A downhole tool used to deliver an impact force to the tool string, usually to operate downhole tools or to dislodge a stuck tool string. Jars of different designs and operating principles are commonly included on slickline, coiled tubing and workover tool strings. Simple slickline jars incorporate an assembly that allows some free travel within the tool to gain momentum for the impact that occurs at the end of the stroke. Larger, more complex jars for coiled tubing or workover strings incorporate a trip or firing mechanism that prevents the jar from operating until the desired tension is applied to the string, thus optimizing the impact delivered. Jars are designed to be reset by simple string manipulation and are capable of repeated operation or firing before being recovered from the well.

Related Terms:

fishing tool