O-rings are an important sealing element in mechanical engineering and are used in many industries. They provide excellent sealing against high pressure fluids and can maintain performance even in harsh working environments.
In this article we clarify the question of what an O-ring is, how O-ring grooves are constructed, what types there are and what materials they are made of. We will also cover some basic gasket calculations.
What are sealing rings?
O-rings are mechanical seals that guarantee a high-quality seal between two mating surfaces. This means they are responsible for blocking the flow of fluids from one side of the seal to the other. For example, consider a bomb. Its outlet contains liquids at high temperature and high pressure, which can easily escape into the low pressure space, reducing the pump's efficiency. Installing o-rings where fluid can leak prevents this, ensuring a strong seal.
Furthermore, O-rings are shaped like a ring, usually with a circular cross-section. They are usually made from a flexible elastomer, as flexibility is the most important material property for O-rings to work efficiently.
Sealing rings can seal statically and dynamically. In a static seal, both mating surfaces are stationary, while in a dynamic seal there is relative movement between them.
O-ring groove/surface working principle
The working principle of the O-ring groove is simple. By connecting the two mating surfaces, the O-ring is clamped between them, creating an airtight seal. To make this possible, a groove is machined into the surface of the inner part at the point where sealing is required. Engineers call this groove the O-ring groove (O-ring stuffing box).
The dimensions of the O-ring groove require slight stretching of the O-ring to obtain a stable fit of the O-ring into the groove, similar to a snap fit. Additionally, the diameter of the O-ring is intentionally kept slightly larger than the depth of the O-ring groove, causing it to protrude beyond the mating surface.
This extended part comes into contact with the outer mating surface during assembly. The mounting pressure compresses the O-ring so much that it partially fills the O-ring groove (O-ring stuffing box) and seals the gap between the two surfaces. This gap is another important design consideration. Engineers use the term “clearance gap” to describe it.
Types of sealing grooves/boxes for sealing rings
The seal ring groove (seal ring seal box) is the most important design feature for producing efficient seal rings. They are available in different shapes and sizes depending on the application. Here we present some of the most important types of grooves for sealing rings.
Mechanical seal flange/groove
Flange seal groove is the most common and simplest type of seal ring groove. The groove is rectangular and has no gap between the contact surfaces. There is therefore direct surface-to-surface contact, avoiding any problems associated with ring extrusion gaps.
Furthermore, the flange seal groove is a static seal ring groove design, suitable for immovable components. The following table shows the recommended O-ring groove dimensions for various sizes according to the AS568 standard.
Dovetail groove
The dovetail groove has sloped side walls, giving it the appearance of a dovetail, hence the name. The slope of the wall creates a groove with a trapezoidal cross section, the main purpose of which is to keep the O-ring in place during installation and maintenance. This design is suitable for static sealing. Additionally, the locking feature makes the dovetail groove ideal for applications that require regular disassembly and maintenance as it allows for less downtime and faster handling.
However, designers must be careful when choosing dovetail groove dimensions because a large rake angle can limit the space within the groove into which the O-ring can expand. In this case, lack of space can damage the O-ring and affect the quality of the seal. The following table summarizes the recommended dimensions of a dovetail o-ring groove (o-ring seal box) for various o-ring sizes.
| CS seal ring | Groove depth (L) | Squeeze (%) | Groove width (G) | Support radius (R) | Groove radius (R₁) |
| 0.07 | 0.053 – 0.055 | 23 | 0.057 – 0.061 | 0.005 | 0.015 |
| ±0.003 | |||||
| 0.103 | 0.081 – 0.083 | 21 | 0.083 – 0.087 | 0.01 | 0.015 |
| ±0.003 | |||||
| 0.139 | 0.111 – 0.113 | 20 | 0.113 – 0.117 | 0.01 | 0.031 |
| ±0.004 | |||||
| 0.21 | 0.171 – 0.173 | 18 | 0.171 – 0.175 | 0.015 | 0.031 |
| ±0.005 | |||||
| 0.275 | 0.231 – 0.234 | 16 | 0.231 – 0.235 | 0.015 | 0.062 |
| ±0.006 | |||||
| 0.375 | 0.315 – 0.319 | 16 | 0.315 – 0.319 | 0.02 | 0.093 |
| ±0.007 |
Half dovetail groove
The half dovetail groove is a special groove with only one inclined side wall. It represents a compromise between the design advantages of the flange sealing groove and the holding capacity of the dovetail groove. It therefore offers the advantages of both, but in a balanced way. For example, in applications where it operates in a vacuum, the O-ring is pulled to one side of the stuffing box. In this case, the flat side of the half dovetail groove is perfect for holding the O-ring while the angled side holds it in the groove.
As before, below you will find the table with recommended dimensions.
| CS seal ring | Groove depth (L) | Squeeze (%) | Groove width (G) | Support radius (R) | Groove radius (R₁) |
| 0.07 | 0.053 – 0.055 | 23 | 0.064 – 0.066 | 0.005 | 0.015 |
| ±0.003 | |||||
| 0.103 | 0.083 – 0.085 | 19 | 0.095 – 0.097 | 0.01 | 0.015 |
| ±0.003 | |||||
| 0.139 | 0.113 – 0.115 | 18 | 0.124 – 0.128 | 0.01 | 0.031 |
| ±0.004 | |||||
| 0.21 | 0.173 – 0.176 | 17 | 0.190 – 0.193 | 0.015 | 0.031 |
| ±0.005 | |||||
| 0.275 | 0.234 – 0.238 | 15 | 0.255 – 0.257 | 0.015 | 0.062 |
| ±0.006 | |||||
| 0.375 | 0.319 – 0.323 | 14 | 0.350 – 0.358 | 0.02 | 0.093 |
| ±0.007 |
Triangular crushing groove
This design has a triangular cross section with the groove machined into one of the counterparts. It enables a simple and economical grooving process with sufficient sealing performance. The seal is static and also not reusable. Extreme and uneven compression can cause the O-ring to become permanently deformed. Designers' recommended O-ring groove dimensions are shown below for various O-ring cross-section sizes.
| Cross section (W) | ± | Groove depth (L) | ± (-0) |
| 0.070″ | ±0.003″ | 0.092″ | +0.003″ |
| 0.103″ | ±0.003″ | 0.136″ | +0.005″ |
| 0.139″ | ±0.004″ | 0.184″ | +0.007″ |
| 0.210″ | ±0.005″ | 0.277″ | +0.010″ |
| 0.275″ | ±0.006″ | 0.363″ | +0.015″ |
| 1.50mm | ±0.08mm | 1.98mm | +0.08mm |
| 2.00mm | ±0.08mm | 2.64mm | +0.08mm |
| 2.50mm | ±0.08mm | 3.30mm | +0.13mm |
| 3.00mm | ±0.10mm | 3.96mm | +0.13mm |
| 4.00mm | ±0.13mm | 5.28mm | +0.18mm |
O-ring groove inner/outer diameter design considerations
The inside diameter (ID) and outside diameter (OD) are important dimensions of the seal ring groove because they determine the nature of the interaction between the seal ring and the mating surfaces. Additionally, they are also useful in controlling the compression and friction forces experienced by the O-ring.
Experts recommend choosing the inner diameter 1 to 5% smaller than the outer diameter of the O-ring groove (O-ring stuffing box). This allows for the slight expansion of the O-ring during installation that we mentioned at the beginning.
The outer diameter should ideally be slightly larger than the depth of the stuffing box. This allows proper seal ring compression to be achieved after installation. The goal is about 1% to 3% compression.
Sealing ring/stuffing box groove depth and width design considerations
The width and depth of the O-ring groove (O-ring seal box) are critical dimensions for O-ring performance as they determine its void volume and overall shape.
The depth should be less than the O-ring diameter by the manufacturer's recommended clearance. This is necessary because a shallow depth can overcompress the bearing, while a greater depth can cause extrusion problems. Extrusion refers to the event where the gap is so large that a significant portion of the gasket is sucked into it, causing unwanted stress and damaging the gasket.
The gap width is also important for the O-ring groove dimensions. A particularly wide groove increases costs and can accumulate fluid, which can cause additional pressure on the sealing surface. A small width has the same effect as a small height: overcompression.
Additionally, both the width and depth of the O-ring groove determine the overall filling of the O-ring assembly housing. Filling is the proportion of the groove cross-section that the O-ring occupies after assembly. The dimensions of the O-ring groove must be determined so that the filling is no more than 85%.
Seal ring cross-sectional area
The cross-sectional area of O-rings depends on their width (diameter of their cross-section). O-rings with a smaller cross-section are generally cheaper to produce and lighter, but are more likely to have a high compression set. This means they deform more in the groove.
Although a larger cross-section is bulkier, it has less compression deformation and is more robust. However, surface friction is high due to a larger contact area with the groove walls.
O-Ring Compression Assembly (Tightening)
Compression (crush) assembly is a critical design consideration in O-ring groove design. Geometrically, this is the difference between the width of the O-ring cross section and the height of the stuffing box. In other words, it is the length that the assembly compresses the O-ring.
If the O-ring groove is too large, there is a risk of excessive compression of the O-ring, which can result in permanent deformation or even an inadequate seal. On the other hand, lower compression can also be problematic as it may not produce enough surface traction to ensure a seal.
O-ring groove materials
Material selection in O-ring design is another important decision. As O-rings largely depend on their flexibility, the catalog of possible material options is limited to elastomers. However, there are many elastomers to choose from and designers must have knowledge about them to make informed decisions.
Nitrile rubber
Nitrile rubber is the most commonly used material for o-rings. It is available in different qualities that differ in the composition of acrylonitrile. It is very resistant to abrasion, has high mechanical resistance and good resistance to permeability to gases and compounds such as mineral oils and fats.
Furthermore, it is suitable for a wide temperature range of up to 130°C.
Silicone rubber
Silicone is another popular material for making o-rings. It is resistant to various harmful environments, such as ozone, UV radiation, oils and hot air. Furthermore, it is very temperature resistant and maintains its mechanical properties, including flexibility, up to 230°C.
However, the disadvantages include low strength and wear resistance. Therefore, it is only suitable for static sealing applications.
thermoplastic polyurethane
Polyurethane is ideal for extreme working conditions, such as high pressure sealing. It is characterized by high toughness, abrasion resistance and thermal stability.
The disadvantage is that it can lose its properties in environments with acids, chlorinated hydrocarbons and humidity.
Fluorelastomer
Fluoroelastomers are a good choice for o-rings because of their chemical inactivity against most potentially harmful compounds. Additionally, fluoroelastomers have a smooth surface with self-lubricating properties, making them perfect for applications where a lubrication system is difficult to set up.
However, they tend to mechanically drag out over time.
Other materials for sealing rings
Due to the length of this article, it is impossible to cover the full range of o-ring materials. Therefore, below is a comprehensive list of other important materials:
- Chlorprene
- Ethylene-propylene
- Perfluoroelastomer
- Polyacrylate Rubber
- Hydrogenated nitrile rubber
- PTFE (Teflon)
- LOOK
Advantages of O-ring groove
The O-ring groove (O-ring stuffing box) offers numerous advantages over other sealing methods. Below we will briefly discuss these advantages.
Reliable seal
O-rings are, without a doubt, extremely reliable seals. With so many gasket groove types and materials, designers can choose the best combination for their gasket applications. O-rings rarely fail, have a long service life and can be used in harsh conditions, which is enough to classify them among high-quality sealing solutions.
Compact installation
The compact geometry of the O-ring groove design stands out among conventional sealing solutions. Seals require a large surface area and labyrinth seals require additional hardware and space, but O-rings take up minimal space in an assembly and are easy to install.
Light
A big advantage of O-rings is their low mass. They are small rubber rings that add little weight to the set, but still guarantee a good seal.
Variety of designs and materials
The huge variety of materials and versatility of the O-ring groove design have already been discussed above, but we would like to emphasize this point again as another big advantage. This variety is not typically available to designers and it is a luxury to have multiple options to choose from for such an important application.
Bonus: How to Calculate the Dimensions of an O-Ring Fitting
In the previous sections, the dimensions of the O-ring groove (O-ring stuffing box) were explained in detail. In this section we will go into detail about these points and provide some tips on how to calculate the most important dimensions of the O-ring groove, namely the height and width of the stuffing box.
Sealing ring groove height
The most important parameters that determine the height of the seal ring stuffing box are the desired compression ratio and the cross-sectional width of the seal ring. Since these two parameters are related to each other, the formula is derived from their common relationship.
Stuffing box height = cross-section width – (compression ratio – cross-section width)
The third input is the compression ratio, which is a design decision. Ideally, engineers try to design a package with a compression ratio of approximately 20%.
O-ring fitting width
The most important parameters affecting the width of the O-ring stuffing box are the cross-sectional area of the O-ring and the desired filling volume. We suggested in a previous section that the O-ring fill should be below 85%. With this in mind, engineers typically aim for a value between 50% and 85%, depending on the application, material selection, temperature conditions, etc.
The generalized formula for the width of an o-ring stuffing box:
Note that this is not the direct formula for O-ring stuffing box width, but rather a general guide for designers to familiarize themselves with the basic geometry of O-ring groove design. In reality, there are numerous correction factors for variables like thermal expansion that refine the calculations.
Concluding
O-rings undoubtedly have a fundamental value in machine elements and are the preferred sealing solution for most technical applications. This article has covered the O-ring groove (O-ring stuffing box) comprehensively, from its purpose to design and materials. To complement this, we also share some basic design tips and formulas.
Common questions
How do I choose a gasket size?
The size of the O-ring is determined based on the dimensions of the components to be sealed. The inner diameter needs to be slightly smaller than the shaft diameter to achieve any stretch, while the outer diameter should be approximately equal to the outer diameter of the O-ring.
What causes an O-ring failure?
There are numerous reasons why O-rings can fail. Physical damage in the form of overcompression, extrusions and general wear and tear can lead to failure. Additionally, if the O-ring material is exposed to chemicals with which it is incompatible, it may break and fail due to chemical reactions.
How do I install o-rings?
The O-ring can be gently and effortlessly pushed into its groove during installation. If it is too tight, some lubrication may be needed. It is also advisable to avoid sharp edges/corners and be careful when sliding the O-ring over threaded sections.
