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A red iron crane at the Baltimore Museum of Industry by Carol M. Highsmith.

Cranes

Stonehenge, the Pyramids, the Empire State Building, the Burj Khalifa. What do these breathtaking structures, these magical monuments to human engineering, have in common? All were built using "simple machines", lever-like mechanisms that hauled heavy materials off the ground and into new positions. We still don't know precisely how ancient structures like the Pyramids were made. But where the wonders of the modern world are concerned, we do: no soaring skyscraper or gleaming football stadium could be built today without the graceful aerial ballet of the crane. The science behind cranes is easy to understand—they're essentially pulleys and levers working together—but it still leaves plenty of interesting questions. Why are there so many different crane types? Why would you choose one rather than another? How much can a crane lift before it buckles or topples over? Let's take a closer look at the wonderful world of cranes and find out more!

Photo: Cranes are an important part of our industrial heritage, as celebrated here at the Baltimore Museum of Industry. Credit: Photographs in the Carol M. Highsmith Archive, Library of Congress.

Contents

  1. What is a crane?
  2. Types of cranes
  3. Choosing a crane
  4. Find out more

What is a crane?

A crane (sometimes referred to as "iron") is a machine that lifts ("picks") heavy objects and moves ("sets") them, often with great accuracy, into a new position. You can probably see that its three essential parts are a hoist (a length of cable or "wire rope" and a winding drum), a boom (a long arm that can pivot up and down to maneuver the load), and some means of moving the load around as it dangles from the boom (maybe a turntable on a static crane, wheels and tracks on a moving one, or both). If a crane can rotate through 360 degrees (some rotate less, some more), you can usually position the load almost anywhere inside a sphere with a radius the length of the boom by raising, lowering, or rotating ("slewing") it while raising or lowering the load.

The science of cranes

In scientific terms, a crane is a complex machine made up of simple machines (devices that multiply force.) At the heart of a crane are two simple machines: a pulley (the hoisting rope usually wraps around multiple rope-guiding wheels or blocks called sheaves) and a lever (the boom). The boom works a bit like an off-center see-saw with its fulcrum (pivot) much closer to one end than the other, so giving huge leverage. Essentially, the pulley (hoist) makes it easy for a crane to lift heavy things (using less force), while the lever (beam) makes it easy to move them (using less force) once they're suspended in mid-air.

A hydraulic truck crane lifts a space module. A crane is made of two simple machiens: a boom (lever) and a hoist (pulley).

Photo: At its most basic, a crane is made of two simple machines: the boom is a lever and the hoist is a pulley. Photo courtesy of NASA.

The simplest crane you can imagine is a length of rope running up over a single pulley, supported on a long beam, and running back down to a hand-cranked hoist. Attach the rope to the load, crank the handle, swing the beam, and off you go. Although this is quite useful, you're limited by your own strength in what you can lift: if the load is too heavy, you won't be able to turn the handle—or maybe the rope will break or the crane will topple forward.

Now add a more complex pulley: add more wheels and wrap the rope around them in a sequence. This multiplies your lifting force (you can lift heavier things with the same force), though it will take you longer: you will need to crank the handle much more, and pull the rope further, though it'll feel a lot easier too. Once you've got the load off the ground, the longer the boom, the easier it is to move the load to a new position (although longer booms, with more leverage, make it correspondingly harder to position things precisely). Replace the hand crank with an engine and gearbox and you can lift even more, but it will take longer again: a pair of gears that can lift twice as much weight take twice as long to do it.

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Cranes are useful because their lever-beams, pulley-hoists, gears, and hydraulics give us what's called a mechanical advantage (they work together to multiply lifting and moving force), but even the biggest, boldest cranes run up hard against two important limits. First, there's a maximum amount of weight they can lift before the cable snaps or the boom buckles. That limit is easy to understand. Second, there's only so far the beam can swing the load from the pivot point (let's call it the fulcrum) without toppling over, which is rather harder to judge. Crane operators use load charts, supplied by the manufacturer or hire company, which set out how much a given machine can lift and how far. The most important point about cranes—more important than anything else—is the need to work safely, so it's vital that operators understand a crane's limits and never push beyond them.

The parts of a crane

We've seen that a simple crane has three essential parts—a boom, a hoist, and a means of moving itself and the load to a new place—but a real-world, practical crane has many more bits than this. Take a look at this typical crane and you can see most of the key parts and what they do.

The key parts of a typical crane, labeled.

Photo: The main parts of a typical crane. Credit: Photographs in Carol M. Highsmith's America Project in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.

Boom: The long, main, lever arm of a crane. For a crane permanently based in a single location, the boom usually stays the same length. With a hired crawler crane, the boom can be assembled to whatever length is needed and typically remains that long for the duration of the job. In a hydraulic truck crane, the boom is telescopic and can be easily adjusted to whatever length is needed. In a crawler crane (one with tracks), the boom is made up of an open lattice of high-strength steel struts (making it lighter, which means it can potentially lift more), constructed in easy-to-assemble, fixed-length sections. In a telescopic crane, the sections of boom are hollow steel beams nested inside one another for easy transportation, just like the antenna on a transistor radio. A crane's boom can be lifted up and down, but it can (usually) also be turned around through a wide angle (sometimes a full 360° or more).

Jib: A relatively short, light, angled extension to the boom that allows a crane to move the load further away, while still operating safely. Sometimes a jib gives more height; sometimes more reach. The jib can be raised or lowered independently of the boom. In a tower crane, the long horizontal arm that picks up the load is (confusingly) referred to as the jib. The short arm that balances it, at the back of the crane, is called the counter jib.

Hoist: The mechanism that raises or lowers the main lifting cable (known as the hoist rope). At its simplest, it's a winding drum that raises or lowers a length of wire rope with something like a hook on the end. It has one or more brakes to lock the load in position when it's been lifted. In a tower crane, the hoist rope is positioned over the load by a trolley that moves along guide rails.

Hoist mechanism on red Ransomes and Rapier steam railroad crane.

Photo: The hoist mechanism on a vintage British Ransomes and Rapier Steam Breakdown Crane dating from the 1940s. The parts you can see here are: 1) the steam-powered crank that powers the winding mechanism; 2) the winding drum; 3) the pulleys; 4) the gearbox; 5) the boom (currently lowered for transportation); 6) the turntable. Cranes like this were designed for railroad breakdowns and repairs in the age of heavy steam engines and war. A machine like this could lift about 45 tonnes.

Cab: The operator's "control room" has large toughened glass (or plexiglass) windows so they have a clear view of the boom at all times. Older cranes were entirely mechanical, operated by levers, pedals, and switches. Modern ones are more likely to be electronic, controlled (like a video game) by one or more small joysticks, with a variety of electronic safety mechanisms, and small computer display panels giving instant feedback on the lift. Cabs in tower cranes often have creature comforts like microwave ovens and stereos, although they don't have toilets (there isn't really room and there's no practical way to empty the waste).

Housing: The main body of a crane consisting of the cab, engine, hydraulics, hoist, and so on.

Gantry: A short tower running up from the housing carrying a cable that can raise or lower the boom.

Sheaves: The pulley wheels or other supports around which the wire rope runs. A big truck-mounted crane might have as many as 10 sheaves and 20 lines supporting the hook.

Hook: A load is typically supported by multiple sheaves/pulleys and lines, but just a single hook. That means the hook needs to be really strong and sturdy. On a large truck crane, even the hook can weigh 200–300kg (400–700lb) and the entire lifting tackle can easily weigh a tonne or more, which is extra weight the crane has to lift on top of the load. If the hook is relatively small and light, a weighted "headache" ball, just above the hook itself, helps to keep proper tension in the heavy lifting lines.

Pulley sheaves and hook on a large crane

Photo: The pulley/sheaves and mighty hook on a typical large crane. The hoisting rope is running around eight wheels here so the load is shared between 16 lengths of wire rope. Photo by Jet Lowe, Historic American Engineering Record (HAER), courtesy of US Library of Congress.

Outriggers: On a mobile crane, a set of (usually four or more) hydraulically powered legs that extend out from the chassis to prevent the whole thing toppling over. They work the same way as the stabilizers on a children's bike, usually coming out at right angles from the chassis but sometimes making an X-shape for added stability.

Diesel engine: Sometimes replaced by an electric motor. It powers the hoists, generates electricity, and drives any hydraulic mechanisms.

Lines and pendants: The cables between the mast and the boom.

Cribbing: The flat surface or mat built under a crane to support it on rough ground.

Lifting device: It could be a simple hook, a clamshell bucket, an electromagnet, a grabbing claw, or a pronged attachment called a spreader for lifting standard-sized shipping containers at their corners.

Counterweight: Most cranes have one or more heavy (often concrete) blocks mounted opposite the boom to stop them toppling over, but also to balance them so they move smoothly. On a crawler or truck crane, the counterweight is positioned at the base on the opposite side of the turntable from the hook. On a tower crane, the counterweights are mounted at the opposite end of the jib from the hoist, and can be moved as necessary to maintain stability. The counterweights are typically adjusted to suit whatever loads the crane needs to lift; if they're too heavy, they make the crane unbalanced.

Turntable: A means of rotating the boom, usually some kind of turntable mounted on bearings powered by the engine or motor. On a tower crane, the turntable is called the slewing unit, because "slewing" is crane-world jargon for horizontal rotation. It's essentially a large gear wheel and motor at the top of the tower.

Types of cranes

A row of port cranes unloading a container ship in Long Beach, California. Photo by Dennis Schroeder, NREL

Photo: Port cranes like these, at Long Beach California, do the same job—unloading containers from ships—over and over again; you can't really use them for anything else. Other cranes, mounted on trucks and driven from site to site, are much more general-purpose and flexible. Photo by Dennis Schroeder courtesy of US DOE/NREL.

You might think a crane... is a crane… is a crane, but there are many different types and quite a few ways of classifying them.

The simplest distinction is between static or fixed cranes (at a port, for example, or in a breaker's yard) and mobile ones (mounted or transported on trucks that drive from place to place). But even mobile cranes divide into crawlers (cranes on tracks that slowly grind around a work site but never, by themselves, drive on a highway) and truck-mounted cranes (with separate driving and crane-operating cabs, which can be driven from site to site).

Some cranes are designed for a single, specialized, but highly repetitive job; port cranes, for example, typically have a permanent "spreader" attachment that only ever lifts containers from ships. Other cranes, such as towers, can be assembled in varying configurations to create city buildings at different heights. Or, like truck cranes for hire, they might be designed for general use in many different places, in many different ways. Just to confuse matters, some cranes mix different "types" on a single machine. For example, you can put tower crane attachments on top of crawler cranes and you can put traditional cranes with angled booms or jibs ("luffing cranes") onto tower bases so they can operate in much more confined urban spaces.

Now let's look in more detail at five of the most common types of everyday cranes.

Crawler cranes

A Link Belt crawler crame with a lattice boom.

Photo: A crawler (track-mounted) crane with a lattice boom, made by Link Belt. Photo by Kimberly A. Brown, US Navy, courtesy of Wikimedia Commons. If you look at the full-size version of this photo, you can see the flatbed truck that the crane is transported on, parked up alongside.

Crawlers are the simplest and easiest to recognize of mobile cranes. There's a basic cab, an open-lattice boom (made of criss-cross struts you can see through), a hoist, and usually a diesel engine mounted on a turntable, itself mounted on wheels or crawler tracks. These help to spread the load, stop the crane sinking into the ground, and allow it to move freely around the work site. Typically, counterweights are mounted at the back of the housing or on a circular base attachment ("ringer") so they always balance the load being lifted as the crane spins around. In the photo below, the counterweights are the large red and white pieces at the back of the white housing.

Although crawlers can't drive on the road, they can be taken to pieces and transported in sections on the backs of flatbed trucks. A really large crane might need anything from three, five, or ten, to as many as 50 truck-loads to transport it all, though most cranes need fewer. (Sometimes the jib of a crane can be slotted inside the boom for easy transportation.) Once all its bits are unloaded, they're laid on the ground in a straight line. Sometimes a second, "assist" crane has to be used to assemble the main crane, although many cranes can assemble themselves. To do this, they pick up and assemble their own parts, eventually lifting them upright to make the finished boom and jib. Typical crawlers lift 250–2000 tonnes, with main boom lengths of up to 75m (250ft), and jibs perhaps 25–45m (75–150ft).

Modern cranes are much sophisticated than older ones and have a variety of computerized aids to help the operator work safely and efficiently. For example, the Liebherr machine pictured below has a control panel that shows the ground pressure (warning the operator if the ground is about to give way beneath the crane), wind speed, and other critical data like hydraulic pressure, power supply details, and so on. It can also be controlled from outside the cab with a handheld, remote control, joystick unit for situations where the operator needs a clearer view of what's going on. Some machines use augmented reality to superimpose helpful computer data on a moment-by-moment view of the lift captured by a camera.

A white Liebherr LR1250 crawler crane parked on the HS2 construction site in Birmingham

Photo: A Liebherr LR1250 crawler crane with a strong, reinforced, lattice boom. Left: The whole crane. Right: A closeup of the housing and crawlers. This crane has a maximum lift capacity of 250 tonnes and can be configured with a 73m (240ft) boom and an 83m (272ft) jib. It has a maximum operating radius of 97m (318ft). That's almost the length of an Olympic sprint! (While you're looking at the picture, note the stack of red counterweights on the left. It's easy to add more or take some away, as necessary.)

Truck cranes

The most versatile, general-purpose cranes are trucks with driving cabs at the front and lattice or telescopic-hydraulic boom cranes on the back (in the trade, hydraulic cranes are often called "hydros" for short). When the booms are solid, they're relatively heavy, which reduces the weight the crane can lift and shift when the boom is fully extended. The smallest truck cranes have just two axles, while really big ones have up to nine (typically five or more of them are steerable).

A truck crane with a lattice book lifts an airplane

Photo: A typical truck-mounted crane with a lattice boom. Note the four outriggers, which are taking the weight. Photo by Danny Webb courtesy of US Air Force.

Truck cranes can be hired by the day and are relatively quick and easy to deploy (a long "hydro" will take maybe 5–7 minutes to fully extend its telescopic boom). While crawlers can move around as they lift, truck cranes are usually parked with their outriggers extended in one place on the work site, so they may not be able to move loads quite so freely. Outriggers might look like a nuisance, but they significantly increase how much a crane can "pick" safely (a truck-mounted crane with outriggers will be able to lift more than a similar crane mounted on wheels or crawlers).

A yellow Liebherr truck crane, preparing to start work, seen from the very rear

Photo: Preparing a hydraulic truck crane ("hydro") for work. Photo by Werner Slocum courtesy of US DOE/NREL.

Most truck cranes are designed to drive on ordinary roads, which imposes weight and size limits on them, and so limits what they can lift. Others, designed to work on particularly rough terrain ("RT"), are fitted with huge tires, like giant construction machines; not roadworthy by themselves, they're designed to be shipped around on the back of a truck. All terrain ("AT") cranes are the best of both worlds: roadworthy, but still with the ability to work on rough terrain. Typical truck cranes might lift 75–300 tonnes and have boom lengths of 90m (300ft) or less.

At the opposite end of the scale, small truck cranes, used for things like delivering building supplies or installing telephone poles, use articulated booms, rather like multi-jointed human arms but operated hydraulically. Cranes like this are typically controlled from instrument panels down at the back or on top of the truck. Depending on the lifting capacity and the weight of the truck, they may or may not have outriggers.

The chassis section of a typical Liebherr hydraulic truck crane, showing the large rear counterweights

Photo: A typical Liebherr hydraulic truck crane. Note the extended outriggers at the side and the large red and white counterweights at the rear (at the base of the white boom). Photo by Dennis Schroeder courtesy of US DOE/NREL.

Tower cranes

Red luffing tower crane.

Delta hire luffing tower crane.

Photo: Luffing tower cranes (ones with angled jibs) are more suited to work in confined urban spaces than hammerheads (ones with long horizontal jibs).

Looking up at city construction site, you might think all tower cranes are exactly the same: picture one in your mind and you'll see a permanent vertical tower, a main jib extending sideways from it (balanced by a short counterweight jib), a cab (or "operator's house"), and a little trolley running back and forth along the jib moving the hoisting line that lifts the load.

In fact, there are quite a few different types of tower cranes. Some are built upward from the ground (anchored in deep concrete foundations) with the help of truck cranes or derricks mounted on top of other buildings. Some can slowly jack themselves up into the air using a hydraulic mechanism that locks new pieces of the tower into place as they're needed. Some tower cranes are constructed inside the buildings they're creating and then torn down again later. Occasionally, cranes are built in elevator shafts for convenience or, as in the case of the fateful World Trade Center, at the four corners of the building, moving upward as the building itself ascends.

Tower cranes with horizontal jibs are often called hammerheads, because that's what they look like. Another popular variety, known as luffing cranes, have angled booms and jibs (like high bent shoulders and elbows), which allow them to work in much more confined urban spaces; since they have much less horizontal reach, typically they can lift more but with a shorter operating radius. Driven by powerful diesel engines or quieter electric motors, they're a bit like ground-based cranes mounted up on towers.

Labeled parts of a tower crane. Photo by Carol M. Highsmith

Photo: Key parts of a hammerhead (straight jib) tower crane. 1 - Counterweight; 2 - Counter-jib tie; 3 - Jib tie; 4 - Main jib; 5 - Trolley; 6 - Hook; 7 - Tower; 8 - Turntable (slewing ring); 9 - Counter jib; 10 - Hoist; 11 Cab. Credit: The Lyda Hill Texas Collection of Photographs in Carol M. Highsmith's America Project, Library of Congress.

You might wonder, as I often used to, how a tower crane manages to lift extraordinary weights over such long horizontal distances without toppling over. You might think it's all down to the concrete counterweights that you can see nestling under one end of the jib (labeled "1" and on the extreme left in the photo above). More simply, it's because tower cranes don't generally lift very heavy things: they're more about lifting modest loads of building materials through considerable heights than shifting really heavy loads—and speedy lifting is often the most important thing where concrete construction is concerned. Basic physics tells us that (like any other crane), they can lift much more weight close to their tower than further away from it. A really big tower crane (such as a classic Kroll K-10000) can lift a maximum of 120 tonnes at a maximum distance of 82m (270ft) from its tower, giving it a rating (multiplying the two numbers) of 9,800 tonne-meters.

Overhead cranes and gantries

Freightiner railroad gantry crane at Birmingham, UK.

Photo: Gantry crane: Two different views of a Liebherr crane that lifts containers on and off railroad cars in Birmingham, England. Cranes like this can manage maximum loads of about 65 tonnes.

Freightiner railroad gantry crane at Birmingham, UK.

Not all cranes have booms and jibs or high, swinging towers. In factories, for example, it's often necessary to lift things only small distances but move them anywhere on the factory floor. If that's a routine part of the production process, the factory probably has an overhead crane with a trolley-mounted hoist that can travel along bridge-like beams. A variation on this is called a gantry crane: it's the same kind of beam-mounted overhead crane but the base has wheels (sometimes running along rails) so it can move from place to place. The world's biggest gantry crane, Taisun, can lift 20,000 tons a height of up to 80m (260ft), using 50km (30 miles) of wire rope to do it!

Derricks

The simplest static cranes are derricks, which are frameworks or towers from which a hoisting pulley can be suspended, with or without booms to help. Derricks come in various shapes and sizes, including vertical towers and tilted A-frames. Oil rigs use simple vertical tower derricks to raise and lower their drills into the ground.

A crane derrick lifts a retired Space Shuttle

Photo: A crane derrick (right) is a kind of giant, usually non-rotating lifting frame. In this photo, the red and white derrick on the right is being assisted by the yellow and black truck crane (on the left). Photo courtesy of NASA.

Choosing a crane

All cranes do the same basic job—lifting and moving—but if you're choosing a machine for a particular purpose, there are quite a few different factors you'll need to weigh up:

All these things help to dictate the use of one type of crane over another. For instance, electric tower cranes are quieter than diesel ones and more suited to working in densely populated cities, while luffing tower cranes need less space than hammerhead ones. Hydraulic, mobile truck cranes are ready to start work as soon as they arrive on site, but might work more slowly. Finally, how many cranes do you need? Awkward picks might need two or more. Sometimes you'll need one crane to build or tear down another (a mobile hydraulic truck to build a tower, for example), which adds to the cost of the lifting operation.

One thing about cranes is always certain: there's always a machine for the job. It's just a matter of picking the right one!

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Woodford, Chris. (2021) Cranes. Retrieved from https://www.explainthatstuff.com/cranes.html. [Accessed (Insert date here)]

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