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counterweight, by its position, and by the geometry of the surface
unit. (Pumping speed is also a factor in the case of beam
counterbalance). A given effect can be obtained by placing
counterweights on the walking beam, on the pitman, or on the
crank. In some cases, particularly on larger units, air pressure is
used to obtain the desired counterbalance effect. The method of
determining desired counterbalance effect is not influenced by the
type of counterbalance used, and in the design and analysis
examples which follow in this chapter, only crank counterbalance
will be considered. In this instance, various counterbalance effects
may be obtained by changing the position of the counterweight
along the crank.
Another member of the surface pumping unit is the gear
reducer. It is necessary to determine quite accurately the probable
peak torque to which the gear reducer will be subjected, since the
American Petroleum Institute uses peak torque as the basis for
rating pumping units. The API assignation for a unit is simply the
maximum permissible torque (on the gear reducer) in thousands of
inch-pounds. For example, an API Size 114unit has a peak torque
rating of 114,000 in.-lb. The API has standardized on 16 peak
torque ratings varying from G.4 to 1824 thousand in.-lb.
The unit sheave is the member which receives power from
the prime mover (through a V-belt drive). The ratio of unit sheave
diameter to engine sheave diameter and the speed reduction at the
gear reducer determine the overall speed reduction from prime
mover to polished rod. The available unit sheave sizes and the
number and type of V-belts to be used can be determined from the
pumping unit manufacturer's literature.

The Prime Mover

The function of the prime mover is to supply to the
installation mechanical energy which is eventually transmitted to
the pump and used to lift fluid. The prime mover selected for a

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