A crane is a type of machine, generally equipped with a hoist rope, wire ropes or chains, and sheaves, that can be used both to lift and lower materials and to move them horizontally.
It is mainly used for lifting heavy things and transporting them to other places.The device uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of a human.
Cranes are commonly employed in the transport industry for the loading and unloading of freight, in the construction industry for the movement of materials and in the manufacturing industry for the assembling of heavy equipment.
The earliest steam crane being introduced in the 18th or 19th century, with many remaining in use well into the late 20th century.
How does a Crane work:
To operate efficiently and maintain its vital stability, every type of crane must obey the laws of physics.
The two most important considerations in this respect are that the crane must not move weights which exceed its rated capacity, and that any stressful movements occurring beyond each machine’s designated plane of operation should be eliminated wherever possible.
A crane is able to lift objects because the load is offset by counterweights which stabilise the crane, allowing it to lift and move its load.
What features & capabilities do different types of Crane have?
The various categories and types of crane have a range of different features. Here are some of the common features in cranes that are often considered when purchasing, using a crane hire company or contract lifting service, which are covered in a Load Chart:
Lifting Capacity: How much weight can the crane lift?
Lifting Range: Where do you need to lift to?
Lift Angle: With a high angle of lift, the lift capacity decreases, so the angle of lift is a key consideration.
Working Radius: What area does the crane need to work over?
Mobility: How much space is there for the Crane to operate? Is a mobile tower crane required?
Weight & Dimensions: Situating the crane itself is a key consideration – the size and mobility of the crane need to fit in with the restrictions of the construction site. With outriggers extended lifting capacity is affected, so a confined space requires a certain type of crane.
Setup Time: Some projects require minimal disruption, so the fast setup time that comes with Mobile Tower Cranes, for example, is a benefit.
Night Working: Quieter operations and appropriate lighting can be a requirement for working at night.
The first known construction cranes were invented by the Ancient Greeks and were powered by men or beasts of burden such as donkeys, these cranes were used for the construction of tall buildings. Later, larger cranes were developed employing the use of human treadwheels, permitting the lifting of heavier weights. Harbour cranes were introduced to load and unload ships and assist with their construction in the High middle ages, they were used to built into stone towers for extra strength and stability. The earliest cranes were constructed from wood, but cast iron, iron and steel took over with the coming of the Industrial Revolution. Modern cranes usually use internal combustion engines or electric motors and hydraulic systems to provide a much greater lifting capability than was previously possible, although manual cranes are still utilized where the provision of power would be uneconomic.
Cranes exist in an enormous variety of forms – each tailored to a specific use. Sizes range from the smallest jib cranes, used inside workshops, to the tallest tower cranes, used for constructing high buildings. Mini-cranes are also used for constructing high buildings, in order to facilitate constructions by reaching tight spaces. Finally, we can find larger floating cranes, generally used to build oil rigs and salvage sunken ships.
History of Cranes
The crane for lifting heavy loads was invented by the Ancient Greeks in the late 6th century BC.The archaeological record shows that no later than c.515 BC distinctive cuttings for both lifting tongs and lewis irons begin to appear on stone blocks of Greek temples. Since these holes point at the use of a lifting device, and since they are to be found either above the center of gravity of the block, or in pairs equidistant from a point over the center of gravity, they are regarded by archaeologists as the positive evidence required for the existence of the crane.The introduction of the winch and pulley hoist soon lead to a widespread replacement of ramps as the main means of vertical motion. For the next two hundred years, Greek building sites witnessed a sharp drop in the weights handled, as the new lifting technique made the use of several smaller stones more practical than of fewer larger ones. In contrast to the archaic period with its tendency to ever-increasing block sizes, Greek temples of the classical age like the Parthenon invariably featured stone blocks weighing less than 15-20 tons.
Also, The practice of erecting large monolithic columns was practically abandoned in favor of using several column drums. Although the exact circumstances of the shift from the ramp to the crane technology remain unclear, it has been argued that the volatile social and political conditions of Greece were more suitable to the employment of small, professional construction teams than of large bodies of unskilled labour, making the crane more preferable to the Greek polis than the more labor-intensive ramp which had been the norm in the autocratic societies of Egypt or Assyria.
The first unequivocal literary evidence for the existence of the compound pulley system appears in the Mechanical Problems (Mech. 18, 853a32-853b13) attributed to Aristotle (384-322 B.C.E.), but perhaps composed at a slightly later date. Around the same time, block sizes at Greek temples began to match their archaic predecessors again, indicating that the more sophisticated compound pulley must have found its way to Greek construction sites by then.
The heyday of crane in ancient times came under the Roman Empire, when construction activity soared and buildings reached enormous dimensions. The Romans adopted the Greek crane and developed it further.
The simplest Roman crane, the Trispastos, consisted of a single-beam jib, a winch, a rope, and a block containing three pulleys. Heavier crane types featured five pulleys or, in case of the largest one, a set of three by five pulleys (Polyspastos) and came with two, three or four masts, depending on the maximum load.
During the High Middle Ages the treadwheel crane was reintroduced on a large scale after the technology had fallen into disuse in western Europe with the demise of the Western Roman Empire. Generally, vertical transport was done more safely and cheaply by cranes than by customary methods. Typical areas of application were harbors, mines, and, in particular, building sites where the treadwheel crane played a pivotal role in the construction of the lofty Gothic cathedrals.
Nevertheless, both archival and pictorial sources of the time suggest that newly introduced machines like treadwheels or wheelbarrows did not completely replace more labor-intensive methods like ladders and hand barrows. Rather, old and new machinery continued to coexist on medieval construction sites and harbors.
Apart from treadwheels, medieval depictions also show cranes to be powered manually by windlasses with radiating spokes, cranks and by the fifteenth century, also by windlasses shaped like a ship’s wheel. To smooth out irregularities of impulse and get over “dead-spots” in the lifting process flywheels are known to be in use as early as 1123.
Structure and Placements:
The medieval treadwheel was a large wooden wheel turning around a central shaft with a treadway wide enough for two workers walking side by side. While the earlier ‘compass -arm wheel had spokes directly driven into the central shaft, the more advanced clasp -arm type featured arms arranged as chords to the wheel rim, giving the possibility of using a thinner shaft and providing thus a greater mechanical advantage. Contrary to a popularly held belief, cranes on medieval building sites were neither placed on the extremely lightweight scaffolding used at the time nor on the thin walls of the Gothic churches, which were incapable of supporting the weight of both hoisting machine and load.
Rather, cranes were placed in the initial stages of construction on the ground, often within the building. When a new floor was completed, and massive tie beams of the roof connected the walls, the crane was dismantled and reassembled on the roof beams from where it was moved from bay to bay during construction of the vaults. Thus, the crane “grew” and “wandered” with the building with the result that today all extant construction cranes in England are found in church towers above the vaulting and below the roof, where they remained after building construction for bringing material for repairs aloft. Less frequently, medieval illuminations also show cranes mounted on the outside of walls with the stand of the machine secured to putlogs.
Mechanics and operation:
Accordingly, lifting work was organized at the workplace in a different way than today. In building construction, for example, it is assumed that either the crane lifted the stone blocks from the bottom directly into place, or from a place opposite the center of the wall from where it could deliver the blocks for two teams working at each end of the wall. Additionally, the crane master who usually gave orders at the treadwheel workers from outside the crane was able to manipulate the movement laterally by a small rope attached to the load. Slewing cranes, which allowed a rotation of the load, were thus particularly suited for dockside work appeared as early as 1340. While ashlar blocks were directly lifted by a sling, lewis, or devil’s clamp (German Teufelskralle), other objects were placed before in containers like pallets, baskets, wooden boxes, or barrels.
It is noteworthy that medieval cranes rarely featured ratchets or brakes to forestall the load from running backward.This curious absence is explained by the high friction force exercised by medieval treadwheels, which normally prevented the wheel from accelerating beyond control.
According to the “present state of knowledge” unknown in antiquity, stationary harbor cranes are considered a new development of the Middle Ages.The typical harbor crane was a pivoting structure equipped with double treadwheels. These cranes were placed dock sides for the loading and unloading of cargo where they replaced or complemented older lifting methods like see-saws, winches, and yards.Two different types of harbor cranes can be identified with a varying geographical distribution: While gantry cranes which pivoted on a central vertical axle were commonly found at the Flemish and Dutch coastside, German sea, and inland harbors typically featured tower cranes where the windlass and treadwheels were situated in a solid tower with only jib arm and roof rotating. Interestingly, dockside cranes were not adopted in the Mediterranean region and the highly developed Italian ports, where authorities continued to rely on the more labor-intensive method of unloading goods by ramps beyond the Middle Ages.
There are two major considerations that are taken into account in the design of cranes. The first is that the crane must be able to lift a load of a specified weight and the second is that the crane must remain stable and not topple over when the load is lifted and moved to another location.
For stability, the sum of all moments about the base of the crane must be close to zero so that the crane does not overturn. In practice, the magnitude of load that is permitted to be lifted (called the “rated load” in the US) is some value less than the load that will cause the crane to tip, thus providing a safety margin.
The lever: A balance crane contains a horizontal beam (the lever) pivoted about a point called the fulcrum. The principle of the lever allows a heavy load attached to the shorter end of the beam to be lifted by a smaller force applied in the opposite direction to the longer end of the beam. The ratio of the load’s weight to the applied force is equal to the ratio of the lengths of the longer arm and the shorter arm, and is called the mechanical advantage.
The pulley: A jib crane contains a tilted strut (the jib) that supports a fixed pulley block. Cables are wrapped multiple times round the fixed block and round another block attached to the load. When the free end of the cable is pulled by hand or by a winding machine, the pulley system delivers a force to the load that is equal to the applied force multiplied by the number of lengths of cable passing between the two blocks. This number is the mechanical advantage.
The hydraulic cylinder: This can be used directly to lift the load (as with a HIAB), or indirectly to move the jib or beam that carries another lifting device.
Types of Crane:
Truck Mounted Crane:
A self-propelled loading-unloading machine mounted on a truck body, with a working section consisting of a rotating cantilevered boom.
It is used for the loading and unloading of motor vehicle rolling stock, for cargoes primarily of a heavy and single-item nature, and also for construction and repair work.
A railroad crane is a crane with flanged wheels, used by railroads. The simplest form is just a crane mounted on a railroad car or on a flatcar. More capable devices are purpose-built. Different types of crane are used for maintenance work, recovery operations and freight loading in goods yards.
Rough Terrain Crane:
A rough terrain crane has a boom mounted on an undercarriage atop four rubber tires that is designed for off-road pick-and-carry operations. Outriggers are used to level and stabilize the crane for hoisting.
These telescopic cranes are single-engine machines, with the same engine powering the undercarriage and the crane, similar to a crawler crane. The engine is usually mounted in the undercarriage rather than in the upper, as with crawler crane.
A crawler crane has its boom mounted on an undercarriage fitted with a set of crawler tracks that provide both stability and mobility. Crawler cranes range in lifting capacity from about 40 to 3,500 short tons (35.7 to 3,125.0 long tons: 36.3 to 3,175.1 t).
The main advantage of a crawler crane is its ready mobility and use, since the crane is able to operate on sites with minimal improvement and stable on its tracks without outriggers.
Floating cranes are used mainly in bridge building and port construction, but they are also used for occasional loading and unloading of especially heavy or awkward loads on and off ships.
Some floating cranes are mounted on pontoons, others are specialized crane barges with a lifting capacity exceeding 10,000 short tons (8,929 long tons; 9,072 t) and have been used to transport entire bridge sections. Floating cranes have also been used to salvage sunken ships.
Mobile cranes are commonly mounted on wheeled vehicles, but cranes used for railway work are adapted to travel on rail tracks, and various floating cranes can be attached to barges when used for construction work on bridges and waterways.
Driven by a hydraulic mechanism, a telescopic crane features a set of concentric tubular steel booms which can be easily extended and retracted to alter the operational height of the crane.
Telescopic cranes are widely used, including by rescue services, and to perform tasks such as launching and retrieving boats at the waterside.
“Static” refers to the requirement that the crane is installed in a certain place, rather than transported in by itself. This is the stark difference between static cranes and mobile cranes.
The term “static cranes” encompasses different types of crane including Tower Cranes.
The top manufacturers of Cranes are: