For "Flight Idle"
Jet Engines and Propulsion Systems For Engineers
Chapter 8-26
(pages 319-323 of pdf)
Jet Engines Engine
AXIAL BEARING THRUST CONTROL
Balancing the.axial thrust loads which develop in the flowpath and internal cavities of jet engines is critical to obtaining acceptable thrust bearing lives. Since the secondary systems air model includes all major cavities, a simple summation of pressure times projected area for all pertinent cavities will give the resultant load on all rotating hardware. Factoring in the compressor and turbine aerodynamic blade loads yields the axial forces on the engine thrust bearings.
HP Rotor Thrust Table 8.1 shows a schematic and tabulation of pressures, areas, and forces involved in obtaining me resultant loads on the high pressure rotor bearings of the CF6-80C engine at takeoff. Note that the resultant load (-4561 lb.) is small relative to the major cavity loads. This is typical of HP rotor bearing axial loads. Also, note that maximum cavity loads are substantially higher than either compressor or turbine total airfoil aerodynamic loads. Compare forces 104 and 114 with force 113 on Table 8.1. The accuracy of predicting these cavity pressures is a critical factor in predicting bearing loads.
Once the bearing load is predicted and determined to be too high for acceptable bearing life it can usually be adjusted to required levels by moving a critical seal to a larger or smaller diameter thus changing its projected area.. For instance, in Table 8.1, changing the diameter of the seal which affects forces 105 and 106 would be used to balance the load on the HP thrust bearing. In drastic cases if more adjustment is necessary, several seals or even turbine airfoil changes may be required to obtain desired bearing loads. Figure 8.24 shows a comparison of four different engines axial HP load and how they change with engine speed.
LP Rotor Thrust Prediction of the low pressure rotor thrust bearing load is generally much easier mainly due to the lower pressure levels involved. The principles remain the same. Figure 8.25 shows the low pressure rotor thrust for the same CF6-80C engine. Note that the predicted load is very close to the measured data presented in the figure.
Table 8.1 Variables Affecting Loads On High Pressure Rotor Bearings
NO Rt RO CHAM AREA PRESS FORCE
101 3.19 3.845 30 14.48 19.69 -285
102 3.845 4.840 33 27.16 33.07 -898
103 4.840 6.944 5 77.89 24.71 -1925
104 1 1 — — — +5783
105 7.640 11.825 7 253.92 364.65 +9332
106 6.375 7.650 36 55.70 102.82 +5727
107 4.650 6.375 59 59.75 22.75 +1359
108 4.000 4.650 41 17.66 26.65 +471
109 4.000 4.075 58 1,90 20.93 +40
110 4.325 5.050 62 21.35 21.32 -451
111 5.050 6.200 39 40.64 105.14 -4273
112 6.200 7.470 40 54.34 316.95 -1130£
113 7.470 4.040 10 448.39 253.99 -118879
114 1 1 — — — -31856
115 12.89 1.825 11 85.04 186.08 -17448
116 12.89 14.00 12 93.77 128.76 -8282
117 6.830 13.660 51 64.23 76.22 +41339
118 4.425 3.543 54 429.66 83.70 +7118
119 2.925 6.830 50 85.04 25.09 + 869