Text 9. A contemporary crane

The basic crane described up to this point didn’t change much since the medieval times or perhaps even since ancient Rome. Materials and power systems have advanced through the Industrial Age up to the present, but the essential arrangement of drum, boom strut, and block and tackle have been fairly constant. Manual or animal power has been replaced by electricity and internal combustion, hemp by metallic rope, and wood spars with steel booms. Lifting gear that was once fixed in place is now mobile, and what was once exclusively custom-built has become modular and standardized. Contemporary cranes and derricks have branched out into many forms to perform varied industrial roles. The state of the art, however, is well represented by the example of a telescoping cantilevered boom crane, commonly called a hydraulic crane.

A telescoping boom is almost always mounted on a wheeled chassis with outriggers, these are two of the key crane adaptations of the twentieth century. The cantilevered telescopic boom is made possible by advances in steel quality and in fabrication techniques as well as by the development of fluid power transmission systems. The boom systems utilize hydraulic power transmissions for extending (telescoping) and elevating (luffing) the boom. The boom is raised and lowered with one or a pair of hydraulic cylinders, also called rams or pistons. Telescoping of the boom sections is powered by one or more hydraulic cylinders hidden within the closed sections.

In both luffing and telescoping hydraulic power circuits, a holding valve is mounted on each cylinder to prevent failure in the event of a loss of pressure; it is combined with a counterbalance valve that equilibrates the flow of hydraulic fluid from one side of the piston to the other so that the movement of the piston is kept steady.

The prime mover of a mobile telescopic crane is usually a diesel engine; power is transmitted through a gearbox to hydraulic pumps and motors that drives the various crane motions. A small crane model is equipped with one engine that powers both the crane and the carriage that drives it; a larger mobile crane has separate engines for driving the vehicle and operating the crane.

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