apart to give a balanced load. When the eccentrics are placed on
the shaft above the gear wheel, the power is termed an overpull type;
when the eccentrics are placed under the gear wheel the power is
termed an underpull type. In some overpull types the eccentrics
are between the gear wheel and the top of the frame; in some they
are on an extension of the shaft above the top of the frame; and in
some one eccentric is between the gear and the top of the frame and
the other is on an extension of the shaft above the top of the frame.
If a third eccentric is used, in some powers it is put above and in
others below the top of the frame. Still other types of both over-
pull and underpull bevel-gear and eccentric powers are designed
without braces and frame work above the gear wheel, and depend
more on the weight and stability of the foundation for rigidity.
The relative merits of the different types of bevel gear and eccentric
powers are not discussed here.
In a rough, hilly country, where conditions require the shackle
lines to be carried some distance from the ground, the overpull
power is often advantageous; in a flat country, without timber and
obstructions to shackle lines, the underpull power may be proved
the more satisfactory.
As a general rule, the longer the shaft carrying the gear and
eccentrics, the more the reliance placed on the braces; the shorter the
shaft, the more the reliance on the mass and rigidity of the founda-
tions. Generally, with other conditions equal, the nearer the gear
and eccentrics are to the ground the more satisfactory will be the
service, provided the foundations are suitable, for there will be less
chance for play and stresses to develop.
Plate XIII, C (p. 63), shows a power of the bevel gear and ec-
centric type but without top braces. It is rigidly held in place by a
heavy concrete foundation.
Band-wheel powers are similar to the bevel gear and eccentric
type, except that a large belt-driven wooden or steel band wheel--16
feet, 18 feet, or 20 feet in diameter and usually with a 14-inch or
15-inch face-is used instead of a bevel gear and pinion drive.
The position of the eccentrics with reference to the band wheel
Series as does that of the eccentrics with reference to the gear in
the bevel gear and eccentric type. If top braces are used, they
have to be long, strong, and firm, because of the larger diameter of
the band wheel and the consequent greater lateral thrust. If no
top braces are used, a heavy, firm concrete foundation is necessary
for the same reason. This latter type of band-wheel power is shown
in plan and vertical section in Plate XVI, where A is the engine
George, H. C. Surface Machinery and Methods for Oil-Well Pumping. Washington D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc12407/. Accessed September 27, 2016.