Electrical Apparatus magazine, November 2005 Applying vertical tilting pad thrust bearings

In horizontal-shaft electric motors, the operating principle of the sleeve or journal bearing is well known, involving formation of a wedge-shaped oil film between the rotating steel shaft and a stationary babbitt-lined supporting structure. Re-shaping those cylindrical components into parallel flat surfaces, separated by a similar wedge of oil, creates a plate-type or tilting pad thrust bearing. Such bearings are used in large vertical-shaft motors when the bearing loading exceeds the capacity of anti-friction bearings such as the spherical roller type.

To maintain the oil wedge, the babbitt surface is divided into a series of segments (pads or shoes) that can tilt slightly. Most bearings are of the "equalizing" type, in which the pads rest on an interlocking ring of leveling blocks such that irregularities that depress one pad cause adjacent pads to rise slightly, equalizing the load carried by each. Thrust load is applied to the pads by a rigid steel or cast iron thrust block or runner rotating with the shaft. Runner faces are precisely machined and polished to form a smooth, flat surface.

Whereas the allowable load on an anti-friction bearing decreases with increasing speed (because higher rpm means more rapid fatigue cycling), the reverse is true for a tilting pad bearing, because higher speed enhances oil wedge formation.

Fluid friction within the oil causes tilting pad bearing heat loss to exceed that in a roller bearing, usually requiring water cooling of the lubricant. Cooling coils consist of multiple turns of either plain or finned copper tubing, depending upon the heat loss, temperature limit (typically between 100 and 120øC), available water temperature, and the space within the bearing oil sump. Bearing pads are usually fitted with temperature sensors.

Because anti-friction thrust bearings will sustain some radial load, they maintain the vertical centering of the shaft. To perform that function when a tilting pad bearing is used, the assembly includes a pilot or guide bearing with similar pads or shoes surrounding the thrust runner. A second guide bearing (which may be a ball bearing) keeps the lower end of the shaft equally centered.

Industry news...

Featured Technical Article
November 2005

On this page is a summary of the Electrical Apparatus November 2005 featured technical article, by Richard L. Nailen, P.E. , "Applying vertical tilting pad thrust bearings" Einsatz vertikaler Axial-Kippsegmentlager ... Application des butées verticales à patins oscillants ... La aplicación de cojinetes de empuje verticales con almohadillas basculantes...

To order backissues which contain the foreign language summaries and the full article, call 312-321-9440 or visit our online webstore. See also links to additional technical summaries, below.

For our international readers, summaries of the technical articles are provided in German, French and Spanish. (Summaries do not appear when the article is U.S.-specific.)

 

Advertising for the March Electrical Apparatus closes February 15. Classified ads close February 20. (Space is sometimes available after the closing date - call 312-321-9440.)