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Transmission Elements for High-Speed Turbomachinery 245

Figure 13.2 Variation of reaction stage efficiency as a function of
the volume coefficient δ.(η o = reference efficiency; δ= volume coef-
˙
2
ficient = V/r u; r = radius, at mean diameter of the blade path;
˙
u = peripheral speed at radius r; V = volume flow.) (Asea Brown-
Boveri, Baden, Switzerland)
either single- or double-helical gears as shown in Figs. 13.4 and 13.5,
but some are designed as epicyclic or planetary gear sets.

13.2 Epicyclic Gears
Epicyclic gearing represents a special case of the family of gear drives
known as split train gears. Power is transmitted through multiple
paths (Fig. 13.6) to reduce the size of the split train drive compared to
that of conventional single-path drives. Figure 13.7 illustrates how in
epicyclic gears the transmitted power is also split through multiple
paths.
The four basic elements of epicyclic gears are an internally toothed
flexible annulus ring, a plant or star carrier, a central sun wheel, and
planet or star wheels. Holding stationary any one of the first three ele-
ments (internally toothed annulus, star carrier, or sun wheel) results in
the three types of epicyclic gear configurations shown in Fig. 13.8a
through c. With the fixed annulus system we obtain what is generally
called a planetary gear unit. The input vs. output shaft speed ratios can