When choosing tilted-pad bearings, users should be aware of certain fundamentals:
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At the same time, thrust bearing rated loads are typically based on a minimum film thickness for low-speed operation, a mechanical limit for moderate-speed operation and the white metal temperature for high-speed operation (Figure 1).
Figure 1: A slim-equalized thrust bearing is primarily used in high-speed machinery with limited free axial space, such as overhung compressors[/caption]
For low-speed operation, the minimum film thickness should be at least twice the thickness required for operation with clean, cool oil and fine machine surface finishes. As the bearing manufacturer has no control over the surface finishes of the thrust collar or the cleanliness of the oil, it is prudent that a safety factor of two be built into the film thickness and load ratings.
For moderate-speed operation, however, the mechanical limit of the bearing is not normally a factor when using typical thrust bearings. The structural design limit of most thrust bearings is five-to-six times that of the rated load value. Thus it can handle instantaneous surge loadings, caused either by process surges or seismic tremors. The bearing can absorb these momentary surges without difficulty even in the oil films. However, it cannot operate continuously under these surge loadings.
White metal temperature limits the load-carrying capability of thrust bearings for high-speed operation. Usually the practical limit of 115ºC is a maximum temperature for most load ratings, measured at the 75/75 location on the white metal face of the thrust shoe. Many tests have been conducted at operating speeds greater than 100 meters per second using various thrust bearings.
The white metal will not start to become plastic or creep until temperatures reach 160ºC. The unit load would have to more than double for the temperature to increase from 115ºC to 160ºC. A safety factor of two will assure safe operation. Such safety factors comply with end-user specifications, such as the American Petroleum Institute.
In some applications, though, specifications restrict white metal temperatures to less than 115ºC. In others, the original equipment manufacturer needs a bearing that will fit in existing space but carry even greater loads than recommended by most manufacturers. For such applications, some alternatives are available.
Chrome-copper backed thrust pads, for example, can be used in place of standard thrust shoes as the backing material’s high thermal conductivity absorbs heat, thus reducing maximum white metal face temperatures. At high loads and moderate to high speeds, this can reduce temperatures by 8 ºC to 25ºC (Figure 2). Further, they can increase load carrying capability on the order of about 20%. If center-pivoted models are selected, they can be used for bidirectional rotation.
Figure 2: Temperature comparison at 3.00 MPa of configurations showing improvements
over center tilt steel pads with offset tilt, chrome copper and a combination of the two[/caption]
However, chrome-copper-backed pads must be applied with caution as they are not nearly as effective at lower operating speeds and in light load applications. In addition, they offer no increase in load capacity during low speed operation.
Thrust bearing theory indicates that tilting thrust pads will not operate if they are flat and centrally pivoted. That is, they do not have load-carrying capability. This is compensated for by thermal and elastic crowning to enhance load carrying capability in centrally pivoted shoes. However, it has been generally recognized that a better way to achieve maximum load capability is to offset the pivot for unidirectional applications.
In recent years, there have been real breakthroughs in optimizing pivot location and pivoted thrust pads. The optimized pivot location increases minimum oil film thickness dramatically. It also achieves almost perfect crowning, thus using the entire surface area of the thrust pad to carry a load.
As a result, pad temperatures at the same unit loadings as the standard bearings are reduced dramatically. At high load and high speed operation, there can often be more than 25ºC reduction in temperature between the standard pivoted thrust pad and the optimized offset pivot thrust pad.
Ultimately, the optimized offset pivot thrust pad can carry up to 50% greater load than the standard center pivot pad and still yield operating temperatures that are less than those of a center pivot steel pad.
It should be noted that the optimized offset pivot thrust pad is effective at all operating speeds, because the minimum oil film thickness is increased over the standard center pivot pad. But they are good only for unidirectional operation. And care must be taken when installing one of these bearing on each side of the thrust collar.
Directed lubrication technology is another possible alternative as it can increase the bearing’s load carrying capacity while decreasing white metal temperature and oil flow rates. Directed lubrication thrust bearings with steel-backed pads can provide a temperature advantage of up to 12ºC, depending on load and shaft speed, as well as a boost in load-carrying capability.
In addition, certain directed lubrication bearings will operate at oil flow rates up to 60% lower than a standard bearing. While not necessary for every turbomachinery application, directed lubrication thrust bearings are typically the best choice for highspeed operation.
Tilting pad thrust bearings are designed to transfer high axial loads from rotating shafts with minimum power loss, while simplifying installation and maintenance. The shaft diameters for which the bearings are designed range from 20 mm to more than 1,000 mm. The maximum loads for the various bearing types range from 0.5 to 500 tons. Bearings of larger size and load capacity are considered nonstandard but can be made to special order.
Each bearing consists of a series of pads supported in a carrier ring; each pad is free to tilt so that it creates a self-sustaining hydrodynamic film. The carrier ring may be in one piece or in halves with various location arrangements.
Two options exist for lubrication. The first is to fully flood the bearing housing. The second, which is more suitable for higher speed applications, directs oil to the thrust face. This oil is then allowed to drain freely from the bearing housing.
Similarly, two geometric options exist. The first option does not use equalizing or leveling links (Figure 1). This option is used in many gear units and other shaft systems where perpendicularity between shaft centerline and bearing faces is assured.
Figure 1. Flooded Lubrication:
Typical Double-thrust Arrangement
Bearings for both flooded and directed lubrication are intended for machines where an equalized thrust bearing is specified by API requirements, or where the bearing may be required for other reasons.
The conventional method of lubricating tilting pad thrust bearings is to flood the housing with oil, using an orifice on the outlet to regulate the flow and maintain pressure. A housing pressure of 0.7 to 1.0 bar (10.1 to 14.5 PSI) is typical, and to minimize leakage, seal rings are required where the shaft passes through the housing.
Although flooded lubrication is simple, it results in high parasitic power loss due to turbulence at high speed. Where mean sliding speeds in excess of 50 meters per second (m/s) are expected, these losses may be largely eliminated by employing the system of directed lubrication. As well as reducing power loss by typically 50 percent, directed lubrication reduces the bearing temperature, and in most cases, oil flow.
Some typical double-thrust bearing arrangements using directed lubrication are shown in Figure 2.
Figure 2. Directed Lubrication: Typical Double-thrust
Arrangements Designed to Prevent Bulk
Oil from Contacting the Collar
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It should be noted that:
Directed and flooded bearings have the same basic sizes and use identical thrust pads.
Preferred oil supply pressure for directed lubrication is 1.4 bar (20.3 PSI).
Oil velocity in the supply passages should not exceed three meters per second (m/s) to ensure full pressure at the bearing.
The bearing housing must be kept free of bulk oil through an ample drain area around the collar periphery.
No seal rings are required on the shaft.
Manufacturers offer a variety of pad materials. Some polymeric materials are capable of operating at temperatures up to 120°C (248°F) higher than conventional white metal or babbitt. Also, pad pivot position can have an effect on thrust pad temperature.
All pads can be supplied with offset pivots, but center-pivoted pads are preferred for bidirectional running, foolproof assembly and minimum stocks. At moderate speeds, the pivot position does not affect load capacity; however, where mean sliding speeds exceed 70 m/s, offset pivots can reduce bearing surface temperatures and thus increase load capacity under running conditions.
Thrust bearings can be fitted with temperature sensors, proximity probes and load cells.
In hydraulic thrust metering systems, a hydraulic piston is located behind each thrust pad and is connected to a high-pressure oil supply. The pressure in the system then gives a measure of the applied thrust load. Figure 3 shows a typical installation of this system complete with control panel, which incorporates the high-pressure oil pump and system pressure gauge calibrated to read thrust load.
Figure 3. Hydraulic Thrust Metering
Arrangement
For systems incorporating load cells or hydraulic pistons, it is typically necessary to increase the overall axial thickness of the thrust ring.
Finally, thrust bearings incorporate hydraulic jacking provisions. These provisions ensure that an appropriate oil film exists between thrust runner and bearing pads while operating at low speeds.
At startup, the load-carrying capacity of tilting pad thrust bearings is restricted to approximately 60 percent of the maximum permissible operating load. If the startup load on a bearing exceeds this figure and a larger bearing is not an option, the manufacturer can supply thrust bearings fitted with a hydrostatic jacking system to allow the bearing to operate with heavy loads at low speeds. This system introduces oil at high pressure (typically 100 to 150 bar (1,450 to 2,175 PSI) between the bearing surfaces to form a hydrostatic oil film.
It should be noted that a similar approach is taken when making hydraulic jacking provisions for radial bearings. A hybrid thrust bearing is offered by Kingsbury and Colherne Company (based in the United Kingdom) under the name KingCole.
The bearing housing requirements for the KingCole LEG bearing are similar to those of standard thrust bearings. Oil seals at the back of the carrier rings are not required because the inlet oil is confined to passages within the base ring assembly. Fresh oil enters the bearing through an annulus located at the bottom of the base ring. The discharge space should be large enough to minimize contact between the discharged oil and the rotating collar. The discharge oil outlet should be sized so that oil can flow freely from the bearing cavity.
The manufacturer recommends a tangential discharge opening, equal in diameter to 80 percent of the recommended collar thickness. If possible, the discharge outlet should be located in the bottom of the bearing housing. Alternatively, it should be located tangential to the collar rotation. The bearing pads and carrier ring are constructed so that cool undiluted inlet oil flows from the leading edge groove in the bearing pad directly into the oil film. The cool oil in the oil film wedge insulates the white metal face from the hot oil carryover that adheres to the rotating collar.
In contrast to the LEG bearing, the oil for spray-fed bearings is injected between the bearing surfaces, not directly on them. This can result in uneven bearing lubrication and the need to supply nonpractical high pressure to achieve true effective scouring of the hot oil carryover adhering to the thrust collar. There is also a possibility for the small jet holes to clog with foreign material.
Friction power loss is claimed to be lower than both flooded and spray-fed bearings due to the reduced oil flow. The flow of cool oil over the leading edge lowers pad surface temperatures and increases the KingCole’s capacity.
The resulting performance improvements are shown in Figure 4.
Figure 4. LEG Bearings vs. Standard Flooded Bearings and Spray-fed BearingsAssuming an oil inlet temperature of 50°C (122.4°F), it is possible to estimate the white metal temperature of KingCole leading edge bearings from Figure 5. These temperatures are a function of surface speed and contact pressure.
Figure 5. LEG White Metal Temperatures at 75/75 Position (6- and 8-pad series, steel pads)Thrust load, shaft RPM, oil viscosity and shaft diameter through the bearing determine the bearing size to be selected.
Leading edge bearings are sized for normal load and speed when transient load and speed are within 20 percent of normal conditions.
All curves are based on an oil viscosity of ISO VG32, with an inlet oil temperature of 50°C (122.4°F). The manufacturer recommends ISO VG32 oil viscosity for moderate- through high-speed applications.
Table 1.
Thrust Bearing Designation
Numbers and Bearing Area
(KingCole 8-pad thrust bearings)
The basic principles of tilting pad journal bearing operation are explained in the selection guides and related literature of many competent manufacturers. One of these is Waukesha Bearings of Waukesha, Wisconsin.
Sources
The Glacier Metal Company in London, England and Mystic, Connecticut; Kingsbury Inc. in Philadelphia, Pennsylvania and Waukesha Bearings in Waukesha, Wisconsin.
Editor’s Note:
This article was published in Heinz Bloch’s book, Practical Lubrication for Industrial Facilities. This and other lubrication-related books are available through Noria's Online Bookstore.
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