7.1 Bearing Types, Designations, and Selection
Key Takeaways
- A plain journal bearing supports the shaft on a hydrodynamic oil film, while a rolling element bearing separates the rings with balls or rollers.
- In a metric bearing part number, bore codes 00, 01, 02, and 03 mean 10, 12, 15, and 17 mm, and codes 04 and above are multiplied by 5 to get the bore in millimeters.
- Ball bearings carry lighter loads at higher speeds; roller bearings carry heavier loads because they contact along a line rather than at a point.
- Spherical roller and self-aligning ball bearings tolerate shaft misalignment; cylindrical and tapered roller bearings do not.
- A C3 internal clearance bearing has more clearance than standard and is specified where an interference fit or elevated temperature would close a normal clearance too far.
Two families of bearing
Domain 6, Bearings and Coupling, contributes 12 of 115 items across modules 32207 Introduction to Bearings, 32303 Installing Bearings, and 32304 Installing Couplings.
A bearing supports a load while permitting relative motion. Industrial practice splits into two families:
- Plain (journal or sleeve) bearings carry the shaft on a hydrodynamic film of oil. At rest the shaft sits on the bearing; as it turns, it drags oil into a converging wedge that lifts it off the surface. The film — not the metal — carries the load. Common linings are babbitt, a soft tin or lead alloy that embeds contaminants and protects the shaft.
- Rolling element bearings put balls or rollers between hardened rings so that sliding friction becomes rolling friction. Starting torque is far lower, so they dominate motor, pump, and gearbox applications.
| Property | Plain bearing | Rolling element bearing |
|---|---|---|
| Starting friction | High until the film forms | Low |
| Load capacity for a given size | Very high | Moderate |
| Shock tolerance | Excellent | Moderate |
| Speed capability | Very high with proper lubrication | Limited by cage and lubricant |
| Lubrication dependence | Absolute — film loss is immediate failure | High, but grease retains some protection |
| Noise and damping | Quiet, damps vibration | Noisier, transmits vibration |
| Replacement | Reline or replace shell | Replace the bearing |
Rolling element types
| Type | Contact | Load direction | Misalignment tolerance |
|---|---|---|---|
| Deep groove ball | Point | Radial plus some thrust | Low |
| Angular contact ball | Point at an angle | Combined radial and one-direction thrust | Low |
| Self-aligning ball | Point, spherical outer race | Radial, light thrust | High |
| Cylindrical roller | Line | Heavy radial only; allows axial float | Very low |
| Tapered roller | Line | Heavy radial and heavy thrust | Low |
| Spherical roller | Line, spherical outer race | Very heavy radial and thrust | High |
| Needle roller | Line, small section | Heavy radial in a small envelope | Very low |
| Thrust ball / thrust roller | Axial | Thrust only | Low |
The organizing idea: point contact means lower load and higher speed; line contact means higher load and lower speed. A ball touches its race at essentially a point, so the stress is concentrated but the friction is small. A roller touches along a line, spreading the load over far more area.
The second organizing idea: only the spherical designs tolerate misalignment. A self-aligning ball bearing and a spherical roller bearing both have a spherical outer raceway, so the inner assembly can swivel. Put a cylindrical roller bearing on a misaligned shaft and it will fail in weeks.
Reading a metric designation
A number like 6208-2RS C3 decodes as follows:
| Position | Example | Meaning |
|---|---|---|
| First digit — type | 6 | Single-row deep groove ball bearing |
| Second digit — dimension series | 2 | Width and outside diameter series (load capacity) |
| Last two digits — bore code | 08 | Bore size |
| Suffix — closure | 2RS | Contact seals on both sides (ZZ or 2Z means shields) |
| Suffix — clearance | C3 | Internal clearance greater than normal |
The bore code rule:
| Bore code | Bore |
|---|---|
| 00 | 10 mm |
| 01 | 12 mm |
| 02 | 15 mm |
| 03 | 17 mm |
| 04 and above | code × 5 mm |
So a 6208 has a bore of 8 × 5 = 40 mm, and a 6312 has a bore of 12 × 5 = 60 mm. Codes below 04 are the memorized exceptions.
Internal clearance
Internal clearance is the amount the inner ring can move relative to the outer before load is applied. Codes run C2 (less than normal), CN or unmarked (normal), C3, C4, and C5 (progressively greater).
Clearance is consumed at assembly and in service:
- Pressing the inner ring onto an interference fit expands it and closes clearance.
- Running temperature raises the inner ring hotter than the outer, which expands it more and closes clearance further.
That is why C3 is standard for electric motors and for any bearing with a heavy interference fit or a hot running condition: the extra cold clearance ends up as correct operating clearance. Installing a normal-clearance bearing where C3 is specified preloads it and drives the temperature up until it fails.
Selecting a bearing
Match four things: load magnitude and direction, speed, misalignment present in the machine, and environment.
- Pure radial load, moderate speed, small misalignment: deep groove ball.
- Heavy radial load, no thrust, need for axial float at one end: cylindrical roller.
- Combined heavy radial and thrust: tapered roller or spherical roller.
- Shaft that deflects or a housing that cannot be bored in line: spherical roller or self-aligning ball.
- Contaminated or wet environment: sealed (2RS) rather than shielded (2Z), and consider a design with a slinger or labyrinth.
Bearings are also arranged in a pair: one fixed (locating) bearing that takes thrust and positions the shaft, and one free (floating) bearing that allows the shaft to grow thermally. Locking both ends axially preloads the bearings as the shaft heats and expands — a very common cause of premature failure in a rebuilt machine.
What is the bore diameter of a 6310 bearing?
A pump shaft runs in a housing that cannot be bored perfectly in line, producing a small but persistent angular misalignment at one bearing. Which bearing type is best suited?
Why is C3 internal clearance commonly specified for electric motor bearings?