Some Babbitting Suggestions
Modified on 2012/04/17 09:26 by Bob Holcombe — Categorized as: Bearings, Maintenance, Restoration Topics
The following article was originally published in the November, 1924 issue of
magazine, page 69.
Some Babbitting Suggestions
By C. L. A.
An intelligent selection of the proper grade of babbitt for each particular use, is wise. Nothing is gained by using costly babbitt everywhere, neither is anything saved through the use of cheap metal in bearings which are subjected to heavy strain or the wear and tear of high-speed shafting. True economy in the use of babbitt is a matter of keen judgment as to shaft-bearing conditions and needs. The wise mechanic uses old babbitt in unimportant bearings and high-grade Babbitt where strength and durability are demanded.
Heat is injurious to babbitt. Even new babbitt is somewhat weakened by its first melting, hence unnecessary heating should be avoided. One can further conserve the strength of the metal by heating it in a thick ladle over a slow fire. The wisest course is to allow the metal to heat slowly while preparing the bearing.
The preparation of the bearing is more a matter of care than of labor. All dirt and oil should be cleared away with the old metal. If there are no lug holes in the bearing for holding the liners solid, this is a good time to put some there, at least three or four at each bearing end of both cap and base; 1/4-in. holes 1/4-in. deep, will answer. Both the number and size can be governed by the size of the bearing, and the mechanic's gray matter. Many a bearing liner has been lost simply because of a lack of these lug holes, a matter of gross carelessness in preparation.
A babbitting mandrel should be used wherever possible, as the heat from the metal is liable to spring the shaft, especially where it is impossible to rid the shaft of all strain, counting that of its own weight. The trouble here is due to the fact that the metal always heats one side of the shaft quickly, thereby springing it, and it does not always come back to a straight line in cooling. I know of several band-wheel shafts, and planer heads which have been junked because of their having been sprung through this reckless practice.
Mandrels are not costly affairs; they can be fashioned from almost any old shafting. Mainly, they should be a fraction larger in circumference than the shaft which they save, and should also be long enough to reach at least two bearings. Then the shaft is given a chance to turn freely in the new bearing, and the bearing itself is in line with its mates. Also, both the mandrel and the bearing should be warm when the babbitt is poured, then the latter has a real chance to form a true and solid liner. This is another good point about the mandrel; it can be warmed up, where, quite often the shafting can not.
Both fire-clay paste and yellow soap are reliable for packing a bearing against leakage of hot babbitt. Either of these is easily worked, yet firm, and when properly applied will safely confine even the hottest of metals. In packing a bearing, always leave plenty of openings at the top, so that the air can escape from under the mandrel as the metal flows in; otherwise the liner will be imperfect. Collars of iron or cardboard at each bearing end are great aids in confining the molten metal to true lines. They leave the bearing neat in appearance when they are backed up by soap or fire clay. The average bearing requires each liner to be about 1/4-in. in thickness. The main thing, however, is to have one-half of the shaft in the cap and the other half in the base. Then there is but little danger of either cap or base liner gripping the shaft and starting a hot bearing.
In centering the shaft in the bearing I never rest the shaft upon blocks, especially away from the ends of the bearing, because of the fact that these blocks, when taken out, leave holes in the liner, through which the oil drains from the bearing. Where I do have to use blocks in this manner, I place one, preferably of leather, at the extreme end of the bearing, then leave it there. If it is next to a tight collar, so much the better. For shims between cap and base I like stiff cardboard. Sheet-iron or tin is more durable, but with them one is running a continual risk. If the cap happens to loosen some, a metal shim may get under the shaft-then the fireworks.
The proper heat for babbitt is mainly governed by commonsense and conditions_ One should at all times have it hot enough so that it will run freely into all corners of the bearing, but no hotter than this. A popular test is that of the softwood sliver. When the metal will char a pine or hemlock, sliver it is hot enough for average pouring conditions. It should be poured steadily, with as much speed as is allowable without wrecking the packing about the bearing. Few good bearings result from timidity, but he is wise who will make his pouring conditions such that no great heat will be required in the babbitt.
The poured bearing, where possible, should be allowed to cool before the mandrel is withdrawn, then the liners will be solid and will fit the shaft. lf the mandrel is withdrawn while the metal is hot, the liners will contract; their edges along the rims of the cap and base will curl inward, then excessive scraping will be necessary before the shaft can turn freely.
Scraping an otherwise perfect liner is, I think wrong. Other than the sides of the liner, I never touch at all, except to cut oilways. I do scrape each side of each liner in order to prevent them from gripping the shaft, and this also allows the oil a freer circulation. These are my only reasons for scraping a new liner. Even the best mechanic cannot improve upon the working surface of a properly-fashioned liner by going over it with a scraper. He cannot make it fit the shaft any truer; he cannot make that metal any smoother, so where is the gain? The only time I scrape the entire working surface of a liner is when an old liner has heated, and I do this merely to clear it of scale and clean the oilways. Scrape a new liner, especially the base, then use it for a couple of days, and see how many humps and hollows you have left there with the scraper. You will see that the shaft is only touching about half of the liner's surface.
The wise course, in starting a newly-run bearing, is to keep the cap just tight enough to steady the shaft. Prevent lost motion, but allow the shaft perfect freedom, then keep it oiled. Good babbitt liners are not the result of much labor; they are gained by a small amount of labor done carefully and judiciously.