What Is an Air Turbine Starter (ATS) Mechanical Seal?
An air turbine starter (ATS) mechanical seal is the rotating face pair on the starter's output shaft that separates gearbox lubricant from the air path and from ambient. It is built around a carbon-graphite face and a metal bellows, so sealing survives intermittent duty, very high speed, rapid temperature change and a thin oil film.
The physics matches that of an industrial pump seal; the boundary conditions do not. A pump sees continuous fluid, steady temperature and reliable lubrication. An ATS run lasts seconds at very high speed, temperature climbs sharply, and in some phases almost no oil reaches the faces. Aerospace applications are a separate discipline.
This guide covers how an ATS works, what loads its seal, the design responses, and the data to supply when requesting one. For the general classification of seal constructions, see mechanical seal types.
How an ATS Works and Where the Seal Sits
An ATS converts compressed air into shaft torque. Air comes from the auxiliary power unit (APU), a ground air cart, or a cross-bleed line from another running engine. It drives a turbine wheel, a reduction gearbox trades speed for torque, and a clutch passes it to the engine until it sustains itself.
Despite their small size and light weight, ATS systems can produce a significant amount of torque to turn the engine until it runs independently. The duty is intermittent: a start is short, then the unit stands idle. The seal is assessed against repeated short, severe cycles, not continuous service.
The seal sits on the gearbox output shaft and separates the lubricant, liquid oil or oil mist, from the air path or ambient. Leakage hurts both ways: oil escaping to the air side is lost lubricant and contamination; air and particles entering the gearbox shorten gear and bearing life.
The Conditions That Load an ATS Seal
Speed comes first. The turbine side runs at speeds on the order of tens of thousands of rpm, and although the gearbox reduces it at the output shaft, sliding velocity at the faces stays well above that of industrial pumps. High sliding velocity raises face heat and the dynamic load from any unbalanced mass.
Transient temperature and lubrication come second. Face temperature rises quickly during a start and falls once the clutch disengages; the seal repeats that cycle on every engine start. Oil arrives as a jet or a mist, so the film at the faces is very thin in the first moments of rotation; near-dry running is a normal phase.
Environment and service life come third. The unit works under vibration and shock, and ambient pressure and temperature swing widely as altitude changes. Weight and envelope limits, long overhaul intervals and traceability to manufacturing history are added on top.
Operating Condition and Seal Design Response
The table below ties each ATS-specific condition to a design decision. The design is the sum of those decisions; improving one component alone does not move it.
| ATS operating condition | Seal design response |
|---|---|
| Turbine-side speed on the order of tens of thousands of rpm; high sliding velocity at the faces | Low-mass moving assembly; precise balancing and centering |
| Sharp temperature rise through a start, cooling after it | Elastomer-free secondary sealing: metal bellows in a high-temperature alloy |
| Very thin oil film in some phases, near-dry running | Self-lubricating carbon-graphite face on a hardened or hard-faced counterface |
| Pressure fluctuation on the air side | Balanced construction; a balance ratio limiting how much the face load follows an air-side pressure change |
| Vibration and shock | Continuous face load from bellows or springs; positive drive rather than friction |
| Weight and envelope limits | Compact layout with few parts; thin-walled body with integrated drive |
| Long overhaul interval and traceability | Batch traceability, acceptance testing and a document set delivered with the part |
The Carbon-Graphite Face and Hard Counterface Pair
Carbon-graphite is the soft face of an ATS seal, and three properties drive that choice: its layered structure leaves a transfer film on the counterface, so it lubricates itself; its density is below that of metals and hard ceramics, so rotating mass stays small; and it tolerates thermal shock.
The counterface is hardened steel or a hard-faced ring, so wear concentrates on the cheaper replaceable carbon part. For a comparison of hard face materials, see silicon carbide and tungsten carbide; in an ATS that choice also weighs mass and thermal expansion match.
Two hard faces running against each other are unsuitable here: frictional heat rises quickly when the oil film thins. The asymmetric pair — soft carbon against a hard counterface — keeps friction and heat low and tolerates near-dry rotation at a start.
Metal Bellows: Elastomer-Free Secondary Sealing
A metal bellows does three jobs in one part: secondary sealing, the spring load that presses the face onto the counterface, and rotational drive. With an elastomer O-ring those jobs are split across parts, and an O-ring that hangs up on the shaft can hold the face back axially.
Removing the elastomer is a direct gain under ATS conditions: nothing is left to age, take a compression set or reach a temperature limit, and the bellows alloy sets the operating range. Elastomer choice still governs industrial duty; see FFKM and elastomer selection.
The bellows is formed from a high-temperature alloy as thin-walled convolutions. Its critical quantities are axial travel allowance and fatigue life: flexible enough to absorb shaft run-out and face wear, durable enough not to fatigue over repeated starts. Rotation is transmitted by positive drive, and low mass lets the face follow at high speed.
The Aerospace Quality Approach: Acceptance Testing and Traceability
In aerospace a part's history is delivered alongside the part. The customer specification defines the quality management system that applies and the acceptance criteria; the applicable set changes from program to program, and the order file governs.
In practice: material lots and the heat treatment and coating processes are recorded, every part traces back to those records, dimensional and surface checks are carried out, a functional and leakage test runs at representative conditions, and results ship with the part.
ATS Seal Specification Checklist
Aerospace seals are built from design data, not picked from a catalog. The headings below are the minimum to supply with a request for quotation; every blank heading forces the designer to assume.
- Speed: the range on the turbine side and at the gearbox output, plus the start peak.
- Temperature: oil inlet, the transient peak through a start, and the lowest ambient.
- Lubrication: oil type, delivery method (jet, mist or splash), and permissible dry-running time.
- Air side: pressure range, the spike at a start, and the contaminants expected.
- Shaft and layout: shaft or sleeve diameter, run-out tolerance, axial movement, bearing arrangement.
- Envelope: permissible outside diameter, axial length and weight limit.
- Life target: number of starts, total running hours and the overhaul interval.
- Documentation: traceability, acceptance testing, delivery file and the customer specification.
Where the Same Design Logic Also Applies
The combination that defines an ATS seal — low-mass carbon face, elastomer-free metal bellows, balanced construction, positive drive — is not exclusive to aviation. It reappears in aero-derivative gas turbines, ground power units and the auxiliary systems of defense platforms.
Industry imposes comparable conditions: high-speed compressors, turbomachinery and test rigs combine intermittent operation with limited lubrication. For how seals fail there and what face marks indicate, see mechanical seal failure and maintenance.
Meccanotecnica Umbra and Aerospace Applications
Meccanotecnica Umbra Turkey is the Turkish company of the Italian mechanical seal manufacturer Meccanotecnica Umbra S.p.A. From Ikitelli OSB, Istanbul, it manufactures and supplies mechanical seals, gland packings, gaskets and rotary joints for automotive, home appliance and industrial pump applications.
The group designs and manufactures mechanical sealing systems specifically for the aerospace sector, fully customized to the design data supplied; alongside air turbine starter (ATS) and auxiliary power unit applications, helicopter gearboxes fall within that scope. Helicopter gearbox sealing systems are developed at the Competence Center in Campello sul Clitunno using technologies such as surface texturing.
The aerospace range is not selected from a catalog; each application is quoted against its design data. For the scope, consult the group's aerospace applications catalog; for the industrial ranges see our product catalog. Send your speed, temperature, lubrication and envelope data and contact us; our Turkey team will route the request to the group's aerospace engineering.
