Thrust Bearings

Centrifugal compressor impellers, with the exception of the open impeller, are thrust unbalanced. The machine also has a requirement for a location device to maintain axial clearances. For these reasons, all centrifugal compressors use some form of thrust bearing.

API 617 recognizes the need for the compressor thrust design to take into account peripheral factors such as the coupling. Gear couplings can transmit thrust to the compressor because of tooth friction. The standard uses an arbitrary friction coefficient of .25, which can be a design basis. Flexible element couplings transmit less thrust because of the lower flexing element axial stiffness.

The basic type of thrust bearing consists of a thrust collar attached to the shaft running against a flat land (see Figure 5-39). The land is normally a steel ring with a babbitted surface. The load-carrying capacity of this bearing is quite limited, making the bearing suitable only for locating purposes. This bearing is commonly used with double helical gear units and is not normally found in centrifugal compressors.

A thrust bearing that physically resembles the flat land bearing is the tapered land bearing. The modification is the construction of the land. The land is grooved radially, dividing the land into segments that are individu-

Thrust Collar Centrifugal Compressor
Figure 5-39. Basic thrust bearing. (Courtesy of Elliott Company

ally tapered to form a wedge. As the collar rotates, relative to the land, oil is carried past the tapered wedges, developing pressure in the oil film resulting in an outward force. This force generates a load-carrying capacity. Theoretically, the tapered land bearing is capable of handling large axial loads, but it can only do so at a limited speed range. The bearing also requires good perpendicular alignment between the shaft and the land to maintain a uniform face gap. This bearing is used only for limited applications in the centrifugal compressor and when used is highly derated.

The tilting pad thrust bearing is available in two forms. The first form is named alternately for one or the other of the inventors, as it was developed by Albert Kingsbury in the United States and A.G.M. Michell in Australia working independently [14]. The bearing consists of a collar or thrust runner, attached to the shaft with the collar either integral or separable, and the stationary carrier in which the pads reside. Various numbers of pads are used, with six or eight being the most common. The pad consists of a babbitted segment, normally made of steel. The load is

Thrust Bearing Collar PhotoTilting Pad Bearing
Figure 5-41. Self-equalizing tilting pad type thrust bearing. (Courtesy of Elliott Company

equalizing bearing is obvious—it can adjust for minor irregularities of the rotor-to-bearing position.

Each bearing described requires a certain amount of axial space, with the simple thrust ring using the least and the self-equalizing bearing the greater amount. The thrust carrying capability of the latter two bearings is theoretically the same, and proponents of the Michell bearing cite deflections in the carrier ring as providing sufficient adjustment to achieve full potential load within the practical limits of bearing misalignment.

API 617 mandates the self-equalizing feature. Steady loads as high as 500 psi can be accommodated with transient loads going higher. However, conservative design practices and some encouragement from API tend to keep the loads on the thrust bearing in the range of 150 psi to 350 psi.

Pads for the multiple pad bearings may be made of a higher heat conducting material such as copper. Load-carrying capability can be increased by use of the copper pads. This option is a good alternate for difficult applications where size limits the use of a standard bearing. Many users do not permit the use of the alternate materials in a new compressor, using the argument that the option should be available for the solution of field problems. It should be mentioned that while the reference to copper is the common usage, in reality the material is a chromium copper alloy.

Compressors built to the API standard are required to have equal thrust capability in both directions; that is, the bearing is to be symmetrically constructed. However, the thrust bearings can be combined using various numbers of pads on each side, or a tapered land can be combined with the multiple-segment design. Other combinations of the four thrust bearings discussed are found in certain isolated applications.

The thrust bearing is responsible for large portions of the mechanical horsepower losses; however, it is the sophisticated multiple-segment bearing that has the highest losses of all the thrust bearing types. The power consumption is due to the churning of the oil in the essentially flooded bearing; therefore, care must be used in sizing the bearing to maintain margins for reliability. Alternative lubrication methods, such as the directed lubrication that uses a spray or jet to apply oil to the pads and eliminate the need for flooding, are available on some designs.

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Responses

  • mandy eberhart
    What is thrust or collar bearing in hindi and images?
    3 years ago
  • wiseman galbassi
    How to sdjust th thrust in centrifugal compressor?
    3 years ago

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