PV Value

Definition and Significance of the PV Value

The PV value (also p·v value) is a tribological parameter used in sealing technology for the initial classification of loading and risk at a sliding contact point. Tribology refers to the study of friction, wear, and lubrication. The PV value therefore practically answers the question: how strongly is a sealing point loaded simultaneously by pressure and motion?

It is defined as the product of surface pressure p and sliding velocity v:

$$PV=p\cdot v$$

Here, p stands for the contact pressure on the effective sealing or sliding surface, and v stands for the relative velocity between the partners (e.g., shaft against sealing lip). In many applications, the PV value is used as a limit parameter to estimate when frictional heat, wear, and material damage become more likely — particularly with polymers (plastics), which can soften as temperature rises.

Important is the classification: the PV value is not a universal material parameter. It depends strongly on operating conditions — for example on lubrication, mating surface, temperature, and heat dissipation. Therefore, PV limits from catalogs are only reliable when the underlying conditions are comparable.

Units and Common Misconceptions

In practice, MPa·m/s and bar·m/s are mainly used. This frequently leads to errors when numerical values are passed on without units.

A PV value “of 2” is therefore not interpretable without a unit. For design, it also counts whether continuous operation, start-stop, or oscillating motion is meant, because lubrication state and temperature profile can differ strongly.

Physical Background: Why PV Correlates with Heat and Wear

The PV value correlates in many cases with frictional power per area, because two central drivers are considered together: high pressure increases contact loading, and high velocity increases the friction work introduced per unit time. As a result, frictional heat frequently rises — and with it the temperature at the sealing point.

As temperature rises, material properties and lubrication conditions change. With plastics, this can lead to accelerated wear, plastic deformation (permanent change in shape), or, in the extreme, to thermal failure such as softening or melting. With elastomers, hardness and recovery properties can decrease, which additionally weakens the sealing function.

Nevertheless, PV describes only part of reality. The actual temperature also depends on factors that are not included in the PV value — for example on the friction coefficient μ (measure of friction), on heat dissipation through metal parts and medium, on roughness and hardness of the mating surface, and on the lubrication state (hydrodynamic, mixed friction, dry). PV is therefore a robust rule of thumb for the operating window but does not replace a service-life model.

Calculation in Practice: Determining p and v at the Sealing Point

For the calculation, two quantities are needed that are often only approximately known in real sealing contacts. p is the mean surface pressure on the relevant contact zone, and v is the mean sliding velocity at this zone. In practice, the first question is usually where exactly the contact acts, because the real contact area at sealing lips and slide rings varies with deformation, temperature, and lubricating film.

As an approximation, the following applies:

• p can be estimated from contact force / effective area.
• v follows from the motion profile (rotating, linear, oscillating) and the mean relative velocity.

At many sealing points, p and v are not constant, because lubricating film, temperature, and contact mechanics change during operation. As a result, PV limits from test rigs cannot be transferred 1:1 to every application.

Differences by Seal Type (Face Seal vs. Linear)

Depending on the seal type, p is interpreted differently, and the determination of v is different in nature:

• Mechanical face seal (face contact): in practice, PV is frequently used as the pressure differential P across the seal times the mean circumferential velocity V at the face. This serves as an operating-window parameter, even though the local contact pressure in the friction zone is more complex.
• Piston and rod seal (linear): v is the translational velocity of the rod or piston. p refers to the effective pressure at the sealing edge or in the contact zone, which can fluctuate due to installation, system pressure, and material deformation.

Particularly with start-stop or oscillation, lubrication is often more critical than in steady-state operation, because the lubricating film cannot build up permanently. As a result, a numerically moderate PV value can still lead to high peak loads in practice.

Limit Values, Safety Factors, and Levers for PV Reduction

PV limit values are frequently stated as PV_max. Such values, however, apply only to the respective test conditions — for example defined mating materials, surface roughness, temperature, lubricant, and heat dissipation. In real systems, these conditions scatter, which is why PV limits are usually used conservatively.

In many cases, it is recommended to remain noticeably below the limit in operation — often with a reserve in the range of roughly 50–60 % of the stated PV_max. This reserve absorbs scatter, heating, media changes, and short-term deterioration of lubrication. It becomes critical particularly when dry running (missing lubricating film) can occur, because friction and temperature then rise very quickly.

To reduce PV loading or failure risk, several levers exist that can be combined:

When PV is used as a screening criterion, it quickly delivers a clear answer to the question of whether a concept is fundamentally plausible. For a reliable service-life or failure estimate, the real operating conditions should then be verified in a targeted way. For critical applications, specialized tribological consultation can be sensible.