Oil seals


Structure of oil seals and workings of each part

The following figure and table show the structure of oil seals and workings of each part, respectively.

Structure of oil seals and workings of each part

Structure of oil seals and workings of each part

Name Workings of each part
Lip Lip tip The lip tip has a wedge-shaped cross section and the function of sealing the fluid as it is pressed against the shaft surface.
Seal lip The seal lip is made of a flexible elastomer, and designed to maintain stable sealing performance against vibration of machinery and pressure change of the sealed fluid. The spring maintains the force of the seal lip pressed against the shaft for a long time.
Dust lip The dust lip is a supplemental lip without spring, and has the function of preventing the entry of dust.
Fitting portion The fitting portion has the function of fixing the oil seal in the housing hole and preventing the leakage of the fluid from the contact surface between the outer circumferential surface of the oil seal and the inner circumferential surface of the housing or the entry of dust. The metallic ring has the function of securing the fitting force to fix the oil seal on the housing.

Main types, characteristics and related standards of oil seals

Shape Type Applications/characteristics JIS B2402 JASO F401 ISO 6194
S type SC SB For oil, no dust lip Type 1
Type 2
T type TC TB For oil, with dust lip Type 4
Type 5
V type VC VB For grease, no dust lip (G) (GM) G GM  
K type KC KB For grease, with dust lip   P PM  

Materials used for oil seals

The following table shows main rubber materials, springs and metallic ring materials used for oil seals.

Main rubber material

Main material Characteristics of rubber material Operating temperature
(heat resistance)
Material cost
Nitrile rubber (NBR) ・ Material costs are low.
・ Offers versatility and most commonly used.
・ The operating temperature (heat resistance) is limited.
~100℃ Material cost
Acrylic rubber (ACM) ・ Superior in heat resistance to nitrile rubber.
・ Resistance to brake fluid and fuel oil is inferior.
Silicone rubber (VMQ) ・ Superior in heat resistance to acrylic rubber.
・ Has best cold resistance.(-60℃~)
・ Inferior in resistance to alkali and water.
Fluororubber (FKM) ・ Has best heat resistance.
・ Most widely suitable for lubricating oils and fuel oils.

Spring and metallic ring material

Fluid to be sealed\
Spring and metallic ring material
Spring Metallic ring
Standard material Dedicated material Standard material Dedicated material
JIS G3521 SW
Dedicated material
(Piano wire)
(Stainless steel wire)
(Cold-rolled steel sheets
and steel strips)
(Hot-rolled steel sheets
and steel strips)
(Cold-rolled stainless steel sheets)
(Cold-rolled stainless steel strips)
304 316 304 316
Lubricating oil/grease
Water × ×
Vapor × ×
Seawater × × × ×
Acid × × × ×
Alkali × ×

○: Usable ×: Not usable

Method of selecting oil seals

To make full use of the function of oil seals, selecting the most suitable shape and material for the use conditions is necessary. Generally, the following steps are followed:

  • Select the shape.
  • Select the lip material.
  • Select the metallic material.
  • Select the dimensions.

Since NOK offers a wide range of lineup that meets various requirements, please see our catalogs or contact the sales office nearest you.

Design of oil seal mounted portion

Since the shaft, housing shape, roughness, etc. influence the performance of oil seals, key points for general shaft and housing design are described below.

Key points for shaft and housing design

Mounted portion Design key points and recommended values Remarks (precautions)
Shaft Material Use of carbon steel pipes for mechanical structures is recommended. Use of cast iron should be avoided as much as possible since pinholes may be generated.
Hardness 30 HRC min.  
Roughness and machining method ・ 0.32—0.1 μmRa and 2.5—0.8 μmRz are recommended.
・ Finish with a grinder or emery paper is recommended.
If shaft machining marks have orientation, leakage may be caused.
Chamfering Chamfer the shaft ends.  
Housing Material Steel or cast iron is recommended. Use of light metal or resign should be avoided as much as possible since they are thermally expanded significantly.
Inner circumferential surface roughness Outer metal: 3.2—0.4 μmRa, 12.5—1.6 μmRz Outer rubber: 3.2—1.6 μmRa, 12.5—6.3 μmRz When the inner circumferential surface roughness is large, a gap is created between the contact faces, causing leakage.