NSK 608 Extra Small Deep Groove Ball Bearing

The 8 mm bore standard bearing for small shafts, fans and precision mechanisms

The NSK 608 is a single-row deep groove ball bearing with an 8 mm bore, 22 mm outside diameter and 7 mm width. In NSK terminology it belongs to the extra small ball bearings, defined as bearings with a bore under 10 mm and an outside diameter of 9 mm or more; below that outside diameter the same family is called miniature. It is a very common bearing number, found in small electric motors, cooling fans, pumps, instrumentation, linear motion carriages and light machinery. It is offered open, with a shield (ZZ1), a non-contact rubber seal (VV) or a contact rubber seal (DD). The catalogue grease speed rating is 34,000 rpm for open and shielded types.

Technical specifications

Size table for common NSK extra small bearings

The rows show the 5 to 9 mm bore bearings in the NSK metric series around the 608. Mass is given in grams; speeds are for grease and oil lubrication of the open design.

Selection and handling notes

The catalogue gives the speed capability by seal type: open, Z and ZZ types and V and VV types share one grease limit, while contact-sealed D and DD types are limited by seal friction (28,000 rpm for the 608). Choose ZZ1 or VV when speed and low torque matter, and DD when dust or splash is the main risk. Because the loads are small, the ratio of load to basic rating is often very low and bearing life is limited by lubricant life and contamination; confirm the grease type and quantity and the operating temperature. For applications with an outside diameter below 9 mm, or special miniature designs, NSK refers readers to its Miniature Ball Bearings catalogue (Cat. No. E126).

Larger sealed bearings such as the SKF 6206-2RS1 are listed on this site, and further drive components are under Electric Motors, Pumps and Turbines.

Features and construction

The 608 is built to the metric extra small series, so its 8 mm bore, 22 mm outside diameter and 7 mm width follow ISO boundary dimensions and are interchangeable with the same size from other bearing makers, subject to the clearance class and seal. It is a non-separable deep groove design, meaning the balls, cage and both rings stay together as a single unit. A deep groove raceway carries radial load and a modest axial load in both directions, which is why the type is widely used in small motors and fans.

Choosing the seal or shield

• Open 608: lowest friction and highest oil speed limit; lubricant is added by the user and the bearing needs housing protection.
• 608ZZ1: metal shields keep larger particles out and grease in, with no seal drag.
• 608VV: non-contact rubber seal, giving better protection against dust than a shield and the same grease limiting speed as the open type.
• 608DD: contact rubber seal, the best barrier against dust and splashes, but friction and heat rise, so the limiting speed falls to 28,000 rpm.

Typical applications

Besides small motors and fans, the 8 mm bore is common in small pumps, printers and office machines, power tools, instruments, skates and linear carriages, and in actuators for valves and dampers. In sealed enclosures in hot and dusty locations, such as the electronics cabinets of outdoor equipment in the Gulf, the sealed variants are preferred because relubrication is rarely possible.

Fitting and handling notes

• The shaft and housing seats should be accurate and clean. Small bearings are thin-walled, and an out-of-round seat or burr can distort the raceway and raise noise.
• Apply force only to the ring being fitted. Pressing through the balls damages the raceways. Use a sleeve that touches the inner ring when fitting to the shaft and the outer ring when fitting to the housing.
• Keep the bearing in its package until the moment of fitting, and avoid washing sealed bearings in solvent, which removes the factory grease.
• Avoid heavy preload or end-float errors in applications with two bearings on one shaft, and allow for thermal growth of the shaft.
• Check for noise and temperature rise after first running; a small bearing that warms quickly may be over-greased, over-loaded or fitted with a contact seal at too high a speed.

Information needed for a quotation

• Complete NSK designation with shield or seal suffix
• Radial internal clearance class and any grease or noise requirement
• Operating speed, load and temperature
• Quantity, packaging and delivery location

Selection checklist

• Confirm the shaft and housing dimensions against 8 mm, 22 mm and 7 mm before looking at alternatives such as the 628 or 627 listed in the table, which differ in outside diameter or width.
• Decide whether the bearing will be relubricated. If it will not, choose a sealed type and confirm that the factory grease suits the operating temperature.
• Check the operating speed against the limiting speed for the chosen seal type, leaving a margin for continuous running.
• Compare the load with the basic dynamic and static ratings in the specification table. At small sizes, shock loads and misalignment are more likely to cause failure than steady load.
• Consider noise requirements, as quiet running in appliances and instruments may call for a specific grade or grease.

Inspection and replacement

Small bearings usually fail from contamination, lubricant loss or poor fits rather than from fatigue. When a unit is dismantled, look for discoloured grease, rust marks on the raceway, flats on the balls or a rough feel when rotated by hand. Replace the bearing as a whole and do not reuse a bearing that has been pressed off, because the removal force passes through the balls. Fit the same designation in the same sealing arrangement unless there is a reason to change it.

Documents and ordering

State the full designation including shield or seal suffix, clearance class, grease and quantity, and any special requirement such as low noise. The attached PDF is a page extract from NSK catalogue Rolling Bearings for Industrial Machinery (Cat. No. E1103b) with the extra small bearing notes and the metric series tables. Seven Star LLC is an independent supplier in Muscat; availability is confirmed per enquiry.