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13 THE PRIME MOVERS

The term prime mover, as defined by Webster, is "the
original force in a series of transmissions of force ". Perhaps, a
more descriptive definition for our purpose is "that unit within a
power system which first converts an energy source into a
mechanical force ".
Typical examples are windmills, water wheels, water
turbines, steam engines, steam turbines, internal combustion
piston-type engines, gas turbines and jet engines. Electric motors,
sometimes referred to as prime movers, are considered in this
discussion as part of a transmission system. The prime mover in
this instance is the unit that drives the electric generator. In
study of drilling engines, the first and at one time the most
important source of power was steam. This was true for two
special reasons, the first being historical. In the early days steam
was the only power used. A second reason was: steam was the best
and most flexible drilling power available. It was the simplest and
most easily maintained; any roughneck could repair a steam
drilling engine or pump.
The steam rig and its boiler plant have undergone radical
changes. Once three pots, usually of less than 50 hp each, furnished
steam at 100 to 125 psig. Then finally, there was a large high speed
engine operating at 300 to 500 psig steam pressure.

Steam Power

Lack of space prevents anything like a comprehensive study
of a steam power plant. Highlights with only a brief explanation
may be mentioned. One of the more popular drilling engines was
a twin with 14-in. x 14-in. cylinders. Maximum crankshaft speeds
were 400 rpm for intermittent service and 250 rpm for constant
speed work. Rated capacity was 1690 bhp at 400 rpm and 400 psig

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