An oil seal is a contact-type rotary shaft seal that closes the gap between a rotating shaft and a stationary housing, retaining lubricant while excluding dust, water and metal particles [S2]. Major designs are specified in ISO 6194-1 and JIS B 2402-1, with Type A (rubber-covered OD), Type B (metal OD), Type C (double metal-OD with auxiliary lip), and Type F (rubber OD with auxiliary lip) as the four anchor designations [S7].
Performance depends on shaft surface finish, alignment, runout, peripheral speed and lubricant chemistry, so the catalogue splits first by construction (single-lip, double-lip, metal vs rubber OD, pressure-rated) and second by elastomer [S4]. For background on radial sealing devices used in pumps, gearboxes and wheel hubs, see the oil seal encyclopedia entry and the broader construction machinery and equipment reference page.
Component Anatomy and the Eight Functional Parts
A typical oil seal is built from eight functional elements that map directly to failure modes [S2]. The main lip (①) is the wedge-shaped contact edge that runs on the shaft and carries the radial load, while the minor lip (②) blocks external contamination and traps a lubricant film between the two lips.
The sealing edge (③) is the precise contact line whose geometry governs peripheral-speed capability. A garter spring (④) supplements the lip radial load to compensate for wear, heat relaxation and elastomer set. The metal case (⑤) provides rigidity for press-in mounting; the OD surface (⑥) creates the static seal against the housing bore; the fluid-side face or nose (⑦) acts as a gasket on the housing shoulder; and the air-side back face (⑧) sits opposite the sealed medium [S2].
Lip Count and Construction: Single, Double, Triple and More
Lip count is the first selection switch, because it trades friction torque against contamination resistance [S1]. Single-lip seals give the lowest drag and suit clean, pressurized housings; double-lip designs add a dust lip and dominate wheel-hub, gearbox-input and electric-motor bearing shields.
Triple-lip and four-or-more-lip configurations are reserved for severe-dust environments such as off-highway axles, agricultural drivelines and tracked construction equipment, where the extra lips stack a labyrinth-like exclusion zone in front of the main sealing edge [S1]. As a rule of thumb on the shop floor, each extra lip adds roughly 5–15 % to breakaway torque at room temperature, so the energy budget of the driven machine sets the upper count. For related static-seal choices, the sealing washer types and classifications: a 2026 spec reference page covers the companion gaskets used in the same assemblies.
ISO/JIS Type Codes: A, B, C, F and Beyond
The four classical ISO/JIS outer-diameter codes answer most procurement calls in 2026 [S7]. Type A is a rubber-covered OD used in aluminium and plastic housings that need tolerance compensation; Type B is a metal-OD seal for rigid cast-iron and steel bores; Type C adds a metal-OD auxiliary dust lip; Type F mirrors C but with a rubber-covered OD for less rigid housings.
Type codes continue through the alphabet (D, E, G, and so on) to cover split seals, metal-cased variants, and pressure-resistant bodies rated above the standard 0.02–0.05 MPa differential [S1]. Identifying an unknown seal in the field starts with measuring the OD (rubber or metal), counting the lips from the air side, and then matching to the type-letter grid on the manufacturer chart. Where the seal sits at the interface of a packed gland rather than a rotary shaft, the gland packing advantages and disadvantages reference is the right cross-check.
Material Selection: NBR, FKM, HNBR, ACM, Silicone, PTFE
Elastomer choice is governed by temperature envelope, media compatibility and shaft speed [S3][S4]. NBR (nitrile) covers roughly -30 °C to +100 °C in mineral oils and standard greases, and remains the default for general mechanical engineering; FKM (fluorocarbon, e.g. Viton) extends to roughly -20 °C to +200 °C in hot oils, fuels and synthetic lubricants, at a 3–8× cost premium over NBR.
HNBR (hydrogenated nitrile) spans roughly -30 °C to +140 °C and resists modern additive-rich engine oils and ozone attack, which is why it has largely replaced NBR in higher-pressure dynamic seals [S4]. Polyacrylate (ACM) targets hot gearbox and transmission oil up to roughly 175 °C; silicone rubber extends the low end to about -60 °C but is limited to roughly 200 °C and low mechanical loads; PTFE (Teflon) handles dry-running, chemically aggressive media and the highest peripheral speeds, often above 20 m/s, where elastomers would thermally fail [S5].
Decision Matrix: NBR vs FKM vs HNBR vs PTFE
The four-way trade-off below captures the bulk of 2026 field decisions and lines up against the most common selection criteria [S3][S4].
NBR: temperature range -30 °C to +100 °C, mineral oils and standard greases, lowest cost, default for general mechanical engineering. FKM (Viton class): -20 °C to +200 °C, hot oils, fuels, synthetic lubricants and chemical media, 3–8× the cost of NBR, heavier industrial duty. HNBR: -30 °C to +140 °C, modern additive-loaded oils and synthetic lubricants, better ageing and ozone resistance than NBR, suited to higher pressure and dynamic loads. PTFE: roughly -60 °C to +260 °C (compound-dependent), chemically aggressive media and dry running, peripheral speeds above 20 m/s, highest unit cost and requires a stainless garter or no spring [S4][S5].
For dry-running or chemically aggressive media where elastomers would swell or harden, PTFE seals are the default; for additive-loaded engine and gearbox oil, HNBR; for general industrial lubricant retention at room to moderate temperature, NBR; for sustained high-temperature oil and chemical service, FKM [S3][S4].
Operating Limits, Failure Modes and Standards Anchors
Standard peripheral-speed limits sit at roughly 10 m/s for NBR, 12–15 m/s for FKM, and 20 m/s or higher for PTFE compounds, while differential pressure capability is generally capped near 0.05 MPa for an unmodified single-lip design and extends to 0.3–1.0 MPa with pressure-rated profiles or back-up rings [S1]. Shaft surface finish should land at Ra 0.2–0.8 µm and hardness at 45–60 HRC for a nitrile lip to survive its full service life.
The dominant failure modes remain lip wear, lip hardening (thermo-oxidative set), spring relaxation, OD static-seal leakage from bore out-of-round, and chemical attack on the elastomer [S2]. Specification traceability should reference ISO 6194-1 for rotary-shaft seal geometry and acceptance criteria, JIS B 2402-1 for the Japanese type code, and a material datasheet that names the compound family, hardness (typically 70–90 Shore A), and tested temperature envelope [S2].
Where Oil Seals Are Specified and Where They Are Not
Oil seals are specified for low-to-medium speed rotary interfaces, moderate differential pressure, and a defined lubricant chemistry: electric-motor bearing ends, gearboxes, pumps, wheel hubs, transmissions, and small electric drives [S1]. They are not the right answer for high-pressure hydraulic cylinders, where compact U-cup or rod seals dominate, nor for ultra-high-speed spindles above 30 m/s where labyrinth or magnetic bearings are preferred.
Aggressive chemical media that would attack both NBR and FKM, such as strong organic solvents or hot steam above 180 °C, also push the design away from elastomeric lip seals toward PTFE or mechanical face seals [S3][S4]. Specifying engineers working on heavy-machinery drivelines can cross-reference the construction machinery and equipment page for the broader housing and bearing environment.
Track the next procurement cycle against two signals in the 2026 elastomer market: FKM spot price movement, which is tied to fluoroelastomer feedstock and historically sets the upper bound on premium seal budgets, and HNBR capacity announcements, which govern the migration path away from NBR in higher-pressure dynamic seals. Shaft-finish inspection, lip-temperature telemetry on test rigs, and ISO 6194-1 lot certificates remain the three field checks that catch a mis-specified seal before it ships.
Spec-level background on the components involved: checking and testing apparatus.