V-Packing

Definition and Basic Principle

A V-packing (also called a chevron packing or cup seal) is a V-shaped lip seal that belongs to the family of V-packings and chevron packings. It is used in hydraulic and pneumatic systems to seal a moving machine part against the escape of pressure medium (for example, hydraulic oil or compressed air). The cross-section resembles a small V — which is also why the German term “Dachmanschette” (literally “roof cuff”) describes the same shape.

The basic principle is pressure-activated. With rising system pressure, the two sealing lips of the V-profile are pushed radially outward. As a result, the contact force at the sealing surfaces increases. This effect is self-energizing, because pressure presses the seal more firmly against the mating surfaces and thereby reduces leakage further. In practice, V-packings often work as multi-lip seals, because several V-rings are combined into one set.

Rod Seal vs. Piston Seal

V-packings (or chevron packings) typically seal reciprocating motion — that is, back-and-forth movement as in a cylinder. In sealing technology, it is important to clarify early on which component is being sealed, because installation position and loading follow from that decision.

In both variants, the V-packing aims to minimize leakage under pressure without slowing the motion excessively.

Build-up of a V-Packing Set (Seal Kit)

In many applications, a chevron packing is installed not as a single part but as a seal kit. The reason is technical: several elements can be deliberately preloaded, better adapted to tolerances, and protected against gap extrusion.

A typical set consists of:

• Pressure ring: transfers the axial assembly force into the seal kit and creates a defined initial preload.
• V-rings (chevron rings): usually several rings (often 3–5) that perform the actual sealing.
• Back-up ring: stabilizes the set and improves extrusion resistance — that is, the resistance against the material being pressed out into gaps.

Multiple V-rings share the loading. As a result, service life can rise, and the seal becomes more tolerant of slight misalignment.

Preload, Pressure Activation, and Extrusion Protection

During installation, the seal kit is axially preloaded. This preload ensures that an initial sealing effect is present even at low or zero system pressure. During operation, pressure activation is added: incoming pressure presses the lips additionally outward, so that the sealing effect rises with the pressure.

The design becomes critical when gap dimensions between moving and stationary parts grow — for example, through tolerances, wear, or side loads. Sealing material can then be pressed into that gap (extrusion). The back-up ring reduces this risk by mechanically supporting the seal kit and stabilizing the edge against gap migration. Particularly with pressure peaks and demanding industrial applications, this combination is a key reason why chevron packings are frequently chosen.

Operating Conditions, Advantages, and Limits

V-packings are particularly useful when a system sees medium to high pressures and the installation situation calls for a robust seal. They are frequently used wherever tolerances, slight misalignment, or pressure peaks occur, and where an adjustable preload is helpful. A practical advantage is that they install well in axially open installation spaces, such as during cylinder cover assembly.

At the same time, the principle has limits. Multiple lips and higher contact pressure can raise friction. This can show up as stick-slip at low speeds — a jerky sliding caused by alternating static and kinetic friction. The environment also matters: in dirty applications, service life drops when particles reach the sealing edge and act abrasively. In such cases, an additional protective seal is often specified.

Materials and Selection Criteria (Brief Overview)

In sealing technology, material choice is driven primarily by medium, temperature, pressure, sliding conditions, and the risk of extrusion. V-packings are frequently elastomer-based, sometimes fabric-reinforced, to increase dimensional stability and extrusion resistance.

In the selection process, the first questions usually concern medium and temperature. Pressure level, gap dimension, and surface quality of the mating surfaces follow, because these factors strongly influence friction, leakage, and extrusion risk. For demanding applications, specialized design and consultation can be advisable.