lip-seal

name	lip-seal
description	Lip seal (radial shaft seal) design — RWSSD selection, shaft speed limits, lip contact pressure, housing/shaft tolerances, materials (NBR/FKM/PTFE), failure modes.
metadata	{"priority":7,"promptSignals":{"phrases":["lip seal","radial shaft seal","rotary shaft seal","oil seal","shaft seal"],"minScore":3}}

Lip Seal (Radial Shaft Seal) — Complete Skill

Types

• Standard (RWSSD / 12-type): single lip, spring-loaded, excludes air side
• Double lip: secondary lip excludes contamination from outside
• With dust lip: primary seal + axial wiper lip
• PTFE lip: contact-free or light contact; highest speed capability
• V-ring (axial lip): slips onto shaft; no housing bore required
• Labyrinth seal: non-contacting; for high speed / low pressure

Shaft Speed Limits

Surface speed v = π d n / 60,000 [m/s; d in mm, n in rpm]

Lip material	Max v [m/s]	Temp range	Notes
NBR (nitrile)	4–6	−40 to +100°C	Most common; mineral oils
FKM (Viton)	6–10	−20 to +180°C	Petroleum, solvents
ACM	4–8	−25 to +150°C	High-temp oil
PTFE (lip)	8–18	−200 to +200°C	Wide range; low friction
HNBR	6–10	−40 to +150°C	Zinc, refrigerants

Typical surface speed limit (NBR, standard): 4–5 m/s Above limit: seal overheats → lip hardens → leakage increases

Shaft Tolerances

Shaft surface finish critical for seal life: Ra = 0.2–0.8 μm (lead-free ground finish; no spiral marks) Hardness: HRC ≥ 45 (carburized/induction hardened) for abrasive conditions Shaft tolerance: h11 or h8 (ISO) — check seal manufacturer spec

Shaft eccentricity and runout: TIR (Total Indicated Runout) ≤ 0.05 mm for v < 4 m/s TIR ≤ 0.025 mm for v > 4 m/s Eccentricity causes dynamic loading on lip → accelerated wear

Housing Bore Tolerances

Housing bore: H8 tolerance standard Interference of OD: 0.2–0.5 mm (press-in fit) Surface finish of bore: Ra ≤ 3.2 μm Housing material: steel or cast iron (stiffer retention); aluminum requires care (thermal expansion)

Lip Contact Pressure

Spring-loaded lip maintains radial contact force: Radial lip load: F_rad = π × d × k × δ_spring [N] k = garter spring rate (from manufacturer) Typical contact pressure: 0.05–0.15 N/mm (linear)

High contact pressure → high friction → overheating Low contact pressure → leakage, poor sealing

Lubrication of Lip

Oil side: wetted lip = self-lubricated; fill with grease before assembly if dry Dry side: pre-fill lip cavity with grease (e.g., NLGI 2) at installation Air-side lip: dry running; limit to low-duty applications

Pressure Rating

Standard lip seal: rated for 0.5 bar (50 kPa) pressure differential maximum Above 0.5 bar: use PTFE lip design, mechanical seal, or labyrinth Higher pressure requires back-up ring or pressure-rated design

Installation

• Check shaft for burrs, chamfer 15° at installation lead
• Apply oil or grease to lip before assembly
• Press housing OD squarely (avoid cocking); use installation tool matching OD
• Never drive lip seal with hammer directly on seal face

Common Failure Modes

Failure	Appearance	Cause
Lip hardening	Cracked/brittle lip	Heat, wrong material, ozone
Lip wear	Groove on shaft	Abrasion, contamination
Lip eversion	Lip turned wrong way	Installation error, pressure
Spring loss	No contact, leaks	Corrosion, over-speed, vibration
Housing leak	Oil at OD	Insufficient press fit, bore oversize

Selection Procedure

1. Determine shaft diameter d [mm], speed n [rpm] → v_surface [m/s]
2. Identify fluid sealed (oil, water, solvent), temperature range
3. Select lip material from table
4. Check v_surface vs. material limit; add dust lip if contamination present
5. Specify shaft tolerance (h11 recommended) and surface roughness (Ra 0.2–0.8 μm)
6. Specify housing bore (H8), check retention
7. Pre-fill with grease if dry-side is air

Output

Provide: seal designation (e.g., 35×62×10 NBR), lip material, max surface speed [m/s] vs. actual, shaft tolerance, housing bore tolerance, pressure rating, installation instructions.