Page 62 - The Tribology Handbook

High speed bearings and rotor dynamics A10

Bearings in high speed machines tend to have high power losses and oil film temperatures.
Machines with high speed rotating parts tend to be prone to vibration and this can be reduced by the use of bearings ofan
appropriate design.
Avoiding problems which can arise from high power losses and temperatures

Possible problem Conditions under which it may occur Possible solutions
-
Loss of operating clearance when starting Designs in which the shaft may heat up 1. Avoid desiens with features that can
the machine fi-om cold and expand more rapidly than the cause the problem
bearing and its housing. Tubular
shafts are prone to this problem 2. Design with diametral clearances towards
the upper limit
Housings of substantial wall thickness 3. Use a lubricant of lower viscosity if
e.g. a housing outside diameter > 3 possible
times the shaft diameter
4. Control the acceleration rate under cold
starting conditions

Housings with a substantial external 5. Preheat the oil system and machine prior
flange member in line with the bearing to starting
Loss of operating clearance caused by Corrosion and deposition rates increase 1. Determine the chemical nature of the
the build up of corrosive deposits on a at higher operating temperatures corrosion and eliminate the cause, which
bearing or seal may be:-
A corrosive material to which the
The deposit usually builds up bearing material is sensitive needs to be (a) an external contaminant mixing with the
preferentially at the highest present in the lubricating oil lubricant
temperature area, such as the position
of minimum oil film thickness (b) an oil additive

2. Change the bearing material to one that
is less affected by the particular corrosion
mechanism
3. Attempt to reduce the operating
temperature
Loss of operating clearance from the The presence of condensation water in 1. Modify the oil system to eliminate any
build up of deposits from the lubricating oil and its build up in static pockets, particularly in the oil tank
microbiological contaminants static pockets in the system

The deposit usually builds up Temperatures in low pressure regions of 2. Occasional treatment of the lubrication
down-stream of the minimum film the oil films which exceed the boiling system with biocides
thickness where any water present in point of water
the lubricant tends to evaporate 3. Raise the oil system temperature if this is
permissible
Increased operating temperatures arising High surface speeds and clearances 1. Check whether reduced bearing diameter
from turbulenlce in the oil film combined with low viscosity lubricants or clearance may be acceptable
(see Fig. 10.1)
2. Accept the turbulence but check that the
temperature rises are satisfactory
Increased operating temperatures arising Thrust bearings are particularly prone to 1, Keep the bearing housing fully drained of
from churning losses in the bearing this problem because they are usually oil
housing of a larger diameter than associated
journal bearings 2. In bearings with separate pads such as
tilting pad thrust bearings, feed the pads
by individual sprays

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