A Fluid Film Thrust Bearing bear axial load, which is force acting in parallel with the shaft axis. It separates two surfaces with a supercharged lubricating film. The film is very thin and is usually measured in microns. Even so, it can withstand several megapascals pressure. It works without rolling elements, magnetic fields or gas injection. Instead, it relies on geometry, relative motion, and the behavior of viscous fluids between converging surfaces. To understand how it works, it is necessary to examine the basic hydrodynamic theory. Then, the theory is related to the geometric characteristics of thrust bearing design.
The Central Problem: Why Thrust Load Is Geometrically Harder Than Radial Load
In ordinary journal bearing that support radial load, the rotation shaft naturally moves to an eccentric position inside the hole. This eccentricity creates a convergence-a converging-diverging gap around the circumference of the circle. The converging section produces hydrodynamic pressure that lifts the shaft from the bearing wall. Geometry develops naturally. Even if the shaft is perfectly cylindrical and the hole is perfectly cylindrical, the load causes eccentricity. As a result, the lubricant gap becomes wedge-shaped.
Axial load creates a very different situation. The wheel is a rotating thrust ring on the shaft that faces the bearing pad and spans two nearly parallel planes. When two parallel surfaces move relative to each other, they cannot generate hydrodynamic pressure on their own. Lubricants cannot be compressed because the gap remains the same throughout the surface. On the contrary, the liquid flows outwards along all radial directions without increasing pressure. Reynolds' lubrication equation, developed from the Navier-Stokes equation in a thin film approximation, makes this clear. Pressure occurs only when there is a a converging gap called a wedge term or a gap height called an squeeze film term term that varies over time. A perfectly parallel surface moving at a steady speed satisfies neither (Reynolds, Philosophical Transactions of the Royal Society, vol 177, 1886).
This is the basic engineering problem that liquid film thrust bearing must solve. It must form a converging wedge between two surfaces and naturally remain parallel under axial load.
The Reynolds Equation: Mathematical Foundation
Thin-film Reynolds equation for an incompressible fluid with constant viscosity can be written as follows:
Estracheit (h3 trail/ trail) + trail/ trail (h3 trail/ trail) = 6U trail/ trail x + 12trail/ trail
In this equation, h is the thickness of the local film depending on location. p is the hydrodynamic pressure. The energy converter is the dynamic viscosity of the lubricant. U is the relative velocity between two surfaces. The residual difference is the rate at which the thickness of the film changes over time. The term is important only when there is an extrusion membrane effect.
The right side of the equation shows two ways of generating pressure.
Eripinte
Pressure occurs when the thickness of the thin film h decreases along the direction of motion. In other words, the gap must be closing. The resulting pressure increases with the viscosity and sliding speed of the lubricant. If viscosity is zero, the pressure disappears. The same would happen if the sliding speed was zero. Under no circumstances can bearings be subjected to hydrodynamic action.
Terminology of compressedfilm (energy conversion efficiency)
Over time, gaps get smaller and can be stressful. This happens even when there is no sliding movement between surfaces. This effect is important in systems with rapidly changing impact bearings, engine startup conditions and loads. In thrust bearings operating at a constant speed, the term is usually very small in stable operation. However, this becomes very important when engineers study the stability of bearings under varying loads.
The practical idea is simple. The thrust bearing works by controlling the h(x) function, which describes the film thickness profile of the film. The residual must be designed to move in a negative direction. This means that the lubricating film thins as the flow path passes through the gasket. Therefore, thrust bearing design focuses on the creation and maintenance of a converging film under actual operating conditions such as temperature, load variations, rotational speed and shaft misalignment.
How the Convergent Wedge Is Created: Three Geometric Approaches
1. Fixed-Geometry Tapered-Land Pads
The simplest method is to process bearing pad surfaces with small cones. That means the edge of the lead, which is first met by a spinning runner, is slightly deeper than the tail edge. The taper angle is small. Usually between 0.5 and 2.0 milliradians. This creates a height difference of only a few microns across the length of the gasket. As the wheel passes through the tapered surface, it pulls lubricant from a deeper area to a shallower one. As a result, pressure increases the converging region.
Terraces are simple in design and low in cost. It also does not require moving parts inside the bearings. However, it has an important limitation. The optimal taper angle changes varies with the operating conditions. The thickness distribution h (x) is affected by temperature increase, wear and velocity change. Because of this, bearings may deviate from optimal working conditions. At low load, the taper can be too large. At high loads and temperatures, thermal expansion alters the shape of the gasket and alters the effective taper profile in unpredictable ways (Khonsari & Booser, Applied Tribology, 2nd ed.., Cambridge University Press, 2017).
2. Tilting-Pad (Pivoted-Pad) Bearings
The tilting-pad design was invented by Anthony Michell and Kingsbury in the early 20th century. In this design, each pad can rotate slightly around a pivot point. When the wheel passes through the pad surface, it drags lubricant through the gap. The front edge is away from the runner and the back edge is closer to the runner because the pad can be tilted. This automatically creates a converging wedge. wedge angle and then adjust themselves to operating conditions.
The process of self-regulation is simple. As the wheel approaches the liner, pressure is generated in the lubricating film. This pressure creates rotational forces around the pivot. The gasket continues to tilt until the force is balanced by the fulcrum's support. In this state of equilibrium, the tilt angle becomes the exact value needed to support the load at the current speed and viscosity of the lubricant. If the load increases, the film becomes thinner and the viscosity increases. Then the gasket tilted more to maintain balance. If the speed increases, the film thickens and the cushions tilt less. In this way, bearings can be adjusted automatically without external control.
Tilting-pad thrust bearings are widely used in high demand industrial equipment. Examples include steam turbines, centrifugal compressors, large hydroelectric generators and ship propulsion shafts. They are popular because they automatically adapt to changes in load, speed and alignment. This helps prevent membrane failure and direct contact between surfaces.
pivot location, also. If the pivot is in the center of the pad, or about 50% of the length of the pad, from the front edge, the bearing works best in a rotational direction and is irreversible. For equipment that must rotate in both directions, pivots are usually placed at about 60% of the length of the liner. This arrangement reduces maximum load capacity in the preferred direction but provides acceptable performance in both rotational directions (Hamrock, Schmid & Jacobson, Mechanics Principles, 3rd ed., CRC Press, 2019).
3. Step-Profile (Rayleigh Step) Bearings
In 1918, Lord Rayleigh showed that the best movie contour load is not a gradual taper. Instead, it's a step profile. The design has a flat deep end at the entrance and a flat shallow end at the exit, separated by a sudden step. The Rayleigh step produces more pressure than linear cones of similar size. This is because pressure forms in the intake region and is maintained in the steps before the film begins to disperse (Rayleigh, Notes on Lubrication Theory, Journal of Philosophy, Series 6, Volume 35, 1918).
Stepper bearings are mainly used where they can be manufactured with precision. This is usually done by precision machining or by laminating surfaces with grooves. These bearings are generally not suitable for bidirectional rotation. They are commonly used in high-speed and light-duty systems such as gyroscopes and precision instruments. They are not common in heavy industrial equipment because it is difficult to maintain the required step geometry when thermal expansion and mechanical deformation occur.
The Physical Mechanism in Detail: What Happens in the Converging Film
Leaving the equation and looking at the physical process, it's easier to understand how the wedge creates pressure.
Consider that the surface of the flattened channel moves at U speed on a tapered pad. The thickness of the film decreases from h_inlet to h_outlet along the direction of motion. Lubricant enters the gap at the inlet into a tapering space. Fluids cannot simply be compressed to accommodate smaller spaces because in most thrust bearing applications, liquids are almost incompressible. This is true for typical mineral oils with a pressure below about 500 MPa (Bair & Winer, Journal of ASME Journal of Tribology, vol. 114, 1992). Therefore, the same amount of liquid must pass through smaller gaps.
As the film thins, the average velocity of lubricating oil must increase. This is because a fluid of the same volume must pass through a smaller cross section volume.
The lubricant inside the film does not move at the same speed anywhere. Fluid velocity varies with the thickness of the film. It is zero on the stationary pad surface and U on the moving runner surface. Therefore, the pressure inside the film must be adjusted to align the flow with the requirements of continuity.
In the converging part of the film, pressure increases along the direction of motion. This helps to control fluid flow and prevents lube from leaving the area too quickly. If there is a dispersal zone, the pressure goes down. Therefore, a pressure peak forms is formed in the polymerization segment of the film. This pressure distribution is commonly known as pressure hill. When the pressure integrates within the pad area, the force required to support the load is generated.
The pressure hill supports the runners. A thin film of lubricating oil separates the runner from the liner. In well-designed industrial thrust bearings, the minimum film thickness, h_min, is usually between 5 and 50 microns. For tapered-land designs, this minimum thickness is usually near the tail edge (Wilcock & Booser, Bearing Design and Applications, McGraw-Hill, 1957).
Rotators and gaskets should not touch each other during normal use. A thin layer of fluid separates them. At the highest pressure, this lubricating film can withstand more compressive stresses than many structural steels can withstand.
Temperature and Viscosity: The Feedback Loop
Energy conversion efficiency, energy conversion efficiency, appears directly in two parts of the Reynolds equation. Because of this, there is a very important feedback process within the bearing. Lubricant produces pressure and therefore load capacity, depending on viscosity. At the same time, friction within the lubricant produces heat, which reduces the viscosity lubricant.
As the bearing temperature increases, the oil becomes thinner. At the same speed and geometry, the thinner oil produces less pressure. If the load remains the same, the lubricating film must be thinner to support the load. The thinner film, the faster the shear rate inside the lubricant. This creates more frictional heat, which further reduces viscosity. Then the cycle continues. If this process is not controlled, it can lead to thermal instability and bearing failure.
The rate of heat generation from viscous shearing in a thrust bearing film can be estimated by:
Q_friction = η (U/h)² × A_pad × h
where A_pad is the pad area.
This equation can be simplified to:
Q_friction = ηU²A_pad/h
This form shows that frictional heat is inversely proportional to the thickness of the film. As the film thins, the heat generated increases rapidly.
There are several ways to remove this heat from bearings. Heat can enter the bearing housing through pad material. Heat can also take lubricant away from the film at the trailing edge. In addition, heat can enter the shaft through the spinner.
Heat generation and heat removal are balanced in reasonably designed bearings. At this point, the temperature of the lubricant is stable and the oil retains enough viscosity to support the load.
Two factors are particularly important for temperature control. The first is the thermal conductivity of the pad material. The second is the flow of lubricant through the bearing. Copper and copper-alloy gaskets transfer heat more efficiently than steel pads ones. Therefore, under the same load and speed conditions, they usually operate at lower membrane temperatures.
The lubrication method also affects temperature. Flooded lubrication provides a lot of oil for the bearing cavity and eliminates more heat. Directed-supply lubrication mainly delivers oil to the front edge of each gasket and usually requires less oil. However, flood systems can cause greater agitation losses in large high-speed machines (Etz, Journal of ASME Tribology, vol 102, 1980).
Minimum Film Thickness and Load Capacity: Engineering the Operating Point
The minimum film thickness, h_min, is the most important operating parameter of a Fluid Film Thrust Bearing. It shows whether the bearing is working safely in the hydrodynamic lubrication regime or moving toward mixed or boundary lubrication, where surface contact can begin.
The relationship between minimum film thickness and operating conditions is often expressed by a thrust-bearing characteristic number:
h_min / L ∝ √(ηUL / W)
In this equation, L is the pad length in the direction of motion, and W is the applied thrust load.
This relationship shows how different operating factors affect film thickness.
Increase speed U
As the revs increase, the lubricating film thickens. This is usually beneficial because it improves surface separation. However, higher speeds can also increase sticky heating, which can cause heat problems.
Increase viscosity η
When viscosity increases, the lubricating film thickens. This increasescarrying capacity. However, higher viscosity, the greater the stirring loss and the higher the working temperature.
Reduce load W
As the load decreases, the lubricating film thickens. This is the most direct way to increase the thickness of the film.
Reduce pad length L
The effect of pad length is complex. Shorter cushions can increase the thickness gradient of the film. This can increase the amount of pressure produced per unit area. However, shorter cushions also provide less support area, resulting in lower overall load capacity.
In most industrial applications, the minimum film thickness is between 15 and 50 microns, which provides sufficient protection from surface contact. This range takes into account surface roughness, dynamic operating conditions and lubricant contamination. The surface roughness of precision bearings is usually between 0.2 and 0.8 micrometers.
When the minimum film thickness less than 10 microns, bearings may enter the mixed lubrication regime. In this condition, some contact may occur between surface asperities. Therefore, engineers must pay close attention to surface finish, contamination control, and on and shutdown procedures (Booser & Khonsari, STLE Life Factors for Plain Bearings, Society of Tribologists and Lubrication Engineers, 2020). Lubrication Engineers, 2020).
Startup and Shutdown: The Unavoidable Mixed-Lubrication Period
Every hydrodynamic thrust bearing, no matter how well designed, is subjected to mixed or boundary lubrication when starting and closing. When the axis is stationary, the velocity U in the Reynolds equation is zero. Therefore, no hydrodynamic pressure cannot be generated. The entire thrust load is directly supported by the pad surface.
As the axis rotates, the lubricating film forms gradually. The bearings are first lubricated at boundary, where the surface is separated by a very thin lubricant layers. As speed increases, the bearing enters a mixed lubrication. At this stage, some of the load is borne by the lubricating film, while some surface asperities remain in contact. As the velocity increases further, a complete hydrodynamic membrane forms. At this point, the lube film separates the surface completely.
The velocity at which complete separation occurs is called transition speed. This speed depends on load, lubricant viscosity and surface finish. In well-designed, surface quality and well-lubricated industrial bearings, the transition speed is typically between 0.5% and 5% of normal operating speed (Stachowiak & Batchelor, Engineering Tribology, 4th ed., Butterworth-Heinemann, 2014).
Below the transition speed, there was still some surface contact. In the meantime, there may be wear and tear. If the bearing goes through multiple starting and stopping cycles, this wear gradually damages the precision surface finish required for efficient hydrodynamic operation.
For this reason, many large turbines and compressors use hydrostatic jacking oil systems. These systems inject high pressure oil, usually between 20 and 70 MPa, through holes in the bearing before the shaft starts to rotate. The pressurized oil separates the surface to prevent direct contact during start-up.
After the shaft reaches high enough speed to produce a stable hydrodynamic film, the top oil supply is shut off. The bearings then operate using its own hydrodynamically generated lubricant film. This method increases equipment costs and requires additional pumping power, but in applications with frequent start and close cycles, it can significantly extend the bearing's service life.
Key Design Variables and Their Effect on Bearing Performance
The following variables are the main design factors for a Fluid Film Thrust Bearing.
Pad Arc Angle (Circumferential Width)
pad arc provides pad area. Because of this, each gasket can support a higher load. However, larger cushions also produce more heat. In multi-liner thrust bearings, pad arc angle is usually between 25° and 60°.
Number of Pads
Using more gaskets can improve load distribution and reduce the maximum pressure on each gasket. However, more gaskets also mean more pivots, greater temperature variation, bearings and higher manufacturing complexity. Most industrial thrust bearings use 6-12 pads.
Pivot Position (Tilting-Pad Designs)
pivot position has a big impact on performance. The pivot, located at 50% of the length of the liner, provides optimal performance in a rotational direction. The pivot, located at 60% of the length of the gasket, slightly reduces peak performance but allows acceptable operation in both rotational directions.
Pad Material
The selection of pad material affects heat transfer and durability. Steel cushions are rigid and can maintain the shape of steel cushions well during use. Copper alloy gaskets can conduct heat more efficiently and help reduce the temperature of lubricating oil.
Many bearing pads also use Babbit-coated bearing surface. Babbitt, which can be tin or lead based, adapts to small surface irregularities and absorbs small particles of pollution. This improves bearing reliability. The fatigue strength of the babbitt layer limits the maximum working pressure. Conventional babbitt materials typically withstand about 10 to 15 MPa pressure. High-tin and aluminum-based overlay materials typically can withstand pressure between 18 and 25 MPa (Khonsari & Booser, 2017).
Lubricant Viscosity Grade
Lubricant viscosity has a major influence on bearing performance. Oil with higher viscosity can withstand a larger load at a lower speed. However, they also produce more heat at high speeds.
ISO VG 32 to ISO VG 68 mineral oils is commonly used in turbomachinery applications. In slower and heavier industrial equipment, such as gear systems and compressors, ISO ISO VG 100 to ISO VG 320 oils are commonly used (ISO 3448:1992, ISO Viscosity Classification industrial liquid lubricants).
Industrial Liquid Lubricants - ISO Viscosity Classification).
Radial Dimensions (Inner and Outer Radius)
Internal and external radii affect the working characteristics of bearings. The average radius determines the sliding velocity of a given shaft speed.
The larger ratio of outer radius to inner radius, the larger pad area. This can increase load capacity. However, it also creates a larger difference in sliding speed across the pad width. As a result, the pressure distribution becomes uneven and the actual carrying capacity may be lower than that indicated by the pad area alone.
Failure Modes and Their Connection to the Working Principle
Understanding how a thrust bearing works makes its failure modes easier to understand.
Film Collapse (Lubricant Starvation)
If the lube oil supply is too low, the oil film on the front edge becomes thinner. As a result, the hydrodynamic wedge donot develop normally. The minimum film thickness then reduced, possibly below the level required for full-film lubrication. When this happens, the bearing enters the mixed-lubrication regime.
As surface contact starts to occur, the flakes or coverings begin to wear off. This contact creates more friction and heat. The higher temperature, the lower viscosity of the lubricant, making the film thinner. The process then continues, potentially leading to heat loss and bearing failure (Booser & Khonsari, 2020).
Thermal Distortion (Crowning)
Temperature on pad surface is not always uniform. If some areas are hotter than others, different parts of the gasket expand by different amounts. This will change the shape of the pad surface and may alter the expected cone or flat profile.
In tilting-pad bearings, thermal crowning can shift the effective pressure center away from the pivot location. This changes the balance of forces acting on the gasket and can reduce the minimum thickness of the film to unsafe levels.
Therefore, thermal deformation analysis is an important part of high-performance thrust bearing design. Engineers often use finite element analysis to study the combined effects of temperature and structural deformation (Ettles, 1980).
Edge Loading (Misalignment)
Shaft misalignment or bearing housing deformation may cause the thrust collar to tilt relative to the bearing pads. When this happens, the load is concentrated on one edge rather than scattered across the pad surface.
This condition is called edge loading. It can add a lot of local pressure. In some cases, the pressure on the edge of the load may be three to five times higher than the average pressure on the entire pad. Therefore, even if the overall bearing load remains within the design specification, the Babbit layer may exceed its fatigue limit.
Pivoting Instability (Tilting-Pad Bearings)
In high speed and low load tilt pad thrust bearings, the pad may not be stable in one position. Instead, the pads can be angled back and forth.
This behavior is called pivoting instability. It is similar to an oil whirl in a journal bearings. Instability produces vibrations at a frequency lower than the axis speed.
The possibility of this problem depends on the relationship between stiffness generated by pivot support and stiffness generated by hydrodynamic forces in the lubricating film. If the balance becomes unfavourable, it becomes difficult to maintain a stable operation (Childs, Turbomachinery Rotordynamics, Wiley, 1993).
Conclusion
The working principle of liquid film thrust bearings is based on a simple physical fact. When the two surfaces move relative to each other, the viscous fluid in the a converging gap creates pressure. This pressure is generated by hydrodynamic action and does not require external pressure sources. For this reason, thrust bearings are designed to create, maintain, and control polymerized lubricant film under different operating conditions, such as load, speed, temperature, and mechanical deformation.
In the manufacturing process, fixed cone bearing and step bearing create the converging geometry. The shape is built on the bearing surface and remains fixed during operation.
Tilting-pad bearings work differently. Each gasket can be adjusted during use. This allows the pad to continuously form a converging wedge that meets current operating conditions. As load, speed, and lubricant properties change, so does the angle of the gasket.
In both types of bearings, lubricating film separates the bearing surface from the rotating thrust collar during normal hydrodynamic operation. The load is achieved by the pressure generated within the lubricating film rather than by direct contact between solid surfaces. The lubricating film is usually only a few micrometers to a few dozen microns thick, depending on operating conditions and bearing design.
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