Contents
Overview
What sling angles are and how they are measured
Sling angles are the lean of each leg of a multi-leg sling, measured from the vertical. They matter because the crane lifts the same weight whatever the angle, so a leaning leg pulls harder than its share. With equal legs, each pulls about 1.4 times its share at 45 degrees from vertical and twice its share at 60.
In UK practice, sling angles are measured between each leg and the vertical, the convention in LEEA’s code of practice. Older charts give the included angle between two legs, which is twice the angle to the vertical, so legs at 45 degrees from vertical have a 90 degree included angle between them.
Some charts measure from the horizontal. Check which one a chart uses before reading a figure off it: 60 degrees from the horizontal is 30 degrees from the vertical.
In short
- Each leg pulls with its share of the load divided by the cosine of its angle from vertical.
- Under the uniform load method used in LEEA’s code, no rating band for a multi-leg sling goes beyond 60 degrees from vertical.
- A leaning leg also pulls sideways, drawing the lifting points on the load in towards each other.
- Longer legs or a spreader beam bring the legs back towards vertical.
On a contract lift, our own appointed person plans the lift and writes the lift plan.
Leg tension
Why each leg pulls harder as the angle widens
The crane only ever lifts the weight of the load, straight up. A sling leg hanging vertically carries its share of that weight and nothing more. Once it leans, it must still give the same upward pull, so the pull along the leg rises, and part of it now pulls sideways.
To find the pull in a leg, divide its share of the load by the cosine of the leg’s angle from vertical. The table shows a 2 t load on a two-leg sling, where each leg’s share is 1 t.
| Angle from vertical | Included angle | Pull in each leg | Sideways pull at each end |
|---|---|---|---|
| 0° | 0° | 1.00 t | None |
| 15° | 30° | 1.04 t | 0.27 t |
| 30° | 60° | 1.15 t | 0.58 t |
| 45° | 90° | 1.41 t | 1.00 t |
| 60° | 120° | 2.00 t | 1.73 t |
Working it out
How to work out a sling angle on site
Measure one leg, from the hook to its lifting point, and the distance between the lifting points. On a two-leg sling, half that distance divided by the leg length is the sine of the angle from vertical.
With legs 2 m long on lifting points 2.8 m apart, for example, 1.4 divided by 2 is 0.7, the sine of about 44 degrees. That is just under 45 degrees, the first limit in the ratings below.
A quicker route to the pull in each leg is to divide the leg length by the height from the hook down to the lifting points. In the same example the hook sits about 1.43 m above the lifting points, and 2 divided by 1.43 is 1.4, so each leg of a 2 t load pulls with 1.4 t.
Without a calculator: when two lifting points are no further apart than one leg is long, the legs are within 30 degrees of vertical, and at about 1.4 times the leg length apart they are at 45 degrees.
On a three- or four-leg sling, use the distance from the point directly below the hook out to each lifting point in place of half the distance between points. For four points on a rectangle, that is half the diagonal. Measuring between neighbouring points understates the angle.
Ratings
How multi-leg slings are rated for angle
A multi-leg sling has one marked rating for the whole sling, and it holds only within a stated range of leg angles. Under the uniform load method in LEEA’s code, a two-leg sling’s marked rating is 1.4 times the single-leg rating for legs up to 45 degrees from vertical, and equal to the single-leg rating from 45 to 60 degrees. A three- or four-leg sling is rated at 2.1 times the single-leg rating up to 45 degrees, and 1.5 times from 45 to 60.
The 1.4 comes from the table above. At 45 degrees each leg pulls with 1.41 times its share, so a leg with a single-leg rating of 1 t can carry a 0.71 t share, and two of them about 1.4 t. A four-leg sling gets the same marked rating as a three-leg sling of the same size, on the assumption that only three of its legs might take the load.
Neither band goes past 60 degrees, and the code says the rated angle must not be exceeded. Some makers still rate slings by the trigonometric method, with a rating that changes with every degree, and LEEA strongly recommends using that method only for single-purpose slings, or where a national standard allows it. Our guide to SWL and WLL explains how the marked rating relates to what a sling may lift on a given job.
Work from the sling’s marking and the maker’s instructions. LOLER requires that a competent person plans the lift, and for a crane lift BS 7121 makes that the appointed person, who plans the lift and is responsible for it.
Lifting points
What wide sling angles do to shackles and lifting points
The sideways part of each leg’s pull acts at both ends. At the load it draws the lifting points in towards each other, and at the top it pulls the fittings on the hook apart. At 45 degrees that sideways pull is as large as the leg’s share of the weight, enough to squeeze a long, light frame out of shape or bend a lifting lug sideways.
A shackle on a leg takes the full leg pull, and it should be loaded in line along its centre line. LEEA’s code says to assume a shackle cannot be side loaded unless the maker’s instructions allow it, and then only at the maker’s reduced rating.
How much angled pull an eyebolt takes depends on its type, so check the maker’s rating for the angle you will use. Dynamo eyebolts are for straight lifting only, and a pair of them must be used with a lifting beam. For other pairs, the code says to use the longest legs possible to keep the angle down, and that a sling should not be reeved (threaded) through the eyebolts or shackles, because that considerably increases the load on each eyebolt.
Uneven legs
Unequal legs and off-centre loads
The ratings assume the legs hang at equal angles, evenly spaced around the load. A crane hook settles directly above the load’s centre of gravity, so when that point is off centre, the legs end up at different angles. The steeper leg then carries more of the load.
In an example in LEEA’s code, with a 30 degree included angle, if one leg hangs 12 degrees steeper than the other, that leg reaches its full single-leg rating when the sling lifts its marked rating.
On a stiff load, a four-leg sling can end up carried by only two diagonally opposite legs, and for that case the code says the sling should be derated to two thirds of its marked rating. Using fewer legs cuts the rating as well: to half the marked rating for two legs of a four-leg sling, and to two thirds for two legs of a three-leg sling.
Spreader beams
Using a spreader beam to keep the legs upright
A spreader beam hangs from two top slings and is pushed together by them, while a lifting beam hangs from one point on its top and takes the load in bending. Either one holds the lower slings apart so they can hang straight down to the load. They then carry only their share of the weight, with no sideways pull on the load or its lifting points. Our guide to spreader beams and lifting beams explains how each type carries the load.
The top slings on a spreader still lean, and their sideways pull now acts on the beam, which is designed as a strut to take that squeeze along its length. For eyebolts, LEEA’s code recommends a lifting beam or spreader where practicable to get their full safe working load.
From our lifts
The angle is set in the lift plan from the leg length and the spacing of the lifting points, and the slinger checks the rig matches it before the crane takes the weight.
Choosing between longer legs and a beam isn’t clear cut, because of the number of variables. Our appointed person balances what is safest against what is practical. A vertical lift is best, but getting one can mean extra beams and more complexity, and sometimes the headroom isn’t there.
Questions
Common questions
What is the best sling angle for lifting?
Under the uniform load method, the best range is within 45 degrees of vertical, where a multi-leg sling keeps its highest marked rating. From 45 to 60 degrees it loses part of that rating. Very narrow angles bring their own risk: LEEA’s code warns that the effect of any tilt of the load grows as the legs close together.
Are sling angles below 30 degrees safe?
Measured from the vertical, angles below 30 degrees are inside a multi-leg sling’s top rating band, and with a level load on equal legs, each pulls up to about 15 per cent more than its share. From the horizontal, they are not: below 30 degrees, the legs lean past 60 degrees from vertical, outside every uniform load rating band.
What is the maximum sling angle?
The maximum is 60 degrees from the vertical for a multi-leg sling rated by the uniform load method, a 120 degree included angle between two legs. From the horizontal, that is 30 degrees. Where the sling’s marking gives a lower angle, that lower angle is the limit, and LEEA’s code forbids using a sling beyond its rated angle.
What is the smallest recommended sling angle?
Measured from the horizontal, the smallest angle a multi-leg sling is rated for is 30 degrees, the same as 60 degrees from the vertical under the uniform load method. From the vertical, no rating sets a minimum, but narrow legs are sensitive to tilt: on a fully loaded sling, a slight tilt can bring the downhill leg to its full rating.
How do you calculate a sling angle?
Divide half the distance between two lifting points by the length of one leg, and the answer is the sine of the angle from vertical. Legs of 3 m on points 3 m apart give 1.5 over 3, or 0.5, the sine of 30 degrees. On three- or four-leg slings, measure from below the hook to each point.
How can you tell before a lift whether the sling angle is too wide?
Compare the spacing of the lifting points with the leg length before the load is slung. On a two-leg sling, spacing equal to the leg length puts the legs at 30 degrees from vertical. At about 1.4 times, they reach 45 degrees and the marked rating drops. Past 1.7 times, they exceed 60 degrees, beyond every uniform load band.
How do you calculate sling capacity at an angle?
Under the uniform load method in LEEA’s code, the marked rating already allows for the angle: a two-leg sling takes 1.4 times one leg’s rating up to 45 degrees from vertical, and one leg’s rating from 45 to 60. For a trigonometric rating, multiply one leg’s rating by the number of legs, then by the cosine of the angle.
What is a sling angle factor?
A sling angle factor is the number you multiply a leg’s share of the load by to find the actual pull in that leg: 1.41 at 45 degrees from vertical and 2 at 60 degrees. Some charts print its reciprocal instead, the cosine of the angle, which is the fraction of a leg’s rating left as upward pull.
What regulations apply to lifting slings?
In Great Britain, slings come under LOLER, which requires them to be marked with their safe working load and safe-use details, and thoroughly examined (see our guide to LOLER inspections). Makers supply them under the Supply of Machinery (Safety) Regulations 2008, and our lifting accessory standards guide covers the standards. Northern Ireland has equivalent regulations of its own.
Get help
Slinging a load at the right angle
Call us on 0800 008 6096 or email office@synergy-lifting.co.uk. We need the load’s weight and the spacing of its lifting points, with photos of them. From that we can work out the leg length and angle the lift needs.
We plan the rigging on a contract lift, as set out under crane hire and contract lifting. Machines lifted onto skates or into a new position come under machine movement.
Get a quote
Ready for a quote?
Send us the details and we will come back with a quote, usually the same day for a desktop survey.
Or call 0800 008 6096
