In this paper dealing with the tilting pad journal bearing, experimental results are
presented which show that, at higher shaft speeds, the leading-edge-groove (LEG)
design has significantly lower operating temperatures to those of the conventional
design of tilting pad journal bearing. Subsequent theoretical analysis has shown that
this reduction in pad operating temperature is the result of feeding cool oil directly
to the leading edge of the pad. This has the effect of reducing the amount of hot
oil carried over from one pad to the next.
Introduction
The leading-edge=groove (LEG) bearing is a term used to
describe a new design of tilting pad bearing that incorporates
an oil distribution groove at the leading edge of each pad. The
principle behind the operation of this bearing is that a controlled
amount of cool lubricant is introduced directly into the
oil film from the groove. In high speed thrust bearings, this
method of oil supply has been found to give significantly reduced
frictional losses and operating temperatures (Mikula,
1988).
This paper describes an experimental and theoretical study
of a leading-edge-groove tilting pad journal bearing. Test results
from this bearing are compared with experimental data
taken from a conventional tilting pad journal bearing. Initial
results are encouraging since they show that the LEG bearing
has significantly lower metal temperatures to those of the conventional
bearing, when operating at higher shaft speeds. A
theoretical study of the LEG bearing indicates that this is the
result of reducing the amount of hot oil carried over from one
Pad to the next.
Hot oil carry over, and the problem of calculating the temperature
at the leading edge of the oil film, has been investigated
only a few times in recent years. In a series of studies
on hot oil carry over in thrust bearings, Ettles (1968, 1970)
analyzed the thermal and velocity boundary layers in the space
(or groove) between the bearing pads and defined a hot oil
carry over factor:
From results presented in these papers, it was shown that a
layer of hot oil adheres to the thrust runner, regardless of oil
flow and temperature conditions within the groove. Providing
both the hot oil carry over factor and the runner temperature
are known, the temperature of the oil film at the leading edge
(t1) can be found. Values of A vary between 0.5 and 0.93, and
were determined experimentally by Ettles (1970), and later by
Ettles and Advani (1980).
Using the Ettles’ hot oil carry over factor, Vohr (1981) analyzed
the behavior of a large thrust bearing, but found that a
small inaccuracy in the assumed value of A could lead to a
large difference in the calculated oil temperature. Vohr then
considered the heat balance of the entire bearing and included
the analysis of the thermal boundary layer across the groove.
However, the complexity of this analysis makes it's application
to the calculation of bearing performance difficult.
Mitsui et al. (1983) presented a modified form of Ettles’ hot
oil carry over factor that was related to both the outlet flow
and the total flow entering the oil film, i.e.,
The values of λ, estimated experimentally, ranged from 0.4
to 1.0.
Heshmat and Pinkus (1986) considered the energy balance
in the bearing groove and obtained the following relationships
for calculating the leading edge oil temperature:
The factor λ is referred to as a mixing coefficient and represents
the amount of heat lost by the trailing edge oil flow
(q2), before entering the leading edge of the next pad. The
mixing coefficient was calculated from the following empirical
formula:
In this paper dealing with the LEG and conventional designs
of tilting pad journal bearing, it is shown that simple' 'mixing"
equations can be used to determine the oil temperature at the
leading edge of the pads.
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