JT8D Bearings & Seals
Revised 25-Mar-2001
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The main bearings support and align the two rotating systems, N1 N2. They are antifrictional types to reduce rolling friction. They are lubricated, cooled and cleaned by the oil. Some bearings may be vibration damped by the oil.

Roller bearings support rotating parts radially, absorbing angular loads, they do not prevent axial movement. All rollers have a one piece cage, a ring with a recess, a ring without a recess, either the inner or outer ring of a bearing may be the recess ring.

Ball bearings hold rotating parts in axial and radial alignment. They absorb both thrust (axial) and angular loads. All ball bearings have a one piece cage, two rings (inner and outer) each with a recess.

Description & Location

  • No1 Bearing
    • Front Compressor Front
    • Roller
    • Inlet case, front of LPC
    • Radial support for front LPC rotor
    • supported by Fan Inlet Case May be oil damped.


  • No2 Bearing

    • Front Compressor Rear
    • Duplex Ball
    • Rear Of LPC
    • Thrust bearing for LPC and LPT
    • Supported by Compressor Intermediate Case


  • No3 Bearing

    • Rear Compressor Front
    • Ball
    • Front of HPC
    • Radial support of front HPC, is also thrust bearing for accessory drive gear
    • Supported by Compressor Intermediate Case


  • No4 Bearing

    • Rear Compressor Rear
    • Duplex Ball
    • Rear of HPC
    • Thrust bearing for HPT and HPC rotor
    • Supported by Diffuser Case


  • No 4 1/2 Bearing
    • Turbine Intershaft
    • Roller
    • In line with the midpoint of combustion chamber case
    • Radial support for the N1 drive shaft to prevent flexing
    • Outer race against inner surfaces of N2 drive shaft, inner race and rollers on N1 drive shaft


  • No 5 Bearing
    • Turbine Front
    • Roller
    • Front of HPT, in line with rear flange of combustion chamber case
    • Radial support for HPT rotor
    • Supported by Inner Combustion Case

  • No 6 Bearing
    • Turbine Rear
    • Roller
    • Rear of LPT
    • Radial support for the LPT Rear Hub
    • Supported by Turbine Exhaust Case

Main Bearing Seals

The bearing seals keep lubricating oil confined to the bearing compartments to reduce oil consumption. They also minimise the leakage of high pressure air from the gaspath into the bearing compartments. Seals are not airtight there is a small flow of high pressure air into the bearing compartments to prevent leakage into the gaspath.

The JT8D has these types of seals installed. Labyrinth, Carbon, split ring and face type, Combination of carbon / labyrinth.

Labyrinth Seals
The seal has two parts. One part is a non rotating ring that is attached to the engine. It has an inner surface that is made in the shape of a series of steps (lands). The other part is a rotating ring that is attached to the N1 and N2 assembly. It's outer surface has knife edge seals that align with the lands. These knife edges do not touch the lands there is a very small clearance between. As compressed air goes through the cascade of seals and lands, it's pressure is reduced. This reduced pressure enters the bearing compartment at a slower rate.

Labyrinth seals on the JT8 can be found at these locations, behind the No1 bearing, in front of the No2 bearing, behind the No3 bearing, in front of the No4 bearing (earlier engins all labyrinth, latter are combination), behind the No5 bearing (combination), in front of the No6 bearing (combination).

Carbon Seals
These are made from a mixture of carbon and graphite powder which are bonded together, heat treated, sealed, and machine ground and lapped. Carbon seal rings make contact with metal surfaces that are also machined. This type of seal permits less leakage of the gaspath air and are used in high pressure areas of the engine for greater efficiency and reduced breather pressure.
Split Ring Type
Contact surfaces are horizontal, carbon seal rings turn, seal seat remain stationary in the case of the No6 bearing or rotate in the No 4 1/2 bearing.

Face Type
Contact surfaces are vertical, carbon seal rings do not turn, metal seal seats turn. Spring loaded carbon seal ring and a smaller pressure loaded backup carbon seal ring. Face type carbon seals are used in combination with knife edge seals.
JT8D uses wet faced carbon seals. Small holes which permit oil to lubricate the contact surfaces.

Locations, in front of the No4 bearing (face type latter models), behind the No4 1/2 bearing (split ring type), behind the No5 bearing (face type), in front of the No6 bearing (split ring type).

That's about as deep as I will go for now, the other point of note is the tower shaft is supported by a roller bearing at the upper end and a ball at the lower end. Oil is supplied by holes in the bevel gear.

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