Shackles for Lifting and Rigging in Singapore: The Complete Guide to Types, Grades, SWL and MOM Compliance

Shackles are the most widely used rigging hardware in Singapore's construction, marine, and industrial sectors. Every lifting operation that connects a sling to a crane hook, a rigging point to a load, or a wire rope to a structural anchor uses at least one shackle — often several.

Yet shackles are also one of the most frequently misused pieces of rigging hardware on Singapore worksites. The wrong type for the application, an unmoused pin, an incorrect grade, or a shackle used beyond its rated SWL can cause catastrophic load drop.

This guide covers everything you need to know — shackle types, grades, SWL ratings, correct use, mousing, inspection, and MOM compliance — so you can specify, use, and manage shackles correctly on every lift.

The Two Main Shackle Types: Bow vs D Shackle

All shackles share the same basic anatomy — a curved steel body and a removable pin that closes the shackle and transmits load. The critical difference between shackle types is the shape of the body and the direction of load each is designed to carry.

Bow Shackle (Anchor Shackle)

A bow shackle has a wide, rounded body — shaped like a traditional anchor, which is why it is also called an anchor shackle. The wide bow creates a larger internal space that allows multiple sling legs, multi-directional loads, and larger diameter fittings to be connected.

Key characteristics:

• Wide rounded body accommodates multiple sling legs simultaneously

• Designed for loads from multiple directions — the wide bow distributes load across a larger area

• Better suited to angled loading — when sling legs pull at an angle to the pin axis

• Available in the largest SWL range of any shackle type

• The standard choice for crane hooks, master links, and multi-leg sling connections

Best for:

• Connecting multi-leg slings to a crane hook

• Any rigging application where the load may shift direction

• Marine lifting and mooring applications

• High-capacity rigging where large fittings must connect

D Shackle (Chain Shackle)

A D shackle has a narrow, D-shaped body — creating a compact, straight profile that suits in-line, single-direction loading. The narrow body means only one fitting can be connected at a time, and load must travel in a straight line through the shackle.

Key characteristics:

• Narrow D-shaped body — compact and in-line

• Designed for straight, in-line loads only — load must pull directly through the pin axis

• Not suitable for side loads or multi-directional loading — side loading a D shackle dramatically reduces its capacity and risks pin failure

• Slightly higher efficiency than a bow shackle for straight in-line pulls

• Common in chain assemblies and straight-line rigging

Best for:

• Connecting single sling legs in a straight vertical pull

• Chain rigging where the shackle sits between chain links in-line

• Straight-line tensioning applications

• Applications where compactness is more important than multi-directional capability

Bow vs D Shackle: Quick Decision Guide

Situation	Use Bow Shackle	Use D Shackle
Multi-leg sling to crane hook	✅ Yes	❌ No
Load may shift direction	✅ Yes	❌ No
Marine and offshore rigging	✅ Yes	Caution — in-line only
Single leg, straight vertical pull	✅ Either	✅ Either
In-line chain assembly	❌ Too wide	✅ Yes
Multiple slings on one pin	✅ Yes	❌ No
Angled sling legs	✅ Yes	❌ Never

The safe default: When in doubt, use a bow shackle. It handles a wider range of loading conditions and is the standard choice for the vast majority of lifting and rigging applications in Singapore.

Shackle Pin Types: Screw Pin vs Bolt Type (Safety Pin)

The pin closes the shackle and is the component that transmits the load. There are two main pin types — and choosing the wrong one for your application is a common and serious mistake.

Screw Pin Shackle

The screw pin is threaded and screwed into the shackle body by hand. It is the most common shackle pin type in Singapore — fast to connect and disconnect, and available across the full range of shackle sizes and grades.

Key requirement: The pin MUST be moused.

In any application involving vibration, load cycling, or movement — which describes almost every lifting and marine application — a screw pin will eventually unscrew under the combined effect of vibration and load-induced pin rotation. An unscrewed pin means an open shackle means a dropped load.

Mousing means securing the screwed-in pin with a wire or cable tie through the pin eye and around the shackle body, preventing the pin from rotating and unscrewing. On a properly moused shackle, the pin cannot unscrew even if it tries.

Mousing is mandatory for:

• All marine and offshore applications

• Any application with vibration — cranes, machinery, vehicles

• Any application where the shackle will be left under load for an extended period

• Any application where the load may rotate or swing

Bolt Type Shackle (Safety Pin Shackle)

A bolt type shackle uses a bolt, nut, and cotter pin in place of the screw pin. The cotter pin through the bolt prevents the nut from unscrewing — making it mechanically secure without requiring mousing wire.

Key characteristics:

• Mechanically secured — nut cannot unscrew without removing the cotter pin

• Slower to connect and disconnect than a screw pin

• Preferred where the shackle will remain in service for extended periods

• Standard for permanent or semi-permanent rigging installations

• Required by many marine class societies and offshore operators for permanent rigging

Best for:

• Permanent or semi-permanent rigging installations

• Marine and offshore applications where regular disconnection is not required

• Any application where mechanical security of the pin is mandatory

• Rigging that will be left under load for extended periods without inspection access

Shackle Grades: What the Numbers Mean

Like chain slings, shackles are manufactured in different grades — with higher grades offering more strength for the same physical size.

Grade S (Sea) Shackles

Grade S shackles are manufactured from higher-strength alloy steel and are specifically designed for marine and offshore applications. They offer:

• Higher SWL than standard carbon steel shackles of the same size

• Better toughness and fatigue resistance for dynamic marine loading

• Hot-dip galvanised or stainless steel finish options for corrosion resistance

• The standard choice for Singapore's shipyard and offshore sectors

  
  

Grade M (Machinery) Shackles

Grade M shackles are the standard industrial grade — suitable for general construction, workshop, and industrial lifting. Most shackles used on Singapore construction sites and in industrial facilities are Grade M.

Stainless Steel Shackles (Grade 316)

Stainless steel shackles offer maximum corrosion resistance for aggressive marine environments, chemical plants, and food processing facilities. Lower SWL than alloy steel shackles of the same size, but unmatched corrosion resistance.

SWL Reference Table: Common Shackle Sizes

The SWL of a shackle depends on its pin diameter, body construction, and grade. The table below gives approximate SWL values for Grade S bow shackles — the most common type used for lifting in Singapore:

Pin Diameter	Approximate SWL
9.5mm (3/8")	1.0T
12.7mm (1/2")	2.0T
16mm (5/8")	3.25T
19mm (3/4")	4.75T
22mm (7/8")	6.5T
25.4mm (1")	8.5T
28.6mm (1-1/8")	9.5T
31.8mm (1-1/4")	12.0T
38.1mm (1-1/2")	17.0T
44.5mm (1-3/4")	25.0T
50.8mm (2")	35.0T

Always verify against your specific shackle's stamped SWL and test certificate. SWL values vary by manufacturer, grade, and construction. Never use a shackle that does not have a legible SWL stamp on the body.

Critical Rules for Safe Shackle Use

Never Side Load a D Shackle

A D shackle loaded from the side — with force applied perpendicular to the pin axis — loses up to 50% of its rated SWL. Side loading a D shackle is one of the most common and dangerous rigging mistakes in Singapore worksites. If your application involves any side loading or directional uncertainty, use a bow shackle.

Never Cross Load a Shackle Pin

Cross loading means the load bears on the pin rather than on the shackle body. This happens when a sling eye or fitting sits on the pin itself rather than inside the bow of the shackle. Cross loading dramatically overloads the pin and can cause sudden pin failure. Always ensure the load-bearing fitting sits inside the shackle body, not on the pin.

Never Apply the Load to the Tip of the Pin

The pin tip is the threaded or cotter-pinned end — the weakest part of the pin. Never rig in a way that places load on the pin tip. The load must bear on the body of the pin, inside the shackle bow.

Match the Shackle to the Sling Fitting

The sling eye or fitting that sits inside the shackle must fit correctly — not too large and not too small. An oversized fitting that bears on the curved corners of the bow rather than sitting flat inside it creates stress concentrations. An undersized fitting that can slide along the pin is also a risk. Always check the physical fit before loading.

Never Use a Shackle as a Permanent Structural Connection Without Engineering Review

Shackles are rated for lifting and temporary rigging — not permanent structural connections. If you are using shackles as part of a permanent or long-term structural connection (such as in a suspended ceiling, permanent façade rigging, or structural tie), an engineer must review the application and confirm suitability.

Shackle Inspection: What to Check

Before Every Use — Visual Inspection

Remove from service immediately if any of the following are found:

Body:

• Cracks — particularly in the area of the pin holes and at the crown of the bow

• Nicks, gouges, or cuts — especially on the inside of the bow where sling contact occurs

• Deformation — bending, twisting, or distortion of the body shape

• Wear at the pin holes — if the pin holes are elongated or worn, the shackle is condemned

• Corrosion — surface rust, monitor and clean; pitting corrosion, condemn

Pin:

• Cracks or deformation

• Thread damage — the pin must screw fully home and seat correctly

• Wear — if the pin diameter has reduced visibly, condemn

• Missing or damaged cotter pin (bolt type shackles)

• Missing mousing wire (screw pin shackles in marine/vibration applications)

General:

• SWL stamp must be legible — if it cannot be read, the shackle cannot be used

• If the shackle has no traceable test certificate and unknown history, do not use it

  
  

Formal Inspection — Every 12 Months Minimum

A MOM-recognised AE or Competent Person must carry out formal inspection of all shackles in service at intervals not exceeding 12 months. In marine environments and high-cycle applications, more frequent formal inspection is warranted.

Retire Immediately If:

• Any crack is found — there is no acceptable repair for a cracked shackle body or pin

• Pin holes are elongated or visibly worn

• The shackle has been involved in a shock load or overload event

• Deformation of any kind — a bent shackle body or bent pin is condemned

• Severe corrosion — pitting, section loss, or internal corrosion

• The SWL stamp is illegible

• The shackle has no test certificate or traceable history

MOM Compliance Requirements for Shackles in Singapore

Under Singapore's WSH (Lifting Operations) Regulations:

Before first use:

• Proof load tested to 2× SWL

• Test certificate issued by the manufacturer or accredited testing body

• SWL permanently stamped on the shackle body

In service:

• Pre-use visual inspection before every lift

• Formal inspection by a Competent Person at least every 12 months

• Inspection records maintained

• Colour coding applied to match current inspection year

• Condemned shackles rendered unusable before disposal — do not leave condemned shackles in stores

After any overload or shock load event:

• Remove from service immediately

• Re-inspect and re-proof test before returning to use

Shackles for Marine and Offshore Use: Additional Requirements

Singapore's shipyards, offshore platforms, and vessel operators face additional shackle requirements beyond standard industrial use:

Bolt type shackles preferred for permanent rigging — screw pin shackles are generally not acceptable for permanent or semi-permanent marine rigging installations without specific engineering justification.

Grade S or higher — Grade M shackles are not suitable for marine and offshore applications. Specify Grade S as a minimum, stainless steel for continuous seawater exposure.

Galvanised or stainless finish — ungalvanised carbon steel shackles corrode rapidly in Singapore's salt-air marine environment. Specify hot-dip galvanised as a minimum for outdoor marine use, 316 stainless steel for continuous seawater exposure.

Certification traceable to individual shackles — many offshore operators and class societies require that each shackle has its own individual test certificate, not just a batch certificate. Confirm requirements with your client before ordering.

Ordering Shackles From FRT in Singapore

FRT supplies a comprehensive range of shackles for Singapore's construction, marine, offshore, and industrial sectors:

• Bow shackles (anchor shackles) — Grade S, galvanised and stainless steel, screw pin and bolt type, wide mouth and wide body shackle

• D shackles (chain shackles) — Grade S and Grade M, galvanised, screw pin and bolt type

• Stainless steel shackles (316) — bow and D types, for maximum corrosion resistance

• Wide range of sizes — from 1T to 200T SWL in standard stock

• All supplied with test certificates meeting MOM requirements and BS EN 13889 / AS 2741

  
  

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FRT has been supplying shackles and rigging hardware to Singapore's construction, marine, and industrial sectors since 1995. All products comply with MOM regulations and BS EN 13889 / AS 2741.