Radial shaft seals (oil seals): types, materials and selection

A radial shaft seal — commonly called an oil seal, or "gufero" in the Czech/Slovak workshop — seals a rotating shaft, keeping oil or grease in and contaminants out. Unlike an O-ring, which has two dimensions, a shaft seal is ordered with three: shaft diameter (d), housing bore diameter (D) and width (b), written as d × D × b in millimetres (for example 25 × 40 × 7). Selecting one comes down to three decisions: the construction type, the lip material, and whether the operating limits are satisfied.

Construction types (DIN 3760)

The DIN 3760 construction forms differ by the number of sealing lips, the presence of a dust lip, the outer-diameter construction and the spring:

• Type A (industry code SC): one lip, rubber-covered OD, with spring. The most common type — the rubber OD seals even an imperfect or aluminium bore.
• Type AS (code TC): Type A plus a dust lip. The globally most-used type for general industry and automotive.
• Type B (code SB): one lip, bare metal OD, with spring. Requires a precise steel bore.
• Type C (code SA): a double metal case for higher rigidity, for large diameters.
• Springless types (AO/VC): no spring, for grease or light-duty service only.

Lip materials and their limits

The lip material sets both the temperature rating and the maximum shaft surface (peripheral) speed:

• NBR (nitrile): the standard, -40 to +100 °C, up to roughly 10–12 m/s. The cheapest, for oils, hydraulics, grease and water.
• FKM (Viton): -20 to +200 °C, up to roughly 35–38 m/s. For hot engines, gearboxes, fuels and aggressive media.
• VMQ (silicone): the widest temperature range from -60 °C, but only a low dynamic speed (~4–6 m/s) — not for fuels or mineral oils.
• PTFE: the lowest friction, dry-running capable, up to ~40 m/s; needs a harder, smoother shaft and higher fitting force.
• HNBR: better heat and ozone resistance than NBR, up to +150 °C.

Operating limits you must respect

A radial shaft seal is a low-pressure seal. Standard types handle up to about 0.05 MPa; above 0.02 MPa reduce the speed, and above 0.05 MPa you need a high-pressure type or an anti-extrusion ring. The shaft should be at least 45 HRC hard (55 HRC above 4 m/s or with abrasives, 60–65 HRC for a PTFE lip). The shaft running-surface finish should be Ra 0.2–0.8 µm for a rubber lip (a finer Ra 0.1–0.4 µm for PTFE) — and critically, no spiral grind lead, which would act as a pumping thread. Keep shaft runout below 0.05 mm TIR, and give the shaft a 15–30° burr-free lead-in chamfer.

The selection process

1. Measure d × D × b, or read the size off the side wall of the old seal.
2. Choose the type by environment: with a dust lip (AS/TC) where contaminants are a risk; a rubber OD (A/SC) for an imperfect bore.
3. Choose the material by medium, temperature and shaft surface speed.
4. Verify the operating limits (pressure, shaft hardness, finish, runout) are in order — most premature failures come from exceeding them.

Note: the everyday workshop term varies by market — "gufero" in Czech/Slovak, "szimmering" in Hungarian, "simmering" in Polish, "radial shaft seal" or "oil seal" in English. Our online shaft-seal reference helps you find a size and its nearest standard neighbour.