How does a crane work? Crane parts and their functions explained.
Although archaeological records show the use of crane-like machines in ancient Mesopotamia, utilised for irrigation, evidence of machines that operate for similar purposes as modern-day cranes dates back to ancient Greece, to around 6.000 B.C.
There are a number of types of cranes nowadays, but they all operate in accordance with the same mechanical principles utilising a lever system, a pulley system or hydraulic cylinders to create the mechanical advantage necessary to move heavy items.
The function of a crane is to lift a heavy load from one location and to place it in another: this requires a crane to have weight-bearing and lifting capacity, but also stability to avoid toppling under the weight of the load.
Although it may seem intuitive, in order for a crane to be stable the magnitude of load that is to be lifted must be some value less than the load that will cause the crane to tip.
In order to prevent cranes from tipping, the stability can be increased by two main methods: by making the crane wider and increasing its centre of gravity, and by making the part of the crane susceptible to tipping heavier.
Looking to hire a crane rather than learn about one? Take a look at our crane hire services or call the team on 020 8548 3900.
Crane parts at a glance
| Part | What it does | Where you’ll find it |
|---|---|---|
| Carrier | Drives the crane to site and supports everything above it | The wheeled lower section |
| Outriggers | Widen the base and stop the crane tipping | Fold out from each corner of the carrier |
| Slewing ring | Lets the top half of the crane rotate | Between the carrier and the superstructure |
| Superstructure | Carries the cab, boom, hoist and counterweight | On top of the slewing ring |
| Counterweight | Balances the load on the hook | At the back of the superstructure |
| Operator’s cab | Where the operator controls the lift | On the superstructure, beside the boom |
| Boom | The main arm that lifts and reaches | Front of the superstructure |
| Jib | Adds extra height or reach | Fitted to the tip of the boom |
| Hydraulic system | Powers the boom, outriggers and slewing | Throughout the crane |
| Hoist and wire rope | Raises and lowers the load | Winch drum on the superstructure, rope over the boom |
| Hook block | Connects the load to the crane | Hangs from the wire rope |
| Rated capacity indicator | Warns the operator when the crane is near its limits | Display in the cab, sensors on the crane |
How does a crane work?
At its simplest, a crane is a lever. The load hangs off one end of the boom and the weight of the crane, plus its counterweight, sits on the other side of the pivot point. As long as the counterweight side wins, the crane stays where it is.
Two things decide what a crane can lift. The first is strength: how much the boom, rope and hook can physically take. The second is stability: how much it can lift before it starts to tip. Stability is the reason a crane can lift far less at a long reach than it can close in. A 100-tonne crane only lifts anything like 100 tonnes at a very short radius. Move the load further out and the capacity drops quickly.
To improve stability you can widen the base, add weight at the back or keep the centre of gravity low. Almost every part below does one of those jobs, or helps the operator move the load safely once it’s in the air.
CRANE PARTS AND FUNCTIONS
Carrier and driving components
The carrier is the part that gets the crane to site. On an all-terrain or truck-mounted crane it’s essentially a heavy-duty lorry chassis with several axles, its own engine and a driver’s cab, built to travel on public roads. Crawler cranes run on tracks instead of wheels. Tracks spread the weight over a bigger area, so a crawler can move around site with a load on the hook, but it has to be brought in on a low-loader.
For most jobs in and around London, a wheeled mobile crane is the practical choice because it can drive itself to site and be set up in a short time. Telescopic crawler cranes come into their own on longer projects and soft ground.
OUTRIGGERS
Outriggers are the legs that fold out from the sides of a mobile crane before it lifts anything. Each one has a beam that slides out sideways and a jack that pushes down onto the ground, raising the tyres clear. That gives the crane a much wider and steadier base than its wheels ever could.
Because the outriggers put a lot of weight through a small area, the ground underneath matters. On soft or uneven ground, which over a British winter means a lot of sites, outrigger pads or crane mats are laid down to spread the load and stop the legs sinking. Our crane mats can be hired with or without one of our cranes. Many modern cranes, including the Liebherr models in our fleet, can also set up with each outrigger extended by a different amount when space is tight.
Slewing ring
The slewing ring is a large bearing between the carrier and the top half of the crane. It allows the whole superstructure, including the boom, cab and counterweight, to rotate. The operator can pick a load up on one side and set it down on the other without moving the crane itself. Turning the crane like this is called slewing, and it’s driven by a hydraulic motor and gearbox.
COUNTERWEIGHTS
Counterweights are heavy slabs, usually cast iron or steel, stacked at the back of the superstructure. They balance the load on the hook so the crane can lift more without tipping forward.
On larger mobile cranes the counterweight comes off. The crane travels with some or none of it fitted to stay within road weight limits, and the rest follows on a separate lorry to be fitted on site. How much goes on depends on the lift.
OPERATOR’S CAB
The cab is where the operator controls everything: slewing, luffing (raising and lowering the boom), telescoping and hoisting. Most all-terrain cranes actually have two cabs, one on the carrier for driving on the road and one on the superstructure for lifting. The crane cab is positioned to give a good view of the hook and load, and its screens show the crane’s set-up, the weight on the hook and the working radius.
When the operator can’t see the load, for example on a lift over a building, they’re guided by a slinger signaller using radio or hand signals.
THE BOOM
The boom is the main arm of the crane. Its length and angle decide how high and how far out the crane can work. There are two main types:
- Telescopic booms are steel sections that slide inside one another and extend hydraulically. They’re quick to set up and are what you’ll find on most mobile cranes.
- Lattice booms are built from a framework of steel tubes and assembled in sections on site. They’re lighter for their length, so they’re used on crawler cranes and for very heavy or very tall lifts.
Raising and lowering the boom is called luffing. Extending it is called telescoping.
Jib
A jib is an extension fitted to the end of the boom for extra height or reach. On mobile cranes it’s usually a lattice section that folds alongside the boom for travel and swings into place on site. Some jibs can be set at an angle to reach over obstacles, such as placing a load on a roof behind a parapet. The trade-off is capacity: a jib adds reach but reduces how much the crane can lift.
Hydraulic system
Most of a modern mobile crane’s movements are powered by hydraulics. A pump driven by the engine pushes oil under high pressure into cylinders and motors. The cylinders extend the boom sections, raise the boom and push the outriggers down, while hydraulic motors turn the hoist drum and slew the crane. Because oil doesn’t compress, hydraulics give the operator smooth, precise control even with very heavy loads.
Hoist and wire rope
The hoist is the winch that raises and lowers the load. It’s a motor-driven drum on the superstructure with steel wire rope wound round it. The rope runs up the boom, over pulleys (called sheaves) at the boom head and down to the hook block. Winding the drum in lifts the load and paying it out lowers it, all without moving the boom. Many cranes have a main hoist and a second, auxiliary hoist for lighter, quicker lifts.
HOOK Block
The hook block hangs from the wire rope, and it’s what the load is actually attached to, using slings, chains, shackles or a lifting beam. Inside the block are more sheaves, and the rope can be threaded back and forth through them several times. This is called reeving. Each extra length of rope (or “fall”) shares the load, so the crane can lift more, though it hoists more slowly. The hook has a safety catch to stop slings slipping off.
Rated capacity indicator
Every modern mobile crane has a rated capacity indicator, often called a load moment indicator or safe load indicator. Sensors measure the boom length, angle and radius and the weight on the hook, and the system checks these against the crane’s load chart for its current set-up. If a lift gets close to the limit it warns the operator, then stops any movement that would make things worse.
It’s an important safety device, but it doesn’t replace proper planning. Every lift should be planned by a competent appointed person before the crane arrives.
Putting it all together: a typical lift
On a typical job, the crane drives onto site and sets up on its outriggers, on mats if the ground needs them. The right amount of counterweight is fitted for the lift, and the operator sets the crane’s configuration in the rated capacity indicator. The slinger signaller attaches the load to the hook block. The operator then hoists, slews and luffs or telescopes the boom to put the load where it needs to go, guided by the signaller.
All of this follows a lift plan drawn up beforehand by the appointed person, in line with LOLER and BS 7121, the British Standard for the safe use of cranes.
Frequently asked questions
What is a crane?
A crane is a machine that lifts, lowers and moves heavy loads using a boom, a wire rope and a hook. Mobile cranes can drive to site on public roads and set up in a few hours, which is why they’re the most common type used for construction and industrial lifting in the UK.
What are the main parts of a crane?
The main parts of a mobile crane are the carrier, outriggers, slewing ring, superstructure, counterweight, operator’s cab, boom, jib, hydraulic system, hoist and wire rope, hook block and rated capacity indicator. Each one either helps the crane lift, keeps it stable, or helps the operator control the load safely.
How does a mobile crane work?
A mobile crane works like a lever. The load hangs from the boom on one side, and the crane’s weight and counterweight balance it on the other. Outriggers widen the base for stability, hydraulics move the boom, and a winch called the hoist raises and lowers the load on a wire rope.
What is the difference between a boom and a jib?
The boom is the crane’s main arm and does most of the lifting. A jib is a lighter extension fitted to the end of the boom to give extra height or reach. Adding a jib lets the crane work higher or further out, but it reduces the weight the crane can lift.
Why does a crane need outriggers and counterweights?
Both stop the crane tipping over. Outriggers widen the base so the crane is far more stable than it would be on its wheels, and counterweights sit at the back to balance the load on the hook. Without them, a mobile crane could only lift a fraction of its rated capacity.
What qualifications do you need to operate a mobile crane in the UK?
Most UK sites ask mobile crane operators to hold a CPCS A60 card. You start with the red Trained Operator card and move up to the blue Competent Operator card once you’ve completed an NVQ. Slingers and signallers need A40. We run CPCS training courses covering both.
Need a crane, or training to operate one?
Emerson Crane Hire supplies mobile, crawler and spider cranes across London and Essex from our depots in Dagenham, Basildon and West London. We offer operated crane hire, or we can take care of the whole job as a contract lift, including the lift plan and appointed person.
We also run lifting and plant courses for operators and site teams, including CPCS , NOCN, NVQ and ALLMI courses.
Get in touch with the team or call 020 8548 3900 to talk through your lift.
