Institute of Fluid Flow Machinery,
Polish Academy of Sciences,
Tribology and Mechanics Laboratory,
National Research Council,
Vancouver, B.C., V6S 2L2
Philadelphia, PA 19154
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.
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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