More Babbitt Basics

In the farm shop

| September 1988

Farm Mechanics

A mold for a bearing that is poured around the shaft.

The following is a reprint of an article by C. T. Schaeffer, which originally appeared in the May, 1930 issue of Farm Mechanics. Article was sent to us courtesy of Dick Hamp, 1772 Conrad Ave., San Jose, CA 95124.

Babbitt metal as used for bearings varies greatly in composition, and its pouring temperature depends upon its composition. Also the grade of babbitt which will be used on repair jobs will depend upon the local stock from which the selection is made. Various babbitt linings contain zinc, lead, aluminum, copper, tin and antimony, but real babbitt is a combination of the last three in various proportions for different grades of service. Where bearings are large and the service is light, the babbitt metal may have a comparatively large percentage of lead and naturally the metal will be comparatively cheap as compared to a high grade metal that will stand high speeds and heavy loads. Lead has many properties which would make it an ideal bearing metal, but it is too soft to stand hard uses in service. However, in the cheaper metals it forms the base when combined with copper, tin, zinc and antimony.

A lead content is generally found in babbitt metal used for general purpose bearings where weight is no object in the design of the unit and parts can be properly proportioned for the service they are to render. Thus, for heavy duty the metal may consist of 40 parts lead, 50 parts tin, 8 parts antimony and 2 parts copper. For light work the specifications may call for 5 parts lead, 44 parts tin, 50 parts antimony and 1 part copper. High grade babbitt used for the bearings of engines generally consists of about 86 percent tin, 7 percent copper and 7 percent antimony. The softer the metal is the less resistant it is and naturally its pouring temperature is lower than that of the harder metals.

Babbitt metal should be placed in a pot or ladle and carefully heated until melted. The metal should also be stirred thoroughly before pouring and care must be taken to melt it slowly. A sufficient quantity of the metal should also be melted to fill the bearing to permit pouring without draining the ladle. This is to insure clean babbitt being used throughout the bearing, leaving the scum, dirt and oxidized metal in the ladle. The molten metal should be kept thoroughly mixed, especially just before pouring, by continually bringing the lower strata up from the bottom of the ladle. If permitted to remain quiet while in the liquid state, the mass may tend to separate into layers. Any lead present, being heaviest sinks to the bottom and the lighter ingredients come to the top. Bearings poured from metal in this state are bound to give trouble.

An old practice is to mix old babbitt from worn bearings with new metal but such practice is not recommended, although in emergencies it may be advisable to use all old metal in preference to waiting for the new metal. Water should not be permitted to drop into the molten metal as this will cause the metal to fly in all directions with danger of burns to the workman. Dross should be removed from the the surface before pouring the metal and upon completion of babbitting operations, the metal, before cooling, should be covered with charcoal or sawdust to retard oxidation and formation of scum.

Various methods have been recommended for determining the correct temperature of the metal for pouring, however, the only safe method is to bring it up to the pouring temperature specified by the maker with allowance sufficient to compensate for the heat loss in transferring the metal to the mould. With lead base babbitt, a fairly accurate method is to heat the metal until it reaches a point where it will just char a soft pine stick. The metal is then ready to pour. The appearance of the metal when skimmed is also used as a means of determining the pouring temperature. When the metal appears like quicksilver and tarnishes slowly after the scum has been skimmed off it is at the correct temperature. Metal that is too hot coats rapidly and the tarnish shows all colors, while metal that is too cold is sluggish and the tarnish takes on a dull appearance. Babbitt metal for bearings must not be overheated.