Timken Double-Row Cylindrical Roller Bearings NNU MAW33

Double-row cylindrical roller bearings for more radial capacity in the same bore

Timken double-row cylindrical roller bearings in the NNU style put two rows of rollers between a solid-ribbed outer ring and a ribless inner ring, which gives more radial load capacity and stiffness than a single-row bearing of the same bore. The inner ring slides axially relative to the outer ring, so the bearing floats and absorbs shaft expansion without adding thrust load. Timken sells them as a complete assembly with dimensions held to the interchangeable ISO and DIN plan: diameter under the rollers for NNU and over the rollers for NN.

The catalogue range printed for the two-row MAW33 design starts at 150 mm bore in the 49 and 41 dimension series. Each bearing has a lubrication groove and holes in the outer ring (the W33 feature) and one solid machined brass cage with drilled pockets. Timken lists cylindrical roller bearings for helical and planetary gearboxes, geared motors, pumps, compressors, electric motors and centrifuges, all places where a stiff, high-capacity radial support is needed.

Technical specifications

Catalogue sizes for the two-row MAW33 range: dimensions d / D / B in mm, DUR (diameter under rollers) F in mm, and static C0 / dynamic C1 load rating in kN:

Selecting a two-row bearing and fitting it correctly

Choose a two-row cylindrical bearing when the radial load exceeds what a single-row bearing in the same envelope can carry and the shaft needs no axial location from this bearing. The designation NNU4930MAW33 reads as NNU two-row type, 49 dimension series, 30 for a 150 mm bore (30 times five), MA brass cage and W33 lubrication holes. The 41 series bearings are wider and carry roughly three times the dynamic rating of the 49 series at the same bore, for example NNU4140 at 1,690 kN against NNU4940 at 550 kN.

Because the bearing is sold as a complete assembly, order it by the full designation rather than by ring. Shaft and housing fits depend on which ring rotates and on the load, so take them from the fit tables in the catalogue and keep the rated clearance in mind when choosing a tolerance.

What to confirm before ordering

Confirm the complete designation, cage code, clearance, shaft and housing tolerances, lubrication method and the speed in relation to the thermal reference value. For lighter radial duty see the single-row range in the related Electric Motors, Pumps and Turbines listing, or the 22216EJW33 spherical roller bearing where misalignment must be absorbed.

Mounting a two-row NNU bearing

Large two-row cylindrical roller bearings reward a clean, well-planned fitting job. Before mounting, measure the shaft and housing seats and compare them with the tolerances selected from the Timken fit tables, check shoulders for squareness and burrs, and keep the bearing in its packaging until the moment it is needed. Workshops and site maintenance areas in Oman are often dusty, so prepare a covered, clean work area and protect the open bearing from blowing sand, which is one of the fastest ways to shorten bearing life.

With an interference fit on the shaft, the inner ring is usually heated for mounting. Use a controlled heating method with temperature monitoring and follow the limits given in Timken documentation; never use an open flame. Because the NNU inner ring is ribless and separable from the outer ring and roller assembly, take care when bringing the two parts together so that rollers are not scratched or skewed. Push the components together squarely rather than forcing them.

Clearance after fitting

An interference fit reduces internal radial clearance. For precision applications such as machine tool spindles or high-speed gearboxes, measure the clearance after mounting where the design calls for it and compare it with the value intended by the machine designer.

Lubrication and start-up

The W33 groove and holes in the outer ring allow lubricant to be fed to the centre of the bearing between the two roller rows. Make sure the housing supply point lines up with this groove, and confirm that the grease or oil selected suits the speed and the operating temperature. During the first hours of running after an overhaul, monitor bearing temperature and noise closely. A temperature that keeps rising rather than settling may indicate too much grease, too tight a fit or a lubrication supply problem.

Service conditions in GCC industry

Gearboxes, pumps, compressors and motors in the Gulf frequently operate in high ambient temperatures, which raises lubricant temperature and shortens relubrication intervals compared with cooler climates. Review the lubricant viscosity at the real operating temperature, not only at the standard reference temperature. Dust ingress through worn housing seals is another common failure route on outdoor equipment and in cement, mining and quarry plants, so inspect sealing arrangements whenever the bearing housing is opened.

Condition monitoring and replacement

Trend vibration and temperature on critical machines, and analyse used lubricant where an oil system is fitted. When a bearing is removed, keep it for inspection: the pattern of wear on rings and rollers often shows whether the cause was misalignment, contamination, lubrication or overload. Order the replacement by the complete designation so that cage, clearance and lubrication feature match the original.

Documents and quotation

The attached Timken Cylindrical Roller Bearing Catalog contains the full two-row table, fits and clearance data. Send the designation and quantity when requesting a quotation from Seven Star LLC, an independent supplier in Muscat.