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FMEA — Rotating Shaft Assembly (4340 Steel, SKF 6208 Bearings)

Failure Mode & Effects Analysis Table

#	Component / Location	Failure Mode	Failure Effect	S	Cause	O	Current Controls	D	RPN	Recommended Action
1	Shaft keyway fillet	Fatigue crack initiation	Sudden shaft fracture, drivetrain seizure	9	Bending + torsion stress concentration at Kt ≈ 2.8–3.2; insufficient fillet radius	5	Visual inspection every 500h	7	315	Increase fillet radius to r ≥ 1.5mm; shot peen keyway; specify Ra ≤ 0.8μm surface finish
2	Bearing inner race (both)	Spalling / fatigue pitting	Vibration, noise, eventual lock-up	8	Inadequate lubrication relubrication interval; misalignment > 0.05mm	4	Oil mist lubrication continuous	5	160	Set relubrication interval ≤ 500h; add vibration monitoring (ISO 10816); verify shaft alignment ≤ 0.02mm TIR
3	Shaft journal surface	Fretting wear at bearing seat	Loose bearing fit, vibration, secondary imbalance	7	Micro-motion between bearing bore and shaft due to interference fit loss at temperature cycling	3	Interference fit H7/r6	6	126	Increase interference to H7/s6; apply Loctite 638 as secondary retention; inspect at each overhaul
4	Shaft under coupling	Torsional overload fracture	Complete drivetrain failure	9	Torque spike from motor controller fault or jammed load; no torque limiter	3	Motor overcurrent protection	6	162	Install mechanical torque limiter rated at 220 N·m (110% of nominal); add shaft encoder for peak torque logging
5	Bearing outer race	Outer race creep	Bearing migration, housing bore damage	6	Insufficient housing bore tolerance; thermal expansion during warmup	3	H7 housing bore tolerance	5	90	Tighten housing to J7 or K7; add axial retention circlip; record bore diameter at overhaul
6	Oil mist lubrication system	Lubricant starvation	Bearing seizure within 30 min	9	Mist generator failure; blocked nozzle; low oil level	4	Low oil level alarm	6	216	Add differential pressure switch on mist line; set alarm + auto-shutdown at 2 min starvation delay

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Critical Items (RPN ≥ 200 or Severity = 9–10)

Item 1 — Keyway fatigue (RPN 315, S=9) Root cause: stress concentration factor Kt = 2.8–3.2 at standard keyway corners combined with fully-reversed bending at 10⁸ cycles falls below the endurance limit for 4340 steel at Ra 1.6μm. Action: Increase fillet radius to r ≥ 1.5mm; specify Ra ≤ 0.8μm at fillet; shot peen to induce +200 MPa compressive residual stress. Verify per ASTM E466 fatigue test on representative specimen.

Item 6 — Lubricant starvation (RPN 216, S=9) Loss of oil mist leads to catastrophic bearing seizure within 30 minutes. Current alarm relies on level only — no flow verification. Action: Install mist flow differential pressure switch (Δp ≥ 0.2 bar) with 2-minute timer-delayed auto-shutdown interlock.

Item 4 — Torsional overload (RPN 162, S=9) Motor controller faults can deliver 4–6× rated torque transiently. Shaft designed to 200 N·m nominal has safety factor < 1.5 at 800 N·m spike. Action: Install torque-limiting coupling rated 220 N·m (slip at 110% nominal). Size motor overcurrent trip at 130% FLA with ≤ 100ms response.

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Design Recommendations (ranked by RPN reduction potential)

1. Increase keyway fillet radius r: 0.5mm → 1.5mm — reduces Kt from ~3.0 to ~1.8, drops RPN from 315 to ~115 (−63%)
2. Add mist flow monitoring interlock — eliminates starvation scenario; RPN 216 → 40 (−81%)
3. Install torque-limiting coupling — caps peak torque exposure; RPN 162 → 54 (−67%)
4. Tighten housing bore to K7 — reduces outer race creep risk; RPN 90 → 42 (−53%)
5. Upgrade to H7/s6 bearing fit with Loctite — eliminates fretting path at journal; RPN 126 → 54 (−57%)

Suggested Controls

Control	Applies To	Target
Vibration monitoring (ISO 10816 Zone C alarm)	Bearings 1 & 2	Detect spalling ≥ 4 weeks before seizure
500h overhaul inspection — keyway dye-penetrant	Shaft keyway	Detect cracks > 0.5mm depth
Oil mist Δp switch with datalogger	Lubrication system	Log all starvation events > 30s
Torque peak logging via shaft encoder	Drive coupling	Flag events > 220 N·m for engineering review