NSK 6208DDU Deep Groove Ball Bearing

A sealed 40 mm deep groove bearing for pumps, motors and gearboxes

The NSK 6208DDU is a single-row deep groove ball bearing from the 62 series with a 40 mm bore, 80 mm outside diameter and 18 mm width. The DDU suffix means a rubber contact seal on both sides, so the bearing leaves the factory greased and closed. NSK describes this seal type as the one with the best dust and water resistance among its shielded and sealed variants, at the price of a lower speed capability than ZZ or VV types. Deep groove ball bearings carry radial loads and moderate axial loads in both directions, which is why the 62 series is a common choice in electric motors, centrifugal pumps, fans, blowers and light gearboxes.

Technical specifications

Size table for the NSK 62 series

The rows below are taken from the NSK bearing table for single-row deep groove ball bearings. Ratings and speeds are catalogue values; the grease column applies to open, Z and ZZ types, and the DDU column to rubber-sealed types.

Reading the designation and choosing the variant

In the NSK system the first digit gives the bearing type (6 = single-row deep groove), the second the dimension series (2 = 02 series), and the next two digits the bore number; from 04 upward the bore is the bore number multiplied by 5. Seal codes are Z and ZZ (steel shield on one or both sides), V and VV (non-contact rubber seal) and DU and DDU (contact rubber seal). Internal clearance is CN when omitted; C3 and C4 are larger, and CM is the clearance NSK offers for electric motors. When shields or seals are fitted on both sides the designation also carries a grease code, for example NS7 for NS HI-LUBE.

NSK warns that a contact-sealed bearing is limited by seal friction, so use a ZZ or VV bearing when speed is the priority and DDU when contamination or splash is the main risk. For a comparable 30 mm bore bearing from another manufacturer see the SKF 6206-2RS1 already listed, and browse the wider range under Electric Motors, Pumps and Turbines.

Key features of the 6208DDU

• Boundary dimensions of 40 × 80 × 18 mm, interchangeable with any ISO 62-series 6208 bearing of the same seal arrangement.
• Rubber contact seals on both faces (DDU), giving NSK’s highest dust and water resistance among its sealed and shielded options.
• Factory-filled with grease and closed, so no relubrication point is needed on the housing.
• Basic dynamic load rating of 32,000 N and static rating of 17,900 N, with a pressed steel cage.
• Accepts radial load plus moderate axial load in either direction, which suits locating and non-locating positions on motor shafts.

Choosing between DDU, ZZ, VV and open types

The seal decision usually comes down to contamination versus speed. A contact seal rubs on the inner ring, so it keeps out fine dust and splashed water better than a non-contact shield, but the friction raises running temperature and lowers the limiting speed to 5,600 rpm in this size, compared with 9,500 rpm for open, Z and ZZ types on grease. On a four-pole motor running near 1,500 rpm, or a pump on a 50 Hz supply, the DDU speed limit leaves a comfortable margin. On a high-speed spindle or a two-pole fan running close to 3,000 rpm the margin is still adequate, but if the shaft is driven faster by a belt or a variable speed drive, compare the actual speed against the table before selecting the sealed type.

Open bearings are the choice where the bearing sits in an oil bath or a regreased housing; a sealed bearing in an oil bath will not benefit from the oil and can trap heat. Where clearance, noise grade or a special grease is required, these are specified with additional suffixes, so the full suffix string on the old bearing should be copied exactly when ordering a replacement.

Installation and handling

• Press the bearing on with force applied only to the ring being fitted; pushing through the balls brinells the raceways and causes noise from the first hour.
• Check the shaft and housing seats for size, roundness and burrs before fitting; an oversize shaft removes internal clearance and raises temperature.
• Do not wash a sealed bearing in solvent, and do not heat it with an open flame; if induction heating is used, keep the temperature within the seal limit.
• Keep the bearing in its original packing until fitting to avoid contamination and corrosion in humid stores.
• After start-up, listen for irregular noise and check housing temperature; a steady temperature after the first running hours is the normal pattern.

Service in Oman and GCC conditions

Fine sand and high humidity near the coast are the two main enemies of rolling bearings in the region. Contact-sealed bearings are often preferred on outdoor pump sets, cooling-tower fans and conveyor drives for exactly this reason, because dust that passes a shield is stopped by the lip. High ambient temperature also matters: the DDU seal operating range of -10 to +100 °C should be compared with the actual bearing temperature, which on an enclosed motor in summer can be well above the surrounding air. Where the bearing will run hotter than that, a shielded ZZ type or an open bearing with a high-temperature grease is the more appropriate path, and the motor or pump manufacturer’s recommendation should take priority.

What to include in your RFQ

• Full designation as marked on the old bearing, including any clearance (for example C3) or noise suffix.
• Quantity, and whether matched lots or a single production batch are needed.
• The machine it fits (motor frame, pump model, gearbox) and running speed.
• Any grease or temperature requirement that differs from standard.
• Required delivery location in Oman or the GCC and any documentation such as certificates of origin.

What to confirm and documents

Before ordering, confirm bore, seal type, clearance class (C3 is common for motors with a hot shaft), grease code, operating speed against the limiting values above, and the radial and axial loads. The attached PDF is a page extract from the NSK catalogue Rolling Bearings for Industrial Machinery (Cat. No. E1103b), covering the deep groove design notes and the 62 series bearing tables. State the full designation and quantity when requesting a quotation. Seven Star LLC is an independent supplier in Muscat; stock and lead time are confirmed per enquiry.