Page:Popular Science Monthly Volume 88.djvu/164

 136

scrape the bearing to a fit before the bearing cap is tightened up. It may be necessary to clean up the crankshaft journals as these may be scored due to not having received clean oil or having had bearings seize upon them. It is not difficult to true up the crank pins or main journals if the score marks are not deep. A fine file and emery cloth may be used, or a lapping tool. The latter is preferable because the file and emery cloth will only tend to smooth the sur-

��^Crdnk ShaFt ^

� �//Zwk /iJ ^®\l

�^

�\ 1 1 Main Bearing Cdp^X

Top Half Crank Ca&e ^

��Fig. 15. Top half of crankcase, showing method of crankshaft re- tention by three main bearing caps

face while the lap will have the effect of restoring the crank to proper con- tour.

If a crank pin is worn out of true to any extent the only method of restoring it is to have it ground down to proper circular form by a competent mechanic having the necessary machine tools to carry on the work accurately.

After the crankshaft is trued the next operation is to fit it to the main bearings or rather to scrape these members to fit the shaft journal. In order to bring the brasses closer together, it may be necessary to remove a little metal from the edges of the caps to compensate for the lost motion. A piece of medium emery cloth is rested on the surface plate and the box or brass is pushed back and forth over that member by hand, the amount of pressure and rapidity of move- ment being determined by the amount of metal it is necessary to remove. This is better than filing because the edges will be flat and there will be no tendency for the bearing caps to rock when placed against the bearing seat. It is important to take enough oiT the edges of the boxes to insure that they will grip the crank tightly. The outer diameter must be

��Popular Science Monthly

checked with a pair of calipers during this operation to make sure that the sur- faces remain parallel. Otherwise the bearing brasses will only grip at one end and with such insuf^cient support they will quickly work loose, both in the bear- ing seat and bearing cap.

Scraping Brasses to Fit

To insure that the bearing brasses will be a good fit on the trued up crank pins or crankshaft journals they must be scraped to fit the various crankshaft journals. The process of scraping, while a tedious one, is not difficult, requiring only patience and some degree of care to do a good job. The surface of the crank pin is smeared with Prussian blue pigment which is spread evenly over the entire surface. The bearings are then clamped together in the usual manner with the proper bolts and the crankshaft revolved several times to indicate the high spots on the bearing cap. The high spots are indicated by blue, as where the shaft does not bear on the bearing there is no color. The high spots are removed by means of a scraping tool, which is easily made from a worn out file. These are forged to shape and ground hollow and are kept properly sharpened by fre- quent rubbing on an ordinary oil stone. To scrape properly, the edge of the scraper must be very keen.

When correcting errors on flat or curved surfaces by hand scraping, it is ' desirable, of course, to obtain an even- ly spotted bearing with as little scraping as possible. When the part to be scraped is first applied to the surface-late or to a journal in the case of a bearing three or four "high" spots may be indi- cated by the marking material. The time required to reduce these high spots and obtain a bearing that is distributed over the entire surface depends largely upon the way the scraping is started. If the first bearing marks indicate a decided rise in the surface, much time can be saved by scraping larger areas than are covered by the bearing marks ; this is especially true of large shaft and engine bearings, etc. An experienced workman will not only remove the heavy marks, but also reduce a larger area ; then, when the bearing is tested again, the marks

�� �