The Sperry Drilling services Sledgehammer™ hydromechanical drilling jar is run in the bottomhole assembly in order to free the drillstring if it should become stuck. It reduces downtime and is suited for hot-hole applications and extended-reach drilling. The drilling jar allows drillers to build strain energy in the drillstring, which, when released, results in high-impact forces, either upwards or downwards. These "jarring" or impact forces can be more than six times higher than the force applied to the drillstring.
Depending on the hole conditions, the jar can be located near the top of the BHA (in tension) or near the bit (in compression). Maximum effectiveness is achieved when jarring occurs close to the stabilizers and bit. Recommended placement in compression is three drill collars above the top stabilizer in the BHA. However, if run in tension, at least two drill collars, or the equivalent weight in heavy wall drillpipe, should be kept above the jar for an effective hammer. In order to minimize the risk of getting the string stuck above the jar, the outer diameter of the collars and drillpipe above the jar should always be equal to or slightly smaller than the outer diameter of the jar.