Tilted-Pad Bearing Selection
DESIGN, RATED LOAD AND WHITE METAL TEMPERATURES SHOULD BE CONSIDERED
When choosing tilted-pad bearings, users should be aware of certain fundamentals:
- Load capacity is based on film thickness and white metal temperature
- Larger thrust bearings for low or moderate speed applications are rated for higher specific loads (load per unit of area) than smaller thrust bearings
- Frictional power loss increases with higher rotational speed, greater oil viscosity, and larger thrust bearings.
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.
High-speed operation
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.
Pivoted Thrust Pads (Shoes)
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.
