Chuck Yung
EASA Senior Technical Support Specialist
When sleeve bearings are rebabbitted or replaced, an important step during assembly is to check the contact between the sleeve bearing and the journal which rides in it. The use of self-aligning sleeve bearings (also called spherical or ball fit) renders this step almost unnecessary. Still, cylindrical sleeve bearings should be inspected to make sure the contact area is sufficient.
Sleeve bearings, also known as babbitt bearings, plain bearings or white metal bearings, have been in use for over 150 years. For a detailed explanation of sleeve bearing design and operation, request the EASA 2007 Convention paper, “Sleeve Bearing Repair Tips,” or see Mechanical Repair Fundamentals of Electric Motors, 2nd Edition.
This article is specific to checking and correcting the wear pattern when installing a new sleeve bearing in an electric motor. Fitting a sleeve bearing is not difficult; it just requires some basic knowledge. An interesting bit of history: the toolkit provided with the old Model A Ford automobile included a babbitt knife for scraping crankshaft bearings. Imagine dismantling your engine alongside the road to remove and fit the babbitt bearings.
Designers of electrical rotating equipment generally keep sleeve bearing load pressure around 145 psi (1000 kPa) compared to 580-725 psi (4000-5000 kPa) for internal combustion engines. Some older motors used even lower bearing load pressure, so vintage machines sometimes have a larger bearing than a modern motor with similar characteristics.
The heavier the rotor and/or the lower the speed rating, the longer the bearing usually is. It follows that the longer the bearing surface, the greater the chance of angular misalignment between the bearing and journal.
Clearance
I am often asked, “How much scraping is too much?” To answer that, we measured sleeve bearings before and after scraping them and found that, in most cases, there was no discernable change in the bearing bore diameter.
Fitting
With the spherical fit design, after both lower halves have been installed, use a hoist to gently lift and “settle” each end so that the bearing aligns to the journal.
Fitting a new sleeve bearing is an important part of the assembly process. Install the bottom half of each bearing and use a finger to wipe a small amount of oil onto the journal. Spin the shaft by hand to establish a wear pattern quickly. You may need a spanner wrench to get it started, but hand pressure is sufficient to keep the shaft spinning. Thrust the shaft axially several times while it rotates. Use a nylon strap with the overhead crane to lift the shaft just high enough to permit the bearing half to be easily rolled out by hand.
High points of contact are readily identifiable by their shiny appearance. (Figure 1) The objective is to obtain a reasonable percentage of contact area across the bottom half. When viewing the contact area as a proportion of the total bottom half area, 30 percent contact is considered reasonable. A realistic goal is that there should be 60 percent contact in the load zone of the bearing.
Hand-fitting involves repetitive adjustment, often referred to as “scraping in the bearings.” This is because the fitting process involves using a babbitt knife or similar tool to shave or scrape small quantities of babbitt from high regions that would otherwise contact the shaft. Regions of high pressure, even small ones, can break through the oil film and elevate the bearing temperature. Scraping is generally done using a babbitt knife or bearing scraper (Figure 2), followed by polishing with a Scotchbrite pad.
The bearing should be thoroughly cleaned after each fitting before being reinstalled for further evaluation. The objective is a minimum of 60 percent contact centered in the bottom half of both bearings. As a practical matter, concentrate on fitting the bottom half of both bearings first. Once you are satisfied with the wear pattern, install the top half of the bearing and the top cap, bolting them securely. With both ends buttoned up, perform one last spin and inspection process.
This allows the technician to verify that no pinch-points exist. Too tight a bearing-to-housing fit may distort the bearing shell and cause bearing-to-shaft contact that was not evident during the initial fitting process. When the shaft centerline is not perpendicular to the stator-bracket fit, the top bracket half may further alter the bearing-to-shaft alignment. There are some older designs utilizing a robust two-piece cast iron end bracket, which were prone to distortion. Installing the top half of the bracket (Figure 3) could change the angle of the bottom half, which in turn can change the wear pattern of the bearing.
Now that I’ve covered the “how to,” let’s cover a few of the “don’ts.” Mechanics sometimes describe the use of Prussian Blue as “bluing.” That has been misinterpreted as being the blue layout fluid used by machinists for parts layout.
Don’t use the blue layout fluid to fit a sleeve bearing. It can act as an abrasive, damaging the bearing or the shaft journal. Also, it is not as effective as the method described in this article.
Don’t use lapping compound to “fit” a sleeve bearing. It will damage the shaft as well as the babbitt.
Don’t use a flapper wheel, emery cloth or steel wool to clean or polish a sleeve bearing.
General Guidelines
Contact between the 4:30 and 7:30 position is preferable to 100 percent contact, or to contact on the sides alone. The shaft should ride on a film of oil. In practice, the shaft will contact the bearing just to one side of the center, depending on the direction of rotation. The shaft tends to “climb” the bearing. That is, if the shaft rotates clockwise when running, the contact is likely to be centered around the 5:00 position, whereas a shaft rotating counterclockwise tends to center around 7:00. The actual position is affected by the speed, rotor weight, shaft diameter and other factors.
Contact on the top half of the bearing is not desirable. Significant change to the bottom contact regions may indicate that the top half of the bearing housing and bracket are not properly aligned. Possible causes include:
- The split-line of the bracket may not be perpendicular to the face of the stator.
- The stator face might not be perpendicular to the shaft axis of rotation. Observe the top half of each bracket for looseness while tightening/loosening the bolts.
- The mechanic might have accidentally turned one bearing top half end-for-end. Or put the bearings in the wrong end. It really does happen.
- Dowel pins (or their holes) may be distorted or damaged. A previous repairer might even have swapped the bearing top halves.
Final Thoughts
Once the bearings are adequately fitted, finish assembling the bearing housings with oil-rings, etc. Fill to the correct oil level and prepare the motor for a test run. For 2-pole machines, it is especially critical to monitor the bearing temperature. If a region of contact remains, the resulting friction can cause bearing temperature to increase so quickly that there is little time to react before bearing damage occurs. If you see smoke, it’s already too late. Use a vibration analyzer, placing the accelerometer on the bearing housing (axially), to monitor in velocity units (not displacement). If the bearing friction increases, the velocity reading will suddenly begin to climb. Consider that as an early warning before the bearing temperature increases. It might save a bearing. When possible, use an accelerometer on each end of the motor, one placed axially on each bearing housing. This early-warning system is local to the end with the failing bearing.
AVAILABLE IN SPANISH
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100- Section 2: Mechanical repair
Print
