Hydrodynamic Bearings

Nov. 15, 2002
Journal bearing design is complex.

Journal bearing design is complex. It involves optimizing clearances, bearing length, minimum film lubricant, viscosity, flow rate, and inlet slots. Design equations are available, but their solution is time consuming unless done on a computer. Fortunately, these equations have been reduced to chart form, and a wide variety of design problems can be solved with various charts in the literature.

Effective viscosity for the bearing should be obtained from the mean oil operating temperature. Using mineral oil-based lubricants, this temperature typically ranges from 120 to 180°F, but should be less than 250°F. As an approximation, an oil temperature rise of 20°F above the inlet can be assumed. But, most accurate results are obtained from actual field experience. Mean oil temperature is not indicative of maximum bearing metal temperatures.

Oil flow rates are determined from the oil temperature rise and power loss. When the required oil flow is determined, an estimate should be made as to whether the required amount of oil is drawn through the clearance space in the bearing.

Minimum film thickness is often shown on design charts and is found from (1 - ec) where e = eccentricity ratio and c = bearing clearance. However, an acceptable minimum value is not shown because it depends on factors such as surface roughness and type of loading. There is no substitute for minimum film values that have proven successful in similar applications.

High pressures and temperatures generated in a hydrodynamic film should be considered when selecting bearing materials. Bearings subjected to cyclic stresses can fail by fatigue.

The bearing material should also be compatible with the journal material so when metal-to-metal contact occurs at starting and stopping, minimal surface damage occurs.

Hydrodynamic instability occurs when a journal does not return to its established equilibrium position or attitude when momentarily displaced. Shocks or vibrations produce an instability in which the journal whirls around the bearing axis at less than one-half journal speed. This instability is known as half-speed whirl and occurs in lightly loaded, high-speed bearings. The problem also occurs in vertical-shaft journal bearings where there is essentially no radial load on the bearing. The extent of shaft whirl ranges from unnoticed to a violent reaction that destroys the bearing.

When the bearing speed, journal diameter, and load are fixed, stability can be increased by reducing bearing length or lubricant viscosity. If whirl cannot be avoided, a lobed bearing may alleviate the situation. A tiltpad bearing is the most stable radial bearing configuration, but it is costly.

Bearing stiffness and damping capacity are important considerations when making machine vibration and critical shaft speed analyses. Stiffness is the reciprocal of the journal displacement with applied load. Damping is the force resisting journal radial motion.

Bearing stiffness can be considered a displacement-force relation. It varies with displacement of the journal, and can be estimated by determining eccentricity ratios for a series of applied loads and calculating the slope of the resulting displacement load curve.

From consideration of the damping capacity, shaft critical speed may be reduced from that predicted on the basis of a rigid metal support system.

Thrust bearing: design is as complicated as the design of a journal bearing. Complete analysis requires consideration of heat generation, oil flow, bearing material, load capacity, and stiffness.

In a tapered-land thrust bearing, design variables include the slope, width, and length of the lands. Slope of the land, usually defined as the ratio of total "rise" to gap thickness, is normally between 0.5 and 2. The running film thickness usually is less than a thousandth of an inch, and land tapers are usually only 0.1 to 0.2 mil/in. of pad length.

Fixed geometry bearings, such as tapered land, are designed for a specific set of operating conditions. Deviations from these conditions result in less than optimum bearing performance.

Unlike fixed geometry bearings, tilting-pad thrust bearings have the ability to self-adjust the slope or tilt to accommodate varying operating conditions. Equalizing tilting pad thrust bearings permits even thrust-load distribution over pads. This equalization handles misalignment and deflections in the housing and seating.

The six types of thrust bearing are flat land, step, taper, tilting pad, spring supported, and hydrostatic. The first five are called hydrodynamic bearings because they generate oil pressure when a thrust face on a rotating shaft pumps oil by shear into a zone of reduced downstream clearance and increased outflow resistance. When thrust load increases, film thickness drops until a new balance between inflow and outflow is reached, raising pressure until the higher bearing load is balanced. The hydrostatic thrust bearing uses a separate oil pump to supply pressurized flow to balance outflow.

Flat-land: bearings are the simplest to fabricate and the least costly. Thus, they are a first choice for simple positioning of a rotor and for light loads in machinery such as electric motors, appliances, pumps, and crankshafts.

However, flat-land bearings carry less load than the others because flat parallel planes do not directly provide the required pumping action. Instead, their action depends on thermal expansion and warping of the bearing material induced by heating from passing oil. The slight oil wedge shape then gives a load rating of about 10 to 20% of that expected with other bearing types.

Step: bearings also have a relatively simple design. With a coined or etched step, they lend themselves to mass production in small sizes in the form of bearings and thrust washers. Step height for about the minimum film thickness is commonly about 0.001 in. Circumferential length of the raised area beyond a step is ideally 28% of the total bearing segment.

Step-thrust bearings are well suited for low-viscosity fluids such as water, gasoline, and solvents. Minimum film thickness in these applications is so small that features such as pivots and tapers are impractical.

Step height must be small enough for good load capacity, yet large enough for the bearing to accommodate some wear without losing its load capacity by becoming smooth and flat. Fortunately, load capacity drops gradually for steps greater than optimum height, so some wear actually improves performance for higher steps. Still, step erosion by trapped contaminants is sometimes a problem in large step bearings.

Tapered-land: bearings are reliable, compact designs for a large variety of mid to large-size high-speed machines such as steam and gas turbines, compressors, and pumps. As with step bearings, taper height normally should be about equal to the minimum film thickness. To minimize wear during starting and stopping, and at low speeds, a flat land is commonly machined at the trailing edge. Load capacity is highest when the land length ranges from 15 to 30% of circumferential length of each bearing segment.

Because the operation of these bearings is sensitive to load, speed, and lubricant, they are typically designed to meet narrow operating conditions in specific machines.

Tilting-pad thrust bearings are used increasingly in marine drives, turbines, compressors, and pumps over much the same range of applications as tapered-land designs. They commonly have a central supporting pivot, and each of the six to 10 or more bearing segments adjusts freely to form a nearly optimum oil wedge even with widely varying loads, speeds, lubricants, and rotation in both directions. Leveling linkages to equalize load carried by individual bearing pads provide a further advantage over tapered-land designs by accommodating some misalignment.

Off-the-shelf units are available to match rotor shaft sizes from about 2 to 12 in., and custom designs can be adapted to a wide variety of applications including diameters up to the 120 to 170-in. range. Recent trends toward offset pivot, substituting copper for steel backing of a babbitt bearing surface, and nonflooded lubrication have significantly increased load capacity of tilting-pad bearing with some cost penalty.

Springs and other flexible supports for thrust segments are used for large bearings ranging to 10 ft or more in diameter and carrying millions of pounds of thrust. Flexible mounting avoids high load concentrations at pivots in large tilting-pad bearings. However, mechanical pivots provide similar advantages. These include automatic adjustment to provide a hydrodynamic supporting oil wedge for varying operating condition and accommodation of misalignment.

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