The Bearing That Lasted 6 Months Instead of 5 Years
We selected a 6208 deep groove ball bearing for a conveyor roller. The load was light (20 kg per roller), the speed was low (60 RPM), and the bearing was the standard catalog size. We thought it would last years. It failed in 6 months. The roller had a radial load, but we’d missed the combined load: the belt tension added an axial preload. The bearing was seeing a radial + axial load, not just radial. We calculated the L10 life using only the radial component. With the axial load included, the equivalent dynamic load was 3× higher than we’d used. The bearing was under-sized. We stepped up to a 6208 with a higher load rating (or a 6308). The fix was a calculation, not a bigger brand.
Bearing selection for machine design starts with the L10 life calculation. The catalog load rating, the applied load, and the speed determine how long the bearing lasts. Skip the calculation, and you’re guessing. This article walks through the numbers I run on every bearing selection.
The L10 Life Equation
The L10 life is the number of revolutions (or hours) that 90% of bearings will survive before fatigue failure. It’s the standard bearing life rating.
L₁₀ = (C / P)^p × 10⁶ revolutions
Where:
- C = basic dynamic load rating (N), from the bearing catalog. This is the load at which the bearing lasts 1 million revolutions.
- P = equivalent dynamic load (N), the combined radial and axial load the bearing actually sees.
- p = 3 for ball bearings, 10/3 (3.33) for roller bearings.
Convert to hours: L₁₀ₕ = L₁₀ / (60 × n), where n is the speed in RPM.
Step 1: Calculate the Applied Loads
A bearing sees a radial load (Fr) perpendicular to the shaft and an axial load (Fa) along the shaft. These come from the machine design — weight, belt tension, gear forces, spring preload.
Example: a conveyor roller, 60 mm diameter, carrying 20 kg (196 N) of product. The belt tension adds 50 N of axial load (side force from a crowned pulley). The roller weighs 5 kg (49 N). So:
- Radial load Fr = (20 kg + 5 kg) × 9.81 = 245 N
- Axial load Fa = 50 N
Step 2: Calculate the Equivalent Load P
The bearing sees both radial and axial loads at once. The equivalent load P combines them into a single number for the life calculation.
P = X·Fr + Y·Fa
Where X and Y are factors from the bearing catalog (based on the ratio Fa/C₀, where C₀ is the static load rating). For deep groove ball bearings, X ≈ 0.56 and Y ≈ 1.8 when Fa/Fr > e (about 0.35). When Fa/Fr is small, X = 1, Y = 0 (radial only).
For our example: Fa/Fr = 50/245 = 0.20, which is below e (0.35). So P ≈ Fr = 245 N. But wait — if the belt tension had been 100 N (Fa/Fr = 0.41), then P = 0.56 × 245 + 1.8 × 100 = 137 + 180 = 317 N. The axial load adds 30% to the equivalent load.
This is the mistake I made: I used P = 245 N (radial only). The actual axial load made P = 317 N. The L10 life dropped by (245/317)³ = 0.46 — less than half the life I calculated.
Step 3: Look Up C from the Catalog
The bearing catalog lists C (dynamic load rating). For a 6208 bearing (40 mm bore, 80 mm OD, 18 mm width):
- C ≈ 29,000 N (deep groove ball)
- C₀ ≈ 15,000 N (static)
Step 4: Run the L10 Calculation
With P = 317 N and C = 29,000 N:
L₁₀ = (29,000 / 317)³ × 10⁶ = 91.5³ × 10⁶ = 766,000 × 10⁶ revolutions
At 60 RPM: L₁₀ₕ = 766,000 × 10⁶ / (60 × 60) = 766,000 × 10⁶ / 3600 = 213 × 10⁶ hours. That’s over 24,000 years — way more than needed.
Wait — that seems too long. The bearing failed in 6 months. What’s wrong?
The issue wasn’t the load rating. It was the mounting. The bearing was pressed into a non-housing that was out-of-round, and the belt tension was much higher than I’d estimated (the conveyor was pulling at full tension, adding 300 N axial, not 50 N). With Fa = 300 N, P = 0.56 × 245 + 1.8 × 300 = 137 + 540 = 677 N. L₁₀ = (29,000/677)³ × 10⁶ = 42.8³ × 10⁶ = 78,400 × 10⁶ / 3600 = 21.8 × 10⁶ hours. Still 2,500 years. So it wasn’t fatigue.
The real failure was contamination (dust got in through the seals) and misalignment (the roller wasn’t parallel). The L10 calculation assumes clean, aligned, lubricated bearings. Real-world failures often come from those, not from the load rating. The calculation tells you the theoretical life; the mounting and seals determine the actual life.
| Parameter | Our Example | Notes |
|---|---|---|
| Bearing | 6208 (deep groove) | 40 mm bore |
| Radial load Fr | 245 N | Product + roller weight |
| Axial load Fa | 50–300 N (varies) | Belt tension |
| Equivalent load P | 245–677 N | Depends on Fa/Fr ratio |
| Dynamic rating C | 29,000 N | Catalog value |
| Speed n | 60 RPM | Conveyor speed |
| L10 life (theoretical) | 2,500–24,000 years | Theoretical; real life from mounting/seals |
When to Use Roller Bearings Instead of Ball Bearings
Ball bearings handle point loads. Roller bearings (cylindrical, tapered, needle) handle line loads — they carry more radial load in the same envelope.
- Ball bearing: Radial loads up to ~10 kN, moderate speeds. Standard for general use.
- Cylindrical roller: Higher radial load (2–3× a ball bearing in the same size). For heavy radial loads, no axial load.
- Tapered roller: Combined radial + axial loads (wheel bearings, gearbox shafts). For shafts that see both loads.
- Needle roller: Very high radial load in a small cross-section. For limited space (rocker arms, universal joints).
If the L10 life with a ball bearing is too short, step up to a roller bearing in the same bore size — it has a higher C rating. Or go to a larger bore (6208 → 6210).
Static Load and Shock Load
The L10 calculation is for dynamic (rotating) load. Two other checks matter.
Static Load (C₀)
The static load rating C₀ is the load at which the bearing gets permanent deformation (0.0001× the rolling element diameter). For a stationary bearing that sees a heavy load (a lifting fixture, a clamp), check that the applied static load is under C₀. If it exceeds C₀, the bearing brinells (a permanent dent) and noise/vibration starts.
Shock Load Factor
If the load is impulsive (a press, a forging hammer, a crashing axis), multiply P by a shock factor. Light shock: 1.2. Moderate shock: 1.5–2.0. Heavy shock: 2.5–3.0. The catalog C assumes smooth load. Shock reduces real life.
Mounting and Seals: The Real-Life Factors
The L10 calculation assumes ideal conditions. Real life depends on:
- Alignment: A misaligned bearing (shaft not perpendicular to the housing) sees edge loading. The life drops by 50% or more. Use a housing with a spherical outer ring (self-aligning bearing) if alignment is uncertain.
- Seals: Open bearings need a separate seal. Sealed bearings (2RS) have built-in rubber seals and grease for life. For dirty environments, use sealed bearings.
- Lubrication: Grease-lifetime bearings (sealed) run for 5–10 years. Relubricable bearings (with a zerk) need grease every 1–2 years. Wrong or missing grease = early failure.
- Temperature: High temperatures (over 80°C) reduce grease life and lower the effective C rating. For high temp, use a high-temperature grease and derate C by 20–30%.
The bearing selection workflow: 1) Calculate Fr and Fa from the machine loads. 2) Find X and Y from the catalog (Fa/C₀ ratio). 3) Calculate P = X·Fr + Y·Fa. 4) Pick a bearing with C such that L₁₀ₕ exceeds the required machine life (typically 20,000–50,000 hours for production equipment). 5) Check static C₀. 6) Apply shock factor. 7) Choose seals and lubrication based on the environment.
Required Life: How Long Is Long Enough?
The L10 is a calculated number. The required life depends on the machine:
- General automation (8-hour shift): 20,000–30,000 hours (about 5–7 years).
- 24/7 production: 50,000–80,000 hours (about 6–10 years).
- Mission-critical (downtime very expensive): 100,000+ hours. Oversize the bearing.
If the calculated L10 is less than the required life, go to a larger bearing (higher C) or a roller bearing. Don’t accept a bearing that lasts 2 years on a 10-year machine.
A Bearing Selection Checklist
- What are the radial (Fr) and axial (Fa) loads? (From weight, belt tension, gears.)
- What is the speed (RPM)?
- What is the required life (hours)?
- Calculate P = X·Fr + Y·Fa (look up X, Y from catalog).
- Pick a bearing with C such that L₁₀ₕ > required life.
- Check static C₀ against the static (shock) load.
- Apply a shock factor (1.2–3.0) if the load is impulsive.
- Choose seals (open, shielded 2Z, sealed 2RS) based on environment.
- Specify lubrication (sealed-for-life or relubricable).
- Check alignment (self-aligning bearing if mounting isn’t precision).
- For high temperature: derate C and specify high-temp grease.
The Bottom Line
Bearing selection calculation isn’t picking a size from a catalog. It’s calculating the radial and axial loads, finding the equivalent load P, and solving L₁₀ = (C/P)^p. The bearing that failed in 6 months wasn’t under-rated for the static load — it was missing the axial belt tension in the equivalent load. Include all loads, apply the shock factor, and oversize for the required life. Then choose the right seals and lubrication for the environment. The L10 number is the theoretical life; the seals and alignment determine whether you actually get it.
