Rotary Seal

Definition and Purpose

A rotary seal is a dynamic seal that seals the gap between a rotating component (usually a shaft) and a stationary housing. “Dynamic” means that the sealing point has a continuous relative motion during operation. The goal is to limit leakage and at the same time to prevent the ingress of dirt, moisture, or dust.

Which media are sealed? In practice, these are primarily liquids such as oil or water as well as gases. How tight a rotary seal has to be depends on the system: some applications tolerate a very low, defined leakage, others require an almost leak-free function.

Rotary seals can work in contact or non-contact. Contacting seals rest with a sealing edge or sealing surface and thereby generate friction and heat. Non-contact concepts use a very small gap or a thin fluid film to reduce friction but often require tighter manufacturing and operating conditions.

Typical applications are gearboxes, pumps, motors, as well as hydraulic and pneumatic drives. Everywhere, the same basic question arises: how do you keep the medium in the system, and how do you protect the environment from contamination — despite a rotating shaft?

Distinction from Linear Seals

The decisive difference compared with linear seals is the form of motion. With rotary seals, rotation is the primary motion, while linear seals seal a translational motion. In many systems, an axial movement is additionally present alongside rotation — for example through shaft play or installation effects. This increases the tribological load, because the sealing edge constantly slides over a mating surface and thereby introduces heat. Therefore, lubrication, surface quality, and permissible misalignments are particularly critical with rotary seals.

Main Designs: Construction, Operating Principle, Typical Limits

In sealing technology, three designs have established themselves particularly. They differ in contact principle, effort, and the typical limits regarding pressure, speed, lubrication, and maintenance.

Radial Shaft Seal (Lip Seal)

The radial shaft seal (often referred to as a lip seal) seals with an elastic sealing lip that rests radially on the shaft surface. Frequently, a garter spring supports the contact force so that the lip rests reliably even with tolerances and aging. This design is very often used in gearboxes and motors because it is compact and can be well integrated into standard dimensions (among others ISO 6194 and DIN 3760).

Its limits usually lie where frictional heat rises strongly, the shaft surface is not suitably manufactured, or concentricity and misalignment are too high. Higher pressure differentials are also only permissible to a limited extent depending on the design, because the lip is then loaded more strongly and wear increases.

Mechanical Face Seal

The mechanical face seal works with two very smooth, ring-shaped sealing surfaces that are pressed axially against each other. One surface rotates with the shaft, the other is fixed in the housing. A spring or pressure force ensures the contact. In operation, a very thin lubricating film often forms, which reduces friction and wear and at the same time stabilizes the sealing effect.

This design is particularly widespread in pumps and with more demanding media, because it can be well adapted in design to different operating conditions. The price for this is a higher effort in design, installation, and operational monitoring than with simple lip seals.

PTFE Profiles with Energizer

With PTFE profiles, a profile ring made of PTFE (polytetrafluoroethylene) takes over the sealing function. PTFE has good sliding properties and is chemically resistant but needs a defined preload. This preload is provided by an energizer, usually an elastomer O-ring or a spring. As a result, this design is often suitable for applications in which low friction, higher circumferential speed, or more difficult lubrication are to be expected.

Important here is the system perspective: groove geometry, edges, lead-in chamfers, and installation state strongly determine function and service life. Small installation errors otherwise quickly lead to damage to the profile.

How Rotary Seals Become “Tight”: Friction, Lubrication, and Mating Surface

With contacting rotary seals, sealing effect does not arise from “absolute” contact, but from a controlled contact zone. There, contact force, micro contacts, and a thin lubricating film act together. This state is called mixed friction: part of the load is carried via solid-body contact, part via the fluid film. Precisely this balance determines leakage, friction torque, and temperature.

When lubrication is missing, dry running arises. Dry running increases friction strongly and leads to heat and wear within a short time. Therefore, when designing, it must always be clarified whether a stable oil film is present in operation, whether limited lubrication is possible, or whether phases without medium can occur — for example at start-up or during shutdowns.

The mating surface (usually the shaft surface) is an active part of the sealing system. Its roughness, hardness, and geometric quality influence whether a load-bearing lubricating film forms and how quickly the sealing edge wears. Concentricity and wobble are also decisive, because they load the contact zone cyclically and can therefore increase leakage and temperature.

Surface Parameters: Ra and Rz (Practical Reference)

Surface quality is frequently described via Ra and Rz. Ra is the arithmetic mean roughness value and describes the average deviation of the surface. Rz is a parameter for the mean roughness depth and reacts more strongly to individual peaks and valleys. Both values help to select a surface so that it can hold lubricant on the one hand and does not mechanically “cut into” the sealing edge on the other hand.

For many radial shaft seals, a rough orientation in the sealing area is often Ra ≈ 0.2 to 0.8 µm, depending on design and operating conditions. In addition, a twist-free machining is important. “Twist” means a helical structure that can act like a conveying geometry and thereby promote leakage.

Selection and Design Check: The Most Important Operating Conditions (10 Guiding Questions)

A suitable rotary seal rarely results from a single parameter. In practice, a short, structured clarification of the operating conditions leads more quickly to a robust solution, because design, materials, mating surface, and installation concept can be derived from it.

When several operating conditions are critical at the same time — for example high speed with poor lubrication or pressure with abrasive contamination — a deeper design effort pays off. In such cases, specialized consultation can be sensible to coordinate seal, mating surface, and installation as an overall system.