US2375224A - Silver-thallium bearing - Google Patents

Description

Patented May 8, 1945 2,375,224 SILVER-THALLIUM BEARING Franz R. Hensel, Indianapolis, Ind., assignor to P. R. Mallory & Co., Inc., Indianapolis, Ind., a

" v corporation of Delaware No Drawing. Application August 14, 1941, Serial No. 406,841

This invention relates 'to an improved bearing 11 Claims.

of high efliciency and durability.

It is an object of the present invention to provide an improved silver base bearing which has a high fatigue resistance, which will not score under excessive loads or at high speeds and which possesses unusual wear resisting qualities. It is a special object to produce a silver base bearing having a greater resistance to oil andacid corrosion than silver base bearings of the present commercial art.

Another object. is the provision of a silver alloy hearing which possesses improved properties over silver alloy bearings previously used or described.

It is a further object of the invention to provide a bearing metal which has a low coefficient of friction.

It is another object of the invention to provide a bearing metal which has a comparatively,

high thermal conductivity.

It is a further object of the invention to provide a bearing which possesses the ability to run in and to conform to local excess pressures.

It is afurther object of the invention to provide a bearing which has the property of imbedding grit.

Other objects of the invention will in part be obvious and in part appear hereinafter. The invention accordingly comprises the features of construction and a combination of elements and arrangements of parts which will be exemplified in the descriptions hereinafter set forth and the scope of the application, of which will be indicated in the claims.

In the prior art, pure silver has been used for bearing purposes. Attempts have also been made to produce silver-lead alloys in the form of bearings or bushings. These bearings have been produced by either casting the silver or silver and lead combinations onto ferrous backings or by electroplating silver and/or silverlead. In the case of pure silver, it was found that the material does not prevent scoring, while in the case of silver and lead, difliculties have been experienced in obtaining a uniform lead distribution.

Certain methods have been suggested to overcome these difficulties. It is general practice now to apply a lead coating to the silver, so that the actual bearing surface is pure lead or indiumlead and not the silver. There is, however, a danger that in thin lead coatings, the shaft might actually wear through the lead and run on pure silver, which might give rise to failures, particularly due to scoring.

The present invention attempts to overcome the difllculties described above by providing a new material which eliminates the scoring tendencies of fine silver and which also eliminates the segregations found in silver-lead.

It is contemplated that alloys of the new invention may have the following approximate composition:

Thallium per cent" 1-20 Silver Substantially the balance The addition of thallium is equally effective in solid solution silver alloys and therefore such elements as cadmium, indium, manganese, lead, tin, zinc, aluminum and magnesium, may be present. Other additions such as nickel, bismuth and. arsenic may also be made, provided that such additions are held within limits which do not affect the bearing performance deleteriously. These additions may have the purpose of increasing the strength and fatigue properties and the corrosion resistance of the silver matrix.

A careful study has been made of the alloys containing from l20% of thallium, and the fact has been established that a considerable amount of thallium will be taken up in solid solution by the silver. The solid solubility of thallium in silver, however, seems to be .afunction of the temperature and seems to decrease with lower temperatures. Extensive experiments have been made with a silver-thallium alloy containing 3V2% thallium. This alloy is normally within the solid solution range; however, in the form to obtain a stable condition, these alloys may be heated for several hours at 500 C. and then the microstructure shows a completely homogeneous grain-structure, typical of the solid solu- The bearing was also tested on the test setup of one of the largest manufacturers of aircraft' engines. This test was run at a speed of 3500 R. P. M, or 1830 surface feet per minute. At this speed, the load on the bearings was If this heat treatment is.

bath temperature, agitation,

' 2290 lb. per. sq. in., considerably higher than automotive bearing loads. The oil used was SAE 30 to which 1% of oleic acid had been added so that corrosive conditions would develop during testing. The bearing temperatures were maintained at 300 F. during the whole of the test. The test was run for 50 hours at the above mentioned test speed, after a short break in run of 1 hour at 1750 R. P. M. and 2 hours at 2450 R. P. M.

The test was completed-uneventfully, with no signs of distress from the bearings. Upon disassembly of the test machine at the conclusion of the run, it was found that the bearings were in excellent condition, there being no cracking or signs of scoring. The shaft on which the bearings were run was also in good condition. It was not scored in spite of the fact that it was made of soft steel, around 250 Brinell.

The bearings showed no signs of being subject to acid corrosion in the oil, since they lost no weight during the test in the acid oil. In fact,

the bearings even gained weight, presumably due to pickup of sulphur from the oil, since there was a,v heavy coating of a material with the appearance of silver sulphide on the surface. There was a pickup of 0.027 gram in the weight of the two bearings, and this would correspond to a sulphide'film of 0.0001" thickness.

Tests were also conducted with the following materials:

Silver and 5% thallium Silver and 10% thallium and 3% lead Silver and 20% thallium These materials were tested and in all cases, the material exhibited good antifriction properties, as determined by the score test developed.

by the research laboratories of the aircraft engine manufacturers above referred to.

A binary alloy containing 3.5% thallium, has a cast ultimate tensile strength of approximately 25,000 p. s. i. The Brinell hardness of these materials is in the range desirable for bearing materials. Any alloy, for instance, with 5% thallium, has a Brinell hardness of 32.4 when tested at 80 F., and a Brinell hardness of 28, when tested at 345 F. By increasing the thallium content to 20%, the Brinell hardness at 80 F. is 28.9, and at 345 F., 20.8.

The bearing metals containing up to 5% thallium lend themselves very readily to hot and cold working, provided that homogenizing treatments are given to the material. They can be produced in the form of strip, rod, cylinders or any other form and in the cold worked condition the tensile strength may be increased to above 50,000 p. s. i.

The alloys may also be produced by means of plating. The silver alloy plating solution which gave satisfactory results was as follows:

- Grams, Silver cyanide 35 Potassium cyanide 19 Potassium carbonate 38 2 drops carbon bisulphide to the litre.

To this was added ml. of thallous perchlorate, containing .4 gram of metallic thallium. This bath was operated at room temperature and with an approximate current density of .75 ampere per square decimeter. The plated samples showed a thallium content of 4.32% thallium. Variations can be made in the solution in the current density and the like, in order to improve the deposits or to speed up the process of plating. The plated silver-thallium alloy did not react with diluted and concentrated nitric,acid. Treatments with almost all acids except hydrochloric acid, did not affect the alloy.

The tests made with the various alloys seem to indicate that both the solid solution type alloys of silver and thallium and the alloys where part of the thallium is in solution and a remaining part is suspended homogeneously in the silver matrix in the form of small particles, which are very soft and ductile, are suited for hearing purposes. The free thallium particles will melt at a temperature of approximately 578 and if the bearings run out hot, will afford liquid lubrication.

It is likewise possible to produce a thallium containing silver by diffusion processes. This latter method usually entails the coating of the silver with thallium by any suitable means, including electrodeposition and then heating same to a temperature where it will penetrate into the silver. v

If the alloys are made by melting, the silver base material is melted first and then the thallium is introduced under a fiux. The melt is thoroughly stirred and then cast into desirable shapes.

The silver-thallium alloys may be bonded to backings of ferrous alloys or copper base alloys by such means as brazing, con-joint extrusion, hot rolling into the form of a bimetal and then forming, casting into a pro-heated shell in a nonoxidizing atmosphere, etc.

For certain applications, it has been found desirable to add a certain percentage of lead to the new thallium alloys, with thallium and lead forming a continuous series of solid solutions with no phases of low melting points. Up to 10% lead may be added.

It has also been found of advantage in certain instances to add to the present materials, small amounts of alkaline metals or alkaline earth metals, such as lithium, sodium, calcium, strontium and barium, in order to completely deoxidize the matrix, effect grain refinement and to increase the hardness of the thallium or thalliumlead phase. The range of thallium covers a number of classes of bearings, to which the addition of thallium has been found useful. Some of the lower percentage alloys may be used in bearings which operate without a steel backing. These al-' loys may be used in the form of castings or forgings. The alloys with a higher thallium content are used for special applications which are more generally backed up with steel or asimilar metal. Since the use of bearings are manifold, the alloy compositions must be adjusted to the specific application. The silver-thallium base alloys do not constitute just one bearing composition, but a new class of bearing materials, and therefore the thallium content necessarily must be proportioned, in order to meet specific requirements.

When silver base thallium bearing alloys are attached to a backing material by means of soldering, it is advantageous to place a layer of fine silver between the steel and the silver-thallium alloy. In order to still further improve the corrosion resistant characteristics of the new alloy, it has been found of advantage to add a certain percentage of indium, such as .l-10% of indium,

- to the alloy or to apply an indium coating by 7 indium and thallium form a series of solid solutions which" are highly corrosion resistant. If

the finished bearings are indium plated and then heated to slightly elevated temperatures, the indium will diifuse into the thallium at a temperature of approximately 150 C. Alloys of indium and thallium, up to 50% thallium,,are com-' pletely homogeneous. There may be certain cases where thesilver-thallium may be used asthe base, onto which a layer of lead or a lead alloy, may be plated. This would give the doulble'assurance in bearing performance that no scoring would occur if the shaft should wear through the lead surface because it would run on the silver-thallium backing without causing any scormg.

In certain cases, the lead coating, which is placed on top of the silver-thallium backing, can be plated with indium and the indium diffused into the lead, in order to improve the corrosion resistance. Instead of lead and indium, it may also be possible to use a tin coating as the soft material, on top of the silver-thallium backing;

This application is a continuation in part of my co-pending application S. N. 235,785, filed October 19, 1938.

While the present invention as to'its objects and advantages has been described herein as car.-

ried out in specific embodiments thereof, it is not 2. A bearing containing 2-'5% thallium, all

ance predominantly silver.

thereto.

3. A bearing containing 1--20% thallium, balance predominantly silver, produced by electrobalance predominantly silver, and containing approximately up to 10% lead.

6. A bearing containing 1-20% thallium, balance predominantly silver, said bearing being strengthened by a ferrous allo backing.

7. A bearing containing 1-20% thallium, balance predominantly silver, to which has been applied a coating of lead.

8. A ibearing containing1-20% thallium, balance predominantly silver, and containing upto 10% of indium.

9. A hearing containin 1-20% thallium, balance predominantly silver, to which has been applied a layer of indium, which is subsequently diffused into the bearing.

10. A hearing containing 1-20% thallium, balance predominantly silver, having a coating of lead and a coating of indium, which has subsequently been diffused at elevated temperatures to improve the corrosion resistance of this bearing combination.

11. A bearing comprising a steel backing anda lining-of bearing'metal consisting of .5 to 6 per cent thallium and the balanc silver, bonded FRANZ R. I-IENSEL.