What are bearings? Bearing selection?
The bearings are usually a high precision, multi piece cylindrical or ball
type roller bearing. The outer race with the ball/roller component is housed in
a frame that is attached to the engine case structure and the inner race is
usually pressed onto the shaft that passes through the engine. The frames are
usually designed with radial "struts" that provide the physical
attachment to the engine case. The struts are hollow to allow for flow of oil
and pressurized air into the compartments. In some designs, the frames are
bolted to the engine case through radial flanges.
Each bearing "area" is designed as a sealed, pressurized
compartment (sometimes called a sump) and the lubricating oil is circulated to
each sump and contained within by labyrinth type carbon seals on either end of
the compartment.
The seals are also a multi piece design with the outer section installed
into the bearing frame and the inner section pressed onto the shaft. Sealing is
accomplished by pressure from springs within the seals and pressurized air
within the engine. A system of retaining clips and rings secure the bearings
and seals while a set of O-rings complete the assembly and keep the compartment
sealed, both to retain the lube oil and to keep out the high pressure gasses
inside the engine.
The A sump held the bearings to support the fan and the front of the
compressor spool. The B sump held the bearings to support the rear of the
compressor spool and the front of the turbine wheel. The C sump held the
bearings that supported the rear of the turbine wheel.
Installing bearings was a very precise effort and the bearings had to be
kept very clean. Even a stray fingerprint can cause heat to build up that leads
to bearing, and ultimately engine failure.
Angular contact is a term used for
radial rolling bearings with a nominal contact angle greater than zero degrees
up to and including 45 degrees. Nominal angle for the No. 1 bearings is 30
degrees. This allows the bearing to take a combination of radial and thrust
loads.
MATERIAL
Materials include M50 tool steel (AMS6491), carbon chrome steel (AMS6444),
the corrosion resistant AMS5930, 440C stainless steel, silicon nitride
(ceramic) and titanium carbide-coated 440C.
Typically special attention is given to the material specification,
non-destructive testing and to the traceability of the bearing (that is, a
system of documents that enables an engineer to trace a bearing, typically back
to its manufacturing batch and material supply).
In order to assure bearing performance it is necessary for the bearing
steel to be of high quality. Jet engine bearings are typically manufactured
from metals manufactured using a vacuum arc remelt to enable material
requirements to be met.
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