How the two differ in kind
A lift has to pass two independent checks, and passing one says nothing about the other.
CRANE CAPACITY is the machine's rated lift for the configuration in use — the boom length, the radius to the load, the counterweight, and whether the outriggers are fully extended — read from the load chart rather than from the crane's headline rating, which applies only at minimum radius. From that figure comes the deduction of everything hanging below the hook: the block, the slings, the spreader beam and any lifting accessory, all of which reduce what is available for the load itself. Utilisation is the load against that remaining capacity, and lift plans state it.
SLING LEG TENSION is a different question and it is the one people get wrong. The tension in each leg of a multi-leg sling is NOT the load divided by the number of legs. Because the legs are inclined rather than vertical, each carries more than its share: the vertical components have to add to the load, so the tension along each leg is larger than its vertical component by a factor set by the angle. As the angle to the horizontal gets SHALLOWER — shorter slings, a wider lifting point spacing — that factor grows sharply, and at low angles the tension in each leg can exceed the total load being lifted.
So a lift comfortably within the crane's chart can overload its rigging, and the failure is at the sling rather than at the machine.
Two further facts compound it. With more than two legs, load sharing cannot be assumed equal unless the geometry guarantees it — a four-leg sling on a rigid load may be carrying most of the weight on two legs, which is why the design commonly assumes only two are effective. And a sling choked or basketed rather than used in a straight vertical pull has its own rated capacity for that configuration, which is lower.
None of this substitutes for a planned lift by competent people with a proper lift plan.
The factors that actually differ
Show
| Sling leg tension | Crane capacity | |
|---|---|---|
| What it checks | What each leg of the sling actually carries. | Whether the machine can lift the load at that radius and configuration. |
| The counter-intuitive part | Leg tension is NOT load divided by legs — inclined legs carry more, and the multiplier grows as the angle falls. | The headline rating applies at minimum radius only; the chart governs. |
| What drives it | The angle of the legs to the horizontal, which depends on sling length and lifting point spacing. | Radius, boom configuration, counterweight and outrigger extension. |
| What is deducted | Not applicable. | Everything below the hook — block, slings, spreader beam, accessories — before the load is considered. |
| Four legs | Do not assume equal sharing on a rigid load; the design commonly credits only two. | Not affected. |
| Sling configuration | Choked and basketed hitches have their own rated capacities, lower and higher respectively than a vertical pull. | Not affected. |
| How to reduce it | Longer slings, so the legs are steeper; or a spreader beam, which makes them vertical. | Reduce the radius, extend the outriggers fully, add counterweight, or use a larger machine. |
| Where it fails | At the rigging — a sling, a shackle, a lifting point on the load. | At the machine — overturning, or structural overload of the boom. |
| Ground bearing | Not applicable. | A separate check again. A crane within its chart can still overturn if the ground under an outrigger fails. |
| Who does it | The lift plan, by a competent appointed person — this page is the arithmetic, not the authorisation. | The same. |
Which one, and when
Choose sling leg tension when…
- Selecting slings and shackles for a load with known lifting points.
- Where the lifting points are widely spaced and the slings are short, which makes the angle shallow.
- Where a spreader beam is being considered, since it is the direct remedy.
- Whenever the legs are visibly far from vertical, which is when the multiplier matters most.
Choose crane capacity when…
- Checking whether the machine can make the lift at the radius required.
- Comparing crane options or configurations for a planned lift.
- Where the radius is large, which is where capacity falls fastest.
- As part of the lift plan, alongside the ground bearing and the rigging checks.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why isn't leg tension just the load divided by the legs?
- Because the legs are inclined, and only the VERTICAL component of each leg's tension holds the load up. The vertical components must add to the total weight, so if a leg is at an angle, its tension along the sling has to be larger than its vertical component — and the shallower the angle to the horizontal, the larger that difference. At a steep angle, near vertical, the tension is close to the shared load. As the angle falls, the multiplier rises, slowly at first and then sharply: below about thirty degrees to the horizontal the tension in each leg exceeds the whole load being lifted, which is why that angle is a common lower limit in guidance. The same geometry also generates a horizontal compression across the load, which can crush or bend it.
- How do you reduce leg tension?
- Make the legs steeper, and there are two ways. Longer slings raise the angle for the same lifting-point spacing, which is the simplest remedy where there is headroom for them — and headroom under the hook is frequently what forces the short slings that caused the problem. A spreader beam does it directly and completely: the beam takes the horizontal component itself, so the slings from the beam to the load hang vertically and each carries its true share, with the beam's own weight added to the lift. Spreader beams also remove the horizontal compression the sling angle applied to the load, which for a long or slender item is often the reason for using one. Moving the lifting points closer together helps the angle and may not suit the load's balance.
- Why can't four legs be assumed to share equally?
- Because on a rigid load they cannot all be exactly the same length, and the shortest pair takes the weight. With four legs on a stiff item, tiny differences in sling length, in lifting point position or in the load's own flatness mean two diagonally opposite legs come tight first and carry the load while the other two are slack or nearly so — and the load can shift between pairs as it swings. Standard practice is therefore to design a four-leg lift on the assumption that only two legs are effective, unless the load is flexible enough or the rigging is arranged so that sharing is guaranteed. Assuming four-way sharing and selecting slings accordingly leaves each of the two working legs carrying twice what it was chosen for.
- Why is the crane's rated capacity not its capacity?
- Because the headline figure applies at minimum radius, in the best configuration, and the actual capacity is read from the load chart for the boom length, the radius, the counterweight and the outrigger extension in use. Capacity falls steeply as the radius increases — reaching further out is what limits a crane far more often than the weight itself. Then everything below the hook is deducted before the load: the hook block, the slings, the shackles, any spreader beam, and any lifting accessory, all of which are lifted along with the load. On a lift with substantial rigging, that deduction is not trivial. Utilisation expresses the result as a percentage, and lift plans set limits on it so that the lift is not run at the very edge of the chart.
- What is a choked hitch and why does it matter?
- A sling passed around the load and back through its own eye so it tightens on the load as it is lifted — used to grip a load that would otherwise slip, such as a bundle of pipe or a single cylindrical item. It matters because the choke reduces the sling's rated capacity compared with a straight vertical pull: the sling is bent sharply at the choke point and the geometry of the grip induces additional stress, so the manufacturer publishes a lower rating for the configuration. A basket hitch, where the sling passes under the load with both eyes on the hook, does the opposite and has a higher rating than a vertical pull, subject to the angle of the legs. Reading the sling's tag for the configuration actually being used is the point, and it is why the tag lists several ratings.
- Does the ground matter to a crane's capacity?
- It is a separate check that can defeat the chart entirely. A crane's load chart assumes the machine is level and properly supported; the loads under its outriggers or tracks are very high and concentrated, and if the ground beneath cannot carry them, the outrigger sinks, the crane goes out of level and the load moves out — which is an overturning mechanism regardless of what the chart said. Mats spread the load over enough area for the ground's bearing capacity, and the size follows from the outrigger load divided by that capacity. The hazards are ground nobody assessed: made ground, a backfilled trench alongside the lift position, a basement or a services duct beneath, and soft ground after rain. It is why ground bearing appears in every lift plan alongside the capacity and the rigging.
- What else belongs in a lift plan?
- Considerably more than these two calculations, and this page is one input rather than the plan. A lift plan identifies the load's weight and its centre of gravity, the lifting points and whether the load is designed to be lifted from them, the crane and configuration with its chart position, the rigging and its ratings in the configuration used, the ground bearing and the mats, the exclusion zone and the control of people beneath and around the lift, the wind limits — since a large light load is governed by wind rather than weight — and the roles: an appointed person responsible for the plan, a competent operator, a qualified slinger and a banksman. It also covers what happens if the lift has to be aborted with the load in the air, which is the scenario improvised plans do not survive.
- How is the centre of gravity relevant?
- It decides how the load behaves once it leaves the ground, and getting it wrong is a common cause of incidents. The hook must be above the centre of gravity, or the load tilts as it lifts and swings until it hangs beneath the hook — which can be sudden, can strike people or structures, and shifts the tension between the sling legs so that one takes far more than the plan assumed. Loads with an offset centre of gravity — a machine with a heavy motor at one end, a tank with residual contents — need the lifting points or the sling lengths arranged so the hook sits above it. The first indication is at the trial lift, where the load is raised just clear and its behaviour observed before committing, which is exactly what that step is for.