Dynamic Seals

Definition and Distinction: Dynamic vs. Static

Dynamic seals are sealing elements that seal even though at least one sealing surface moves relative to the other. This relative motion can be linear (back and forth), rotary (turning), or oscillating (small angles back and forth). The goal is almost always to achieve a technically allowable leakage while keeping friction, wear, and heat generation low. This trade-off shapes the design: stronger contact pressure usually reduces leakage but often raises friction and therefore wear.

Static seals, by contrast, work without relative motion at the sealing point. They typically seal through surface pressure in a stationary joint. Because no sliding motion is present, no continuous frictional heat develops there and no tribologically driven abrasion occurs (tribology = the science of friction, lubrication, and wear). Dynamic seals are therefore more sensitive in practice to assembly errors, surface quality, and changing operating conditions such as pressure, temperature, and speed.

Motion Types and Typical Applications

The motion type determines how lubrication builds up and how strong friction peaks can become. In sealing technology, a simple classification has become established:

Construction and Function in Hydraulic and Pneumatic Cylinders (Sealing System)

In cylinders, dynamic sealing points are rarely solved with a single component. A sealing system is the norm, because several requirements must be met simultaneously: separating pressure chambers, holding the medium inside the cylinder, repelling contamination, and controlling side loads. This matters because side loads and tilting can locally widen the sealing gap and overload the seal at edges.

A typical cylinder system consists of a rod seal, piston seal, wiper (or scraper), and guide ring. The rod seal seals between the piston rod and the cylinder head toward the outside. The piston seal separates the pressure chambers on either side of the piston. The wiper keeps contamination and moisture from outside, so that particles cannot enter the dynamic contact zone. Guide rings absorb side loads, guide piston or rod, and thereby reduce edge load and unfavorable gaps that would otherwise damage the seal.

Main Tasks: Leakage, Contamination, Lubrication

Dynamic seals must solve three tasks simultaneously, and these tasks influence one another. First, they limit leakage — although “absolutely tight” is often unrealistic or even disadvantageous in dynamic operation, because a minimal fluid film can contribute to lubrication. Second, wipers protect the sealing point against particles, because abrasive contamination quickly leads to score marks and increased leakage. Third, seals indirectly steer the lubricating-film management: a load-bearing oil or fluid film between the seal and the mating surface reduces friction and wear, but it can raise leakage if it becomes too thick or unstable.

Tribology: Friction, Lubricating Film, and Typical Dynamic Problems

Wear of dynamic seals arises primarily from the interplay of friction and lubrication. Under motion, a thin lubricating film can build up that partially separates the contact partners. Depending on speed, viscosity of the medium (viscosity = “thickness”), temperature, and elastic deformation of the sealing lip, a friction regime is established. In boundary and mixed friction, the surface portions touch more strongly, so friction and abrasion increase. Under stronger fluid friction, the lubricating film carries more load, which can lower friction but tightens the leakage requirement.

Friction generates heat. This frictional heat can age the sealing material, alter the hardness, or make the medium locally less viscous, which further shifts friction and leakage behavior. Dynamic seals therefore often respond sensitively to changes in speed and temperature, even when pressure and geometry remain unchanged.

Stick-Slip and Low Speeds

Stick-slip describes a jerky motion at very low speeds. The sealing point alternates between sticking (stick) and sliding (slip), because the lubricating film does not remain stably built up. During the stick phase, the friction force rises until it “releases” the motion, followed by a short slip impulse. As a result, axis control quality decreases and local friction peaks rise. In many applications, this accelerates wear, because the sealing edge repeatedly works under unfavorable boundary-friction conditions.

Design and Damage Mechanisms: Gap, Surface, Material, Failure Patterns

The design of dynamic seals is, in practice, dominated by a few but strongly acting variables. Pressure influences contact pressure and the risk of extrusion. Speed influences lubricating-film build-up and heat. Temperature changes material properties and the viscosity of the medium. The medium itself decides on chemical resistance and lubricity. Hardware factors such as gap dimension and mating surface act as “multipliers” here, because small deviations quickly lead to leakage or wear.

Common failure patterns are rising leakage, score marks on the mating surface, edge break-out, hardening, and compression set (permanent deformation that causes loss of recovery force). The causes often lie in a combination of poor gap management, unsuitable surface, assembly damage, or wrong material choice. For materials, sealing technology often uses elastomers such as NBR, HNBR, FKM, or EPDM, because they provide preload and conformability to tolerances. PTFE and PTFE compounds are used when very low friction or higher temperature and chemical resistance are required. Which combination is suitable depends on medium, temperature, pressure, and motion profile.

Extrusion (Pushing into the Gap) and Gap Management

Extrusion is the pushing of sealing material into the gap between two components under pressure. Under dynamic motion, the extruded material can additionally be dragged along. As a result, friction and heat rise, and pieces can break out, which ultimately leads to leakage.

Technical countermeasures follow a clear logic: the gap is kept small by design, tolerances and guidance are arranged so that tilting remains limited, and back-up rings are used where needed. In addition, a material with sufficient extrusion resistance is selected, which in practice often means higher hardness or suitable compounding.

Mating Surface in Rotary Applications: Roughness and Lead-Free Finish

For rotary seals, the mating surface of the shaft determines function and service life. Roughness must be “appropriate”, because surfaces that are too rough load the sealing edge abrasively, while surfaces that are too smooth can unfavorably influence lubricant supply and film formation. A second critical point is lead — a helical surface structure resulting from machining. Lead can produce a pumping effect that conveys medium along the shaft and encourages leakage. Therefore, a lead-free surface structure is targeted in the sealing zone.

When requirements are high or operating conditions vary strongly, specialized design and consultation pay off, because small hardware or material changes can have large effects on leakage, friction, and service life.