SMALL BEARINGS

machine shop work includes bench work fitting and scraping small bearings is an important part of bench work many machines have rotating parts supported by bearings these vary in size from very large to very small let us look at the bearings on an ordinary automobile engine when this engine is running the crankshaft rotates or turns on three bearings for convenience in assembling these bearings are split here the cap of each bearing has been removed with the crankshaft removed we can see the lower half or body of each of the supporting bearings all of these bearings must be in alignment to provide full and true support for the crankshaft oil grooves in each bearing provide lubrication for the turning shaft bearings must support a turning shaft in animation we see the three requirements for a good bearing first the bearing must make full contact with the shaft on both top and bottom to provide full support second the bearing must be in perfect alignment with the shaft to enable the shaft to run true and free and third some means of lubricating the bearing must be provided usually this is done in larger bearings by an oil hole and oil grooves if the bearing is correctly fitted oftentimes by hand scraping it will provide full support be in correct alignment and have oil grooves correctly cut then oil will flow from the oil hole to all parts of the bearing and provide a smooth running trouble-free shaft here is a shaft supported by two split bearings the fitting of these bearings by hand scraping is a typical bench work job these two caps are babbitted bearings with the oil hole already drilled several forms of hand scrapers are used for scraping the curved surfaces of bearings the three cornered scraper is used to relieve small solid bearings the half round curved scraper is more convenient to use on larger split bearings this half round scraper is used for scraping circular bearings scrapers must be sharp to leave a true surface after grinding all cutting edges should be oil stoned first make certain there are no sharp edges or dirt in the bearing he cleans the shaft and then coats those portions that will rest in the bearing with prussian blue or other marking compound since the body or lower half of the bearing will be scraped first the shaft is laid in the bearings and turned two or three times notice the markings appear high up on both sides of the bearing indicating that the bearing is too small to correct this the bearing surfaces indicated by the blue markings must be scraped the half round straight scraper is selected for scraping this size of bearing the scraper is worked from the bottom up following the surface of the bearing removing the metal indicated by the blue markings with a slight increase in pressure as a scraper approaches the top repeat the scraping on the other bearing notice most of the cutting action comes near the end of the scraper only a very thin film of metal should be removed at first with experience the bench worker will know how much material to remove to free a bearing wipe both bearings clean lay shaft in and turn when the shaft is removed this time blue markings appear in the bottom of the bearing indicating that the bearing is now properly fitted prussian blue must always be applied evenly and spread very thinly with the fingers repeat the same scraping and fitting to the top half or cap of these split bearings with the proper shims and top half of the bearing in place tighten the bolts lightly now turn the shaft two or three times and remove the caps again blue markings high up on the sides indicate where the cap must be scraped with the cap held in a vise these markings are scraped in the same manner as the lower half blue markings now appear in the top of this cap as well as on the sides in order to make this bearing run free all of the high spots indicated by the blue must be scraped in cases where the bearing is not aligned with the shaft there will be blue spots at opposite corners the scraping of these points on both halves of the bearings will bring the bearings into alignment the final step in the fitting and scraping of hand oil bearings is relieving the bearings to aid lubrication starting 30 degrees down from the side and about half inch from the end use the half round scraper to remove approximately 10 000 of an inch of metal from each side of the bearing this cross section shows how this relieving forms oil pockets on both sides of the bearing these oil pockets distribute oil evenly over the entire surface of the bearing to properly distribute oil over the entire length of the bearing and to the oil pockets oil grooves in the cap are necessary these oil grooves should start about a half inch from each end to lay out these oil grooves a piece of flexible metal is used to guide a scriber then with a small round nose chisel of the bent type chip a groove about 3 30 seconds deep following the lines as laid out the burrs raised by the chisel cutting these oil grooves are removed with a half round straight scraper the shaft turns freely and the fitting and scraping of these split bearings is completed this shaft which has two diameters runs in another type of bearing called a solid bearing these bearings must be fitted to the shaft by hand scraping the shaft fits tightly in both bearings and the bearings must be relieved by scraping he tries the shaft in the other end of the small bearing to check for size this tells him whether he is to scrape for size or alignment prussian blue is applied to both portions of the shaft which will be supported by the two solid bearings the bluing is quite heavy on only one end of the bearing indicating that this end of the bearing is small for this small solid bearing a three-cornered scraper is used notice the scraping action of this three-cornered scraper the bearings are checked again and this time the blue marks show high spots the entire length of the bearing the bearings are clean and the shaft is inserted once more now the shaft turns freely and the fitting is completed we have learned that the fitting and scraping of bearings is an important part of bench work to do this work properly the bench worker must locate the high spots select the proper scraper and use it with skill according to the job and continue scraping until results show the bearing provides full support is in perfect alignment and provides for lubrication the worker can then be proud of his job for his skill craftsmanship has contributed a bearing that will give long trouble-free useful service you