Page 162 - Compression Machinery for Oil and Gas

150 SECTION II Types of Equipment

FIG. 4.11 Gear tooth loading (A) on one flank and (B) on both flanks.

Thrust Management
It is extremely rare to use a double helical gear in an IG turbomachine because
the net pinion thrust (i.e., sum of axial impeller gas forces) would act to load one
face of the helical gear more than the other. Therefore, almost all IG turboma-
chinery use a single helical gear. Furthermore, thrust bearings are almost exclu-
sively fluid film-type configurations with the oil and gas industry. To control
the axial position of the large bull gear, a double-acting tapered-land thrust bear-
ing is most commonly applied. The relatively low speed of the bull gear means
that minimal fluid shear losses are present in the bull gear shaft thrust bearing.
To control the axial position of the pinion shafts, either thrust collars or thrust
bearings are applied. Fig. 4.12 shows a pinion with thrust collars. Thrust collars
transmit the net axial force from the aerodynamics and the gear mesh to the bull
gear disk axial surface. The net residual thrust is then reacted against the bull gear
thrust bearing (a lower speed and lower loss mechanism). The area of the load
transmissionisrelativelysmallasitisformedbyoverlappingsectionsoftheouter
diameters from the bull gear and a thrust collar on the pinion gear (Fig. 4.13).
A hydrodynamic oil film is established to keep the bull gear face separate from
thepinionthrustcollar.Thisissimilartoaplainthrustbearing,withthedifference
being that relative motion between the bull gear face and the thrust collar face
must be considered as both surfaces are in rotation about different axes.
San Andres et al. [10] show an approach to thermal-mechanical and dynamic
assessment of thrust collars to understand their performance characteristics.
Fig. 4.14 shows a pinion shaft and combination journal/thrust bearings. The
thrust bearings on the pinion shaft transmit the net axial load from the aerody-
namics and gear axial force through the thrust bearing to the static bearing hous-
ing. The advantage of the high-speed thrust bearing is that large axial loads can
be mitigated; however, this comes at the cost of higher fluid shear losses than a
thrust collar.