Basic Knowledge [Sealing Business]

O-Ring Sealing Principles

When the O-ring is fitted into the groove, the O-ring is squeezed since the depth of the groove is less than the thickness of the O-ring (thickness of the O-ring minus the depth of the groove). This amount of compression is called the squeeze margin. At minute pressure, as shown in Figure 1, the sealing action is performed by the rubber elasticity of the O-ring itself. When hydraulic pressure is applied from one side, a reaction force higher than the hydraulic load pressure is generated as shown in Figure 2. This is called the self-sealing action of the O-ring.

Figure 1. Reaction force at installation
Figure 2. Reaction force at hydraulic load

O-Ring Features

Because of the volume of the mounting section is small, only a small space is required.
Simple structure and easy installation and removal
Many material types available that can be used over a wide temperature range (-60°C to +220°C)
Excellent sealing function even when the pressure alternates between both sides
Reasonably priced compared to other packing types
Abundant stock and short delivery time

O-Ring Groove Design

Squeeze margin

When using O-rings in dynamic applications, the squeeze margin must be kept to a minimum because it greatly affects the sealing effect, sliding resistance, compression set, and other characteristics. For dynamic O-rings used at especially high speeds or with poor lubricating conditions, keep the squeeze margin smaller than that of regular dynamic O-rings. For static applications, there are two usage methods: Static cylindrical surface use, which compresses in the radial direction as shown in Figure 3; and static flat surface (flange) use, which compresses in the direction perpendicular to the radial direction as shown in Figure 4. For static cylindrical surface use, the same kind of squeeze margin as dynamic O-rings is generally provided. For static flat surface use, the decrease in squeeze margin caused by the elongation of the bolt and distortion of the flange must be considered and the squeeze margin must be increased compared to that of dynamic and static cylindrical surface use. Figure 5 shows the squeeze margin of O-ring by application.

Figure 3. For static cylindrical surface use
Figure 4. For static flat surface use

Figure 5. O-ring squeeze margin by application

Gap

Under low pressure conditions, the O-ring is hermetically sealed by the squeeze margin. It deforms as the pressure increases, using the resulting tightening force to create the seal. Under high pressure, the O-ring starts to extrude from the gap between the piston and cylinder, and from between the rod and sleeve as shown in Figure 6-3. This can lead to damage, so it is important to keep this gap as small as possible. An effective way to prevent extrusion is to use a backup ring.

Figure 6-1.
Figure 6-2.
Figure 6-3.

Using backup rings

When high pressure is applied to the O-ring, it will extrude from the gap, but this can be prevented with a backup ring. If pressure is applied from both sides, install a backup ring on both sides of the O-ring. If pressure is applied from only one direction, install a backup ring on the opposite side of the pressure as shown in Figure 7-1. If there is extra space, installing two pieces (as shown in Figure 7-2) is recommended even when pressure is applied from only one direction. Various synthetic resins are used as materials for the backup rings, but Teflon (polytetrafluoroethylene resin) is generally used for most products. There are three types of backup ring shapes: endless, bias cut, and spiral. Endless backup rings are the most effective during use, while bias cut and spiral types are more convenient in terms of mounting.

Figure 7-1.
Figure 7-2.

Oil pressure, O-ring rubber hardness, and extrusion causing play limit

O-Ring Handling and Storage Precautions

O-ring handling precautions

When installing the O-ring, use a jig like the one shown in Figure 8 to prevent scratches caused by sharp edges, screw threads, etc.
- Figure 8.O-ring handling precautions
When inserting the piston into the cylinder, the edges of the small holes may scratch the O-ring as shown in Figure 9.
Design as shown in Figure 10.
- Figure 9.
- Figure 10.
When installing the O-ring, make sure it does not get twisted.
When installing the O-ring, apply sufficient grease or sealing liquid to the O-ring’s groove.
As a rule, do not reuse an O-ring.
Once the O-ring is installed and when washing with a washing solution, make sure that the washing solution is compatible with the O-ring material.
Do not incinerate O-rings, as some materials generate harmful gases by combustion.

O-ring storage precautions

Avoid direct sunlight and keep in a place with low humidity.
Do not open until immediately before use.
Keep in a place where the temperature does not become high and there is little air circulation.
Do not exert load on the O-ring.
Do no hang, as partial deformation will occur when doing so.
Store in a place with little dust and make sure no oil adheres.
Check the label affixed to the outer side of the polyethylene bag containing the O-ring when receiving a delivery.
Also, record the lot number as it is important for maintaining traceability.

Examples of O-Ring Failures and Countermeasures

Name	Phenomenon	Cause	Countermeasures
Swelling	The rubber softens and the entire O-ring enlarges.	The operating liquid penetrates the rubber material of the O-ring.	Change to a suitable rubber material.
Extraction	The rubber hardens and the entire O-ring shrinks.	The softening agent contained in the rubber material of the O-ring is extracted by the operating liquid.	Change to a suitable rubber material.
Permanent deformation	The O-ring is squeezed and does not return to its original shape.	1.The usage temperature is too high.	1.Change to a rubber material with good heat resistance.
Permanent deformation		2.Squeeze margin is too large.	2.Review the groove dimension or the O-ring size.
Extrusion	Partial tearing or around the entire circumference of the O-ring	The O-ring extrudes into the clearance between the groove and the opposite surface, tearing at the edge of the groove.	1.Reduce the clearance with the groove.
Extrusion			2.Use a very hard O-ring or use it together with a backup ring.
Ozone cracks	There are cracks perpendicular to the direction of the stress.	1. A rubber material with poor ozone resistance was used in a strong ozone atmosphere.	1.Change to a rubber material with good ozone resistance.
		2.The O-ring was used stretched unnecessarily.	2.Change the groove dimension or O-ring size.
		3.Insufficient amount of grease applied.	3.Apply sufficient grease when installing.
Twisting	O-ring is twisted and deformed.	1.The depth and width of the grooves are not equal.	1. Make the depth and width of the grooves equal.
		2.Inappropriate finish for the inner surface and the bottom groove surface of the cylinder.	2.Change the finish of the inner surface of the cylinder to 1.5S and the groove bottom surface to 3S.
		3.There is an eccentric motion.	3.Eliminate the eccentric motion.
		4.The O-ring was installed twisted.	4.Use grease to prevent twisting during installation.
Nibbling	Small bits are taken off the O-ring in several places	1.Bits were torn off by the edges of the small hole of the cylinder.	1.Make a design change, such as chamfering the small holes.
Nibbling	Small bits are taken off the O-ring in several places	2.Bits were torn off by sharp edges and screw threads during installation.	2Use a jig when installing.
Abrasion	The O-ring shows wear in the direction of the motion.	1.The surface finish of the inner surface of the cylinder is too rough.	1.Change the surface finish of the cylinder’s inner surface to 1.5S.
		2.Poor lubrication	2.Lubricate better.
		3.Squeeze margin is too large.	3.Change to a thicker O-ring to provide appropriate squeeze margin.
		4.Contamination such as dust, metal objects, etc.	4.Use a filter to prevent foreign objects from entering.
Hardening	The O-ring becomes hard and cracks when bent.	The operating temperature exceeds the heat resistant temperature of the rubber material of the O-ring.	1.Change to a rubber material with good heat resistance.
Hardening	The O-ring becomes hard and cracks when bent.		2.Lower the temperature.

Relationship between the Squeeze Margin and the Squeezing Force of O-Ring

Hardness (Shore A)50

Hardness (Shore A)60

Hardness (Shore A)70

Hardness (Shore A)80

Hardness (Shore A)90