MOS

Technical Information

Defects and Countermeasures

Oil seal

Causes of oil seal leakage

When oil seal leakage is detected, first identify the exact area of the leak. The leakage may not originate from the oil seal itself, and oil from other attached parts may be mistaken for oil seal leakage. Oil seal leakage is generally classified into leakage from the seal lip and leakage from the bore seat.

Leakage factors from the lip

01 Excessive lip wear
Insufficient lubrication, foreign matter intrusion, high internal pressure, rough shaft surface
02 Lip wear
Significant handling misalignment, tilted installation
03 Lip hardening
Abnormal high temperature, high internal pressure, insufficient lubrication
04 Lip softening
Inappropriate lip material
05 Lip damage
Improper assembly, mishan dling, poor shaft chamfering, foreign matter intrusion
06 Lip recess breakage
Improper assembly, excessive internal pressure
07 Lip inversion
Poor shaft chamfering, improper assembly, high internal pressure
08 Spring dislodgement
Poor shaft chamfering, Improper assembly
09 No defect in the oil seal
Shaft damage, shaft directiona lity, excessive shaft eccentricity, shaft wear, reverse installation
10 Oil seal deformation
Improper assembly

Leakage at the lip (inner circumference)

Factors		Failure mode	Causes	Countermeasures
Excessive lip wear	Lack of lubrication	Significant wear is evident at the lip edge, and the worn surface has lost its gloss.	When the lubricant level is below the specified amount, oil does not reach the lip, resulting in dry operation and abnormal wear. The structure near the oil seal is poor, preventing oil from reaching the lip.	Operate by supplying lubricant up to the specified amount.
	Lack of lubrication		Examples A slinger is located in front of the seal lip. A large drain is located in front of the seal lip. With splash lubrication, oil does not circulate at all for a few seconds during startup.	Examples As an emergency measure, change to a double lip type and apply grease between the lips. As a permanent countermeasure, change the structure near the oil seal so that oil flows to the lip.
	Foreign matter intrusion	Significant wear is present at the lip edge, characterized by scratches and indentations.	Using a shaft or oil seal with attached cutting debris causes the cutting powder to embed in the lip. Using an oil seal on a shaft with dust or particles causes them to embed in the lip. Using a liquid gasket attached to the lip or shaft causes it to catch at the lip edge. During equipment painting, if paint adheres to the lip or shaft, it causes chewing damage.	Assemble so that dust or particles do not adhere to the oil seal or shaft. When cleaning equipment, use the lubricant intended for use.
	High internal pressure	Significant wear is present at the lip edge, characterized by scratches and indentations.	The pressure at the oil seal exceeds the design value.	Change to a high-pressure oil seal. Design a breather to create a structure that does not apply pressure.
	Excessive shaft surface roughness	Significant wear is present at the lip edge, and the worn surface shows circumferential scratches.	Abnormal wear occurs due to using a shaft rougher than the specified surface roughness of Rmax 0.8 to 2.5 μm.	Correct the shaft surface roughness to Rmax 0.8 to 2.5 μm using emery paper (around #240 grit). Never move the emery paper in the shaft’s axial direction. Replace with a shaft that meets the specified roughness.
Uneven lip wear	Large handling eccentricity	The lip operating width is not uniform around the circumference, with the positions of the minimum and maximum widths almost opposite each other.	Operated with the shaft and housing misaligned during assembly.	Increase the concentricity accuracy between the shaft and housing.
	Large handling eccentricity		Operated with the shaft bent in one direction.	Increase the shaft strength against bending.
	Tilted installation	The lip operating width is uneven around the circumference, with the positions of the minimum and maximum widths nearly opposite each other. The relationship between the operating widths of the seal lip and dust lip is reversed.	The housing's inner diameter was finished smaller than the specified size, forcing the oil seal to be inserted at an angle.	Use housing that meets the specified standards.
			The housing chamfering was either not done properly or was insufficient, or the oil seal was forcibly inserted, causing the oil seal to be installed at an angle.	Perform or correct the housing chamfering to the proper size.
			The assembly jig was tilted, resulting in an angled installation.	Improve the assembly jig.
Lip hardening	Abnormally high temperature	The lip operating surface is smooth and glossy, but the entire lip has hardened, resulting in lip cracks.	The oil temperature near the seal lip rose due to some cause, exceeding the rubber's heat resistance limit.	Investigate the cause and prevent the temperature rise.
	Abnormally high temperature		The oil temperature exceeded the heat resistance limit due to conditions differing from those assumed during design.	Examples Nitrile rubber → Acrylic rubber (NBR → ACM) Acrylic rubber → Fluoro rubber (ACM → NBR) Be cautious when changing the lip material, as it affects oil resistance.
	High internal pressure	The pressure exceeded the oil seal's pressure resistance limit.	The pressure exceeded the oil seal's pressure resistance limit.	Replace with a high-pressure oil seal. Design a breather to prevent pressure buildup.
	Lack of lubrication	The lip contact area is smooth and glossy, with cracks occurring on the lip contact surface or forming when pressed with a finger. In many cases, only the contact surface becomes hardened.	The lubricant was used in an amount below the specified amount, resulting in insufficient oil flow to the lip and causing a lack of lubrication.	Supply lubricant to the specified level before operation.
	Lack of lubrication		Due to splash lubrication, only a small amount of oil reached the lip, resulting in a lack of lubrication.	As an emergency measure, switch to a dust lip type and apply grease between the lips. As a permanent solution, modify the structure near the oil seal to ensure sufficient oil reaches the lip.
Lip softening	Inappropriate lip material	The lip is swollen and softened.	The lip material was not properly selected for the lubricant, resulting in swelling of the lip.	Replace with an oil seal made of a lip materialthat does not swell in the lubricant. Alternatively, change to a lubricant that does not cause the lip material to swell.
Lip softening	Inappropriate lip material	The lip is swollen and softened.	Swelling also occurred when the seal was immersed in cleaning oil or gasoline or when cleaning fluid remained on the seal after washing and it was left unattended.	Do not clean the oil seal.
Lip damage	Improper assembly	The lip sliding surface is smooth and glossy, and the entire lip has hardened, showing cracks.	When the oil seal passed over a keyway or spline, it came into contact with a sharp edge, causing scratches.	Cover the keyway or spline with a cap to prevent scratches.
	Improper assembly		Scratches occurred because the oil seal was installed while burrs remained on the chamfered section of the shaft.	Remove burrs and other residues.
	Mishandling		During transport or storage, the lip came into contact with a sharp metal part, causing damage to the lip edge.	Improve transport and storage methods.
	Mishandling		Damage to the lip edge occurred due to handling the oil seal with gloves contaminated with metal shavings.	Do not touch the lip edge.
	Improper shaft chamfering	Visible scratches are present at the lip edge.	The chamfer dimension and angle at the shaft end were inaccurate, causing the lip to catch on the shaft end and become scratched.	Properly chamfer the shaft.
	Foreign matter	Foreign matter is attached to the lip edge. There are indentations on the lip sliding surface.	Metal shavings entered the lip edge when a shaft contaminated with cutting debris was used. Metal shavings became embedded in the lip edge due to the use of parts contaminated with metal powder. Foreign matter entered the lip edge from prolonged use of shafts or oil seals left in dusty environments.	Clean the equipment.
Lip reversal	Poor shaft chamfering	Part of the lip twists in the opposite direction when the oil seal is inserted onto the shaft.	The chamfer dimensions and angle at the shaft end were not appropriate, causing the lip to catch on the shaft end and flip over.	Assemble by properly adjusting the chamfer dimensions and angle of the shaft end and applying grease to the chamfered area.
	Improper assembly		The lip twisted due to careless assembly that failed to align the centers of the shaft and housing.	Remove burrs and carefully assemble the lip by aligning the centers of the shaft and housing holes. In this case, apply grease to the shaft end.
	High internal pressure	A portion or the entire circumference of the lip is turned outward (reversed).	During the operation, abnormally high pressure applied a large force to the lip area, causing the lip to twist.	Modify the structure so that pressure is not applied, and use a high-pressure oil seal.
Lip recess breakage	Improper assembly	There are cracks in the lip recess.	During assembly, the lip was deformed, causing cracks in the recess.	Assemble carefully by aligning the centers of the shaft and housing.
	Excessive internal pressure		After assembly, excessive pressure during the pressure test (airtightness test) caused cracks in the recess.	Do not perform tests at pressures exceeding the oil seal’s rated pressure.
	Excessive internal pressure		During the operation, high pressure exceeding the design expectation occurred, causing cracks in the recess.	Change to a high-pressure oil seal and modify the structure to prevent excessive pressure.
Spring dislodgement	Poor shaft chamfering	The spring is partially or completely dislodged.	The chamfer dimensions and angle at the shaft end are improper, causing the lip to catch on the shaft end and the spring to dislodge.	Assemble by properly adjusting the chamfer dimensions and angle of the shaft end and applying grease to the chamfered area.
Spring dislodgement	Improper assembly	The spring is partially or completely dislodged.	During assembly, careless assembly without aligning the centers of the shaft and housing holes caused the spring to dislodge.	Carefully assemble by aligning the centers of the shaft and housing holes. In this case, also apply grease to the shaft end.
Oil seal deformation	Improper assembly	The oil seal is deformed, causing a change in the lip operating width at the deformed area.	The oil seal assembly jig is inappropriate, causing deformation of the oil seal.	Improve the assembly jig.
No abnormalities in the oil seal	Shaft scratches		Visible scratches are present on the shaft's operating surface.	Insert a support into the oil seal to shift the operating position.Rework the damaged area.
	Shaft directionality		The shaft was used as-is after turning (lathe) machining.	Carefully finish only the shaft lip operating surface with emery paper (#240) to avoid scratches.
	Shaft directionality		Scratches occurred due to grinding or the use of emery paper during finishing machining.	Change the finishing method to one that doesnot cause scratches in the shaft direction.
	Shaft eccentricity		Shaft eccentricity increased beyond the design value due to bearing abnormalities.	Replace the bearing.
	Shaft eccentricity		A general-purpose oil seal was used despite significant shaft eccentricity in the mechanism.	Replace with a special oil seal designed for high eccentricity.
	Shaft wear		An oil seal contaminated with dust and cutting chips was installed. Oil deterioration and foreign matter intrusion caused external contaminants to enter the lip operating area.	Clean the equipment and insert a support into the oil seal during assembly to prevent damage to the operating area. If the dust amount is minor, use an oil seal with a dust lip or attach a dust cover.
	Shaft wear		A general-purpose oil seal was used despite significant shaft eccentricity in the mechanism.	Use appropriate shaft materials.
	Shaft handling in the opposite direction		Incorrect installation during assembly.	Install the seal lip with its direction facing the sealed side.

Leakage at the bore seat (outer circumference)

Factors	Failure mode	Causes	Countermeasures
Tilted installation	Before removing the oil seal the oil seal is tilted on the housing and shaft. After removing the oil seal the bore seat is found to be uneven	The housing bore was finished smaller than the specified dimension, and the oil seal was forcibly inserted, causing it to be installed at an angle.	Ensure that the housing bore inner diameter meets the specified dimension.
		The housing bore was either not chamfered or improperly chamfered, and the oil seal was forcibly inserted, resulting in a tilted installation.	Chamfer the housing bore or ensure it is made to the proper dimension.
		The assembly jig was tilted, leading to a misaligned installation.	Improve the assembly jig.
Deformation	Part of the outer diameter (OD) surface shows localized breakage.	The oil seal was deformed due to an inappropriate assembly jig.	Improve the assembly jig.
Deformation		An oil seal that had been partially deformed during handling was assembled, resulting in a gap at the outer diameter (OD) section.	Take care during handling to avoid dropping the oil seal or striking it against hard objects.
Galling or tearing of the outer diameter (OD) section	After removing the oil seal axial damage or tearing of the rubber is observed on the outer diameter (OD) section.	The inner diameter of the housing bore was finished smaller than the specified dimension, and the oil seal was forcibly inserted, causing it to be installed at an angle and resulting in damage to the outer diameter (OD) section.	Finish the housing bore to the specified inner diameter dimension.
		The housing bore was either not chamfered or improperly chamfered, and the oil seal was forcibly inserted, resulting in damage to the outer diameter (OD) of the oil seal.	Chamfer the housing bore or adjust it to the appropriate dimension.
		The oil seal was installed without proper parallel alignment between the oil seal assembly jig and the housing, causing damage to the outer diameter (OD) section of the oil seal.	Ensure parallel alignment between the oil seal assembly jig and the housing.
No abnormalities in the oil seal		Damage occurred on the housing bore inner surface due to the installation of the oil seal with foreign matter, such as cutting chips attached to the housing bore inner surface or oil seal bore seat. Damage occurred on the inner surface of the housing bore due to repeated assembly and disassembly of the oil seal. There is significant damage to the inner surface of the housing bore.	Apply a thin layer of liquid gasket to the inner surface of the housing bore to fill the damage. However, take care to avoid the liquid gasket attaching to the oil seal lip or the shaft.
		Scratches occurred on the housing bore inner surface due to installation without removing burrs from the housing bore surface treatment area.	Remove the oil seal and check for burrs on the chamfered part of the housing bore. If burrs are present, remove them with emery paper and apply a liquid gasket to the inner surface of the housing bore.
		The roughness of the housing bore inner surface is severe.	Emergency measure Apply a liquid gasket to the inner surface of the housing bore. Permanent measure Adjust the thickness of the housing bore inner surface to the appropriate thickness.

Examples of oil seal leakage and misidentification

01 Leakage at the equipment mating surface
Permanent deformation of the gasket, loose bolts, damage to assembled parts
02 Cracks in the equipment body or cover parts, casting leakage
03 Oil contamination on the mating surface of the housing or oil seal during assembly
04 Leakage of the initial lubricant for the oil seal

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