Babbitt - Pouring High Speed Bearings
Modified on 2008/03/29 21:54 by krucker — Categorized as: Bearings, Restoration Topics
Babbitting High Speed Bearings
¶
taken from Machine Moulder Practice, by W. H. Rohr, 1919.
Babbitting the boxes, which carry high-speed spindles, is very particular work and calls for the exercise of both skill and good judgment. On some machines there is a brass name-plate bearing the words, "Never babbitt on a cutterhead journal; you may spring it, and once sprung it cannot be permanently repaired ; use a babbitting mandrel." This is good advice and should always be followed when possible. The greatest danger in springing a journal is when the hot metal is poured directly onto it, and only on one side, to cast a half box. The sudden expansion during the time of pouring, and slow contraction later, is likely to produce a permanent set or slight bend in the journal which will cause trouble by heating and running badly.
When a journal, however, is wrapped with two thicknesses of thin paper and both top and bottom boxes are poured at the same time, there is scarcely any danger of springing it. Still there is an element of risk in the latter, hence the safest course is to always use a babbitting mandrel made of an old spindle, shaft, or stick of hardwood turned to the same diameter as the journal.
A mandrel should be wrapped with two thicknesses of thin paper and the ends pasted down with mucilage or photo paste. The paper enlarges the mandrel just enough to take care of the shrinkage of babbitt in cooling. Babbitt also casts more smoothly around paper than it does around a naked metal shaft. The mandrel must be carefully and firmly secured in exactly the same position that the cutterhead spindle assumes while running. In other words, if one or both boxes for a top, bottom, or profile spindle are to be babbitted, the mandrel is placed level and square with the machine bed. If only one box is being cast, it must line up with its mate. When both are to be cast, the mandrel should always be placed as nearly in the center of the boxes as possible. In adjusting a mandrel for side-head boxes, set it plumb with the machine bed.
It is generally much more convenient, when preparing to babbitt side-head boxes, to detach the yoke in which the boxes are mounted and take it to the repair room where the work can be done in good light near the forge. Align the mandrel parallel with the planed ways of the yokes. Clean out all old babbitt, especially that in the anchor holes and plug up any oil holes or chambers which may be in the bottom box. Oil holes can be easily plugged with bits of wood whittled round to fit them. Oil chambers, however, should be packed with waste and the opening covered with a piece of belting thick enough to fit in snugly between the box and the babbitting mandrel. This piece of belting backed with waste, not only keeps the hot metal from running into the oil chambers or between the mouth of the chamber and the mandrel, but also serves as a shim which assists in centering and supporting the mandrel in the box.
A very simple and easy method of centering and supporting a babbitting mandrel is to place a narrow strip of belting crosswise of the box, letting it extend less than half way around the mandrel to leave plenty of room for melted babbitt to flow in all parts of the box. After the box has been cast, these strips of leather ,may either be left in position or replaced with felt. When both top and bottom boxes require new babbitt lining, considerable time can be saved by arranging to pour both boxes at the same time. As in all cases, the edges of the boxes must be separated by liners to provide enough take-up adjustment for wear. A plan or top view of the bottom half of a bearing ready to be poured in the manner recommended appears in Fig. 82. The liners L, L are made of cardboard or thin wood and have small V-notches, as shown, to permit the melted babbitt to flow from the top half of the box down to the lower half. When the metal cools, it will be joined together at the V-notches, but these slender connections are easily broken apart by a few taps of the hammer on the end of the top box, or the prying action of a chisel. If only the top half of the Box is to be cast, tho liners are not notched at all.
After the oil holes and chambers are plugged and the babbitting mandrel is properly centered and clamped in place between liners, see Fig. 82, the cap (top half of box) is set over the mandrel and clamped or bolted down against the liners. Leather or cardboard washers W, W, Fig. 82, are then placed at each end and the end openings sealed with putty, clay, dough, or a mixture of asbestos and oil, see E, E, Fig. 82. Two open holes should be left in the top of the cap, one for pouring the metal and the other for a vent to permit the escape of entrapped air. Care must be taken that no moisture is present in the boxes when the metal is poured or a "blow-out" will result from sudden formation of steam. If the work is done in a cold atmosphere, the boxes and mandrels should be warmed before the metal is poured. This avoids chilling the hot metal and permits the box to shrink somewhat with the metal when both cool, thereby preserving a firmer connection between the two.
Fig. 82
When everything is in readiness, the babbitt is melted in a regular ladle. In the absence of a proper instrument for taking the temperature, one must simply use good sense in heating the metal, not too hot, but hot enough that it will run freely into all parts of the box before it begins cooling. The temperature at which babbitt should be poured is about 450 to 460 degrees C. Pour the metal in a steady stream, being careful not to let any of the top slag enter the box and be sure to have enough metal to fill the entire box with one pouring. Do not disturb the newly cast box until it has cooled ; then take the boxes apart and smooth off all rough or sharp edges with a chisel or rasp, and babbitt scraper. Open the oil holes, and chambers if any, and cut oil channels from them to points near the ends of the box, as shown in Fig. 83. The function of the oil channels is to facilitate the flow of oil to all parts of the bearing, therefore, they should lead outward from the supply holes in the direction of rotation. Their edges and those of the boxes should be rounded because sharp edges tend to scrape oil off the journal and prevent its proper distribution.
Fig. 83
Every babbitted bearing must be carefully scraped to fit its journal perfectly. Otherwise, the journal will bear only on high spots and all lubricant will be forced into the low places where it will not do any good. Under such conditions, overheating is certain to occur, even tho the bearing is flooded with oil. In order to know where to scrape, give the journal a very thin coating of red lead paint and turn it around in the boxes. The bright spots show where the surface must be scraped. When the fit between journal and boxes appears to be perfect, bolt the latter in place, put on the belts, and run the spindle up to speed for a few minutes. Take the boxes apart again, and the bright places caused by friction show exactly where to do the light, final scraping. This all takes time, but is worth it, because it produces smooth-running, non-troublesome bearings which are so essential to good molder work.
Fig. 84
When adjusting new boxes to a high-speed journal, do not make the mistake of clamping them down too tight. Leave a little play at first and run the spindle at full speed for a while to let it warm up. The expansion from heating may be enough to take up all lost motion, but if it is not, then tighten the boxes just a trifle while they are warm.
In babbitting a box which receives a grooved journal (grooved to prevent side play in the spindle) the usual practice is to give the journal a good coat of white lead paint, and then use it as a mandrel for molding the babbitt lining. A wood mandrel with corresponding grooves can be turned by an expert turner if a template is provided, to fit accurately into the grooves. The wood mandrel is recommended because, if a grooved journal is sprung the least bit, it is practically ruined forever The lubrication of high-speed bearings is of great importance.
To run properly, a journal and its bearings must always be separated by a thin film of oil. It is quite imperative that good oil be used and the supply be kept constant in order to maintain perfect lubrication. It is often necessary, especially on large bearings, to introduce oil from the bottom as well as the top. If there is no provision for an underfeed oil system, and one is desired, drill a small hole through the bottom of the box and tap it for a small U-shaped pipe. The short leg of the U should then be screwed into the bottom casting a