W. Dmochowski and K. Brockwell
Institute for Machinery Research, National Research Council Canada, 3650 Wesbrook Mall, Vancouver, B.C., V6S 2L2, Canada
S. DeCamillo
Kingsbury, Inc., 10385 Drummond Road, Philadelphia, PA 19154, U.S.A.
Abstract
Because of it's superior anti-vibration characteristics, the tilting pad journal bearing is a popular
choice for supporting the shafts of rotating machinery. However, recent increases in shaft
speed are causing concern about the high levels of bearing temperatures and frictional losses.
Accordingly, a new design of bearing has been developed in which cool oil is supplied directly
to the leading edge of each pad.
This paper presents results from the testing of a 0.098 m diameter, five pad bearing operating
with a load of 11.1 kN and speeds up to 27S Hz. These experimental data are compared with
results obtained from a three-dimensional thermohydrodynamic model of the tilting pad journal
bearing. This model includes oil mixing at the leading edge of each pad. Good correlation
between the experimental and calculated results has confirmed the hypothesis that hot-oil-carryover
may be significantly reduced by supplying cool oU directly to the leading-edge-groove.
Also, a comparison of results from the conventional and LEG bearings shows that the latter has
substantially lower operating temperatures. Furthermore, calculations indicate that the power
loss of the LEG bearing may be reduced significantly.
Introduction
The tilting pad bearing, in either the thrust or journal configuration, is a popular choice of
bearing for supporting the shafts of rotating machinery. However, machine operating speeds
have now increased to the point where power loss and bearing metal temperature may be
unacceptably high. A number of studies have, therefore. been aimed at improving bearing
steady state performance [1,2]. Basically, apart from reducing the churning losses of the
tilting pad thrust bearings. effort was focused on directing "fresh" oil to the leading edge of the
pads. Bielec and Leopard [3] demonstrated the beneficial effect of supplying oil to nozzles
located between the pads of a thrust bearing and a similar method was used for a tilting pad
journal bearing [4]. The authors of both papers claimed that by supplying oil under pressure
to the oil film gap, the effect of hot-oil-carry-over was reduced.
The leading-edge-groove journal bearing offers a different approach. In this design, each pad
is extended to accommodate an axial oil distribution groove, from which cool lubricant is
introduced directly into the oil film. It has been shown that there are reductions in temperature
and friction losses of both thrust [2, 5] and journal [6, 7] bearings when this method of oil
supply is used.
Harangozo et al. [8] concluded that these oil supply methods had a greater effect on the
performance of the thrust bearing and were less important in the case of the journal bearing.
However, this conclusion was based on test results where the oil flow was fixed, regardless of
bearing operating conditions. Recently, Fillon et al. [9] concluded that directed lubrication is
effective at higher operating speeds (above 30 m/s).
In most cases, results from investigations into oil supply methods for tilting pad bearings have not been supported by theoretical considerations of the flow and heat exchange in the space between two adjacent pads. Some authors [4, 9] attributed the benefits of directed lubrication to a reduction in hot-oil-carry-over, but in their computer models the mixing coefficients were chosen without consideration of the bearing operating conditions.
Previous attempts to both explain and model the effect of the leading-edge-groove have been
presented by the authors [6, 7]. In this paper, additional test results and theoretical data are presented to support the finding that the decrease in the operating temperature of the LEG
bearing is through hot-oil-carry-over reduction. Furthermore, it will be shown that this effect is dependent on the oil flow rate. The results from the LEG bearing are compared with data from a conventional tilting pad journal bearing, and also with theoretical data from quasi three dimensional thermohydrodynamic computer models of the bearings. In these models, oil film turbulence and pad thermal and elastic distortions are accounted for. Full details are given in reference [6].
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