Oil seals | General-purpose seal technology

Description

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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

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 B JASO F401 ISO SC SB For oil, no dust lip Type 1
Type 2 S SM TYPE1 TYPE2 TC TB For oil, with dust lip Type 4
Type 5 D DM TYPE3 TYPE4 VC VB For grease, no dust lip (G) (GM) G GM   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&#; Acrylic rubber (ACM) &#; Superior in heat resistance to nitrile rubber.
&#; Resistance to brake fluid and fuel oil is inferior. &#;130&#; Silicone rubber (VMQ) &#; Superior in heat resistance to acrylic rubber.
&#; Has best cold resistance.(-60&#;&#;)
&#; Inferior in resistance to alkali and water. &#;150&#; Fluororubber (FKM) &#; Has best heat resistance.
&#; Most widely suitable for lubricating oils and fuel oils. &#;180&#;

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 G SW
Dedicated material
JIS G SWP
&#;Piano wire&#; JIS G SUS
(Stainless steel wire) JIS G SPCC
(Cold-rolled steel sheets
and steel strips)
JIS G SPHC
(Hot-rolled steel sheets
and steel strips) JIS G SUS
&#;Cold-rolled stainless steel sheets)
JIS G SUS
&#;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.

What is an Oil Seal?

Oil seals, also referred to as shaft seals, are widely used to prevent the leakage of medium (such as oils and grease) along a rotating shaft. This leak prevention is primarily achieved by the sealing element which can be made from a wide range of materials that are chosen according to each application. They are commonly used in gearboxes, hydraulic cylinders, and related components.

Additional reading:
Is it normal for power steering fluid to leak?

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Purpose of an Oil Seal

An oil seal is designed to perform three major functions: to prevent lubricants from leaking outside the seal even under high pressure, to act as a barrier to retain the lubricating oil, and to prevent dirt and other contaminants from entering the unit.

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Construction of an Oil Seal

Oil seals normally consist of three basic components: the sealing element, the metal case, and garter spring.

1. Sealing Element

The sealing element makes up the interior of the oil seal, and the materials commonly used are:

• a. Nitrile Rubber (NBR) &#; this is the most commonly used material. It has good heat resistance properties and has good resistance to salt solutions, oils, hydraulic oils, and gasoline. Operating temperatures are recommended from -40 to 248&#; F (-40 to 120 deg.C). Nitrile also functions well in a dry environment, but only for intermittent periods. The disadvantage of this material is poor chemical resistance.
• b. Polyacrylate Rubber (PA) &#; also known as acrylic rubber, this material has better heat resistance than nitrile. PA is also recommended for a high surface speed environment. Operation temperatures are recommended from -4 to 302&#; F (-20 to 150&#; C). Polyacrylate rubber should not be used with water or in temperature below -4&#; F (20&#; C).
• c. Silicone Rubber (SI) &#; these compounds operate effectively in a broad temperature range of -58&#; F to 356&#; F (-50 to 180&#; C). Silicone rubber is a leading choice for its resistance to both low temperatures and heat. The high lubricant absorbency of the material minimizes friction and wear. These oil seals are usually used as crankshaft seals. Silicone has poor resistance to hydrolysis and should not be used in oxidized or hypoid oils.
• d. Fluorocarbon Rubber (FKM) &#; is widely known under the Chemours (formerly Dupont&#;) trade name of Viton® and offers the best resistance to chemicals and superior performance to high temperatures.

2. Metal Case

The metal case is the exterior (or frame) of the oil seal, the principal function of which is to give rigidity and strength to the seal. The material of the case must be selected depending on the environment where the seals are to be used. Often the metal case is covered by the same rubber material used in the sealing element, which also helps seal the exterior of the oil seal in the housing bore. Common case material types are:

• a. Carbon Steel &#; the most common material used in oil seals.
• b. Stainless Steel &#; for applications that require resistance to water, chemicals, or corrosion. (Stainless steel metal cases are also recommended for many FDA applications.)

3. Garter Spring

The garter spring is located at the end of the primary sealing lip and used to apply pressure to the sealing lip against the shaft. Common garter spring material types are:

• a. Carbon Steel &#; which is used in conjunction with regular lubricants.
• b. Stainless Steel &#; which is used when resistance to water, seawater, and chemicals are involved.

Oil Seals From Global O-Ring and Seal

Global O-Ring and Seal carries a full line of oil seals in all industry standard sizes and has the ability to create custom oil seals. If you are a distributor or user of oil seals who may have need for our services, contact us today to speak with one of our team members, request a no-obligation quote, or place an order from our extensive inventory at GlobalOring.com.

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Want to learn more? Read &#;Oil Seals 101, Part 2&#;.

If you are looking for more details, kindly visit Automotive Oil Seal.