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: Two "luffing" tower cranes with their heads in the clouds. The main one, in the center,
is an electrically powered Terex CTL. Cranes like this have a typical jib length of 30–50m (100–160 ft) and can lift up to 1.7 tonnes (1.9 tons) operating at their maximum reach (radius). They can be built to heights of up to 80m (260 ft).
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.
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.
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.
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.
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
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
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.
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).
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).
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.
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.
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.
Photo: A small hydraulic truck crane lifts a bag of building supplies to the sidewalk.
This particular model is a British-made Perry PH180, capable of lifting 1500kg (3300lb) to a
maximum reach of 5m (16ft).
Tower cranes
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.
Photo: A typical self-building tower crane.
Credit: The Lyda Hill Texas Collection of Photographs in Carol M. Highsmith's America Project, Library of Congress. Animation: How it builds itself by lifting new sections into place and jacking
the top section up hydraulically.
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 rotate in two different ways. Some turn ("slew") on a gear mechanism mounted at the top of the tower,
while the main part of the tower (below the jib) is anchored firmly into the ground and doesn't move. These are called high-slewing or top-slewing cranes. Bottom-slewing cranes turn the entire jib and tower around their base.
Top-slewing cranes tend to be used for bigger, higher projects where they can remain in place for longer.
Hammerheads
Tower cranes with horizontal jibs are often called hammerheads,
because that's what they look like. You can see most of the main bits in the photo below:
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.
One thing you can't see here is that there are two separate winches on the jib. There's
the main hoist (sometimes called the hoist winch) for raising and lowering the load, but there's also
a second, trolley winch that moves the trolley up and down the jib using a pulley system.
Greatly simplified, it works something like this:
Artwork: Hammerhead tower cranes have two separate winches on the jib: a hoist for raising and lowering the load (blue) and a pulley and winch system (red) for moving the trolley back and forth.
Luffing cranes
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. Unlike hammerheads, luffing cranes have no trolley: all the lifting is done by
raising and lowering the jib. This means they're often slower to move things
(it takes more time to lift and lower a jib than for a trolley to slide along
one) and more difficult to operate.
Photo: A closer look at a luffing jib tower crane. This one is an all-electric Terex CTL. The inset photo shows a similar crane with the jib raised.
How much can they lift?
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)—and a big pile of other
counterweights you can't see at the base of the tower.
That's the crux of it, but it's also important to note that tower cranes don't generally
lift very heavy things: they're more about lifting modest loads of building
materials through considerable heights and distances 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. A small tower crane like the Terex pictured above
can lift a maximum of about 10 tonnes (near to the tower) or just 1.8 tonnes (at its maximum reach).
Overhead cranes and gantries
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.
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.
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
Photo: Luffing tower cranes (ones with angled jibs) are more suited to work in confined urban spaces than hammerheads (ones with long horizontal jibs).
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:
How high will the crane need to operate and how much weight will it need to lift?
How far will it need to reach (what is the operating "lift radius")?
What kind of terrain and ground conditions will the crane work in?
What sort of access is there to the site and what sort of clearance between
neighboring buildings, trees, power lines, or other obstructions?
What exactly is the load (or loads) and how easy will it be to
maneuver? (Apart from other obstacles on the site, there has to be
enough clearance between the load, the crane's boom, and the
ground.)
How long will the crane take to assemble on site and how fast will it work?
Will it have to make many lifts in a fixed time period or maybe just one, critical "pick"?
How long will it take?
How precisely will the loads need to be positioned? How many people will need
to be on the ground to help?
What kind of crane power is most suitable for the job—diesel, electric, hydraulic?
How noisy will the crane be for local residents? Will that limit the operating hours?
How much will it cost to buy or hire, assemble, take down, and transport?
What about environmental conditions, like wind, that will stress high tower cranes when they
reach their maximum height and make it harder to position loads more accurately?
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!
Photo: A hydraulic truck crane with a clamshell attachment used for road construction and maintenance. This is a Palfinger Epsilon M125L Classic, which can lift a load of about 1–4 tonnes over a reach of around 3–10m (10–33ft) and swing it through angles of up to 425 degrees. Note the outriggers just between the two front wheels.
Cranes and Derricks by Lawrence K. Shapiro, Jay P. Shapiro, McGraw-Hill Education, 2010. A comprehensive, 700-page handbook authored by crane specialists.
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