Page 108 - The Tribology Handbook

Gas bearings AI9

EXTERNALLY PRESSURISED GAS BEARINGS
Externally pressurised gas bearings have the same principle of operation as hydrostatic liquid lubricated bearings.
There are three forms of external flow restrictor in use.

Pocketed on&e (simple Unfiocketed orifice (annular
Restrictor type Slot
onifice) orifice)

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1 1
Flow restriction 01 __ a- a -Y
xd2/4 ndh 2
d2 rd2 y is generally large for
<
xdh
Geometricel resfrzctions - < d6 When 7 practical values of z
4
resulting in large overall
d i.e. d < 4h orifice reverts bearing size
Le. 6 > -
4 to simple type giving
increased bearing stiffness
nd2
<
- Rdph
4
Journal and Generally the highest 33% less than pocketed Highest for very low b/D
back-to-back journals
thrust
Steady state
load Singlle thrust Can approach that of slot Same as pocketed Generally the highest
type by spreading pressure
with surface grooves
Jouriraal and Generally the highest 33% less than pocketed Highest for very low b/D
back-to-back journals
Steady state thrust
stafiess K -
Singlle thrust Highest 33% less than pocketed
Damping Both pockets and surface
grooves will reduce
damping

Stability Both pockets and surface
grooves can result in
instabilities
Dynamics Overall damping decreases with rotation in journal bearings. All bearings are subject to resonance
at the natural frequency o, = ,/(K/m) : symmetrical rigidly supported journal systems have ‘whirl’
at a speed 2 x o,

Lubricant Piny clean gas: design charts presented are for air.

AI 9.1