Many references recommend to do a blue-check or dry-roll check of new sleeve bearings upon installation into an electric motor which determines the need for scraping (to be done only by experienced qualified people). That used to be a little mysterious to me, but I think I have figured it out.
The purpose of scraping new bearings upon installation into a motor is NOT to make them rounder or smoother or to the correct radius (as some people seem to think) .... the bearing manufacturer is perfectly capable of doing that as long as correct specs are used.
The purpose of scraping bearings upon bearing installation into electric motors is to compensate for any misalignment between as-installed position of the shaft/bearing induced by misalignment between the two bearing housings or perhaps gravity sag of shaft. (sleeve bearings are not particularly tolerant of misalignment, especially as ratio of length to clearance increases).
If you have misalignment in the horizontal plane between bearings of horizontal motor, then you may see diagonal pattern. (the angle shows up much more dramatically than you’d think... if you have 3” diameter shaft with for simplicity 3 mils clearance and a horizontal angle misalignement of 3 mils per three inches, then angle you see is NOT 1/1000 which would be invisible.... it is closer to 1:1 since the pattern goes from one side of bearing to the other over the length of the bearing). [I added a new message below with example of this].
If you have misalignment in the vertical plane between bearings of horizontal motor, then the blue-check/dry-roll check pattern will be only on one end of the bearing, and not the other. That is the idea of the 80% contact spec for blue pattern/roll pattern.... it should be at least 80% of the LENGTH of the bearing (NOT the width!)
There seems to be a common mis-conception that 80% means you want 80% contact over the entire bearing (including width). That is just plain silly because the journal has smaller radius than the bearing and digging out the bottom of the bearing to form similar radius of curvature for full width would destroy the ability to form a convergent wedge.
How common is this misconception? See attached excerpt from article “Sleeve Bearings: A Modern Use for an Old Technology” by Richard Nailen (normally very accurate imo), published in Electrical Apparatus magazine, February 2008. On the 2nd page of the pdf, he says
Figure 2 is on the first page of the pdf and shows contact angle extending for 120 degrees centered on bottom of bearing.
The accompanying box [Figure 15] is dead wrong. The “before” of Figure 15a looks like pretty good contact to me. The after Figure 15b is full contact (ouch!). The caption for figure 15b says:
All I can say is ouch.. dead wrong. The difference in radius is a design feature and we don't want to change that.
He goes on to talk about various repair specifications with imprecise terms [I agree], but he never tells us that Figure 15 is dead wrong and in fact the entire context of how he refers to figure 15 and the caption of Figure 15 suggest this is supposed to tell us the correct pattern in Figure b.
I think I have an idea why there misperception that you want 80% or 90% over the entire bearing (rather than just a narrow strip down the center whose length is 80 – 90% for vertical alignment as it should be). When machinists work with FLAT surfaces they use percentage of contact of the ENTIRE AREA as determined by blue check to describe how flat and smooth the surfaces are. Sleeve bearing 80% contact should never be confused with that type of check... it is completely different. The after shown in Figure 15b would kill a bearing.This message has been edited. Last edited by: electricpete,
SleeveBearings_ExcerptMisinterprettedScraping.pdf (1,286 Kb, 120 downloads)
By the way, not really related, but here is a photo of blue check from horizontal sleeve bearing motor which I believe represents horizontal misalignment (diagonal pattern of blue removal and slight wear from lower left to upper right of the photo). After about 30 minutes working on it with a gray scotch-bright and WD-40, put it back and tried again and the patern was right down the middle.
I don't know what the WD-40 was for. That's what the mechanic used. If anyone wants to explain it to me, please do.This message has been edited. Last edited by: electricpete,
Thanks Pete... always learn from you.
During bearing installation, I always hear our millwrights discuss "twist and tilt"... and adjustments they make to the bearing to compensate for these misalignment characteristics by either shimming or making minor adjustments to the lateral axis to tweak the bearing position right in. I believe they use either plastigage or dial indicator swing-checks to make these adjustments. If these type of adjustments are possible, I assume it would be preferable to align the bearing in this fashion rather than scraping a bearing. If my assumption is true... do some installations have limitations to the millwright's ability to adjust a bearing's alignment position such that scraping is necessary?
Thanks for sharing your insight Pete. I always get smarter when I take time to read your posts.
WD40 to scotch brite pad is like adding water to a piece of emery paper Pete. It allows you to get a much smoother finish than dry wiping. there will be no scratches when you get done, no matter how miniscule the depth of them may have been had you not used it. I've used and seen used about any lubricant or penetrating oil to help the scotch brite pad.
Thanks for the information on the installation. I used to get in some arguments with a knot headed supervisor I had in a strip mining job when we would pour babbitt bearings, and he wanted the full 80% contact.
thank you epete for sharing...
My experience with ScotchBrite is it breaks down physically and creates debris. I assume the debris includes some abrasive material.
I'm more than a little scared about using abrasives around bearings made of stuff that has embedability, like most any grade of babbitt.
Many Bronzes and aluminums can be successfully finished by honing even though the honing stones break down in use and produce lots of grit.
Finishing babbitt (and aluminum and bronze) is best done using a cutting tool even though the finishes the produce lack honing's benefits of crosshatch and plateaued finishes.
Most of our industrial bearings are pretty lightly loaded compared to some engine bearings, so we can "get away with" a lot.
The concern raised by Dan is both valid and critical. I have seen too many inexperienced millwrights using emery cloth and/or crocus cloth in the absence of the proper materials. Industrial grade emery consists of the mineral corundum (aluminium oxide) often mixed with other compounds such as magnesia, mullite, and silica (sand). These particles can get embedded in a bearing surface and do tremendous damage to the journal when the machine is placed in service.
Thanks for all the comments and interest.
MachineryWatch – Thanks for your comments. I interpret that most of your discussion applied to clearance at shaft-to-bearing and fit at bearing to housing. Plastigage checks these clearances. Swing check for vertical machines and lift check for vertical machines are also indirect ways of checking the combined clearance shaft-to-bearing plus beairng to housing. Shims can play various roles: If crush/interfernce exists at bearing to housing fit, then shims can be used between splitline for plastigage check.... shims might possibly be used on top of bearing to compensate for excess clearance between bearing and housing although we have to be careful if bearing is insulated and also have to ensure shim does not come loose to interfere with oil ring (I'm not positive whether this is good practice but I've seen it done). I have not heard of using shim for adjusting bearing alignment on a horizontal sleeve bearing motor....it is not mentioned anywhere in EASA or EPRI documents. If shimming is an acceptable method to improve contact pattern for horizontal motors (I’m not saying it is), then we would have to add shim at the rib(s) of the housing beneath the bearing and it would only provide an ability to compensate for vertical plane misalignment, not horizontal and probably not sag.
Dan and John – I definitely appreciate your input and respect your opinion.
At a minimum it highlights the need for very careful inspection of the babbit surface after using Scotchbrite. Whether or not it is acceptable to use Scotchbrite to begin with is something I’d like to discuss more.
Here are two references to consider:
Electric Power Research Institute (EPRI) Report GS-7352 “Manual of Bearing Failures and Repair in Power Plant Rotating Equipment” states:
Electrical Apparature Service Association (EASA) “Mechanical Repair Fundamentals” states
Do you think both EASA and EPRI completely missed the boat in their recommendation to use scotchbrite on babbit? Maybe it is a new concern identified by recent operating experience they were not aware of? Or maybe the concern is different because they are suggesting to use it in a slighly different way that I described (they suggest it as a finishing stage rather than the sole means to re-shape the babbit) ?
I did notice most of the references talk about using a scraping tool. Personally I am a little bit more nervous about a scraping tool than scotchbrite since I picture the scraping tool as a quick / rough change in geometry and the scotchbrite as a slower finer finish. (I realize it also depends alot on who holds the scraping tool). For the same reason, I would be a little bit nervous about using a cutting tool, although I guess it’s not such a big deal if you have a spare bearing ready to go in case there’s a “whoops” moment. If we decide not to use scotchbrite, then what type of cutting tool or other tool would be used? And is the tool incapable of creating rogue embedable particles?
Also I know there are different colors of scotchbrite. I assume these give different roughness and perhaps different hazards for embedable particles.. Any comments on whether grey would be a good grade to use if we use scotchbrite?This message has been edited. Last edited by: electricpete,
electripete, at my former employer (the gear manufacturer) we used a material that resembled a piece of screening, not unlike the mesh of window screening. We called it "screen cloth" although I'm not certain if that was a trade name and Google doesn't come up with anything by that name. One of the objections I would have with Scotch-Bright pads is that they are made with ultra fine grade silicon carbide (see http://solutions.3m.com/wps/po...WS904glLP8NVZTQJBbl). This presents a scenario exactly like, in my experience you want to avoid; the embedding of abrasive material into the babbitt surface. This is supported by what Dan Timberlake states..."My experience with ScotchBrite is it breaks down physically and creates debris. I assume the debris includes some abrasive material. I'm more than a little scared about using abrasives around bearings made of stuff that has embedability, like most any grade of babbitt." Dan's link to information concerning a Tri-metal bearing is also interesting in that we made a line of gearboxes that used these bearings at rotor speeds from about 14000 RPM and up. They were selected because of very high babbitt fatique resistance, which as you know declines rapidly with thicker babbitt. As the aritcle points out, the babbitt was 1 mil thick, so virtually any abrasive product would remove the top layer (babbitt) and leave the next layer of copper.
By the way, a related interesting thread exists at http://maintenanceforums.com/e...451/m/2151038503/p/1
Scotch Brite.This message has been edited. Last edited by: John from PA,
I do know that your (and my) former employer from which you recently retired has used 'Scotlchbrite' to clean bearing surfaces. I would hesitate to call this scrapping.
If you limit the material removal to less than 0.0001 inch you may be ok. 'Scotchbrite' can clean the surface without changing the geometry. Actual scraping can significantly alter the bearing geometry. This is not the best practice for critical machinery.
My advisor for mechanical engineering worked at a machinery company. He related a story about him picking up a scaping tool (around the late 50's early 60's - times have changed), and a senior technican informing him that only certain people were allowed to scrape. This was because of the companies experience with getting machines to run. In all likelyhood, the mechanic was adding a similar benificial profile to his bearings.
There was also a story about bearing crush, which adds a preload. Not everyone could do this well. Today, these items are taken care of by the design and manufacturing of the bearing, not by trial and error scrapping.
I haven’t heard of tri-metal or bearings with babbit that is 1 mil thick. That is different equipment than I work on.
My personal interest is sleeve bearings in electric motors.
If I understand what you and Dan are saying, it is that the concern is based on your own intuition, not an actual experience where using abrasives on babbit caused a particle to become embedded in the babbit and/or resulted in shaft damage, right?
It’s still good input for consideration either way... I just wanted to clarify the basis.
Let me add some more references:
EPRI TR 100897 – “Repair and Reconditioning Specification for AC Squirrel-Cage Motors with Voltage Ratings of 2.3 to 13.2 kV” [December 2000]
The exact same requirement as above appears again in the December 2008 rev of the EPRI motor repair specification document (re-numbered as EPRI 1016679). The EPRI document GS-7352 referenced earlier dates back to 1991.
The quote above from EASA Mechanical Repair Fundamentals also appears in the EASA “Technical Manual” as well as EASA Technical Note 38 – “Repairing Sleeve Bearings”
EASA and EPRI are pretty reputable resources in my opinion. EPRI supports the US power industry which probably has more industrial sleeve bearing machines than any industry other in the US. EASA sets the standard for motor repair practices (for customers from all industries) across the US.
These documents are the closest thing that I know of to “standards” for this particular type of activity. I assume it is the same for many other people in the power and motor repair industry. I assume therefore many people follow the practices recommended in these standard-like documents (EASA repair shops are supposed to follow EASA practices, all repair shops are supposed to follow customer specs).
Despite what I would presume to be widespread usage of these practices over a period of time based on documents published 1991 thru 2008, I have not heard of any problems attributed to the practice of using scotchbrite on babbit.
That’s not proof of anything. But it’s something to think about and it weighs pretty heavily in my own personal thoughts about the subject. Everyone is free to draw their own conclusions.
The more information, analysis and opinion that is offered on any side, the better all of us can do.This message has been edited. Last edited by: electricpete,
A dry waller's screen is basically what John is referring to or so I think.
I have scraped bearings via the ole time various styles and shapes of scrapping tools that use to be available. Most or all of these jobs were due to the hand pouring technique.
Some large ball mills were poured on-site and hand scraped on-site.
I have hand scraped turbine bearings as well. This is not common practice and generally done to get things running vs waiting on a new bearing.
My comment was just personal opinion.
I hold API, EPRI, AGMA and other groups composed of practicing, practical folks in real high regard.
If they are comfortable with applying Scotchbrite to produce a finish a motor bearing's babbited surface, I guess I would be too.
I'll still be concerned about machines with projected area bearing loading over 200 psi.
The Scotchbrite I've used are the commonly available green and dark red grades.
I'd like some of 3M's rotor-peen flappers for shaft straightening.
Thanks Dan. That is something to think about. If Dan/John or anyone wants to add any final thoughts about abrasives or scotch-brite on babbit, please do. I’ll let you have the last word on that (I promise I won’t post any more references or use the “we’ve always done it that way” argument again.)
I think you are looking at the purpose of scraping differently than me. My viewpoint as stated in my original post was:
As-installed misalignment is obviously something the bearing manufacturer cannot address.This message has been edited. Last edited by: electricpete,
We poured many a babbitt bearing on draglines, scraped them, and smoothed them with various means, scotchbrite being one of them. Our application was very low speed . We never poured them for the MG sets. When we poured the last ones I can remember (Marion 7800), we poured 1000# at a time with a crane. We always cleaned the shells with various liquids from varsol to diesel fuel and rags. If the rags would not slide over the babbitt surface without snagging, then it needed more polishing. Never used a microscope to look at them for embedded particles because again, it was slow speed and huge. I could see where the concern is though, as scotchbrite does break down as you use it, even when you use a liquid to fine it down.
I would probably be more concerned with a turbine/generator, but unless I used a lot of force during the scotchbriting (new word) and forced the particles way down in the material, I would count on the oil flow taking small loose ones away.
In an oil field one may have many recips, engines and compressors. There can be many sleeve bearings in these applications (tri-metal may be used). I assume industrial means other than vehicles, because someone like the post office would have many bearings.
OK Bill. I withdraw the suggestion that EPRI is largest user of sleeve bearings.
In view of my post dated 19 December 2011 03:05 PM, would you care to withdraw or explain your comment "Today, these items are taken care of by the design and manufacturing of the bearing, not by trial and error scrapping." ?This message has been edited. Last edited by: electricpete,
Technique and methodology holds tolerances to 0.000n" (even if not an OEM shop or related); while,
Design locates the bearing on concentric center-lines with other machine components on concentric center-lines.
Remove setup distortion and you should be where you want.
It's been a long time since I've seen anyone scrape motor bearings. The large one's in ball mills for example are basically 3' radius and hand poured on-site, then fitted.
Per adventure and hopefully I haven't over-stepped here with this answer. But, this is what and how I see it.
But; still some drop a wrench on the babbit - and scrape the fault or similar incident. Ouch!
As you know, machines don’t always meet design. And we have frame distortion from foot problems, foundation settling, heavy teminal boxes. And these things can make the real world different than the paper world.
That’s why we do the blue check, to find out what we've got.
If blue check is fine, you’re right, concentricity is close enough that we don’t expect any effect on the bearing.
If blue check fails, the condition of our machine doesn’t meet industry standards, we have to decide what to do about it. Scraping (possibly Scotchbrite if you’re comfortable with it) seems like a reasonable thing to do about it if you have someone available that is qualified and experienced in that task. As stated in the EPRI quote above, applies to plain circular journal bearings, not tilting pad or lemon bore etc.
Just a side comment – The mechanical foreman at one of our local motor repair shops described for me their practice that if the two sleeve bearings on a given motor differ from each other by more than 5C during uncoupled run, they stop the run and scrape the bearing on the hot side until the difference becomes less than 5C ....if time allows (usually means if there’s nothing waiting behind it to use the test stand... they just leave the motor on the stand, do the scraping and re-try the run). He said it’s not uncommon for motors not to meet this on initial run and he’s never had one that he couldn’t bring to within 5C. (I guess if the two bearings are equally stressed from the misalignment, the comparison doesn't work) There may have been a little salesmanship involved in that story... take it with a grain of salt.This message has been edited. Last edited by: electricpete,
There are other means to remove distortion and induced stresses and those should be employed.
A motor's foot should never be enlarged is an example. IMHO; neither should the bearing's geometry changed to correct an underlying fault. Address the fault.
The motor in question; is it of a poor design? Can it be improved? Can engineering correct the problem? Can engineering get out of the way and let the skilled craftsmen do their job - sometimes that is THE solution;-D
|Powered by Social Strata||Page 1 2 3|