What Are Mechanical Face Seals (Duo Cone Seals) and How Do They Work?

In an excavator's final drive, a bulldozer's track roller, or a tunnel boring machine's cutter head, the bearing that carries the load is protected by one small component doing a very big job: the mechanical face seal.

Also known as floating seals, duo cone seals or heavy duty seals, these components are the standard answer wherever conventional lip seals give up — in mud, sand, rock dust and water, at slow speeds and heavy loads, for years at a time without maintenance.

This guide explains what a mechanical face seal actually is, how it works, what it is made of, and how to decide whether your application needs one.

What Is a Mechanical Face Seal?

A mechanical face seal is a metal-to-metal rotary seal built from just four parts:

• Two identical, precision-lapped metal rings that seal against each other

• Two elastomeric energizers (O-rings or rubber rings) that center the rings in the housing and maintain the face load

That's the whole assembly. No springs, no garter springs, no separate wear plates.

The two metal rings meet on lapped sealing faces that run at right angles to the rotating shaft — a vertical face, not a cylindrical one like a lip seal. The lubricating oil that the seal is protecting forms a thin film between the two faces, so the rings run against each other on oil, not on bare metal.

This is why these seals survive where lip seals fail.

→ See the full mechanical face seal range — cast iron and forged bearing steel rings, 64 mm to 1,200 mm bore.

A lip seal is a relatively soft rubber edge rubbing on a rotating shaft; abrasive contamination chews that edge away quickly. A mechanical face seal has hard metal faces instead, and the sealing interface is the hardest, most wear-resistant surface in the housing.

How Does a Floating Seal Work?

The working principle has two halves: one mechanical, one hydraulic.

The metal face interface

Each ring's sealing face is ground and lapped to a flatness measured in light bands — far flatter than any machined surface on the housing. When the two faces are pressed together, they touch everywhere except for microscopic gaps that hold a lubricant film roughly one micrometer thin.

That film does the actual sealing job in both directions at once:

• Oil stays in: the film bridges the gap between the faces, so oil inside the bearing chamber cannot migrate outward

• Contamination stays out: dirt particles cannot pass through the film, and anything that does try travels through a gap narrower than the abrasive particles themselves

The elastomer energizer

The rubber ring behind each metal ring does three jobs at once:

1. Centering — it seats the metal ring in the housing and self-centers it, compensating for shaft eccentricity and misalignment so the face load stays uniform around the whole circumference

1. Axial load — it pushes the ring against its partner with a constant, defined force

1. Static sealing — it seals the gap between the metal ring and the housing bore

Because the elastomer is in compression in its housing seat, the face load stays stable over the whole service life — no re-greasing, no adjustment, no scheduled seal replacement.

This is where the nickname "lifetime seal" comes from: properly installed, the seal is expected to last as long as the component it protects.

Where Are Mechanical Face Seals Used?

Anywhere bearings face heavy contamination, heavy loads, or both — typically in rotary applications in construction, mining and heavy machinery:

• Tracked vehicles — excavators, bulldozers, track rollers and idlers

• Final drives and travel gearboxes of construction machinery

• Tunnel boring machines and cutter heads

• Conveyor systems in mining and ports

• Agricultural machinery — wheel hubs, harvesters, tillers

• Heavy trucks — wheel hub and axle sealing

• Mixers, stirrers and gearboxes in process industries

• Wind power stations — pitch and yaw bearings

The common thread: the seal must keep water, mud, sand or rock dust out of a bearing that cannot be opened for maintenance on any reasonable schedule.

→ Not every rotary sealing point calls for a face seal — compare the seal types we make before you specify.

What Are Mechanical Face Seals Made Of?

A floating seal is two material choices: the metal ring and the elastomer.

The metal seal rings

Both materials seal metal-to-metal on a lapped face; the choice is mainly about the environment the machine operates in.

The elastomer energizers

The elastomer defines the temperature window of the assembly —

→ The energizer itself is an O-ring — NBR, HNBR, FKM or VQM — which is why the temperature window follows the elastomer, not the metal.

from -45 °C up to +150 °C depending on the O-ring material — so in practice the material choice usually starts with the operating temperature, then the media.

What Sizes Are Available?

Mechanical face seals are specified by bore diameter:

• Standard range: 64 mm up to 1,200 mm bore diameter

• The standard tapered housing seat can normally be machined on an NC machine tool — no special tooling required for installation

• Pressure capability: up to 0.3 MPa

• Sliding speed: up to 3 m/s

These limits are the reason mechanical face seals dominate in slow-running, heavily loaded rotary applications: they are built for torque and contamination, not for speed or pressure.

Mechanical Face Seal vs. Lip Seal: When Do You Need a Floating Seal?

→ For reciprocating hydraulic duties where a floating seal does not apply, the pressure side is usually handled by PTFE cup seals.

The rule of thumb: if the bearing sees mud, sand, rock dust or water

→ If the part that failed is a cylinder seal rather than a face seal, work through the 12 causes of hydraulic seal failure first.

and cannot be maintained regularly, a mechanical face seal is usually the right answer. If speed is the dominant requirement, a lip seal design may still win.

FAQ

Why are they called "duo cone" seals? The name describes the geometry: two (duo) metal rings with conical (cone) seating faces clamped back-to-back by elastomer rings. "Floating seal" and "heavy duty seal" describe the same product from different angles — the rings float in their housing, and the duty is heavy.

Can a mechanical face seal run dry? No. The lapped faces need the thin oil film to carry the load between them. Without lubrication, the faces overheat and the seal fails quickly. The housing design must ensure the oil level covers the sealing interface.

Which is better: cast iron or bearing steel rings?

Neither is universally better. Cast iron resists corrosion and is the standard in wet, salty or corrosive environments; forged bearing steel is a cost-effective choice in less aggressive conditions. The elastomer choice matters just as much, because it sets the temperature window.

Why does the housing seat matter so much? The elastomer ring compresses between the housing seat and the metal ring. If the seat dimensions deviate from the specification, the face load changes — too little and the seal leaks, too much and the film between the faces collapses. That is why the tapered seat should be machined to the seal supplier's specification, not adapted from a worn part.

Need Help Specifying a Floating Seal?

Tell us the bore diameter, the application (machine type and sealing position), and the operating conditions — temperature, media, speed. Our engineers will recommend the right ring material and elastomer combination, standard or custom.

Talk to a sealing engineer →