Jar 2
Jar 2 : (n.) A percussion tool that operates on a mechanical or hydraulic principle and is designed to deliver a heavy hammer blow to objects in the borehole to which it is attached. Jars are used for such purposes as freeing stuck objects in the hole in fishing operations or imparting a jarring motion to stuck tools for the purpose of freeing them. The design of the jar often permits blow to be delivered in either a downward or upward direction, with control being effected at the surface. v. To apply a heavy upward or downward blow to the drill stem, or a heavy upward or downward blow to wireline tools, by use of a jar. (WLOP)
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