Drag bits were the first bits used in rotary drilling, but are no longer in common use. A drag bit consists of rigid steel blades shaped like a fish-tail which rotate as a single unit. These simple designs were used up to 1900 to successfully drill through soft formations. The introduction of hardfacing to the surface of the blades and the design of fluid passageways greatly improved its performance. Due to the dragging/scraping action of this type of bit, high RPM and low WOB are applied.
The decline in the use of drag bits was due to:
– The introduction of roller cone bits, which could drill soft formations more efficiently
– If too much WOB was applied, excessive torque led to bit failure or drill pipe failure
– Drag bits tend to drill crooked hole, therefore some means of controlling deviation was required
– Drag bits were limited to drilling through uniformly, soft, unconsolidated formations where there were no hard abrasive layers.