PROPERTIES OF TIN FOR PLAIN BEARING MATERIALS

PPROPERTIES OF TIN
FOR SLEEVE BEARING MATERIALS

The tin base bearing materials knows commercially as “Babbitt metals” or “Babbitt”, are substantially alloys of tin, antimony and copper. In these alloys, Zinc, Aluminum, arsenic, bismuth and iron content should be limited.
The composition of tin base bearing alloys according to ASTM Specification B 23 and SAE specification are shown in table 1.
The presence of zinc in these bearing metals is not generally favored, arsenic increases their resistance to deformation at all temperatures, while zinc has a similar effect at 100 F, but causes little or no change at room temperature. Zinc has a marked effect on the microstructure of certain of these alloys. Small quantities of aluminum, even less than 1 % will modify their microstructure.
Bismuth is objectionable because, in combination with tin, it form a eutectic that melt at 279 F. at temperature above this eutectic, the strength of the alloys is decreased appreciably. In high tin alloys such as ASTM grade 1, 2 and 3 and SAE 11 and 12, lead content is limited to 0.50 % or less because of the deleterious effect of higher percentages on the strength f these alloys at temperature of 300 F or more. Lead and tin form a eutectic that melt at 361 F. Bearing at temperature exceeding this point naturally becomes fragile as a result of the formation of a liquid phase within them.
The mechanical properties of ASTM Grade 1 to 5 are shown in table 2.
Value for brinell harness limit of proportionally, compressive yield strength and ultimate compressive strength (load per unit initial area required to compress test pieces 0.250 inch per inch of length) of representative tin-antimony-copper alloys are given in table 3.
The total deformation of the sample in the compression testing of Babbitt is function of the duration of application of the load, a point sometimes overlooked by those reporting and studying result of tests on materials as plastic as these alloys. What is true of compression test is also true of other types of static test. Still, the know limitation of static test do not discount their value. A brinell tes, for example, wherein the load is applied for a standard tine (for instance, 30 sec) is of considerable value for purpose of manufacturing control. Between the brinell hardness of the tin-base bearing materials and their properties in compression, there is a relationship close enough to permit making a fair estimate of the properties in compression on the basis of brinell tests alone.
A knowledge of the properties of these alloys in compression, although usefull, is less important than an understanding of the beneficial effect on bearing life brought about by reducing the thickness of liners in bearing, particularly in those which are to be repeatedly stresses. The relationship that exists between bearing life and thickness of Babbitt is shown in fig. 1.
Compare with others bearing materials, the tin-base alloys have relative low resistance to fatigue, but their resistance is sufficient to warrant their use under low load condition. These alloys are commercially easy to bond and to handle, and they have excellent antiseizure qualities. Further, they resist corrosion much better than lead-base bearing alloys.
Of the above alloys, SAE 12 (ASTM grade 2) is most widely used in the automotive field at the present time. ASTM grade 4 and 5 have a rather limited application and relative unimportant. The microstructure of these alloys varies in accordance with their composition. Alloys that contain from about 0.5 to 8 % Cu and less than about 8 % Sb are characterized by a solid solution matrix in which needles of a copper rich constituent and fine rounded particles of precipitated SbSn are distributed. The proportion of the copper-rich constituent increases with copper content. SAE 12 (ASTM grade 2) has a structure of this type in which the needle often assume a characteristic hexagonal star-like pattern.
Alloys that contain from about 0.5 to 8 % Cu and more than about 8 % Sb exhibit primary cuboids of SbSn, as well as needle of the copper rich constituent in the matrix solid solution. In alloys that contain about 8 % Sb and from about 0.5 tom 8 % Cu, rapid cooling has the effect of suppressing the formation of the cuboids of SbSn. This is particularly true of alloys containing the lower percentages of copper.
Some of the mechanical properties of ASTM grade 2 and 3 are shown in Fig.2. A standar 0.505 inch. Bar of chill cast columnar-grained metal was use as the tensile test speciment, the columnar crystal being perpendicular to the axis of the bar. The speed of testing was 0.015 inch per minute to the yield point and 0.250 inch per minute beyond.
The harness test were made with 125 or 500 kg load applied for 30 second. In the compression test, the specimens, 0.5 inch in diameter by 1.5 inch long, were oriented so that the grain structure was parallel to the axis. The testing speed was 0.015 inch per minute, the strength being taken at 1 % deformation.
Bar with diameter from 0.330 to 0.375 inch were use in a rotating cantilever-beam machine to determine the fatigue properties, the speed of rotation varying from 2000 to 8000 rpm. The columnar in the chill cast were perpendicular to the axis, and the fatigue strength was taken at 100 million cycles. There was no noticeable effect that could be attributed to the variation in diameter of specimens or testing speed.
Creep data for SAE 12 (Fig. 3) were obtained from a tubular specimen, 0.5 inch OD, 0.25 inch ID and 1 inch long, immersed in an oil bath at 250 F, the grain were parallel to the axis of the specimen. Compressive stress was applied at several level from 200 to 1200 psi long enough to produce a steady creep rate. Some specimen were in test as long as 1900 hours.