Which diagnostic method helps identify bearing faults by analyzing frequency content?

• A. Thermography infrared imaging to map surface temperatures.
• B. Oil analysis for viscosity and contaminant levels.
• C. Electrical resistance testing of windings.
• D. Vibration analysis identifying inner/outer race and ball bearing fault frequencies.

Answer: (D)

Analyzing the frequency content of vibration signals lets you detect bearing faults because defects create repeating impulses that generate specific, identifiable frequencies in the spectrum. When a bearing has an inner race, outer race, or ball fault, the passing of each defect produces bursts of vibration at characteristic rates. By measuring vibration with an accelerometer and applying a spectrum analysis (often with envelope detection to highlight the high-frequency bursts), you look for peaks at the known fault frequencies: inner race, outer race, ball pass, and other bearing-related frequencies. Matching these spectral peaks to the bearing’s geometry (number of balls, pitch diameter, contact angle, etc.) tells you which part is faulty.

Other methods don’t provide this frequency-based signature of bearing defects. Thermography shows surface temperatures and heat patterns, which can indicate abnormal operation but not specific bearing fault frequencies. Oil analysis looks at lubricant condition and metal wear debris, not vibration frequencies. Electrical resistance testing checks winding faults, not mechanical faults in bearings.