Page 55 - The Tribology Handbook

A9 Steady load pressure fed journal bearings

BEARING DESIGN LIMITS
Figure 9.3 shows the concept of a safe operating region and Fig. 9.6 gives practical general guidance (also shows how the
recommended operating region changes with different variables).

HIGH BEARING TEMPERATURE LIMIT
danger of lining material
wiping if load lies above this line
THIN OIL FILM LIMIT
danger of metal-to-metal
contact if load lies
above this line OIL OXIDATION LIMIT
danger of excessive
oil oxidation if
speed lies beyond
this line
REGION OF
SAFE OPERATION
OIL FILM WHIRL LIMIT
danger of unacceptable
vibration if speed lies
beyond this line

JOURNAL SPEED

Fig. 9.3. Limits of safe operation for hydrodynamic journal bearings

Thin film limit - danger of metal to metal contact of the in
surfaces resulting in wear. 1 2 4 6 810 20 40
.
Background Safe limit taken as three times the peak-to- , . , ... I.. I a I I
valley (R,,,=) value of surface finish on the journal. The MINIMUM ALLOWABLE
factor of three, allowing for small unintentional misalign- 40 . OIL FILMTHICKNESS
ment and contamination of the oil is used in the general
guide, Fig. 9.6. A factor of two may be satisfactory for very 20 ' PEAK-TO-VA LLEY 690
SURFACE FINISH 400
high standards of build and cleanliness. R,,, depends on 10. /RmaX / SURFACE FINISH - 400
the trend in R, values for different journal diameters as 8:
shown in Fig. 9.4 together with the associated machining 6. 200 C
4.
process. E , '
2.
High bearing temperature limit - danger of bearing 1
wiping at high speed conditions resulting in 'creep' or 0.8 j
plastic flow of the material when subjected to hydrodyna- 0.6
mic pressure. Narrow bearings operating at high speed are 0.4
particularly prone to this limit.
0.2
Background The safe limit is well below the melting point of
the bearing lining material. In the general guide, Fig. 9.6, 10 20 40 60 100 200 400~6001000
whitemetal bearings are considered, with the bearing JOURNAL DIAMETER, mm
maximum temperature limited to 120°C. For higher tem-
peratures other materials can be used: aluminium-tin
(40% tin) up to 150°C and copper-lead up to 200°C. The Fig. 9.4. Guidance on allowable oil film thickness
former has the ability to withstand seizure conditions and dependent on surface finish
dirt, and the latter is less tolerant so a thin soft overlay
plate is recommended, togetjer with a hardened shaft and
good filtration.

A9.2