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Most rotaries take their power through the drawworks
(Fig. 10.3), by means of chains and sprockets, from the primary
engines. This results in an obstruction, in the form of a chain and
guard, extending halfway across the derrick floor. In recent years,
with the trend toward deeper drilling, there has been a move
toward using a separate engine to drive the rotary table.

Figure 10.3 – Drawworks

The first truly independent rotary drive was developed
during the steam era. It consisted of a small 2-cylinder steam
engine mounted on skids with the rotary table and speed increaser
gears, in line with the pinion shaft. Next came drilling units with
internal combustion engines mounted in like manner but with a
gear box, having two speeds forward and one reverse, between
engine and rotary pinion shaft.
The large diesel electric rigs, currently in use, employ a
separate d.c. motor of 500 hp to 800 hp capacity, connected to the
pinion shaft of the rotary table through a 2-speed gear box.
The principal advantage of the auxiliary power is that, with
the proper layout, it may be used in an emergency in place of the

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