Handbook of Turbomachinery Second Edition Revised - Ventech!

assembly ‘‘straddles’’ the two bearing supports and is often referred to as a
straddle-mount rotor. The rotor consists of a single solid steel shaft
[r ¼ 7,832 kg/m 3 (0.283 lbm/in. 3 ), E ¼ 200 GPa (29 ? 10 6 lbf/in. 2 ), G ¼ 77 GPa
(11.17 ? 10 6 lbf/in. 2 )] of length equal to 30 cm (11.81 in.) and diameter equal
to 5 cm (1.97 in.). Three rigid disks of equal size [m ¼ 3.3 kg (7.28 lbm),
Id ¼ 0.006 kg-m 2
(20.50 lbm-in. 2 ), Ip ¼ 0.011 kg-m 2
(37.59 lbm-in. 2 )] are
attached to the shaft—one nominally at each end and one at the center of
the shaft. The isotropic bearings each have a stiffness k ¼ 10 MN/m
(57,100 lbf/in.) and viscous damping coefficient c ¼ 1.0 kN-s/m (5.71 lbf-s/
in.).
A graph of the natural frequencies of whirl versus the rotor spin-speed
(whirl speed map) is presented in Fig. 26. The first forward and first
backward whirl speeds are nearly identical; thus, their values appear
essentially as a single line on the graph. These modes are nearly cylindrical
(i.e., small rotatory motion of the disks) and, as a result, the gyroscopic
effects are negligible, providing essentially constant values for the whirl
speeds as the rotor spin-speed changes. The second and third forward and
Figure 26 Whirl speed map: straddle-mount rotor.
Copyright © 2003 Marcel Dekker, Inc.