good slinging practice - Site Safe

NOTE: This is Part 2 of a 4-part 

module set covering Slinging. 

Part 1 covers Sections 1 to 3. 

Part 3 covers Sections 8 and 9. 

Part 4 covers Sections 10 to 12. 

CRANEAGE: SLINGING - INTRODUCTION 2 

TRAINING MODULE 

SLINGING 

Modules 4 to 7 

Sling Loads 

Reeving and Choking 

Dunnage 

Common Mistakes

SLINGING 

Modules 4 to 7 

This Training Module comes to you courtesy of: 

Site Safe would like to acknowledge Fletcher Construction’s on-going support 

CRANEAGE: SLINGING - INTRODUCTION 2

CRANEAGE - SLINGING BASICS 

SECTION 4: 

KNOW YOUR SLING LOADS 

4.1 Know how to work out Sling Loads 

4.2 Sling Loads Formula 

4.3 Two-Legged Slings 

4.4 Three-Legged Slings 

4.5 Four-Legged Slings 

4.6 Four-Legged Slings: worked examples 

CRANEAGE - SLINGING: BASICS PAGE 1

GOOD SLINGING PRACTICE 

BASIC RULES ... 4 

4.1. KNOW HOW TO WORK OUT SLING LOADS. 

You now know the importance of the included angle in slinging. 

It then becomes necessary to establish the loads actually being carried by slings. 

This can be done with nothing more complicated than a tape measure and a simple 

pocket calculator, and not having to become involved in degrees and angles. 

Not more 

than 45° Angle up to 90° Slings 

Load 

S.W.L. x 1.4 = Safe Working Load. 

(Don't crowd the 

hook!) 

L = Length 

W = weight 

Angle to the 

vertical 

V = 

Height 




4.2. KNOW HOW TO WORK OUT SLING LOADS. 

This formula is very accurate, and allows you to use a tape measure for calculations, 

rather than having to use degrees and angles. 

The formula is: T = W x L 

N V 

T = Tension per leg in kg 

W = weight in kg 

N = number of legs 

L = sling length in metres 

V = vertical height in metres 

Example: 

W = weight = 5,000 kg 

N = No. of legs = 2 

V = vertical height = 2 metres 

L = sling length = 3.5 metres 

T = 5000 kg x 3.5 

2 2 

= 

17,500 

4 

L = Length 

= 4,375 kg per leg. 

You would need a two-legged sling rated at 5,000kg WLL capacity per leg. 

W = weight 

This formula will work for any number of sling legs, but as we all know, a four leg sling 

system can often mean that only two legs are carrying all the weight, with the other two 

legs doing little more than acting as balancing legs. 

Refer to Sheets 4.5 and 4.6 for details of four-leg slings. 

Therefore, use the formula based on TWO legs only, to make sure that you are on the 

safe side. 

It's too late to be checking your calculations when the load has 

fallen to the ground. 

CRANEAGE - SLINGING: BASICS PAGE 3 

V = 

Height



4.3. SAFE WORKING LOADS - TWO-LEG SLINGS. 

The modern method of rating slings is by their working load limit (WLL), which 

rates the sling in pure tension (that is, vertical). The safe working load (SWL)includrefers ed angles on sling loads. 

to the rigging system being used - that is, sling angles, reeving, number of sling legs, 

and the like. 

Using our Riggers' Guide, we find for 

this system: 

1. the SWL = 4500 kg. 

2. the WLL of each sling = 3200 kg. 

3. the load in each sling leg will be 

the actual load x 0.7. 

4. the SWL = WLL of ONE sling leg 

divided by 0.7. 

Note: Older slings will be 

rated as SWL for straight lift 

or pull. In this case, use the 

SWL in your calculations. 

As an example: 

1. Each leg of a two-leg sling has a WLL of 100 kg. 

The included angle between the legs of the sling 

is not more than 60°. 

Safe working load = (100 / 0.58) = 172 kg. 

2. Should the included angle between legs be 

increased to 120°; 

the S.W.L. = (100 / 1) = 100 kg. 

3. The load is 100 kg, and the included angle is 30°. 

Sling load = 100 kg x 0.52 = 52 kg per leg. 

Refer to the chart at right for the effect of 

included angles on sling loads 

90° 

Load 

10mm 

Chain 

Slings 

This chart is in the Riggers Guide 

EFFECT OF INCLUDED ANGLE 

ON SLING LOADS 

0.5 t 0.5 t 

Load 

in 

tonnes 

30° 

0.52 

60° 

0.58 

90° 

120° 

0.7 

1.0 

151° 2.0 

171° 6.0 

1 tonne load



4.4. SAFE WORKING LOADS - THREE-LEG SLINGS. 

Three-leg slings need special consideration, because rarely do all three legs take an 

equal share of the load. It is best to assume that only TWO legs are sharing the load, 

with the third leg only acting as a "balancer". 

Using our Riggers' Guide, we find for 

this system: 

1. the SWL = 4500 kg. 

2. the WLL of each sling = 3200 kg. 

3. the load in each sling leg will be 

the actual load x 0.7. 

4. the SWL = WLL of ONE sling leg 

divided by 0.7. 

If loads are critical and equally shared we can find accurate 

sling loads by calculation - e.g; using the same scenario, 

10mm chain slings with a WLL of 3200 kg and effective 90° 

included angle has a SWL of (3200 / 0.7) = 2286 kg per leg. 

= 6857 kg for the three legs. 

As an example: 

Sling legs are 3m long. 

Pitch diameter of the sling attachments is 3m. 

The effective included angle is 60° (30° from 

vertical). 

Using the approximation method, we assume the 

load is taken on two legs only. Therefore, from our 

Riggers Chart, we find the sling load at 60° 

included angle will be 100 kg x 0.58 (60° load 

factor) = 58 kg. 

Assuming the load is equally distributed between 

the three legs, we find that the sling load = 

100 kg x 0.58 (60° load factor) x 

2 legs 

3 legs 

= 39 kg. 

Assumed included 

angle = 90°. 

= 45° to vertical. 

Load = 100 kg. 

10mm chain 

slings 




4.5. SAFE WORKING LOADS - FOUR-LEG SLINGS. 

You will have noticed that the 

Riggers Chart treats 2, 3 and 4 leg 

slings as having the same SWL for 

various configurations. 

With a 4-leg sling, unless a 

load-sharing device exists in the 

system, the load will ALWAYS be 

taken by TWO slings only. 

The other two will only serve to 

balance the load. 

Angle to 

vertical 

Included angle is 

between two 

opposite legs. 

The reason for this situation is that it is virtually impossible for the length of the slings to be 

exactly equal, and the placement of the slings to load attachment points to be such that the 

distance from their underside to the crane hook is exactly equal. 

If the load is flexible, some sharing of the load will take place as the load twists. 

The degree of sharing is dependent on the stiffness of the load. If the effect of this stiffness is 

unknown, we MUST assume it to be perfectly stiff, and NO load sharing takes place. 

Sling calculations are therefore the same as for two-leg slings (see Sheet 4.3). 

Of special concern with precast slung from four points, 

is the capacity of the pin anchors. 

Let's see, now. Ten tons to pick up - 

4 x 2.5 tonne pins = 10 tonnes. 

10mm chains ought to do. 

Sweet, eh! 

OH, REALLY? 

Remember that one leg at least 

will be slack, and the load will 

then be carried by only two 

slings. Therefore, the included 

angle must be 90°. 

Sling angle = 90°. 

Sling load must be 7.0 tonnes. 

Therefore, 4 x 10-tonne pins 

are required, and 16mm chains. 

Some difference! 

2.5 pin 

anchors 

10 TONNES 

8.5m 

across diagonals 


6m 

10mm 

chain 

slings



4.6 LOAD SHARING DEVICES 

These are examples of load sharing devices which distribute a load equally among 

sling legs. 

One side slung with 

fixed slings, the other 

side using a delta 

plate to ensure all 

slings are equally 

loaded. 

SWL 6.0 T 

Use a lifting beam 

with two sets of two 

leg slings, to ensure 

all legs are equally 

loaded. 

Alternative - 

use a snatch 

block. 

Use short 

two-leg slings 

and a further 

set of 

two-legs, to 

ensure equal 

load sharing. 

CRANEAGE - SLINGING BASICS PAGE 7

GENERALLY LIMIT FOR SAFE CRANE OPERATION. REFER TO SPECIFIC CRANE MANUAL. 

BEAUFORT WIND SCALE 

Beaufort 

Number 

0 

1 

2 

3 

4 

Description Observed Features 

Calm Smoke rises vertically 

Light 

Air 

Light 

Breeze 

Gentle 

Breeze 

Moderate 

Breeze 

Smoke drifts downwind. Wind 

does not move wind vane. 

Wind felt on face. Leaves rustle. 

Wind vane moved by wind. 

Leaves and twigs in constant 

motion. Wind extends light flag. 

Raises dust and loose paper. 

Small branches are moved. 

Velocity 

in MPH 

Less 

than 1 

Velocity 

in KPH 

Less 

than 1.6 

1 - 3 2 - 5 

4 - 7 6 - 11 

8 -12 13 -19 

13 - 18 21 - 29 

Small trees in leaf begin tomsway. 

Fresh 

5 Crested wavelets form on inland 19 - 24 31 - 39 

Breeze 

waters. 

Strong Large branches in motion; 

6 25 - 31 40 - 50 

Breeze whistling heard in power wires. 

Whole trees in motion. 

Moderate 

7 Inconvenience felt in walking 

32 - 38 52 - 61 

Gale 

against wind. 

Fresh Twigs break off trees, progress 

8 39 - 46 63 - 74 

Gale generally impeded. 

Slight structural damage occurs 

Strong 

9 (roof tiles and chimney pots 

47 - 54 76 - 87 

Gale 

removed). 

Seldom experienced inland; 

Whole 

10 trees uprooted, considerable 

55 - 63 88 - 101 

Gale 

structural damage. 

Very rarely experienced; causes 

11 Storm 

64 - 75 103 - 121 

widespread destruction. 

12 

Hurricane 

Note: Velocity is measured approx. 6 metres (20ft) above ground level. 

BEAUFORT WIND SCALE PAGE 8 

Over 

75 

Over 

121


SECTION 5: 

REEVING AND CHOKING 

5.1 Know the Effect of Reeving on Slings 

5.2 The Nip Angle 




5.1. KNOW THE EFFECT OF REEVING ON SLINGS. 

"Reeving" is the practice of wrapping a sling around an object, and either doubling it 

back on the hook, or passing one eye of the sling through the other and then to the 

hook. 

This practice is also known as "choking", or "nipping". 

This practice is perfectly safe when done properly, BUT you MUST realise that the act 

of reeving creates an "included angle" within the sling which reduces the permissible 

load on that sling. 

And NEVER bang the hook down on to the load. You are NOT making the sling more 

secure - in fact, you are creating larger loads in the sling by increasing the included 

angle. 

It also increases the crushing force on the load. 

Any sling reeved around a load needs to have a SWL which is 

TWICE that of a vertical sling. 

To work out the sling loads for a reeved sling, use the following formula. 

This will work for any reeved sling with an included angle LESS that 

120°, which must be the absolute maximum angle. 

An example has been shown. 

R 

Load = W. 

Note protection on sharp corners. 

L 

T = Tension in each slinging leg. 

L = Length of each leg. 

R = Rise. 

W = Load to be lifted. 


T = 

1/2 W x L 

R



5.2. KNOW THE EFFECT OF REEVING ON SLINGS. 

When a load is lifted by a pair of equal-length slings, the load in each leg of a sling 

increases as the angle between the legs increases. 

We then have the seemingly ridiculous situation where the load on a sling can be 

many times the actual weight of the load - a frequent cause of sling failure. 

1,000 kg 

2,000 kg 

1,000 kg 

1 tonne 

Single 

leg sling 

1,000 kg 

1 

tonne 

2,000 kg 

60° 

90° 

120° 

1,155 kg 1,414 kg 2,000 kg 11,470 kg 

LOAD ON ANGLED SLINGS 

Reeving a sling around a load with one eye (or bight) through the other, 

actually halves the safe working load of that sling. 

Keep in mind also, that the angle of the sling has already reduced the capacity of the sling. 

If it were possible to reeve the sling and keep the nip well above the load, this load would 

not be so great, but safety requires the nip to be well down on the load. 

1 tonne 

Reeved, nip, or 

choker sling 

Angle 

A 

Basket or 

cradle sling 

A sling which is to be reeved around a load needs to have a safe working 

load of AT LEAST TWICE that of a sling which would otherwise be used. 

2,000 kg 

2,000 kg 2,000 kg 2,000 kg 2,000 kg 

2,000 kg 

0.5 

tonne 

1 tonne 1 tonne 

Bridled 

sling 

2,000 kg 

170° 

0.5 

tonne 

PAGE 11


SECTION 6: 

KNOW YOUR DUNNAGE 

6.1 Know how to use Dunnage properly 




6.1. KNOW HOW TO USE DUNNAGE. 

ALWAYS use dunnage to allow the slings to be removed after lowering the load 

into place. 

Use the "fifth-point" rule as shown. 

The common materials for dunnage are timber offcuts, which are generally fine. 

NEVER use hollow light-wall tubes or pipes which could collapse under the 

load weight, and NEVER use material which is to be used on the job - the load 

could damage them beyond use. 

1/5 length 

NO!! 

NEVER use dunnage on its edge. 

This is unstable, and the load could 

easily shift and drop. 

CRANEAGE - SLINGING: BASICS 

3/5 length 

Note dunnage is kept aligned. 

1/5 length 

Example of simple but effective 

dunnage for storage of pipes or 

reinforcing bars. The inclined sides 

prevent the material from rolling about.


SECTION 7: 

SOME COMMON MISTAKES 

7.1 3- and 4-leg Slings 

7.2 Load sharing traps 




7.1. KNOW HOW TO SLING SAFELY. 

Slinging, as you are well aware now, is more than just attaching slings to a load and 

waving it away. There are however some wrong assumptions which can be made in 

common slinging situations, and some of them are put right: 

(What hidden 

hazard exists 

in this lift?) 

! 

! 

A. DO THREE- OR 

FOUR-LEGGED SLINGS 

SHARE THE LOAD 

EQUALLY? 

We repeat - whenever four (or 

often, three) legged slings are 

used for a load such as a precast 

slab, it will often be the case that 

only TWO of them are taking the 

load, and the other sling or legs 

only serve to balance the load. 

Always assume this will be 

the case. 

B. SURELY, 

THREE-LEGGED SLINGS 

WILL TAKE EQUAL LOADS? 

In this typical lift of a site fuel tank, 

the front rope will often only be 

acting as a balance. Do NOT 

assume that the load is being 

divided three ways - often, only 

TWO legs are actually taking the 

load. This has a massive 

significance on sling selection. 




7.2. KNOW HOW TO SLING SAFELY. 

Another wrong assumption is that a four-leg sling set will be twice as strong as a 

two-leg set. Here's why it is not. 

Let's see, now. Ten tonnes to pick 

up - 4 x 2.5 tonne pins = 10 tonnes. 

10mm chains ought to do. 

Sweet, eh! 

OH, REALLY? 

Remember that one leg at least 

will be slack, and the load will 

then be carried by only two 

slings. Therefore, the included 

angle must be 90°. 

Sling angle = 90°. 

Sling load must be 7.0 tonnes. 

Therefore, 4 x 10-tonne pins 

are required, and 16mm chains. 

Some difference! 

One side slung with fixed 

slings, the other side using 

a delta plate to ensure all 

slings are equally loaded. 

Use a lifting beam 

with two sets of two 

leg slings, to ensure 

all legs are equally 

loaded. 

10 TONNES 

8.5m 

across diagonals 


6m 

Use short 

two-leg 

slings and 

a further 

set of 

two-legs, 

to ensure 

equal 

load 

sharing. 

90° 

Measured 

between 

diagonals! 

At least 

one leg 

will be 

slack! 

Alternative - 

use a snatch 

block.

good slinging practice - Site Safe

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