Bearing Analysis & Fault Detection Flashcards

What is the primary advantage of using acceleration for high-frequency bearing analysis compared to velocity?

Answer: Acceleration is more sensitive to high-frequency impulsive events that characterize early-stage bearing faults

Accelerometers have frequency response extending to tens of kHz, making them well-suited for detecting the high-frequency stress waves generated by early-stage bearing impacts, whereas velocity is preferred for lower-frequency general machinery monitoring.

Which bearing defect frequency is most affected by shaft speed variation?

Answer: All bearing defect frequencies scale linearly with shaft speed

All four bearing defect frequencies (BPFO, BPFI, BSF, FTF) are directly proportional to shaft rotational speed, so any change in operating speed shifts all defect frequencies proportionally.

What does 'bearing clearance' affect in terms of vibration characteristics?

Answer: Load distribution, contact stress, and vibration amplitude; excessive clearance increases vibration and reduces bearing life

Excessive bearing clearance increases load zone width, raises contact stress, produces higher vibration amplitudes, and reduces bearing life; insufficient clearance causes preloading, heat generation, and premature fatigue.

In rolling element bearing failure analysis, what does 'spalling' refer to?

Answer: Fatigue cracking and flaking of material from bearing raceways or rolling element surfaces

Spalling is subsurface fatigue cracking that propagates to the surface, causing flakes or chips to break away from raceways or rolling elements; it produces the impulsive vibration signatures detected by bearing analysis techniques.

What is the typical shape of a shaft orbit for a machine running with pure rotor unbalance?

Answer: A circular or slightly elliptical loop rotating at 1× shaft speed

Pure unbalance generates a nearly circular or elliptical orbit because the centrifugal force is constant in magnitude and rotates at shaft speed with a consistent phase relationship in both measurement directions.

What does the term 'bearing frequency' BPFI typically equal for a standard bearing with 8 balls at 1800 RPM (30 Hz shaft speed) and a pitch diameter to ball diameter ratio where Bd/Pd cosθ ≈ 0.3?

Answer: Approximately 4.9× shaft speed

BPFI = (N/2) × RPM × (1 + Bd/Pd × cosθ) = (8/2) × 30 × (1 + 0.3) = 4 × 30 × 1.3 ≈ 156 Hz ≈ 4.87× shaft speed (approximately 4.9×).