O-ring Groove - Static - Triangular Groove

What is a triangular groove?

A triangular groove, also called a V-groove, has a triangular cross-section instead of the usual rectangular shape. The groove tapers toward the bottom. When the two parts are joined, the O-ring is pressed into the corner of the triangle and the contact takes on a special geometry. In practice, the triangular groove is used almost exclusively for face seals, such as lids on housings or flanges on drain openings. Its applicability is therefore narrower than that of the rectangular groove. The reason manufacturers still choose this groove shape is almost always structural: there is insufficient depth for a full rectangular groove, or the geometry of the part makes a rectangular groove difficult to mill. Never use the triangular groove by preference, only when the design truly requires it.

When is a triangular groove the right choice?

The triangular groove is a fallback solution with a clearly defined range of application. Outside that range, the rectangular groove is always the better choice: easier to mill, more tolerant of dimensional deviations and better able to withstand variations in assembly force. If you still choose the triangular groove, the tolerances must be respected. This is a design with little margin.

• Lids and flanges where insufficient depth is available for a rectangular groove.
• Designs in which the groove wall also has a load-bearing function and milling a rectangular groove is structurally undesirable.
• Applications where the groove must be added to an existing part that has already been partially machined.

The groove parameters for the triangular groove

The triangular groove has only three parameters: cross-section diameter (d2), groove width (b) and base radius (r3). There is no separate groove depth t in the table, because in a triangular groove the depth follows directly from the groove width and the groove angle. The tolerance on b is only positive and increases with the cross-section diameter. That is the only free element in the design: slightly wider is allowed, narrower is not.

d2: cross-section diameter

The cross-section diameter is also the starting point here. The triangular groove is available for cross-section diameters from 1.50 mm to 15.00 mm. The width b and the base radius r3 are derived directly from d2. Always use the ring for which the groove was designed: a different cross-section diameter completely changes the contact geometry and results in an unreliable seal.

b: groove width (tolerance +0.1 to +0.4 mm)

The groove width is the only parameter shown directly in the table for the triangular groove. The tolerance varies per cross-section diameter: +0.1 mm for small sizes (d2 up to 1.80 mm), increasing to +0.4 mm for larger sizes (d2 above 8.40 mm). Note: this tolerance is asymmetrical and only positive. A groove that is too narrow causes excessive deformation of the ring. A groove that is too wide results in insufficient contact pressure.

r3: base radius

The base radius r3 increases from 0.3 mm for small cross-section diameters to 3.0 mm at d2 = 15.00 mm. This is a significantly larger radius than in rectangular grooves. The reason is the contact geometry: in a triangular groove, the ring rests on the sloping flanks, and the bottom serves as a guide surface. A base that is too sharp creates point loading in the rubber, which leads to premature damage.

Critical points in the triangular groove

The triangular groove places higher demands on machining accuracy than the rectangular groove. The angle of the groove walls must be correct: a deviating angle creates an asymmetrical contact zone on the O-ring and therefore uneven sealing around the circumference. After machining, always check the angle of the groove flanks, the base radius and the width b. Burrs at the transition from the groove flank to the sealing surface are a direct risk to the ring during assembly.

Sealing surface finish: Ra max. 1.6 µm (Rz max. 6.3 µm). This applies to the flat sealing surface outside the groove.

Groove base: Ra max. 3.2 µm. The bottom of the V-groove may be slightly rougher.

Groove flanks: Ra max. 6.3 µm. The sloping flanks are the least critical in terms of roughness.

Three mistakes with the triangular groove

1. Incorrect groove angle milled. The tolerance on the V angle is tight. An incorrect angle causes asymmetrical contact on the ring, resulting in uneven sealing. Check it with a profile measuring instrument.

2. Groove made too narrow. The tolerance on b is only positive. Too narrow causes excessive deformation of the ring and damage during assembly. Measuring before assembly prevents this.

3. Triangular groove used where a rectangular groove was possible. The triangular groove is a fallback solution. Use it only if the design truly rules out a rectangular groove.

Material and temperature

The material choice for the triangular groove is the same as for other static applications: NBR for standard media, EPDM for water and steam, FKM for aggressive chemicals up to +200 °C. Keep in mind that the contact zone in a triangular groove is narrower than in a rectangular groove. At higher operating pressures, the contact pressure on the rubber is therefore higher, which makes the material requirements slightly more demanding.

Consult the chemical resistance guide for compatibility with your specific medium.