The following comes from a recent topic on SmokStak, which can be found on the Internet at: www.engineads.com/smokstak.cgi. As ever, various individuals started, commented on and concluded the following bulletin board thread.
Scraping babbitt is something I have never seen or done, and I'd like to hear from people who have so I can learn how it's done. I have bought some tools I believe to be scraper tools -they look like files with no teeth. Let's see if we all can learn something new. - Kevin
I have seen it done by skilled craftsman, and it's not a process for the impatient. I have watched it done during the rebuild of a precision surface grinder, and there are no shortcuts. The craftsman uses a straight surface such as a granite bar or plate, and he covers the surface with bluing paste and slides the part on the plate. He then scrapes off the clean or high spots that show up on the part until he gets a desired number of contact spots per square inch. He may go back to the surface plate many times to get the contact area he needs.
With round shafts running on babbitt he uses the shaft that runs on the surface. For that process the tool looks like a three-corner file with no teeth. He puts the shaft in and takes it out many times before he gets the desired bearing contact area. This process takes volumes of patience, coffee or whiskey - and determination. Try it on a new set of cast babbitt main bearings and you will be amazed at how close you feel to your newly restored engine. Many of the large precision engines we see have had their bearings scraped, and most steam engines had the main guide box and bearings scraped. It takes a lot of time, but what fun it is. - Al
About 30 years ago I scraped the mains on an American LaFrance 570-cubic-inch T-head engine. It was fun, except for lifting the crankshaft into the mains about 100 times. My buddy on the project and I had sore arms and shoulders for about a week. My scraper was made from a half round file that I ground on a belt sander. It was so sharp I could peel the babbitt off like thin snowflakes. I also could cut the heck out of myself with the scraper. I've still got that scraper and it's wrapped up in an oiled rag so I don't cut myself when groping around in that particular toolbox. - Elden
I worked at a shipyard for 20 years and one of my jobs was on a wheel gang made up of eight workers. Our job was to fit the propeller wheel onto the output shaft. The shaft diameter was usually 34 inches and the taper was about four feet long. The propeller wheel was usually about 20 feet in diameter. After using about a half a pint of Prussian blue to cover the taper we would slide the wheel on by using four 75-ton chain falls, then put on the nut (thread size 24-inch by about 8-tpi) and tighten it. After that we would take off the nut, break the taper and remove the wheel with the four 75-ton chain falls. You'd stick your head in the wheel and grind the high spots with a 4-inch hand grinder, clean the shaft, clean the wheel, re-blue and repeat the whole process again. After about five tries we would have a fit the Coast Guard and chief engineer would approve. All of this took about 10 hours. I have done this so many times I lost count. Pretty soon I'm going to try the bearings on my Monitor -where can I find tiny chain falls? - Patrick
I am in the process of restoring a 1913 Associated air-cooled engine that had a real bad main bearing. After checking on having it poured I decided to pour it myself. The pour went well and I ended up with a good bearing that just needed finishing touches. I have a set of bearing scrapers that were given to me, so I went at it. It took a little time to get used to what I was doing, but I ended up with a great fit and doing it myself felt good.
The hardest part of pouring and fitting a babbitt bearing is getting over the feeling that you might get it wrong. Don't worry about it, if you make a mistake it's nothing to remove all the babbitt and go at it again. I'll do it again in the blink of an eye if the need comes up. - Bob
Scraping bearings differs from scraping ways and other flat surfaces. You can make a good bearing scraper from a triangular file by grinding off all of the teeth and finish honing the tool on an oilstone. The trick is using Prussian blue dye, test-fitting the shaft carefully, and not hogging off too much metal at once. Take your time and you won't be disappointed. Be sure to add your shim packs each time you try the shaft so that your final fit will be done with all the shims in place.
Scraping ways and other steel surfaces is a different game. The scraper resembles a rectangular file (again, without any teeth) and with the tip (opposite end from the handle, or tang) ground to a slight radius and to a slight angle with the scraper body - in other words the tip is not dead square with the scraper body. The surface is blued and checked with a precision flat, and high spots are carefully removed using a combination twisting and pushing action.
A person skilled in this art is known as a 'scraper hand.' As you might expect, it takes good training and lots of experience to master this work. We have all marveled at the ornate 'frosting' applied to some work, which actually serves to slightly roughen the surface to hold lubrication. There is a scraping school on the east coast, I believe in the Carolinas, and I know a gentleman who sells instructional tapes and scraping equipment. This would be a wonderful skill for any machinist to acquire, but I don't think it is being taught in schools anymore. You can probably find info on scraping on-line. I'd search for 'hand scraping' or 'scraper hand.' - Harvey
Most millwrights and millwright helpers in chemical plants learn to scrape. The large multi-rotor compressor and turbines use seals between the wheels in the lower and upper case halves that need to be scraped to exact specifications. As a crane operator, I learned this skill between lifting duties.
The rotor assembly was carefully measured, then the seal assemblies in the upper and lower case were measured, then the scraping would start. When the millwrights had the tolerances close they would blue the rotor assembly and set it in the lower half. One or two rotations by hand and it would be removed and checked for proper clearance. They continued this process until all tolerances were to specifications - it could take up to 36 hours of assembly and disassembly before they had it right.
In the course of 32 years of doing this kind of work the millwrights in my plant never had a rework. Keep in mind the upper and lower cases could weigh up to 100 tons each and the rotor assembly with the wheels could weigh up to 50 tons. - Russell
Kevin, I poured new bearings for the transmission shafts on my IHC 10-20 Titan and scraped them. It was a first-time attempt, but it worked fine - just don't hurry. I recently poured and fitted bearings on an old wood planer whose shafts turn at around 5,000 rpm, and with a wide range of pressure and vibration. The shafts are about 4 to 5 inches in length.
I also found that Permatex makes an aerosol can of blue marking dye, which really helps a lot. - Doc
For shafts of 1 -1 /2 inches or smaller, I have used a large black magic marker. I coated the shaft with black magic marker and it left a nice black spot on the babbitt surface.
I have a factory three-corner scraper, but I have also used a pocket knife honed to a very sharp edge. I can carefully scrape off the dark spots holding one hand on the point and the other on the handle, keeping the blade perpendicular to the bearing surface. I take off maybe two or three thousandths at a time. Clean the shaft, reapply blue/black and repeat until you have the desired contact. One book I read said a 50 percent contact area is acceptable on these old engines.
Also, when you pour new bearings it is sometimes easier to use another shaft the same size as the one you want to fit. I make a wooden jig to hold the 'sample' shaft with the right spacing from the main cap, rod cap or the rod. To avoid a lot of scraping, take your time setting up the jig, making sure the cap is square and parallel to your shaft. You are better off to set it up so you will need some shims to start with. About 1/8-inch will do. Remember, it takes patience - it took me the better part of eight hours to scrape and fit the three bearings on my 1934 Bell sawmill. - Don
One final word. When pouring babbitt, make certain your form has been heated so it's completely free of moisture - steam explodes! Safety First! - Harry
The above messages and many more can be found by visiting SmokStak on the Internet at www.enginads.com SmokStak is an engine conversation bulletin board with over 43,000 messages on file and is part of the Old Engine series of Web sites that started in 1995 as 'Harry's Old Engine.' Harry Matthews is a retired electronic engineer and gas engine collector from Oswego, N.Y., now residing in Sarasota, Fla.