Hydraulic drifters are percussive top-head devices that are very efficient to drill in hard rock as well as overburden. They fall under two main categories of Rockdrills and Overburden drills. Rockdrills are primarily used for drilling holes in hard rock and typical hole diameters are in the 60 - 90 mm ( 2-1/2" - 3-1/2") range. They are characterized by low torque, high rpm and high frequency. Mainly used in quarrying and mining.
Overburden drifters are used typically in hole diameters of 100 to 200 mm (4" to 8") in strata that is a combination of soft as well as hard. They are very efficient in drilling though gravel and boulders. They typically use dual inner and outer drillstring. They are characterized by high torque, low rpm and medium to low frequency. The machines HD90MKIV
are very versatile drills that can use all types of hydraulic drifters. Also see Anchor Drilling