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Preload of Seals
Definition and Distinction
Preload of seals refers to a deliberate deformation through installation that creates an initial contact force. This contact force leads to contact pressure between seal and sealing surfaces — that is, to a surface pressure in the contact zone. Decisive is that the seal can thereby seal even at operating pressure — for example at start, during pressure fluctuations, or under vacuum.
How is preload described? In practice, different parameters are used depending on the seal type: displacement (installation interference or installation deformation), deformation (e.g., squeeze in percent), or force (installation force/bolt force with flat seals). These perspectives describe the same principle: a defined installation state generates a defined initial loading.
Preload is frequently considered together with pressure activation. Pressure activation means that operating pressure additionally presses the seal. This effect only sets in during operation and increases the contact pressure where the pressure acts. In many sealing concepts, pressure activation acts as a supplement to installation preload, yet it does not reliably replace it, because tightness is often already required before pressure build-up.
| Term | When does it act? | What does it create? | What is it for? |
|---|---|---|---|
| Preload (pre-compression) | During installation, pressure | Initial contact pressure | Tightness without operating pressure, tolerance compensation |
| Pressure activation | In operation under applied pressure | Additional contact force | Sealing reserve under pressure, adaptation to pressure directions |
How Preload Arises with O-Rings and Is Described (Squeeze)
With O-rings, preload is usually specified as squeeze. Squeeze is the percentage reduction of the cross-section diameter through installation in the groove. An O-ring is therefore deliberately installed too large for the groove, so that it deforms elastically and loads the sealing surfaces from the start.
What concretely creates this squeeze? The most important geometric lever is groove depth (gland depth). It defines how strongly the O-ring is compressed between the components. Groove width, edge radii, surfaces, and component clearance (gap) also influence the installation state. In design, what therefore counts is not only the nominal dimension but the actually achieved squeeze under consideration of tolerances.
A compact reference for classification:
| Parameter | Meaning in practice | Influence on preload |
|---|---|---|
| Cross-section diameter | Cross-section of the O-ring | Reference for squeeze in |
| Groove depth | Distance that remains | Determines squeeze most strongly |
| Groove width | Lateral space for deformation | Influences fill ratio, friction, installation |
| Radii/chamfers | Edge shape in the groove | Reduces notch effect, improves installation |
| Gap/component clearance | Gap between parts | Critical under pressure (extrusion), influences contact |
Static vs. Dynamic: Typical Target Ranges
How much squeeze is sensible? With static applications (no relative motion at the sealing point), a higher squeeze is frequently chosen, because friction and wear are less critical. With dynamic applications (sliding or oscillating motion), squeeze is often reduced to limit friction, heat generation, and wear.
As rough orientation, the following ranges are frequently named:
- static: about squeeze
- dynamic: about squeeze
Why are these only guideline values? Because medium, temperature, surface roughness, lubrication, tolerance situation, and elastomer material strongly influence the effective contact pressure and wear.
Why Preload Is Necessary and What Happens with Faulty Design
Preload is necessary because tightness is almost always required before pressure build-up. Without sufficient contact pressure, micro gaps arise along the sealing surface through which liquids or gases can pass even at small pressure differentials. Preload also acts as tolerance compensation: it bridges dimensional deviations, settling, and small deformations of the components, as long as enough elastic reserve is available.
What happens with too low preload? Then contact pressure in the installation state is low. Leakage frequently occurs first at low pressure, under vibrations, or during temperature cycles, because the sealing point then opens relatively easily. In vacuum applications, this also shows early, because even the smallest leak paths are relevant.
What happens with too high preload? Then friction and installation forces rise noticeably. In dynamic sealing points, this frequently leads to increased wear and to temperature rise through friction work. In static applications, too, excessive compression can load the seal more strongly, which promotes permanent deformation (setting) and reduces the long-term sealing reserve.
| Design error | Typical consequence | Why it happens |
|---|---|---|
| Preload too low | Leakage at small pressures, start-up leakage | Contact pressure is not sufficient |
| Preload too high | High friction, installation problems, rapid wear | Excessive surface pressure and material loading |
Influencing Factors for the Maintenance of Preload: Material, Temperature, Time, Pressure, and Gap
Whether preload is maintained over service life depends strongly on material and operating conditions. With elastomers, primarily hardness (often in Shore A) and recovery behavior determine how much force builds up at a given deformation and how well the seal returns to its original shape after unloading. A higher hardness frequently increases the resistance to deformation toward the gap but also requires higher installation forces and can, with unfavorable design, reduce the adaptation to roughness.
Temperature and time act as main drivers for aging and setting behavior. With rising temperature, relaxation and aging processes proceed faster. As a result, recovery force drops, and the effective contact pressure can decrease over time even when the geometric installation position remains unchanged.
Pressure brings additional loading into the sealing point. On the one hand, sealing effect frequently rises through pressure activation. On the other hand, mechanical loading rises, particularly toward an existing extrusion gap (gap between components). With an unfavorable combination of pressure, gap, and material, material can be pressed into the gap, which leads to extrusion and damage. In such cases, back-up rings are frequently used to close the gap and support the seal. For O-ring dimensions and basic size assignment, ISO 3601 is often used as orientation in practice, although the specific groove and operating design goes beyond it.
Compression Set as a Test Criterion
A central test criterion for preload loss is compression set. It describes how much permanent shape change remains after prolonged compression. A high compression set means that the seal recovers only little after unloading. As a result, the preload reserve drops, and the sealing point becomes more sensitive to tolerances, settling, and temperature cycles.
In practice, compression set is frequently determined according to ASTM D395, Method B. A specimen is held over a defined time at a defined temperature in constant deformation, and the permanent deformation is then assessed. The parameter does not replace design but makes materials well comparable.
Preload, Operating Pressure, and Extrusion Gap
In a stable sealing point, preload and operating pressure act together. Preload ensures the initial tightness. Operating pressure then frequently increases contact pressure but at the same time generates a driving force toward the gap. Whether extrusion occurs typically depends on three quantities: pressure level, gap dimension, and material stiffness (including hardness and temperature behavior). When one of these quantities is unfavorable, a back-up ring or a low-gap design can noticeably increase robustness.
In the end, the specific application always decides. With high requirements on service life, temperature range, or pressure cycling, a specialized design or material consultation can be sensible.
