The five-letter code stamped on a radial spherical plain bearing, d, D, B, C, dk, is the shorthand every design engineer and bearing supplier reads first, and DIN ISO 12240-1 defines exactly what each value represents on the drawing [S4]. Bore d is the inner ring bore, outer diameter D is the outer ring OD, B is the inner ring width, C is the outer ring width, and dk is the sphere diameter of the inner ring, the dimension that sets the contact geometry with the outer ring race [S2][S4].
For a typical metric radial spherical plain bearing such as the size with d = 17 mm, the catalogue lists D = 35 mm, B = 20 mm, C = 12 mm, dk = 29 mm and a misalignment angle alpha of 19 degrees, with abutment and chamfer dimensions called out separately [S6]. That single line of numbers tells the design engineer the envelope, the load-tilting capability, and the mating shaft and housing dimensions, which is why the d/D/B/C/dk quintet is treated as a unit rather than five independent values.
DIN ISO 12240-1 dimensional series and what each code letter fixes
DIN ISO 12240-1 is the standard that locks the geometry down. It specifies dimensional series, dimensions, tolerances, and radial internal clearance for spherical plain bearings, while leaving the choice of friction pairing (steel/steel, steel/bronze, steel/PTFE) and the load rating methodology to the manufacturer [S4]. The four commonly used metric series are K, E, G, and W, where W is the wide-inner-ring variant used where an extended inner ring is needed to clear a shaft shoulder or a locking feature [S2][S4].
Inside any one series, a given bore d forces a fixed D, B, C and dk, which is what makes manufacturer-to-manufacturer interchange workable. Schaeffler Catalogue 238 (formerly INA Elges) and the SKF spherical plain bearings catalogue both publish full d/D/B/C/dk tables for steel/steel metric, steel/steel inch, extended-inner-ring metric, and extended-inner-ring inch variants, and the rows in those tables line up with the ISO series codes [S2][S3]. Maintenance-free PTFE-lined versions are tabulated in a parallel product table within the same catalogues, so the same d/D/B/C/dk envelope is kept while only the sliding-contact material and the load ratings change [S3].
Reading a sample row: GE 25 ES and similar metric series E sizes
SKF's worked example uses a GE 25 ES, with the static load rating C stated as 48 kN and the sphere diameter dk as 35.5 mm, and the resulting specific bearing load is then checked against the permissible operating range I chart for that size [S3]. The published envelope for that family is d = 25 mm, D = 42 mm, B = 20 mm, C = 16 mm, dk = 35.5 mm, with an alpha of 4 degrees, the small tilt angle typical of series E bearings intended for heavy radial load with limited misalignment [S3][S6].
Stepping down one size in the same E series to the row with d = 17 mm gives D = 35 mm, B = 20 mm, C = 12 mm and dk = 29 mm at alpha = 19 degrees, illustrating that within a single series the dk-to-d ratio and the B-to-C ratio are not constant, they shift to balance load capacity against tilt [S6]. For inch-series radial spherical plain bearings, the row structure is identical, but the bore, OD, and widths come in 1/8-inch increments from 0.25 inch through larger aerospace sizes, with seals available across the standard cross sections [S1].
Tilt angle alpha, radial internal clearance, and why C is wider than B on heavy series
On most radial spherical plain bearings the outer ring width C is less than the inner ring width B, by design, so the inner ring can rock inside the outer race to absorb misalignment, with the resulting tilt angle alpha listed in the catalogue row alongside dk [S6]. The DIN ISO 12240-1 envelope sets a maximum permissible alpha for each series, and exceeding that angle forces the spherical contact to ride up onto the edge of the outer race, accelerating wear and collapsing the load rating [S4][S6].
Radial internal clearance is also controlled in DIN ISO 12240-1, separate from the d/D/B/C/dk dimensions, and is the value the design engineer adjusts to control how much the bearing can flex under load versus how much play sits in the joint at rest [S4]. For maintenance-free steel/PTFE designs the PTFE liner thickness is built into the C and dk values rather than left to the installer, which is one reason interchange sheets such as the National Precision industrial spherical plain bearing interchange tabulate Bore, OD, Width, and ball diameter side by side across brands [S5].
Friction pairings and how they leave the d/D/B/C/dk envelope unchanged
Across steel/steel, steel/bronze and steel/PTFE the d, D, B, C and dk dimensions are held to the ISO standard so that a maintenance-free PTFE bearing can drop into the same housing as a relubricated steel/steel one, while only the load rating, the friction coefficient, and the relubrication interval change [S4]. Steel/steel and steel/bronze are the pairings that need a lubricant film and periodic relubrication, and they tolerate alternating loads plus medium-to-large swivel movements, while steel/PTFE is the maintenance-free pairing used for one-sided load with small-to-medium impact [S4].
SKF and Schaeffler publish separate rows for the maintenance and maintenance-free versions in the same bore size, so a designer can pick a GE..PW (maintenance) or GE..DO (steel/steel, requiring lubrication) and a PTFE-lined equivalent with identical envelope dimensions but a different suffix code and a different C-rating [S2][S3]. Seals, where fitted, sit in the C-face groove and add no length to B, which keeps the d/D/B envelope unchanged from the unsealed part number [S1][S2].
Selection rules: which series for which load and tilt
Series E is the general-purpose radial spherical plain bearing for moderate-to-heavy static load with limited misalignment, typically an alpha in the 4 to 9 degree range on small-to-medium bores [S3][S6]. Series G carries a wider outer ring and a higher tilt capability, and series K is the heavy-section radial bearing for the largest hydraulic and off-highway pin joints, where the d/D/B/C/dk values are large and the C-load ratings are quoted in the hundreds of kN range [S2][S3].
Series W is the extended-inner-ring variant used where the shaft has a shoulder or where a locking collar has to sit on the inner ring, and it shares its D, C, and dk with the matching series E or G bearing while B is grown to clear the shaft feature [S2][S3]. Where alignment is severe, high-misalignment spherical plain bearings from suppliers such as RBC use a fractured outer race or extended race geometry to push the permissible alpha well past the DIN ISO 12240-1 series-E limit, at the cost of a non-standard dk-to-d ratio that breaks interchangeability with ISO series E, G, or K rows [S1].
Comparison of the four common metric series on d/D/B/C/dk
Putting the four metric series side by side on fixed criteria helps a designer pick a series from the d/D/B/C/dk row alone, without reading the full load-rating page. Series E is the baseline, series G widens C and increases alpha at the cost of some C-load, series K pushes D and B up for the highest radial load at the cost of envelope and weight, and series W keeps the D, C, and dk of E or G while lengthening B to clear a shaft shoulder [S2][S3][S4].
For a fixed bore d, the dk-to-d ratio is highest on series G and on the high-misalignment designs, which is the geometric reason those series tolerate a larger alpha, while series K typically holds dk closer to the d value because the contact is sized for pure radial load with minimal tilt [S2][S3]. The C (outer width) is also a tell: on series E and K, C is roughly 0.6 to 0.8 of B, while on series G and on high-misalignment parts, C is closer to 0.9 of B because more outer-ring material is needed to spread the contact over the wider tilt arc [S2][S3][S6].
Common failure modes when the d/D/B/C/dk envelope is misread
Misreading dk and treating it as the bore is a frequent drafting error that puts a bearing on the wrong shaft, since dk is always larger than d on radial spherical plain bearings, and is closer to the mean of d and D than to either extreme [S6]. Specifying the C dimension where B is required, or vice versa, causes the inner ring to bind before the design tilt is reached, because the outer ring width C sets the maximum contact arc and the inner ring width B sets the shoulder engagement [S2][S4].
Exceeding the catalogue alpha forces the spherical contact toward the edge of the outer race, raising contact pressure past the C-rating and burning the bronze or PTFE liner; this is the dominant failure mode in articulated off-highway joints when the original series E or G is replaced with a high-misalignment aftermarket part that does not match the housing dimensions [S1][S4]. Where the joint runs at high sliding speed and continuous rotation, a spherical plain bearing is the wrong element altogether and a needle-bearing-based spherical plain bearing (the ASKUROL-style construction) should be selected, even though the d/D/B/dk envelope still comes from the same DIN ISO 12240-1 series row [S4].
Standards, interchange sheets, and how to verify a candidate part
The governing geometry standard is DIN ISO 12240-1, which fixes dimensional series, dimensions, tolerances, and radial internal clearance, while load ratings, suffixes, and the friction-pairing choice are not standardized and vary by manufacturer [S4]. For interchange between brands, the National Precision industrial spherical plain bearing interchange sheet lines up bore, OD, width, and ball diameter across manufacturers, which lets a buyer confirm that a candidate part matches the original d/D/B/C/dk before approving it [S5].
For a specific candidate, the fastest verification is to compare the five d/D/B/C/dk values from the OEM catalogue row against the interchange sheet and against the housing drawing; if any one of the five is off, the bearing is not interchangeable regardless of C-rating parity. Track for the next quarter: the next issue of DIN ISO 12240-1 and any 2026-vintage updates to supplier interchange tables, plus the release of new high-misalignment sizes that fall outside the current K, E, G, W series rows. For a related read on coupling misalignment that uses a parallel geometric envelope, see sliding disc coupling parallel offset limits and spacer drop-out jaw coupling service practice.
Detailed specification references: spherical plain bearing, steel section, and checking and testing apparatus.