The Bearing That Lasted Three Months
A rotary indexing station we built ran fine for the first three months. Then the customer started hearing a low grinding noise from the main bearing. We pulled it out. The bearing was a 6208 deep groove ball bearing — the default size the designer picked because it matched the shaft diameter. The load calculation? Never done. The expected L10 life? Never calculated. It looked fine in the CAD model, it fit the housing, the price was right.
The bearing failed because it was the wrong bearing for the application, not because it was a bad bearing. It was a perfectly good 6208. It just wasn’t carrying a 6208-sized load at a 6208-sized speed. The station was pushing 4,000 N of combined radial and axial load through it at 30 RPM, 24 hours a day. That’s not a 6208’s job.
Bearing selection for custom machinery is where engineers who can do everything else well still cut corners. We pick bearings from catalogs based on bore diameter and move on. The result is the bearing that lasts three months, the one that runs hot, or the one that fails in a way that costs more to fix than the bearing itself.
This article is the bearing selection process I actually use — the one that came after the 6208 incident and every bearing-related punch-list since.
Step 1: Know the Load Before You Pick the Bearing
Every bearing selection starts with the load. Not the shaft diameter. Not the mounting configuration. The load. If you don’t know what forces are going through the bearing, every bearing you pick is a guess.
What Loads Does the Bearing Actually See?
For a machine application, the bearing sees:
- Radial load (Fr): Force perpendicular to the shaft axis. This is the weight of whatever is mounted on the shaft, plus any side force from belts, gears, or tool reaction.
- Axial load (Fa): Force parallel to the shaft. This comes from clamp forces, spring preload, belt tension, or any actuator that pushes along the shaft axis.
- Moment load: If the load is applied offset from the bearing, it creates a moment. A cantilevered load on a single bearing creates a moment that the bearing has to resist. Deep groove ball bearings can’t resist moment well — they need a second bearing to support the couple.
The trick is that the “rated load” on the bearing’s catalog page is a dynamic load rating (C), not the actual load the bearing sees. You need to calculate the actual equivalent dynamic load (P), which combines Fr and Fa depending on what type of bearing you’re using.
The Equivalent Load Formula (and Why It Matters)
For a deep groove ball bearing, the equivalent load is:
P = X·Fr + Y·Fa
Where X and Y are factors from the bearing table, based on the ratio Fa/C0 (where C0 is the static load rating). If Fa/Fr is small enough (typically less than e, a value from the table), then X=1, Y=0 and P = Fr. If the axial load is significant, you need to look up X and Y in the manufacturer’s table.
This is where most people stop reading. They pick a bearing with a bore that fits the shaft, check that the static load rating is above the actual load, and call it done. The equivalent load calculation is what tells you whether the bearing will last — and it’s where the wrong bearing shows up.
Step 2: Calculate the Required Life, Not Just the Load Rating
A bearing’s dynamic load rating (C) is the load at which it lasts 1 million revolutions. That’s the catalog number. But your machine doesn’t run for 1 million revolutions. It runs for years.
L10 Life: The Number That Matters
The L10 life is the number of revolutions (or hours) that 90% of bearings will survive before fatigue spalling starts. The formula is:
L10 = (C / P)^p (in millions of revolutions)
Where p = 3 for ball bearings and p = 10/3 for roller bearings.
Convert to hours by dividing by speed (RPM) × 60.
For a machine that runs 24/7 at 30 RPM, you want an L10 life of at least 40,000–50,000 hours (about 5–6 years). For a machine that runs one shift at 1,800 RPM, the same number of hours means a lot more revolutions, so the bearing needs to be bigger relative to the load.
| Application | Target L10 Life | Why |
|---|---|---|
| Continuous process, 24/7 | 50,000–100,000 hours | Unplanned downtime is expensive |
| Single-shift production | 20,000–40,000 hours | Preventive maintenance windows exist |
| Intermittent / manual machine | 10,000–20,000 hours | Low duty cycle, easy to replace |
| Instrument / precision axis | 20,000+ hours, but precision matters more | Runout and friction are critical |
The 6208 example: C = 29.1 kN for a 6208. If P = 4 kN and speed = 30 RPM, L10 = (29.1/4)^3 ≈ 383 million revolutions = 212,000 hours. That’s fine. But if P = 8 kN, L10 = (29.1/8)^3 ≈ 48 million = 27,000 hours. At 24/7 that’s about 3 years — not enough. At 1,800 RPM it’s 445 hours. That’s the three-month failure.
Step 3: Pick the Bearing Type, Not Just the Size
Size is necessary but not sufficient. The bearing type determines what loads it can handle, how much misalignment it tolerates, and how much it costs.
Which Bearing for Which Job?
| Bearing Type | Best For | Not Good For |
|---|---|---|
| Deep groove ball (6000, 6200, 6300) | General radial + moderate axial, high speed, low cost | Pure axial load, heavy moment load |
| Angular contact ball (7000, 7200) | Combined radial + axial, precision, preload | Pure radial (wastes capability), low speed with heavy load |
| Cylindrical roller (NU, N, NJ) | Heavy radial load, moderate speed | Axial load (unless combined with a ball bearing) |
| Tapered roller (30200, 32200) | Heavy combined radial + axial, low speed | High speed (generates heat), requires preload |
| Self-aligning ball (1200, 2200) | Shaft misalignment (long shafts, poor housing alignment) | Precision applications (less stiff) |
| Thrust ball (51100, 51200) | Pure axial load, low speed | Radial load (it won’t support any) |
For most custom machine applications, the default is deep groove ball bearings — they’re cheap, available, and handle reasonable combined loads. The exceptions are when:
- The axial load is significant (more than ~20% of the radial load): use angular contact or tapered roller.
- The load is heavy and speed is low (indexing tables, rotary actuators): use tapered roller or cylindrical roller.
- The shaft is long and the housing alignment may be off: use self-aligning.
- The load is purely axial (a vertical shaft under weight): use a thrust bearing.
Step 4: The Arrangement Is as Important as the Bearing
Two identical bearings in two different arrangements behave very differently. This is where bearing selection becomes shaft bearing arrangement design.
The Fixed-Floating Principle
A shaft with two bearings needs one bearing that “locates” the shaft axially (fixed) and one bearing that allows axial movement (floating). Without this, thermal expansion of the shaft creates axial preload that generates heat and shortens life.
The fixed bearing takes both radial and axial load. The floating bearing takes only radial load and slides axially in its housing.
Common arrangements:
- One deep groove ball + one cylindrical roller (NU type): The ball bearing is fixed, the NU bearing floats (the inner ring slides on the rollers). This is the standard for most horizontal shafts.
- Two deep groove ball bearings, one with a loose outer ring: Cheaper, but less precise. The floating bearing needs clearance in the housing so it can move axially.
- Two angular contact bearings in back-to-back or face-to-face: Used for precision and stiffness (spindles, rotary tables). Both bearings are fixed; the preload controls the stiffness. This is a precision arrangement, not a general-purpose one.
Bearing Fits: The Detail That Kills Bearings
A bearing that’s mounted wrong fails regardless of how well you sized it. The fit between the bearing and the shaft, and between the bearing and the housing, matters:
- Rotating inner ring (shaft rotates): The inner ring needs an interference fit (k5, m5, j5). If it’s too loose, the inner ring spins on the shaft and wears the journal.
- Stationary inner ring (housing rotates): The inner ring can be a transition fit. The outer ring needs interference.
- Outer ring in housing: Usually a transition or clearance fit (H7, J7). The floating bearing needs a clearance fit so it can slide.
Use the manufacturer’s fit recommendations. They’re based on decades of failure data. If you put a loose bearing on a rotating shaft, it will fail — no matter how good the bearing is.
Step 5: The Two Mistakes That Bite Every Engineer
Mistake 1: Ignoring the Static Load Rating (C0)
The static load rating C0 is the load at which the bearing develops a permanent deformation of 0.0001 times the rolling element diameter. That sounds tiny. But if the bearing sees a shock load — a sudden impact, a misalignment during assembly, a jam — the static load can exceed C0, and the bearing develops a dent that causes noise and premature failure even if the dynamic load is within limits.
For applications with shock loads (presses, crushers, indexing with hard stops), check the static safety factor: C0 / actual static load should be at least 1.5–2.0. If it’s lower, step up to a heavier series (e.g., from 6208 to 6308).
Mistake 2: Forgetting Lubrication and Sealing
A bearing’s catalog life assumes proper lubrication. If the grease is wrong, the seals are missing, or the environment is dirty, the real life is a fraction of the calculated life.
For most machine applications, shielded or sealed bearings (2RS or ZZ on the part number) are the default. They’re greased for life from the factory. Don’t overthink it. For high-temperature applications (over 100°C) or dirty environments, you need a relubricatable bearing with proper seals — and a regreasing interval in the maintenance schedule.
A Bearing Selection Checklist You Can Use Today
- What is the actual radial load (Fr)? (Weight, reaction forces, belt tension)
- What is the actual axial load (Fa)? (Clamp force, spring preload, gear thrust)
- What is the operating speed (RPM)?
- What L10 life do you need? (Hours × duty cycle)
- What bearing type matches the load combination? (Radial, axial, combined)
- Does the arrangement need a fixed-floating setup? Which bearing is fixed?
- Are the fits correct for which ring rotates?
- Is the static safety factor adequate for shock loads?
- Is the sealing and lubrication appropriate for the environment?
- Cross-check: does the chosen bearing’s catalog life meet your target? If not, step up a series.
The Bottom Line
Custom machine bearing calculation isn’t about doing the math perfectly. It’s about not guessing. The catalog size that “looks right” is a guess. The bearing that comes out of a load calculation — even a rough one — is a decision you can defend when the customer asks why it failed after three months.
Pick the bearing type to match the load, calculate the life to match the duty cycle, arrange the shaft so one bearing floats, and specify the right fits. That’s it. The 6208 that failed wasn’t a bad bearing. It was a bearing doing a job it was never sized for. Don’t repeat that on your next machine.
