Page 260 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 4 .2
Be st Practice 4 .3 Gear and Coupling Best Practices

Best Practice 4.2Practice 4.2
Best
Design audit gears when pitch line velocities exceed 6,000 greater than 6,000 m/min (20,000 ft/min) that required gear
meters/min (20,000 ft/min) to ensure optimum safety and box internal redesign.
reliability. Redesign of gearbox internals, if possible, can take a long time, and
Review all vendor pitch line velocity experience in the engineering may require a larger gearbox which will in turn necessitate baseplate
phase of the project before acceptance. redesign.
If a minimum of 2 year field operating experience for gears with
a higher pitch line velocity is not shown, design audit the proposed Benchmarks
gear box for preventive measures to prevent gearbox flooding. This best practice has been used since the mid 1970s to ensure
Gearbox flooding (drain oil contacting the gear teeth) can cause trouble-free operation and maximum gearbox reliability (above
catastrophic gearbox failure. 99.5%).
Lessons Learned
I have been involved with severe gearbox flooding issues
during factory acceptance tests, at pitch line velocities

Best Practice 4.3Practice 4.3Practice 4.3
Best
Best
Ensure proper anti-friction bearing selection (life and DN) bearing systems in multi-stage, high torque reduction
for low speed, high torque gear applications. gears.
Confirm the B-10 bearing life calculations with the vendors during
the quoting phase of the project. Benchmarks
Confirm vendor D-N (bore diameter times operating speed) for all This best practice has been used since the mid 1980s for all low speed
bearings, and especially for indeterminate (3 bearing) systems. gear applications using anti-friction bearings.
Carefully evaluate low loaded bearings in three bearing systems
and review vendor bearing load experience to eliminate the possibility
of bearing skidding.
Lessons Learned
Complete gearbox damage has been caused by the use of
high DN, low loaded anti-friction bearings in indeterminate

B.P. 4.3. Supporting Material
Where: P ¼ Pressure on the bearing elements
Anti-friction bearings F ¼ The total of all static and dynamic forces acting
on the bearing
Anti-friction radial bearings support the rotor, by using rolling A ¼ Contact area
elements to reduce friction losses. They are used in low For anti-friction bearing applications, the pressure, P is the
horsepower applications (below 375 kW (500 H.P.)), high point contact or ‘Hertzian’ stress on the bearing elements and
torque gear boxes and in aero-derivative gas turbines. Examples rings or ‘races’. For an anti-friction bearing to be properly
of roller and ball type anti-friction bearings are shown in designed, its D-N number and bearing life must be determined.
Figure 4.3.1. Figure 4.3.2 presents the definition of D-N number and its uses.
As previously mentioned, all bearings are designed to have Each type of anti-friction bearing has a maximum operating
sufficient bearing area to support all the forces acting on the D-N number. If this value is exceeded, rapid bearing failure can
bearing. occur. In addition, D-N numbers are typically used to determine
That is: the type of lubrication required for bearings. A common practice
in the turbo-machinery industry has been to use hydrodynamic
F pressurized bearings when the D-N number exceeds approxi-
P ¼
A mately 500,000.
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