O-RING GROOVES | EXPLANATION SIMPLY EXPLAINED
What makes an optimum groove design?
The shape of the sealing element itself is determined by the inner diameter and cord thickness.
Although there are some factors in the operating conditions that can speak in favor of a larger or smaller cord thickness, the most important factor is the interaction of the O-ring with the installation space.
This refers to the grooves in which the O-ring is installed. The shape, depth and width of the groove are the most important parameters.
If the O-ring and groove are optimally matched, low wear, reliable compression and secure installation can be achieved. And these are key factors for a permanently durable seal.
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#1 What are O-ring grooves?
Grooves are the installation space for O-rings. Let’s think of a piston seal: Here, a piston slides over the running surface in the cylinder.
The lateral surface of the piston must be sealed against the cylinder running surface. This is done by the O-ring, which sits in a groove in the piston.
Temperature and pressure differences between the two areas in the cylinder must be maintained, as must the separation of different media. In addition to the piston seal, this also applies to other types of installation.
1. Flange gasket
A flange gasket is used for mostly stationary connections such as pipe joints or inspection openings and closures.
The compression therefore acts in an axial direction. The groove is arranged accordingly. The necessary pressure is created by screwing on a lid.
2. Piston seal
With this type of installation, the O-ring sits in a groove that extends over the outer diameter of the piston. On the one hand, the groove creates space for the sealing element and, on the other hand, ensures the sealing effect effect via the correct elastic compression between the contact surfaces.
In addition to the dimensional ratios of the inner and outer sealing surfaces, the depth of the groove and the thickness of the O-ring are also decisive factors.
3. Rod seal
As with the piston seal, the groove of the rod seal is designed so that the O-ring is pressed along the radius. However, the groove is not located in the inner part of the groove (piston seal), but in the outer part.
#2 These dimensions define the groove
The groove depth has the most direct effect on the compression of the O-ring. However, there are other parameters that can have just as much influence on the long-term durability of a seal.
Depending on the load profile of the individual operating conditions, they have a greater or lesser effect.
1. Groove depth
The distance between the inner and outer surface, groove depth and cord thickness of the O-ring interact to produce the compression of the sealing element. It must lie within a target range that depends on the sealing material.
The correct dimension for grouting also varies depending on whether it is a static or dyamic seal.
For static seals, the compression should be between 15 and 30 percent of the cross-section. In contrast, the maximum value in dynamic use is 20 percent, which is intended to limit frictional stress.
The absolute minimum is 6 percent. With a piston seal, the groove depth should be selected so that the O-ring is stretched approx. 1 to 6 percent over the groove base diameter.
With a rod seal, on the other hand, the O-ring must have an oversize of approx. 1 to 3 percent compared to the outer diameter of the installation space. This must also be taken into account when determining the groove depth.
2. Groove width
First and foremost, the width of the groove channel must be wide enough to accommodate an O-ring with the appropriate cord thickness.
Designers note that the O-ring deforms elliptically due to the compression and takes up more space in the groove. For pressurized applications a sufficient groove width should also ensure that the pressurized medium can enter the installation space.
This ensures an even pressure load on the O-ring. However, a groove that is too wide is particularly disadvantageous when pressure conditions change. As a result, the O-ring always moves to the edge of the groove facing away from the pressure and can suffer mechanical damage.
3. Groove radius
Edges that are touched by the O-ring during assembly and when installed must be rounded. Otherwise they pose a major risk of mechanical damage.
Especially when pressure is applied to the O-ring, it is pressed against the upper edge of the groove. If it is sharp, material can be sheared off very easily.
This behavior is part of the damage pattern of gap extrusion. It directly impairs the sealing effect because the lost material reduces the compression.
4. Sealing gap
Also primarily in connection with the operating pressure is the sealing gap is decisive. The distance between the two sealing surfaces defines this gap dimension.
As a general rule, the sealing gap should be smaller as the pressure increases. Depending on the tolerances, this can push the manufacturability of the components to their limits.
Back-up rings are therefore also used for particularly stressed seals. If the sealing gap is already reduced to the technically and economically feasible minimum, they provide additional safety.
They are made of harder material and prevent the O-ring from being pressed into the sealing gap.
#3 Three groove shapes determine the practice
The arrangement of the groove depends on the type of installation. There are additional design options for the shape of the groove.
1. Rectangular groove
Rectangular grooves are the standard case for installation spaces for O-rings. They are manufactured by turning or milling into metallic components. The simple geometric rectangular shape makes this a particularly economical option.
2. Trapezoidal groove
Due to the more complex production process, a trapezoidal groove is only used when the installation space must have special properties.
For example, the dovetail shape can hold the O-ring in position when the sealing surfaces are separated for maintenance. Important detail: The groove width is measured before the radii are attached to the edges.
Sufficient radii are particularly important to prevent damage to the O-ring. There is also a half trapezoidal groove version.
3. Triangular groove
The triangular groove is an option for flange and cover seals. Due to the shape of the groove, the O-ring presses against three contact surfaces when deformed.
One disadvantage of this groove shape is that it can result in varying compression. However, it is suitable for confined spaces.
#4 Many influencing factors: This is what matters when designing the groove
In many cases, the installation space can only be adapted to changing requirements with some additional effort. This makes it all the more important to consider as many influencing factors as possible when designing the groove.
1. Type of sealing
With the piston seal, the O-ring is stretched over the groove base diameter. The other way round with the rod seal: Here, the groove in the outer part should be made so that the O-ring has an oversize on the outer diameter.
Ensuring minimum compression and taking account of static or dynamic use: these are decisive criteria for calculating the groove. The ideal dimension for compression also depends on the O-ring material.
3. Groove filling
O-rings can swell in contact with certain media. The groove must provide sufficient space so that the O-ring does not swell out. This would increase the compression and thus increase friction.
Piston seal example: O-rings must be stretched during assembly in order to snap into the installation space. It is important that the maximum elongation of the O-ring is not exceeded. Split grooves can prevent excessive elongation and are particularly suitable for O-rings with low elasticity.
5. Insertion chamfers
Just as important as limiting the maximum elongation is protecting O-rings from contact with sharp workpiece edges. With piston and rod seals, insertion chamfers are used for this purpose. If they are long enough, the inner and outer parts can be positioned neatly in relation to each other during installation.
6. System pressure and sealing gap
A smaller sealing gap requires tighter manufacturing tolerances, but pays off under high system pressure. Designers should therefore take the pressure conditions into account when designing the installation space and provide additional support rings if necessary.
O-rings subjected to expansion contract under the effect of heat. This happens, for example, with a piston seal where the O-ring is stretched beyond the groove base diameter. This potentially increasing elongation should be taken into account when dimensioning the groove.
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