NSK 7208A Single-Row Angular Contact Ball Bearing

A 30 degree single-row angular contact bearing for combined radial and axial load

The NSK 7208A is a single-row angular contact ball bearing with a 40 mm bore, 80 mm outside diameter and 18 mm width. Its A contact angle is 30 degrees, so each bearing takes radial load together with axial load in one direction only. Because the axial load path is one-sided, the bearing is normally mounted in pairs or against another bearing that takes the opposite thrust. Typical uses listed by NSK for angular contact bearings include machine tool spindles, vertical pump motors and worm gear reducers, and in industrial plant they appear in pump shafts, gearbox shafts and compressor drives.

Technical specifications

Size table for the NSK 72A series

Rows below are the 30 degree A versions of the NSK 72 series from the angular contact bearing table. Limiting speeds are for single bearings with the standard cage.

Designation, arrangements and selection notes

NSK builds the designation from bearing type, dimension series, bore number, contact angle, cage, arrangement, preload or clearance. Contact angles are C = 15 degrees, A5 = 25 degrees, A = 30 degrees and B = 40 degrees. Cage codes include W for pressed steel, TYN for a polyamide resin cage and no code for a machined brass cage. Arrangement symbols are DB (back-to-back), DF (face-to-face) and DT (tandem); SU means a universal single-row bearing that can be paired in any of those layouts. For clearance, CN is standard and CNB marks the CN-equivalent clearance for universal arrangements.

A B version (40 degrees) trades a lower speed limit for higher axial capacity: the 7208B is listed at 35,500 N and 5,300 rpm with grease. NSK also states that if the axial to radial load ratio of a matched set exceeds the e value, or the load is very small, balls can slip and smear the raceway, so such cases should be discussed with the manufacturer.

Checking the 7208A against the shaft and the loads

Start by confirming that a 40 mm shaft and an 80 mm housing bore suit the design, then look at the direction and size of the thrust. A single 7208A only supports axial load from one side, so the first question is whether the thrust reverses. A vertical pump motor carrying the weight of the impeller pushes mainly one way, while a worm gear reducer that runs in both directions loads the shaft alternately, and needs a pair or an opposing bearing to cover both cases.

Compare the expected equivalent load with the basic dynamic load rating Cr of 39,000 N to estimate life, and use the static rating C0r of 25,100 N to check shock loads at standstill or start-up. If a single bearing is not enough, the matched-pair figures of 63,000 N and 50,500 N show how much a DB, DF or DT set adds. Where the shaft speed is close to the 7,500 rpm grease limit, consider oil lubrication, which raises the limit to 10,000 rpm for a single bearing.

Choosing the arrangement

• DB (back-to-back) gives a wide effective load centre spread and good resistance to tilting moments, common on spindles and pump shafts.
• DF (face-to-face) is more tolerant of small misalignment between shaft and housing.
• DT (tandem) shares a large one-direction thrust between two bearings.
• SU universal bearings let maintenance stores keep single bearings that can be paired in any of these layouts.

Mounting and running in

Angular contact bearings must be fitted the right way round, so mark the direction of thrust on the drawing and check the wider shoulder of the outer ring before pressing it in. Apply mounting force only to the ring being fitted, never through the balls, and use heating rather than hammering for an interference fit on the shaft. Pairs should stay together as supplied, with the matching marks aligned, because mixing bearings from different sets alters the preload. After assembly, rotate the shaft by hand to check for smooth running, then run the machine at reduced speed while watching temperature before bringing it to full speed.

Maintenance and signs of trouble

Rising temperature, new noise or increased vibration are the usual early warnings. Overheating soon after start-up often points to excess preload or over-greasing. Smearing on the raceways, which NSK links to very light load or a high axial to radial ratio in matched sets, is visible when a failed bearing is opened and should prompt a review of the arrangement. Record vibration readings on pump and gearbox shafts so trends can be compared over time.

Bearing service in Oman and the GCC

In pumping stations, desalination plants and industrial sites across the region, the main threats to bearings are fine sand entering through worn seals and high ambient temperatures that shorten grease life. Keep housing seals in good condition, use clean tools and covered workbenches when fitting, and re-lubricate on a schedule that reflects the actual running temperature. Store spare 7208A bearings in their original packaging in a dry, air-conditioned store, because humidity at coastal sites can mark unprotected rings with corrosion before they are ever fitted.

Documents and ordering

Confirm the contact angle, cage, arrangement (single, universal or matched), preload or clearance, tolerance class, speed and the direction of the thrust before requesting a price. The attached PDF is a page extract from the NSK catalogue Rolling Bearings for Industrial Machinery (Cat. No. E1103b) with the angular contact designation notes and bearing tables. A deep groove alternative is the SKF 6206-2RS1; see also Electric Motors, Pumps and Turbines. Seven Star LLC is an independent supplier in Muscat.