/EC of the European Parliament and of the Council of 17 May 2006 on Machinery, and Amending Directive 95/16/EC (recast)
  4. The Supply of Machinery (Safety) Regulations 2008 No. 1597
  5. Safe use of lifting equipment – Lifting Operations and Lifting Equipment Regulations (LOLER) 1998 – Approved Code of Practice and guidance – HSE.
  6. EN 13135 +A1:2018 – Cranes – Safety – Design – Requirements for equipment
  7. ASME B30.2-2016 – Overhead and Gantry Cranes (Top Running Bridge, Single or Multiple Girder, Top Running Trolley Hoist). Safety Standard for Cableways, Cranes, Derricks, Hoists, Hooks, Jacks, and Slings
  8. Lloyd’s Register Code for Lifting Appliances in a Marine Environment
  9. http://www.hoistmagazine.com/features/has-european-based-legislation-worked-for-the-lifting-industry-/
  10. ISO 4306-1:2007 – Cranes – Vocabulary, Part 1: General
  11. EN 15011:2011+A1:2014 – Cranes – Bridge and gantry cranes
  12. EN 14439 + A2:2010 – Cranes – Safety – Tower cranes
  13. BS 7121-1:2006 – Code of Practice for Safe Use of Cranes – Part 1: General
  14. FEM 1.001 – Rules for the Design of Hoisting Appliances – Booklet 7: Safety Rules
  15. KTA 3902:2012 – Design of Lifting Equipment in Nuclear Power Plants
  16. NORSOK Standard R-002 – Lifting Equipment
  17. ASME B30.2-2016 – Overhead and Gantry Cranes (Top Running Bridge, Single or Multiple Girder, Top Running Trolley Hoist). Safety Standard for Cableways, Cranes, Derricks, Hoists, Hooks, Jacks, and Slings
  18. ASME NUM-1-2004 – Rules for Construction of Cranes, Monorails and Hoists (with Bridge or Trolley or Hoist of the Underslung Type)
  19. ASME NOG-1-2004 – Rules for Construction of Overhead and Gantry Cranes (Top-Running Bridge, Multiple Girder)
  20. CMAA 70 – Specifications for Top Running Bridge & Gantry Type Multiple Girder Electric Overhead Traveling Cranes
  21. EN ISO 3266:2010+A1:2015 – Forged steel eyebolts grade 4 for general lifting purposes
  22. ASME B30.9-2018 – Slings. Safety Standard for Cableways, Cranes, Derricks, Hoists, Hooks, Jacks, and Slings
  23. ASME B18.15-2015 – Forged Eyebolts
  24. DNV Standard for Certification No. 2.22: 2008 – Lifting Appliances
  25. Lloyd’s Register Code for Lifting Appliances in a Marine Environment
  26. API Recommended Practice 2D:2014 – Operation and Maintenance of Offshore Cranes
  27. HSE HSG-221:2007 – Technical guidance on the safe use of lifting equipment offshore
  28. https://leeaint.com/uk/Frequently-Asked-Questions
  29. The Association of Oil & Gas Producers Lifting & Hoisting Safety Recommended Practice – Report No: 376 – April 2006
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Mechanical Handling/Lifting

1. Introduction

Most engineers and laypeople will be familiar with the term Safe Working Load (SWL) in relation to the ability of lifting equipment to retain a suspended load. However, there are a number of similar terms currently in use, including, but not limited to:

  • Working Load Limit (WLL);
  • rated capacity;
  • rated load;
  • maximum working load;
  • lifting capacity;
  • capacity.

Many sources tell us that use of the term Safe Working Load (SWL) has been completely discontinued due to the legal implications of the word “safe”; with the Americans leading the charge in the 90s, followed closely by the Europeans. Terms such as “rated capacity” and WLL are now considered to be more favourable and are certainly more aligned with current harmonised European standards. For example, you will not find the term SWL defined in any of the current harmonised European standards related to lifting.

However, if this is the case, then why do we still hear the term SWL used so often and why do we still see it presented on the nameplates of newly manufactured cranes and hoists? If SWL is to be replaced, exactly which term should we replace it with? Is it ever acceptable to use SWL in technical documentation? These are the questions I hope to answer in this article.

I present my own research on the topic and provide an overview of the different terms in-use with a summary of their appropriate application, depending on geographical location, hardware and industry. All definitions in this article are taken from legislation or industry standards/codes. As a general rule, I try to avoid making reference to definitions made in textbooks where possible, as it is easier to demonstrate reasonable skill and care when validating work against codes and standards.

The conclusion of my research shows that each of the above listed terms can be considered correct, depending on the hardware, geographical location and industry of application. In fact, all of the listed terms are equivalent. The important point which then remains is to ensure that we are applying the correct term in relation to the standard we are working with.

1.1. Some Important Definitions

Before investigating the terms related to SWL, we must first recognise some basic definitions associated with lifting and handling hardware.

1.1.1. Lifting Equipment

Lifting equipment is a general expression applicable to all lifting hardware which includes: lifting appliances, lifting gear, and other lifting attachments; used separately or in combination. It can apply to the full suite of lifting paraphernalia, such as cranes, hoists, lifting beams, slings, shackles, hooks and eyebolts etc.

1.1.1. Lifting Appliance

A lifting appliance is a machine of appliance used for the purpose of lifting goods and materials, or in special cases, personnel. Examples include cranes, hoists, winches and lifts etc.

1.1.1. Lifting Accessories

Lifting accessories are load carrying accessories used in combination with a lifting appliance. However, lifting accessories are not necessarily a part of the permanent arrangement of the lifting appliance. Examples include: attachment rings, shackles, swivels, eyebolts, pins, sheaves, hook-blocks, hooks, load cells and slings etc.

Lifting gear is an equivalent term for lifting accessories.

SWL, WLL, Rated Capacity

1.1.1. Non-Fixed Load Lifting Attachments

EN 13155:2003 [1] defines non-fixed load lifting attachments as:

“…items which can be fitted directly or indirectly to the hook or any other coupling device of a crane, hoist or manually controlled manipulating device by the user without affecting the integrity of the crane, hoist or manually controlled manipulating device”.

Examples include: lifting clamps, lifting beams, lifting forks and lifting magnets etc.

1.1.1. Below the Hook Lifting Device

Below-the-hook lifting device is an alternative, but equivalent American term for a non-fixed load lifting attachment which is defined in ASME BTH.1-2017 [2] as:

“…a device used for attaching a load to a hoist. The device may contain components such as slings, hooks, and rigging hardware that are addressed by ASME B30 volumes or other standards.”

2. SWL, WLL, Rated Capacity, Rated Load, what else?

The main source of confusion on the subject is the sheer number of different terms currently in-use which relate to the ability of lifting equipment to retain a load. The cornerstone regulations and design codes/standards which represent the interests of various industries, geographical locations and hardware applications incorporate a variety of terms to refer to the same concepts. For example, the EU and UK Machinery Directives [3], [4], LOLER [5], EN 13135 [6], ASME B30.2-2016 [7] and the Lloyd’s Register Code for Lifting Appliances in a Marine Environment [8] each define a different term to represent what is commonly understood as being the SWL. To shed light on this issue, I researched the terms currently in use across a large sample of industry codes, standards and regulations and present my findings in an easy to understand table format. Before presenting the results of the research, it is useful to consolidate some definitions of the most prevalent terms in current use.

2.1. SWL

The term Safe Working Load (SWL) is widely recognised. The HSE Lifting Operations and Lifting Equipment Regulations (LOLER) [5] are the primary piece of legislation related to lifting and handling in the UK and provide a solid definition of the term SWL. It should be noted that whilst it has no official status outside of the UK, LOLER has been adopted as good practice in many parts of the world and is regarded as an important benchmark, particularly in the oil and gas industry [9]. We shall consider the LOLER definition of SWL as the benchmark against which we will compare all other definitions.

LOLER defines the SWL as follows:

“…a value or set of values based on the strength and/or stability of the equipment when lifting. A range of safe working loads can be specified for the same equipment when used in different configurations. The SWL is usually expressed in terms of the maximum load that the equipment can safely lift, for cranes and lifting attachments, or the actual capacity of the equipment in the case of forklift trucks.”

The SWL can therefore vary, depending on the configuration of the lifting equipment. The maximum SWL is that which applies to the equipment configured in the optimal operating conditions considered during design.

2.1.1. De-Rating the SWL

In less favourable operating conditions, the SWL of an item of lifting equipment must be reduced. This reduction in SWL is referred to as de-rating. Examples of less favourable conditions which require de-rating of equipment include, but are not limited to the following:

  • The use of lifting accessories, such as slings, chains or eyebolts, where the line of load action is not perpendicular to the line of action of the sling.
  • The use of ageing lifting appliances which are approaching the end of useful life, or have been modified or de-rated to extend their useful life.
  • The prevailing wind conditions at the time of lift, in the case of external cranes.
  • The use of lifting appliances offshore or on sea-faring vessels, where the dynamic effects of specific sea-states must be taken into account when planning the lift. The lifting appliance will have been designed to achieve a nominal SWL on-shore, on firm, level ground. This SWL must be de-rated relative to normal land-based duties to take into account varying sea states.
  • The re-certification of an existing piece of lifting equipment to handle items that present a greater safety risk if dropped. For example, the derating of crane that is used for the assembly of plant during the construction of a nuclear facility, which is then re-certified to handle nuclear loads during plant operation.
  • The inability to deploy out-riggers on a mobile crane due to local surface conditions.
  • The handling of loads at an increased radius in the case of lifting appliances with varying radii of operation; such as telescopic jib cranes or cranes capable of luffing (raising or lowering the jib).

2.2. Working Load Limit (WLL)

EN 13155 [1] provides a clear definition of the WLL:

“… [The] …maximum load that the non-fixed load lifting attachment is designed to lift under the conditions specified by the manufacturer.”

By including the phrase “conditions specified by the manufacturer” the standard provides the possibility for varying or de-rating the WLL depending on a range of equipment configurations. Under this definition, the WLL is therefore equivalent to the SWL defined in LOLER and the ability to de-rate is retained.

2.3. Rated Capacity/Rated Load

Clause 3.19 of EN 13135 [6] provides a clear definition of rated capacity and rated load

“… [The] …maximum net load that the crane is designed to lift for a given crane configuration, load location and operating condition…”

2.3.1. Net Load

The net load referred to inEN 13135 [6] is built up by the mass of the payload and the mass of the non-fixed load lifting attachment,each of which are defined in Clause 6.1 of ISO 4306-1:2007 [10] as follows:

2.3.1.1. Net Load

The load, having mass mNL, which is lifted by the crane and suspended from the fixed load-lifting attachment(s). Mass mNL is the sum of the payload mPL and the non-fixed load-lifting attachment(s) mNA.

mNL = mPL + mNA

2.3.1.2. Payload

The load, having mass mPL, which is lifted by the crane and suspended from the non-fixed load-lifting attachment(s) or, if such an attachment is not used, directly from the fixed load-lifting attachment(s).

2.3.1.3. Non-Fixed Load Lifting Attachment

Any equipment, having mass mNA, which connects the payload mpL with the crane and which is a part of neither the crane, nor the payload. Non-fixed load-lifting attachments are easily detachable from the crane and from the payload. For example, lifting slings and spreader beams between the hook and the payload are considered non-fixed load lifting attachments By including the phrase “for a given crane configuration, load location and operating condition” the standard provides the possibility for varying or de-rating the rated capacity/rated load depending on a range of equipment configurations. Rated capacity and rated load are therefore equivalent to our previous definitions of SWL and WLL.

3. Detailed List of Terms Currently in Use

Table 1 provides a detailed summary of research conducted across a selection of cornerstone regulations and design codes/standards which represent the interests of various industries, geographical locations and hardware applications. The list is not exhaustive; however, it provides an adequate sample size. The definitions of relevant terms used in each document are provided and these are related back to the benchmark LOLER definition of SWL provided in Section 3.1.

Table 1 – Summary of research – terms in use related to SWL

4. So, is SWL Out of Date?

As we can see from the research, the answer to this question is both yes, and no. It depends on the industry or application. In relation to industries and applications on dry land, the term SWL has been phased-out and in most case replaced with rated capacity or WLL, with only a few exceptions. In relation to the marine and offshore industries, SWL is still used to some extent; however, it is clear from the definitions provided in the marine and offshore standards that SWL is considered equivalent to other terms such as rated capacity and WLL.

4.1. In Summary

4.1.1. Equivalency

All terms related to the LOLER [5] definition of SWL defined in Table 4‑1 are equivalent, i.e.

SWL  ≡  WLL  ≡  rated capacity  ≡  rated load  ≡  maximum working load  ≡  capacity  ≡  lifting capacity

Each of the values represented by the terms may be varied or de-rated depending on the operating conditions or equipment configuration.

4.1.2. Approximate Boundaries of Use

Table 2 presents a summary recommendation of the preferred terms for use in relation to hardware application, geographical location and industry.

Table 2 – Recommended terms for use

4.2. I still see SWL Everywhere, is this a Problem?

Not particularly, although all reasonable efforts should be made to replace SWL with an appropriate alternative for new and updated designs. To this day, many lifting appliances in the EU are still furnished with a nameplate bearing the “SWL” of the appliance, and this is unlikely to change any time soon.

In fact, many Clients will request that nameplates of lifting appliances bear a marking of SWL rather than, for example, rated capacity, as SWL is still the most widely recognised term. This may not be strictly in accordance with the design standard applicable to the lifting appliance; however, it still complies with the spirit of intent of such standards; which is the provision of legible loading information to assist safe lifting. After all, it’s better to have a recognisable marking on the crane than nothing at all, or a marking that is potentially ambiguous.

5. Where are Some Sources of Confusion?

5.1. The Internet

(I appreciate the irony).

Some sources state that SWL is not used at all anymore and has been completely replaced by WLL. This is simply not true. This statement oversimplifies the situation and neglects the use of rated capacity: the most commonly defined term in relation to lifting appliances. It could be argued that as all the terms are effectively equivalent, it doesn’t necessarily matter which term is used as long as the intent is clear and the situation is understandable. However; it is always worth making best efforts to adopt the appropriate language for your specific application.

5.1. The Machinery Directive (!)

For most Mechanical Engineers in Europe, the Machinery Directive [3], as the fundamental piece of legislation dictating the essential requirements for mechanical equipment placed on the European market, is the first port of call for guidance. Unfortunately, as it was issued in 2006, and is the precursor to many of the recent harmonised European standards, the directive uniquely uses the term Maximum Working Load instead of any other sensible current term such as rated capacity or WLL, and does not provide an immediately clear definition the term. It is necessary to refer to the relevant standards.

5.1. LOLER (!)

As discussed in Section 2.2, the UK LOLER approach has been adopted as good practice in many parts of the world and is regarded as an important benchmark. It therefore makes sense to refer to LOLER for definitions of the relevant term. However, LOLER was issued in 1998, pre-dating the Machinery Directive and the successive harmonised European standards. Cosnequently, the use of SWL in LOLER could be considered out-of-date.

5.1. LEEA

I suspect this might be a controversial position; however the Lifting Equipment Engineers Association (LEAA) website [28] provides the following answer to a FAQ, which I believe to be slightly misleading.

Question: Which is the correct term to describe the lifting capacity of lifting equipment, i.e. Safe Working Load (SWL), Working Load Limit (WLL) or Rated Capacity?

Answer: The answer to this is that they are all correct and the choice of which to use is really dependant on the use of the equipment, as the following information will explain.

The working load limit, or maximum working load as defined in the Supply of Machinery (Safety) Regulations 2008, is the load value assigned to the ‘maximum’ safe working load under ideal conditions and in most cases the working load limit and the safe working load will be the same.

However, depending upon the conditions of use, it may be necessary for the competent person to reduce this to a lower safe working value and it is in these cases that the working load limit and safe working load will differ. The rated capacity is the same as safe working load and is a term used for some cranes where the rating varies with configuration.

My humble opinion is that this assessment is not strictly true and is not compatible with the Machinery Directive, or the harmonised European standards that were released in response to the Directive. The Machinery Directive does not recognise the creation of an SWL distinct from the WLL. In effect, the SWL mentioned in the LEAA answer does not exist as a defined term in the Directive. Clause 4.1.2.5(e) of the Machinery Directive [3] states the following:

“…the maximum working load of a multilegged sling is determined on the basis of the working coefficient of the weakest leg, the number of legs and a reduction factor which depends on the slinging configuration;”

Therefore, the maximum working load (identified by the LEAA as being equivalent to the WLL) already takes into account the conditions of use by the application of a reduction factor. Further, Clause 4.3.3 states that:

“Where the maximum working load depends on the configuration of the machinery, each operating position must be provided with a load plate indicating, preferably in diagrammatic form or by means of tables, the working load permitted for each configuration.”

Again, the maximum working load is already defined based on the configuration of the equipment. The maximum working load or WLL is therefore not explicitly tied to the “ideal conditions” of operation. Yes, the working load can be de-rated further by the competent person, based on other emerging conditions; however this would then just constitute a reduced WLL. There is no separate SWL. I believe that my position is corroborated by the research presented in Table 1.

5.5. OGP Report No. 376

One current Code of Practice: The Association of Oil and Gas Producers (OGP) Report No: 376 [29], defines the WLL in terms of the SWL. The relevant definitions are as follows:

“Safe Working Load (SWL)

The maximum load (as determined by a competent person) that an item of lifting equipment may raise, lower or suspend under particular service conditions, e.g. the SWL can be lower than, but can never exceed, the WLL.

Normally SWL = WLL unless the lifting equipment has been de-rated.”

Working Load Limit (WLL)

The maximum load, determined by the manufacturer, that an item of lifting equipment is designed to raise, lower or suspend. Some standards and documents refer to WLL as the ‘maximum SWL.”

This is very similar to the position taken by the LEAA in Section 7.3. Again, I suspect this may be a controversial position; however my opinion is that the statements made in this Code of Practice are not compatible with the Machinery Directive, or the harmonised European standards and may be more confusing than helpful. If we want to remain in accordance with the Machinery Directive and the harmonised European standards, then we either use SWL, or WLL, but not both. Strictly speaking, the terms SWL and WLL should not co-exist.

6. Do you have any Examples of Litigation Referring to the Safe in SWL?

Nope. I couldn’t find any. Of course, that doesn’t mean they are non-existent.

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