NOK Oil Seals — Profiles and Material Options

A NOK rotary shaft seal is fully described by three things: the type code (SC, TC, TB, VC, KB, TCJ, HRE …), the lip compound code (A727, T303, S728, F585 …) and the housing/shaft geometry it was designed against. Get any one of them wrong and the seal leaks — usually blamed on the rubber, usually caused by the shaft. This page is the working reference we use when reading a Japanese oil seal specification: what each profile letter means, how NOK types map onto ISO 6194 / JIS B 2402 / JASO, which lip material to specify, and the shaft and bore data the seal assumes.

Anatomy of a rotary lip seal

An oil seal is three components bonded into one part: a moulded elastomer body, a stamped steel case that provides hoop stiffness and the interference fit into the bore, and a garter spring that adds a controlled radial load at the lip edge.

Why the lip pumps

NOK published its sealing theory in 1959 and the model still holds: the lip runs on a thin, coherent oil film, and the friction coefficient follows f = Φ·G^1/3, where G = μ·u·b / P_r — viscosity times surface speed times contact width, divided by radial lip load. Two engineering consequences follow. A seal running dry has no film, so friction and lip temperature rise sharply and the edge carbonises. And an over-loaded lip (excess interference, wrong spring, over-thick oil) drives G down, thins the film and wears a groove into the shaft. Correct rotary sealing is a lubrication problem, not a squeeze problem.

NOK type codes and what the letters mean

NOK profile letters describe the sealing function; the second letter describes the outer diameter construction. C = rubber-covered OD (single metal case, elastomer OD), B = metal-clad OD (assembled case, exposed steel OD). Rubber OD tolerates rougher or non-ferrous housings and thermal mismatch; metal OD is more rigid, better for thin-wall or split housings and easier to press without cocking.

The four-way SC / SB / TC / TB matrix covers the overwhelming majority of Japanese machine-tool, gearbox, pump and hydraulic-motor applications. If a machine parts list shows a bare size such as TC 45 65 8, read it as: dust-lipped, spring-loaded, rubber OD, 45 mm shaft, 65 mm bore, 8 mm width, standard nitrile unless a material suffix says otherwise.

Spring-less V and K types are not oil seals

VC/VB and KC/KB have no garter spring. They are grease retainers and dust excluders. They must not be used against fluid pressure and will not hold a low-viscosity oil at speed. Substituting a KB for a TB because the dimensions match is one of the most common breakdown-shop errors.

General and special profiles

Lip materials — NOK compound codes

The lip compound decides temperature range, fluid compatibility and wear life. NOK codes are letter-plus-number: A nitrile, G hydrogenated nitrile, T acrylic, S silicone, F fluorocarbon. Temperatures below are catalogue guide ranges at the lip.

Continuous vs peak temperature

Note the gap between the catalogue "range" figure and the normal continuous figure. The upper range value is a short-excursion capability; sustained duty should sit at or below the normal continuous column. Below the minimum, TR10 governs — the temperature at which a compound at 50 % elongation recovers only 10 % of its deformation, per JIS K 6261. Below TR10 the lip stops behaving elastically and the seal leaks on cold start even though nothing has failed.

Fluid compatibility at a glance

PTFE is chemically indifferent to everything in the table, which is why TCJ-family seals appear wherever the chemistry defeats an elastomer or the shaft runs dry. The trade is cost, a much stricter shaft finish requirement and far lower tolerance of shaft run-out.

Case and spring materials

The metal parts are specified separately from the lip and are frequently the thing that fails first in a wet or salt environment. Standard cases are cold-rolled steel (SPCC); stainless (SUS304/SUS430) cases and springs are available where the seal sees water, washdown, food processing or marine air. A stainless spring in a carbon-steel case is a common half-measure: it protects the load path but leaves the OD to corrode and lose its press fit.

Shaft design — the part that actually decides seal life

Two details are routinely missed. First, the shaft must be plunge ground — traverse grinding or any lead-producing operation machines a shallow screw thread into the surface which pumps oil out from under the lip regardless of the compound. Second, a silicone lip requires a finer finish: 1.6–0.6 µm Rz. Surface hardness of 30 HRC generally implies heat treatment; an untreated mild-steel shaft will be grooved by a spring-loaded lip within months.

Shaft chamfer

The lead-in chamfer exists so the lip is not turned back or the garter spring dislodged during assembly. NOK specifies a 15–25° chamfer with an R0.3–R0.5 root radius, finished at Ra 1.6 or better, with the small diameter d₁ below the free lip ID.

Run-out and misalignment

Total run-out at the lip is the sum of dynamic shaft run-out plus twice the shaft-to-bore misalignment (the centre-to-centre offset is measured once but acts on both sides). Allowable total run-out falls steeply with speed: for a 50 mm shaft at 2000 rpm the limit is about 0.35 mm TIR, so a shaft running 0.10 mm TIR dynamic leaves only 0.25 mm TIR for misalignment. At 5000–7000 rpm the budget drops to roughly 0.1–0.2 mm TIR across most of the diameter range. Shafts that leak "for no reason" after a bearing change are usually failing this arithmetic.

Allowable operating conditions

• Pressure. Standard S and T types are rated to 0.03 MPa (0.3 kgf/cm²) only. Anything above that needs a pressure-resistant construction — a lip seal is not a pressure seal, and a blocked breather is the most common cause of "sudden" oil seal failure.
• Speed. Surface speed, not rpm, is the limit. Elastomer lips are generally applied up to roughly 15–20 m/s with good lubrication and a ground shaft; above that, or where dry running occurs, move to a PTFE lip type.
• V and K types must never see applied pressure.
• Direction. Rotary lip seals are unidirectional in their pumping behaviour when hydrodynamic aids are moulded into the lip. Confirm the intended rotation before substituting a hydrodynamic variant for a plain lip.

Fitting and storage

1. Check the bore for burrs, and the shaft for wear steps, lead marks and nicks at the chamfer.
2. Lightly oil or grease the lip and fill the space between lip and dust lip with the operating grease — never install dry.
3. Press on the case face with a flat driver that contacts the full OD; never strike the rubber or the lip.
4. Use a fitting sleeve over splines, keyways, cross-holes and threads.
5. Check the garter spring is seated after fitting — a dislodged spring is the single most common installation fault.
6. Store flat, in the dark, below 25 °C, away from ozone sources (motors, welding sets) and hydrocarbons. Do not hang seals on hooks or stack heavy boxes on them: a permanently ovalised seal will not recover.

Reading a failed seal

Specifying and sourcing

A complete enquiry is: type code, shaft × bore × width in mm, lip compound (or the fluid, temperature and speed so we can select it), case and spring material, shaft surface condition, and whether the housing is blind or through-bored. Where a machine plate gives only a NOK part number we can work back from the classification digits; where only a worn seal exists we measure the bore and shaft rather than the seal, because a used seal has shrunk or swelled.

Sealparts supplies Japanese rotary shaft seals against NOK type codes and dimensions, in the standard compounds above and in equivalent European materials where lead time matters, including machined and short-run parts for obsolete large diameters. Send the type, size and duty to our enquiry page and we will confirm the material and availability.