Most modern drilling jars are hydraulic. They are also usually double acting, meaning they can deliver an extra-heavy impact should the bottom hole assembly become stuck. They are intended to work as an integral part of the drill string, and can withstand high pressures and temperatures over a long period of time, making them suitable for long-term use.
With almost the same length and diameter specifications as standard drill collars, and with a similar connection strength and slip setting area, they may be used as a component of a stand of drill collars without difficulty.
Usually, jars will be used alongside accelerators, which are run above the jar and work automatically. They serve to amplify the impact force of the jar, and can even double it in some cases. They commonly use the compression of silicon to give added stored energy and optimize jar impact and free-travel distance in both directions. They also have the added benefit of dampening the dynamic load in the drillpipe, since they transmit shock waves poorly, thus helping reduce damage to both string and surface equipment.
Drilling Jars Diagram (Slideshare, 2017)
