Lip seals oil seals types
Lip seals, also known as radial shaft seals, are dynamic sealing elements designed to retain lubricant and exclude contaminants in applications with rotary or oscillating motion. The typical design consists of an outer metal case, an elastomeric element with one or more sealing lips, and a garter spring that maintains a controlled contact pressure on the shaft. They are press-fitted into a housing and rub against the shaft surface, creating a lubricating film that minimizes wear. Standard seals operate under low pressure conditions (typically below 0.5 bar / 7 psi) and peripheral speeds that can reach 15 m/s (3000 fpm) in conventional elastomers and up to 25 m/s (5000 fpm) in PTFE.
Types of lip seals and oil seals
Section titled “Types of lip seals and oil seals”The commercial classification, according to supplier categories such as McMaster-Carr, includes shaft seals, spring-loaded seals, rod seals, piston seals, and wiper seals. From a functional and construction point of view, the main types are:
| Type | Description | Typical application |
|---|---|---|
| Single lip seal (type A) | One sealing lip with spring and one secondary dust lip. | Transmissions, electric motors, bearings. |
| Double lip seal (type B) | Two sealing lips back-to-back with spring on the fluid retention side. | Low viscosity fluids, media separation. |
| Double lip seal with double spring | Each lip has its own spring; bidirectional sealing. | Shafts with reverse rotation or pressure on both sides. |
| Springless seal (V-profile) | Thick or short lip without spring, used as a wiper or at low speed. | Hydraulic cylinder rods, dirty environments. |
| PTFE seal (cassette type) | PTFE lip with labyrinth design, high speed and chemical resistance. | High speed (up to 25 m/s), aggressive fluids, food industry. |
Materials
Section titled “Materials”The materials of the sealing element determine chemical compatibility, temperature range, and wear resistance. Selection is made based on the fluid to be retained, the service temperature, and the surface speed.
| Material (abbreviation) | Common name | Temperature range | Main characteristics |
|---|---|---|---|
| NBR | Nitrile / Buna-N | −40 °C to +120 °C / −40 °F to +248 °F | Good resistance to mineral oils and greases. Most widely used. Low cost. |
| FKM | Fluoroelastomer / Viton® | −20 °C to +200 °C / −4 °F to +392 °F | Excellent chemical and high temperature resistance. Suitable for synthetic oils and fuels. |
| VMQ | Silicone | −60 °C to +180 °C / −76 °F to +356 °F | Excellent low temperature flexibility. Low mechanical strength, do not use in oils with EP additives. |
| PTFE | Polytetrafluoroethylene / Teflon® | −80 °C to +250 °C / −112 °F to +482 °F | Very low coefficient of friction, high speed, chemically inert. Used in cassette design. |
| ACM | Polyacrylate | −20 °C to +150 °C / −4 °F to +302 °F | Good resistance to engine and transmission oils with additives. Limited water resistance. |
The metal cases are normally carbon steel with anti-corrosion coating (phosphated or zinc plated). The springs are stainless steel or spring steel with surface protection.
Standard dimensions
Section titled “Standard dimensions”The dimensions of lip seals generally follow the standard series of ISO 6194 and DIN 3760. Below are the most common combinations for type A single lip seals with shaft diameter between 6 mm and 100 mm.
| Shaft diameter (mm / in) | Housing diameter (mm / in) | Nominal width (mm / in) |
|---|---|---|
| 6 mm / 0.236 in | 16 mm / 0.630 in | 7 mm / 0.276 in |
| 8 mm / 0.315 in | 18 mm / 0.709 in | 7 mm / 0.276 in |
| 10 mm / 0.394 in | 22 mm / 0.866 in | 8 mm / 0.315 in |
| 12 mm / 0.472 in | 24 mm / 0.945 in | 8 mm / 0.315 in |
| 15 mm / 0.591 in | 30 mm / 1.181 in | 10 mm / 0.394 in |
| 20 mm / 0.787 in | 35 mm / 1.378 in | 10 mm / 0.394 in |
| 25 mm / 0.984 in | 40 mm / 1.575 in | 10 mm / 0.394 in |
| 30 mm / 1.181 in | 47 mm / 1.850 in | 10 mm / 0.394 in |
| 35 mm / 1.378 in | 52 mm / 2.047 in | 12 mm / 0.472 in |
| 40 mm / 1.575 in | 62 mm / 2.441 in | 12 mm / 0.472 in |
| 50 mm / 1.969 in | 72 mm / 2.835 in | 12 mm / 0.472 in |
| 60 mm / 2.362 in | 85 mm / 3.346 in | 13 mm / 0.512 in |
| 70 mm / 2.756 in | 95 mm / 3.740 in | 13 mm / 0.512 in |
| 80 mm / 3.150 in | 110 mm / 4.331 in | 13 mm / 0.512 in |
| 90 mm / 3.543 in | 120 mm / 4.724 in | 13 mm / 0.512 in |
| 100 mm / 3.937 in | 130 mm / 5.118 in | 13 mm / 0.512 in |
The recommended tolerances for the housing are H7 (ISO) and for the shaft h8 or h9, depending on speed. An interference fit between the seal outer diameter and the housing ensures static sealing.
Operating capabilities
Section titled “Operating capabilities”Lip seals are limited by fluid pressure, surface speed, and temperature. Typical capabilities for single lip seals with metal case and NBR material are as follows:
| Parameter | Typical value |
|---|---|
| Maximum pressure without backup ring | ≤ 0.5 bar / ≤ 7 psi |
| Maximum pressure with rigid backup | ≤ 10 bar / ≤ 145 psi (consult manufacturer) |
| Maximum peripheral speed (NBR) | 15 m/s / 3 000 fpm |
| Maximum peripheral speed (FKM / PTFE) | 25 m/s / 5 000 fpm |
| Continuous operating temperature (NBR) | −40 °C to +120 °C / −40 °F to +248 °F |
| Continuous operating temperature (FKM) | −20 °C to +200 °C / −4 °F to +392 °F |
| Maximum allowable misalignment (STBM) | ≤ 0.3 mm / ≤ 0.012 in (shaft to housing) |
| Dynamic eccentricity (shaft) | ≤ 0.1 mm / ≤ 0.004 in (for normal speeds) |
These values are indicative for standard applications with oil lubrication and shafts with suitable surface finish. Severe conditions require detailed analysis.
Selection criteria
Section titled “Selection criteria”The selection of the appropriate seal is based on the combination of the following factors:
- Fluid to retain and contaminants to exclude: Determines the chemical compatibility of the lip material and its resistance to degradation.
- Service temperature: Defines the elastomeric material and its continuous operating range.
- Surface speed: Influences the material type and the need for lubrication systems or special lip profiles.
- Pressure: For pressures above 0.5 bar / 7 psi, a backup ring or specific design is required.
- Environment: Presence of dust, water, mud or other external elements that recommend the use of a secondary exclusion lip.
The following table shows an indicative selection summary according to the most common application.
| Application | Fluid / medium | Recommended material | Max. speed | Additional characteristic |
|---|---|---|---|---|
| Gearbox, electric motor | Mineral oil | NBR | 15 m/s / 3 000 fpm | Single lip with spring |
| Automatic transmission | ATF, high temperature | ACM or FKM | 18 m/s / 3 540 fpm | Good resistance to additives |
| Hydraulic pump | Hydraulic oil | NBR or FKM | 10 m/s / 2 000 fpm | Often with backup ring |
| Food / pharmaceutical | CIP products, hot water | PTFE or VMQ | 25 m/s / 5 000 fpm | Sanitary design, contactless in some cases |
| Shafts with external dirt | Oil + dirt/water | NBR with dust lip | 10 m/s / 2 000 fpm | Double lip or wiper profile |
| High temperature (>150 °C) | Synthetic oil | FKM | 20 m/s / 3 940 fpm | Thermal stability against hot oils |
Mounting considerations
Section titled “Mounting considerations”Correct mounting is essential to avoid premature failures. The main recommended practices include:
- Use a mandrel or concentric mounting tool that seats the seal uniformly in the housing, applying pressure only on the metal case.
- Ensure the shaft has an entry chamfer (15°–30°) with rounded edges and free of burrs; the shaft surface hardness should be ≥ 30 HRC to reduce wear.
- Lightly lubricate the sealing lip with the same fluid it will retain or with compatible grease before installation.
- The shaft surface roughness should be maintained between 0.2 µm Ra and 0.4 µm Ra (8–16 µin Ra). Finer finishes do not retain lubricant and rougher ones accelerate wear.
- Control alignment: static misalignment (STBM) between housing and shaft should not exceed 0.3 mm / 0.012 in; dynamic eccentricity of the shaft should be less than 0.1 mm / 0.004 in.
- Verify that the housing meets H7 tolerance and that the inner diameter is free of scratches. The surface should have a finish Ra ≤ 1.6 µm (63 µin).
Selection table by application
Section titled “Selection table by application”Below is a compact direct selection table for the most demanded seal configurations, considering moderate operating conditions with mineral oil.
| Shaft diameter (mm) | Lip type | Material | Standard part number | Max. speed (m/s / fpm) | Max. pressure without support (bar / psi) |
|---|---|---|---|---|---|
| 10–25 mm / 0.39–0.98 in | Single with spring | NBR | A 1x… | 12 m/s / 2 360 fpm | 0.5 bar / 7 psi |
| 25–50 mm / 0.98–1.97 in | Single with spring and dust lip | NBR | A 2x… AD | 15 m/s / 3 000 fpm | 0.5 bar / 7 psi |
| 50–80 mm / 1.97–3.15 in | Single with spring | FKM | A 1x… V | 20 m/s / 3 940 fpm | 0.5 bar / 7 psi |
| 80–100 mm / 3.15–3.94 in | Double lip with double spring | NBR | B 2x… | 10 m/s / 2 000 fpm | 1.0 bar / 14.5 psi |
| all diameters | PTFE cassette | PTFE | CS… | 25 m/s / 5 000 fpm | 0.5 bar / 7 psi |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum peripheral speed of a standard lip seal?
Section titled “What is the maximum peripheral speed of a standard lip seal?”The maximum speed for a standard nitrile (NBR) seal is 15 m/s / 3 000 fpm, while PTFE seals can reach 25 m/s / 5 000 fpm under controlled conditions.
What pressure can an unreinforced lip seal withstand?
Section titled “What pressure can an unreinforced lip seal withstand?”The maximum allowable pressure without backup ring or external support is 0.5 bar / 7 psi for a conventional single lip design.
What is the temperature range of a fluoroelastomer seal?
Section titled “What is the temperature range of a fluoroelastomer seal?”FKM operates continuously from −20 °C / −4 °F to 200 °C / 392 °F, with short peaks up to 220 °C / 428 °F.
What surface roughness should the shaft have for a lip seal?
Section titled “What surface roughness should the shaft have for a lip seal?”The optimal Ra roughness of the shaft is between 0.2 µm / 8 µin and 0.4 µm / 16 µin, measured according to ISO 4287.
What housing tolerance is recommended for mounting a seal?
Section titled “What housing tolerance is recommended for mounting a seal?”The housing should be machined with H7 tolerance, which for a diameter of 52 mm / 2.047 in corresponds to a range of +0.030 mm / +0.0012 in to 0 mm / 0 in.
What minimum hardness should the shaft have to avoid wear?
Section titled “What minimum hardness should the shaft have to avoid wear?”The shaft surface should have a minimum hardness of 30 HRC (approximately 280 HV) to ensure adequate service life of the sealing lip.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/metric-threads-d_777.html
- mcmaster.com: https://www.mcmaster.com/products/lip-seals/