The 3 mm bore of the 600 and 6900 Extra Small Series Miniature Ball Bearings is the 693: 3 mm bore, 8 mm outside diameter, 3 mm wide. This page gives the dimensional row from the table, the shaft and housing fits that suit it, the internal clearance to expect and the mounting, lubrication and failure notes that go with the type.
3 mm Bore Dimensional Row (ISO 15 / DIN 625-1 / GB/T 276)
| Designation | Bore d (mm) | Outer diameter D (mm) | Width B (mm) | Chamfer r min (mm) |
|---|---|---|---|---|
| 693 | 3 | 8 | 3 | 0.15 |
| 623 | 3 | 10 | 4 | 0.15 |
More than one designation in this series shares the 3 mm bore — they differ in outside diameter, width or sealing, not in the shaft they fit.
What a 600 and 6900 Extra Small Series Miniature Ball Bearings Does
A single row of balls runs in deep, uninterrupted grooves in both rings. That geometry gives low friction, quiet running and a useful axial capacity in both directions alongside the radial load, which is why the deep groove ball bearing is the default choice for electric motors, pumps, fans, gearbox layshafts and small machinery wherever the load is moderate and the shaft is stiff.
Primarily radial load, with a real but secondary axial capacity in both directions taken by the groove shoulders. Combined loading is normal service for this type; pure heavy thrust is not, and a bearing asked to work as a thrust bearing runs its contact ellipse over the groove edge.
Speed at 3 mm Bore
No limiting speed is tabulated for this product. Where speed matters, the figure to work to is the one the bearing supplier states for the exact design, cage and lubricant supplied — not a family value.
Shaft & Housing Fit for a 3 mm Seat
With the inner ring rotating under a load whose direction is fixed, the inner ring sees a circumferential load and must be an interference fit, or it will creep on the shaft and fret it. The outer ring sees a point load and takes a transition or clearance fit so that it can shift slowly and spread the wear — and so that one of the two bearings on the shaft is free to take up axial expansion.
| Shaft, light load (P ≤ 0.06 C) | h5 |
|---|---|
| Shaft, normal load (P = 0.06–0.12 C) | j5 |
| Housing — Axial displacement of the outer ring required | H7 |
| Housing — Normal load, outer ring may still shift | J7 |
| Housing — Normal to heavy load, quiet running, outer ring not displaceable | K7 |
ISO 286-2 tolerance grades, applied per the conventional bearing seat recommendations of ISO 1132-2 and DIN 5425-1, for a SOLID STEEL shaft in a cast iron or steel housing. A hollow shaft, a thin-walled housing or a light alloy housing needs a tighter fit than these, because the seat itself deflects.
Radial Internal Clearance at 3 mm Bore
Radial internal clearance is the total distance one ring can be moved relative to the other, across the bearing, before it is mounted. It is not a tolerance and not a quality grade: it is a quantity that gets USED UP on installation. An interference fit on the shaft expands the inner ring; a tight housing fit compresses the outer ring; and a shaft that runs hotter than its housing grows more than the housing does. All three subtract from the clearance the bearing was made with. C3 exists for that reason. Specify it when the shaft fit is heavier than normal, when the inner ring will run 10 °C or more above the outer, when a hollow or heated shaft is involved, or when the bearing sits on an adapter sleeve that is driven up hard. Specify CN — normal, and usually left out of the designation entirely — for the ordinary case of a moderate fit on a shaft close to housing temperature. The failure this prevents is specific and quick: fit a CN bearing with a heavy interference on a shaft that then runs hot, and the residual clearance goes negative. The bearing is now preloaded by its own installation, the rolling elements are squeezed, friction and temperature rise together, and the temperature rise tightens the fit further. It runs hot, then it runs blue, and the raceways spall long before the calculated life.
| CN (normal), bore over 2.5 up to 6 mm | 2–13 µm |
|---|---|
| C3 (greater than normal) | 8–23 µm |
| C2 (smaller than normal) | 0–7 µm |
| C4 (greater than C3) | 14–29 µm |
Pre-mounting values per ISO 5753-1 / GB/T 4604 for single row deep groove ball bearings with a cylindrical bore. Other bearing types have their own tables in the same standard with different values.
Abutment Shoulder & Fillet Radius for 3 mm
The chamfer dimension r min for this size is 0.15 mm. That figure is a limit on YOUR machining, not just on the bearing: the fillet radius in the corner of the shaft or the housing must be smaller than the bearing's corner radius, or the ring seats on the fillet instead of on the shoulder face and is loaded on its corner.
| Chamfer dimension r min (from the table) | 0.15 mm |
|---|---|
| Maximum fillet radius on the shaft or housing | 0.15 mm |
| Shaft shoulder must exceed | 3.3 mm diameter |
| Housing shoulder bore must be under | 7.7 mm diameter |
The two shoulder figures are the geometric floor and ceiling: a shoulder outside them fouls the corner radius instead of bearing on the ring face. The practical values sit inside them — the shoulder must contact the ring face over a useful area without touching the seal, the cage or the opposite ring, and it must still leave room for a puller when the bearing is dismounted. Take the working da and Da from the abutment table for the bearing; the ring shoulder height they are built on is not a boundary dimension and is not in this table.
Mounting a 3 mm Bore 600 and 6900 Extra Small Series Miniature Ball Bearings
The ring that carries the rotating load takes an interference fit; the ring under a stationary point load takes a transition or clearance fit so that it can creep slowly and spread the wear, and so that one ring is free to take up axial expansion. With a rotating shaft under a fixed load direction, that means an interference fit on the shaft and a looser fit in the housing. Press only against the ring being fitted — force driven through the rolling elements leaves brinell marks in the raceway before the bearing has run a revolution.
At a 3 mm bore the interference is small enough that a cold press or a light hydraulic press is normally enough; heating the bearing in an induction heater or an oil bath to 80–100 °C above the shaft is the alternative, and never above 120 °C for a standard-stabilized bearing. Keep the heated bearing pressed against the shoulder while it cools, because it shrinks axially as well as radially.
Lubrication & Relubrication
Grease is the normal lubricant for this type: it stays where it is put, helps to keep contamination out, and needs no circulation system. Fill roughly 30% to 50% of the free space in the bearing arrangement — a bearing packed full runs hot, because the excess grease is churned rather than used. The relubrication interval is not a fixed figure; it falls with speed, with temperature and with load, and it halves for roughly every 15 °C of temperature rise above about 70 °C. Where speed, temperature or the required interval put grease out of reach, change to oil.
Sealed-for-life versions carry their whole service life of grease inside the seals and cannot be relubricated. That is the right choice where the bearing is inaccessible and the duty is mild; it is the wrong one where the temperature is high, the environment is wet, or the required life is longer than the grease life — and the grease, not the steel, is what ends it.
Where a 3 mm Bore 600 and 6900 Extra Small Series Miniature Ball Bearings Is Used
At a 3 mm bore, 600 and 6900 extra small series miniature ball bearings are typically found in small motors and fans, instruments, dental and hand tools, printers, encoders and model machinery — sizes at which the bearing is usually chosen for smooth, quiet running and low starting torque rather than for its rating.
Failure Modes and What They Look Like
- Subsurface fatigue (spalling) — a flake of raceway lifts out and the bearing gets noisy and rough. This is the failure the L10 life calculation predicts — if it arrives near the calculated life the bearing did its job, and if it arrives early the load, the fit or the lubrication was not what the calculation assumed.
- Inadequate lubrication — a polished, then discoloured, then torn raceway, running hot with a darkened grease. It is the most common cause of early failure by a wide margin, and the interval, not the grease, is usually what was wrong.
- Contamination — dents in the raceway with raised edges, then premature spalling from those dents. Wear particles, casting sand and washdown water all do it; sealing and filtration are what prevent it.
- False brinelling and fretting — evenly spaced dull marks at ball or roller pitch, produced by vibration while the shaft is stationary — a machine that travels on a truck, or a standby pump beside a running one. It is not caused by overload and cannot be cured by fitting a bigger bearing.
- Loss of internal clearance — the whole raceway path is loaded, the bearing runs hot from installation and it discolours evenly. The fit was too heavy, the shaft ran hotter than the housing, or a CN bearing was fitted where C3 was needed.
- Skidding under light load — smeared, matte patches on the raceway. A ball or thrust bearing that is not loaded enough to make its rolling elements roll lets them slide instead, and the film breaks down where they scuff.
Where NOT to Use a 600 and 6900 Extra Small Series Miniature Ball Bearings
- Do not use a deep groove ball bearing as a thrust bearing. Its axial capacity is a secondary property of the groove shoulders, and under pure heavy axial load the contact ellipse rides over the shoulder edge and the raceway breaks up there.
- Do not use one where the shaft misaligns. A deep groove bearing tolerates only a few minutes of arc; beyond that it edge-loads. Use a self-aligning ball bearing, a spherical roller bearing or a housed unit instead.
- Do not expect a 2RS bearing to survive a hot washdown. A standard nitrile contact seal is a lip running on steel, not a pressure seal: a 80 °C detergent wash drives past it and into the grease, and the bearing fails from water, not from load.
