In some cases cast-iron boxes are cast with a receptacle for some soft metal, such as the various compound metals known under the general name of Babbitt metal.
Babbitt metal is composed of tin, antimony, and copper, mixed in varying proportions. A good mixture for general use where the duty is light is composed of 50 parts tin, 5 parts antimony, and 1 part copper. A harder composition, sometimes termed white metal, is composed of tin 96 parts, copper 4 parts, and antimony 8 parts. This mixture is especially suitable for journal boxes or bearings. It is mixed as follows: Twelve parts of copper are first melted, and then 36 parts of tin are added; 24 parts of antimony are put in, and then 36 parts of tin, the temperature being lowered as soon as the copper is melted in order not to oxidize the tin and antimony; the surface of the bath being protected from contact with the air. The alloy thus made is subsequently remelted in the proportion of 50 parts of alloy to 100 tin.
For brass bearings or boxes a mixture of 64 parts copper, 8 parts tin, and 1 part zinc is found to answer well; but for bearings not requiring so hard a metal, the quantity of zinc is increased, and that of the tin diminished.
Fig. 2503.
Bearings or boxes that are to be babbitted are usually cast as in [Fig. 2503], there being a rib at a, b, and c, forming a cavity at d, into which the melted metal is poured. The ribs (in new boxes) are sometimes bored out, or for rougher work may be chipped and filed out to fit the shaft, and hold it in line; and to prevent the ribs a, b, &c., from bearing and cutting the shaft, a piece of pasteboard is laid on ribs a and b, thus confining the journal bearing to the babbitt. The best method is to pour the bearing and then rivet the babbitt well into the cavity d, which is made wider at the bottom, to prevent the babbitt from coming loose, and then bore out the bearing in the usual manner.
The principal advantage of a babbitted bearing is the ease with which it can be renewed, and the fact that the metal will soon bed itself to the journal. This is of great advantage in the case of solid bearings in the framing of fast-running machines, and in situations where it would be awkward or difficult to take brasses or bushes out to fit them, or align them to the shaft, which in many cases would also require to be taken out to remove the brasses. On the other hand, any particles of grit that may find ingress to babbitted boxes are apt to become bedded into the babbitt metal and cut or grind away the journal.
Since the babbitt metal in a bearing is apt to close across the bore when cooling after being poured, a mandrel of slightly larger diameter than the diameter of the journal should be used in place of the working journal to run the bearing on. Some effect the same purpose by wrapping writing paper around the journal; but it is wrong to use the journal, for the following reasons: To get a good, sound, well-fitting babbitt metal box, the metal should be poured as cool as possible, for if made red hot it contracts so much in cooling that it does not fit well in the box or frame of the machine. On the other hand unless the metal be well hot it is apt to cool and set too soon and be unsound. To remedy this the journal, or whatever represents it, must be heated. The heating is very apt to bend it. It is obvious then that instead of the journal a temporary bar of iron of slightly larger diameter than the working journal should be used, heating it to a good black hot heat, so that the babbitt metal may be poured less hot than would otherwise be permissible, and the contraction of the babbitt in the box reduced to a minimum. A little powdered resin sprinkled in the box will help the babbitt to flow easily and make a sound casting.
The temporary spindle, or journal, should also be oiled, and as soon as the metal has well set, the temporary journal should be revolved to free it. Babbitt bearings cast in two halves should be fitted to the journal as already described for brasses, which will well repay the cost and trouble.