Chuck Yung
EASA Senior Technical Support Specialist
One aspect of our repair industry that seems to cause a lot of confusion is the bearing arrangements of vertical motors. There is quite an assortment of thrust bearing arrangements, and we tend to try to apply the same method for adjusting end play to all of them. That is often a bad idea, so this article will take a closer look specifically at those fitted with a spherical roller thrust bearing and repair tips unique to these designs.
As with any vertical machine fitted with one or more thrust bearings, it is important to position the bottom guide bearing (Figure 1) as high as practical in the bottom bracket housing to provide as much room as possible beneath it for thermal expansion of the shaft.
Spherical Roller Thrust Bearings
Spherical roller thrust bearings are used for higher thrust ratings when the speed rating of the bearing is compatible with the machine rpm. A typical construction is shown in Figure 2. The rollers have a much larger contact area than balls. The roller axis is at 45° to the shaft and the bearing can carry both moderate- and high-thrust loads. The rollers must be guided, however, and therefore they develop more friction than the ball type. At high speeds, water cooling of the lubricant may be necessary, and lubrication is more critical. The bearing must also be constantly loaded, or it may separate due to centrifugal forces if no thrust is present. That is one purpose of the springs often installed beneath the outer race. The other reason for those springs is to prevent impact damage when the thrust load is restored after an up-thrust event.
The procedure to adjust end play for large motors with spherical roller thrust bearings not fitted with springs is illustrated in Figure 3.
For motors fitted with preload springs, there are two acceptable methods to set the end play/thrust. The preferred method is to fully compress the springs by placing a jack on top of the bearing carrier (Figure 4) and by using chains or cable from the lifting ears of the motor across the jack to press the bearing carrier down and compress the springs. At that point, adjust the thrust as normal.
The second method is to assemble the motor without the springs and set it using the same procedure shown in Figure 3. Use a depth mic or Vernier caliper to document the required distance between the top of the shaft and the bearing carrier. The top end can then be reassembled with the springs.
The reason for using this second method is practicality. In most designs, the springs are sufficiently strong to support 30-40% of the thrust load. In other words, a large spring set such as that shown in Figure 5 would require an impractically large jack to compress the springs.
Additional Issues
The springs should match; they must be of the same compressive strength and height. Spring height should be measured using micrometers or Vernier Calipers. The depth from top of springs in Figure 5 to top of screws varied from 0.023” to 0.160” (1/2 to 4mm) before surface grinding.
If spring heights differ more than 0.010” (0.25mm), place them upright on a magnetic chuck and use a surface grinder to trim them all to the same height. An alternative is to sort the springs by height and evenly distribute the tallest springs, then sort the medium height springs between the tallest springs and then shortest springs. The purpose is to evenly distribute the springs to avoid tilting of the outer race, which could lead to premature bearing failure (Figure 6).
There are designs where the thrust bearing cage may have obstructed oil flow, as in Figure 7. Vertical oil passages positioned beneath the spherical roller thrust bearing are located to guide oil flow directly into the roller path. In at least two recent cases, changes to the bearing cage geometry have obstructed oil flow. This has resulted in satisfactory bearing temperature during relatively brief in-shop test runs, with rapid bearing failure when the motor is installed.
A minimum down thrust, based on bearing size, is required during operation. When test running a motor fitted with springs, either a device must be used to provide the requisite down thrust, or the motor should not be run for more than 3-5 minutes to prevent damage to the lower guide bearing. The same caution should be conveyed to your customer, as it is common practice for many of them to run a returned motor uncoupled for some period of time.
Because the springs lift the spherical roller thrust bearing during a solo run, there is ample gap above the oil injection holes for oil to flow. In operation, the thrust load compresses the springs, reducing the clearance and constricting oil flow. This results in elevated bearing temperatures and premature bearing failure.
AVAILABLE IN SPANISH
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100- Section 2: Mechanical repair
Print