Definition - What does Mechanical Jar mean?
A Mechanical jar is a versatile jar which incorporates a firing mechanism or a mechanical trip which gets activated only when the required compression or tension is applied to a running string. It is a term which is often used to describe a jar which does not contain any hydraulic trip mechanism like tubular jars and links which never incorporate the firing mechanism. These jars are operated using numerous locks, springs and release mechanisms. They offer a simple way of avoiding the expensive and time consuming fishing and stuck string jobs in the wireline applications.
Petropedia explains Mechanical Jar
A Mechanical Jar trips down at preselected compressional force and up at the preselected tensile force. The jar is tripped only at set threshold which is generally beyond the forces which are reached during drilling. With the tripped down jar, drilling cannot be done as the down jarring can damage measurement while drilling equipment and the drill bit as well. During drilling, the position of the jar in either extended or cocked in accordance with the driller’s preference. The released threshold of the jar is either set at the surface or downhole in accordance with the jar design.