NSK 7304AWDT Matched Angular Contact Ball Bearings

Matched 30 degree angular contact bearings for heavier thrust and stiffer shafts

The NSK 7304AWDT is a matched pair of 7304A angular contact ball bearings arranged in tandem. Each bearing has a 20 mm bore, 52 mm outside diameter and 15 mm width, and a 30 degree contact angle. Tandem (DT) mounting shares an axial load in one direction between both bearings, so the pair carries more thrust than one bearing. The two bearings are supplied as a matched set, and the same 7304A bearings can also be supplied as a DB (back-to-back) or DF (face-to-face) pair when a rigid fixed bearing or moment capacity is needed. The 03 series is the heavier of the two dimension series commonly used for pump and gearbox shafts.

Technical specifications

Size table for the NSK 73A matched series

The table lists the 30 degree 73 series in single and matched form, as printed in the NSK angular contact bearing table. Matched speeds are lower than single-bearing speeds because two bearings share one lubricant volume.

Choosing DB, DF or DT

A back-to-back pair resists moment loads and tilting because its load lines diverge, which is why it is the usual choice for fixed bearings on pump and compressor shafts. A face-to-face pair tolerates some misalignment but has less moment stiffness. Tandem mounting has no moment capability worth relying on and is used where the thrust is large and always acts in the same direction, with a third bearing or another set opposing it. NSK notes that the internal clearance or preload of matched bearings is obtained by axial tightening, and that bearings above ISO class P5 are mainly used in machine tool spindles with a preload.

Check that the axial to radial load ratio stays below the e value in the table when running a matched set, otherwise ball slip can smear the raceway. For a sealed radial-only alternative see the SKF 6206-2RS1, or browse Electric Motors, Pumps and Turbines.

Selection checks for a 7304AWDT tandem pair

Confirm the direction and size of the axial load first. A tandem pair only makes sense when thrust acts mainly in one direction, and the shaft must have another bearing or set that locates it in the opposite direction, even if that load is small. Compare the radial and axial loads, including start-up and shock loads, with the matched-pair ratings in the table, and check the operating speed against the matched limiting speeds for the lubrication method you will use. Check the shaft and housing fits recommended by NSK for the load type, and confirm that the shaft shoulder and housing shoulder heights suit the chamfer dimensions so the bearings are fully supported.

When replacing a pair in existing equipment, read the markings on the old bearings and check the drawing for the arrangement. Fitting a DT pair where the design calls for DB or DF will change how the shaft is located and can lead to early failure.

Mounting the matched pair

Keep the two bearings together as supplied, because they are ground and matched as a set; mixing bearings from different pairs removes the controlled load sharing. Before mounting, check the orientation guidance in the NSK documentation and keep both bearings facing the same way for tandem use. Heat the inner rings evenly with an induction heater or oil bath rather than a flame, and push them fully against the shoulder before they cool. Tighten the locknut or clamping arrangement to the value given in the machine documentation, then turn the shaft by hand to check that it runs smoothly without tight spots.

Lubrication and running-in

Use the grease or oil specified by the equipment maker and avoid overfilling, which raises running temperature in angular contact bearings. During the first hours of operation, monitor bearing housing temperature and noise; a gradual rise that then stabilises is normal, while a steady climb points to excess preload, poor fits or too much lubricant. Set up regular vibration and temperature checks on critical pumps and gearboxes so that wear is noticed well before the bearing fails.

Service conditions in Oman and the GCC

High ambient temperatures in Omani pump stations and outdoor gearboxes reduce the margin between normal operating temperature and the point where grease life falls sharply, so relubrication intervals may need to be shorter than in cooler climates. Sand and fine dust are the main contamination risk; check that housing seals and labyrinths are in good condition and clean the area around the housing before opening it. Store spare pairs in their original packaging in a dry room, away from floor-level humidity, and do not wash off the factory preservative until the bearings are ready to be mounted.

Documents and ordering

State whether you need DB, DF or DT, the preload or clearance class, cage, tolerance class and quantity of pairs. The attached PDF is a page extract from the NSK catalogue Rolling Bearings for Industrial Machinery (Cat. No. E1103b) with the designation key, the notes on matched bearings and the 73 series tables. Seven Star LLC is an independent supplier in Muscat; availability is confirmed per enquiry.