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wrecking ball
A wrecking ball is a heavy steel ball, usually hung from a crane, that is used for demolishing large buildings. It was most commonly in use during the 1950s and 1960s. Several wrecking companies claim to have invented the wrecking ball. An early documented use was in the breaking up of the in 1888-1889, by Henry Bath and Co, at Rock Ferry on the river Mersey.
In 1999, the wrecking ball was described as "one of the most common forms of large-scale coarse demolition." Although the wrecking ball is still the most efficient way to raze a concrete frame structure, its use is decreasing. With the invention of hydraulic excavators and other machinery, the wrecking ball has become less common at demolition sites as its working efficiency is less than that of high reach excavators.
==Construction and design==
Modern wrecking balls have had a slight re-shaping, with the metal sphere changed into a pear shape with a portion of the top cut off. This shape allows the ball to be more easily pulled back through a roof or concrete slab after it has broken through.
Wrecking balls range from about to around . The ball is made from forged steel, which means the steel is not cast into a mold in a molten state. It is formed under very high pressure while the steel is red hot (soft but not molten) to compress and to strengthen it.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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