How to Explain a Kingsbury Thrust Bearing

Place three 8-inch by 10-inch pieces of cardboard on the table in front of you. Place a coffee cup mouth down in the middle of one of the pieces of cardboard and draw around the circumference of the cup with a pencil to make a round circle on the cardboard. Place a saucer on each of the other two pieces of cardboard and draw around the circumference of the saucer with a pencil to make a round circle on each piece of cardboard.

Cut along the lines you have drawn using scissors. You now have three disks of cardboard. Label each of the two larger disks with the words "Thrust Bearing." Label the smaller disk "Thrust Collar."

Push the point of your pencil through the center of one of the larger disks. Push the pencil next through the center of the smaller disk. Push the pencil finally through the center of the remaining larger disk. The three disks are now on your pencil and the smaller disk is sandwiched between the two larger disks. This is a model of a thrust-bearing assembly.

Hold the two large disks stationary on your pencil with the fingers of one hand while turning the pencil with your other hand. Note that the small disk, which is the thrust collar, turns between the two fixed larger disks, which are the thrust bearings. Your pencil represents the shaft of a turbine. As the shaft of the turbine turns, the thrust collar of the shaft turns with it but the thrust bearings do not.

Recognize that the function of a thrust bearing, including a Kingsbury thrust bearing, is to prevent the thrust collar of a shaft from moving laterally, or back and forth. Because the thrust collar is sandwiched between two thrust bearings, the bearings hold the thrust collar and the turbine shaft in position.

Imagine each thrust bearing of your model divided into triangular segments, like a cherry pie. Behind each segment on the side opposite the thrust collar is a screw and a flat piece of metal called a shim. This is the basic design of a Kingsbury thrust bearing.

Think of each pie-shaped segment of a Kingsbury thrust bearing as tilted by the screw and shim behind it. The leading edge of each segment is tilted slightly away from the thrust collar next to it and the trailing edge of each segment is tilted slightly toward the thrust collar. The leading edge of a segment is the edge of the segment facing away from the direction in which the thrust collar turns; the trailing edge faces in the direction in which the thrust collar turns. For example, if a thrust collar is turning in a clockwise direction, the leading edge of a segment will be on the left and the trailing edge will be on the right.

Picture a Kingsbury thrust-bearing assembly completely submerged in lubricating oil. As the thrust collar spins with the shaft to which it is attached, it draws oil under the leading edge of each pie-shaped bearing's segment and expels oil at each segment's trailing edge. Because the trailing edge of each segment is closer to the collar than the leading edge, oil cannot escape as quickly as it is drawn in. Accumulated oil creates pressure between each thrust-bearing segment and the thrust collar.