Radial Shaft Seal (Oil Seal) Size Guide — Designations, Lip Types and Material Limits
Selection guide compiled by cBallast. 3-page PDF; the full content is on this page.
Compiled by
cBallast
Subject
Rotary shaft seals
Document type
Selection guide
Reference
2026 edition
Issued
2026-10-04
Pages
3
Format
Web page and PDF
How radial shaft seals are designated (shaft × housing × width), what HMS5/HMSA10 and TC/SC lip codes mean, NBR vs FKM speed and temperature limits, installation rules and how to measure an unknown seal.
Designation = shaft × bore × width
Every standard radial shaft seal is named by three numbers: shaft diameter × housing-bore diameter × width, all in millimetres. A 25×47×7 seal runs on a 25 mm shaft, presses into a 47 mm bore and is 7 mm wide — the same triple every manufacturer uses, so dimensionally the brands interchange (SKF 25×47×10 HMS5 RG and HMSA10 RG are catalogue examples). Metric seal sizes follow ISO 6194/DIN 3760; inch seals use the same logic in inches.
Lip codes
Industry code
SKF code
Construction
Use
SC / type A
HMS5
Single lip, spring loaded, rubber OD
Oil retention, clean environment
TC / type AS
HMSA10
Main lip + auxiliary dust lip, rubber OD
Oil retention with dust/splash outside — the default
SB/TB
CRW1/CRW5
Metal-cased OD versions
Thin-wall or heat-dissipating housings
VR/VA
CT/V-ring
All-rubber axial seal on the shaft
Secondary flinger in front of the main seal
Material limits
Lip material
Temp. range
Shaft speed*
Media
NBR (R suffix)
−40…+100 °C (peaks 120)
to ~14 m/s
Mineral oils, greases, water
FKM (V suffix)
−25…+200 °C
to ~32 m/s
Hot oil, EP additives, fuels, chemicals
PTFE lip
−70…+200 °C
dry-running capable
Poor lubrication, chemicals
*Surface speed = π × shaft Ø × rpm; a 50 mm shaft at 3,000 rpm runs 7.9 m/s — NBR territory. Above ~120 °C sump temperature or with EP gear oils, step to FKM.
Installation rules that save the seal
Shaft surface: ground to Ra 0.2–0.8 µm, hardness ≥ 45 HRC where dirt is present, no spiral grinding lead.
Chamfers: shaft lead-in 15–25°, burr-free; never push a lip over a keyway or spline without a fitting sleeve.
Orientation: main lip faces the medium to be retained; the spring side looks at the oil.
Lubricate the lip and fill 30–50 % of the space between main and dust lip with grease at assembly; a dry start scores the lip in seconds.
Worn groove on the shaft? Press the new seal to a different depth, or fit a thin repair sleeve, rather than letting the lip run in the old track.
Measure an unknown seal with calipers across OD, bore of the old housing and shaft; the stamped face usually carries the triple and material code anyway. Hydraulic cylinder sealing (rod/piston/wiper profiles) is a different family — see the hydraulic seal size finder; bearing housings and their seatings are covered by the bearing dimension chart.
Frequently asked questions
How do I read oil seal size 25x47x7?
Shaft diameter 25 mm, housing bore 47 mm, seal width 7 mm — every radial shaft seal is designated by this triple.
What is the difference between SC and TC oil seals?
SC (SKF HMS5) has a single spring-loaded sealing lip; TC (SKF HMSA10) adds an auxiliary dust lip for contaminated surroundings — TC is the safer default outside clean gearboxes.
When should an FKM (Viton) seal replace NBR?
Above roughly 100–120 °C continuous, at shaft surface speeds beyond ~14 m/s, or with aggressive media and EP-additive gear oils — FKM handles 200 °C and ~32 m/s.
Which way should the seal lip face?
The main spring-loaded lip faces the medium being sealed in (the oil side); the dust lip, when present, faces outward.
Can different brands of shaft seal be interchanged?
Yes — metric seals follow ISO 6194/DIN 3760 dimensions, so a 35×62×10 from any major maker fits the same seat; match lip type and material, not just size.