What Is FTF? — Bearing Glossary

FTF (Fundamental Train Frequency — the rotational frequency of the bearing cage) is the frequency at which the cage revolves around the bearing axis — it appears in the vibration spectrum when the cage is worn, cracked, or deformed, causing irregular impacts between the cage and rolling elements.

FTF is the lowest of the four bearing characteristic frequencies (FTF < BSF < BPFO < BPFI) per ISO 13373, typically only 0.3–0.5 times shaft rotational speed. See the complete four-frequency monitoring approach in bearing vibration monitoring and browse deep groove ball bearings with the cage type suited to your operating speed. Cage failure is particularly dangerous because it occurs suddenly — a fractured cage releases the rolling elements to collide freely with each other, seizing the bearing completely within seconds. There is no gradual vibration increase as with spalling. Common causes: inadequate lubrication (steel cage wears under metal-to-metal friction when grease is insufficient), hard particle contamination lodged between cage and rolling element, or a cage design inadequate for the operating speed.

FTF Formula and Calculation

FTF = (n/2) × (1 − d/D × cos α)

For bearing 6205 (d=7.94 mm, D=38.5 mm, α=0°) at 1,450 rpm (24.17 Hz): FTF = (24.17/2) × (1 − 7.94/38.5) = 12.085 × 0.794 = 9.6 Hz (≈ 576 rpm cage speed)

The cage rotates 60% slower than the shaft. FTF factor for 6205 = 0.397. At 2,900 rpm: FTF = 0.397 × (2,900/60) = 19.2 Hz. This low frequency range is easily masked by machine structural vibration. Detecting FTF reliably requires instruments with high frequency resolution (≤ 0.1 Hz at 10 Hz) such as SKF Microlog or equivalent.

Practical Example: Cage Fault in 22220 EK/C3

At a concrete mixing station in Da Nang, a spherical roller bearing 22220 EK/C3 (d=100, D=180, B=46 mm) on the drum shaft ran at 480 rpm. FTF = 0.41 × (480/60) = 3.28 Hz. Spectrum analysis showed a strong peak at 3.3 Hz and harmonics at 6.6 Hz, 9.9 Hz — confirming cage damage. Physical inspection found the steel cage worn at 3 rolling element pockets, with hardened grease throughout. The bearing was replaced before peak production season.

Cage Materials and Failure Modes

Cage material affects both failure mode and the vibration signature. Steel and brass cages fail primarily through abrasive wear from contamination and inadequate lubrication. Polyamide (nylon) cages fail through thermal softening or centrifugal deformation at high speed.

For speeds above 5,000 rpm, copper alloy or high-quality polyamide cages are preferred over pressed steel — centrifugal force on the cage pockets accelerates steel cage wear at high speed.