Document VGMQDby2dQz4a56x3grQyzV3w

SOME TYPICAL DIESEL-ENGINE BEARING FAILURES EXCESSIVE DIRT in the ot>gtno assem bly or In the lubtlcotlng oil caused rapid shaft wear and this beoring failure filing condition! distort engine struetures. This may produce edge contact between' bearing and shaft Heat gen* erated by the metal-to-metal contact nay be sufficient to cause bearing fall* ure. If the metal can eonform, It ad* justs itself to the new condition. Even If the bearing fails, ability to cooform will often prevent shaft damage. The qualities of embedability and conformability have been combined by Uoderwood as "delormability" and he lists bearing materials in increasing order of deformability as follows: (1) bronzes (2) silver and copper (3) alu minum alloys (4) copperhead with tin or silver (5) thio babbitt overlays (0.003-in. or less) (6) copperhead (?) cadmium alloys and (8) lead* and tin-base babbitts. Setsura Resistance. More time has been spent testing bearing materials for seizure resistance than for any other property. Unfortunately, laboratory bearing-test rigs are so rigid end accu rately made, to secure reprodocible re sults, that interpreting their data in actual serviceability is extremely diffi cult Such testa serve, however, to rente materials roughly and to eliminate those clearly unsuitable. Seimre-resiiUnce data can be of value when measured in periods of high overstress in actual engine service. II bearing engineers and users could com pare notes when sciture does occur, it would add gTeally to our knowledge. Unfortunately a crankshaft is damaged fn the process ond since the informa tion applies to only one engine it Is expensive knowledge to obtain. On the basis of seizure resistance of an actual- bearing in on engine, 76 (S0| FATIGUE FAILURE shown here resulted from ovciloodinp the bearing material. Note the typicol condition of the surface without any overlay of lead-tin (or "run* In," Esarey ranks the materials during period* ol overstress as follows: (1) lead-base babbitt (2) tin-base babbitt (3) copper-lead on steel back (4) een* trifugally cast tin-lead bronze (5) silver with small additions, eleetro deposited (6) cadmium-silver (?) copper with small silver additions. Cridding the surface and filling it with babbitt and providing a lead-tin run-in surface markedly improves seizure resistance of silver, centrifugally cast lead-bronze and copper-clad steel-back bearings.' Corrosion Reslstooce. Studies of cor rosion resistance bring out the point that nearly all bearing moteriala can be corroded by some set of conditioos, depending on typo of acid and tempera ture. In general, operating tempera* tures ond changes in oil conditions of present-day engines are such that bear ings with a lead-tin or lead-indium over lay give satisfactory corrosion resis tance, or have this resistance os an inherent characteristic. Originally de veloped as a run-in surface for silver beariags in aircraft engines, the lead-tin end lead-indium overlays provide com plete coverage of bearings for surpris* Ingly long service periods. Bengabillrv. Ability to obtain con* tlstently strong bonds between lining and bock Is largely a matter ol careful ness and manufacturing procedure If suitable materials are used. With modern methods In which the human element is removed as much as possible (for example, by centrifugal casting under electrical control) good bonds r* suit consistently. The Kolene process for steel or cast-iron backs gives per fectly satisfactory bonds If care Is alloy* require hsrder shafts, ?;i cleaner lubricant, better jour t finish snd greater construe- \tfce^al a4ccNuraucy-Beoring** Several gen- Jump* ol hare found WH` ,, bearing linings where loads "*w fcjgb for babbitts. One group H-riso the lead bronzes, with IB to 22 \ui added to the copper-tin alloy 7? u bronze. Development of cen- trifalrily cast Jead-bronte came about joss before the war period when steel became increasingly short in mppfr- i( wu believed that a material 8f good conductivity and uniform Ououghout. would eliminate the weak. OIL-FILM FAILURE allowed metotjl motot contact In this illustration, till suiting seizure coutod condition stn3l! taken to follow the instructions. Akift) ~ to obtain strong chemical beads pel ! mils the present-day elimination e dovetails and other mechanical boaqj \ Over-all suitability of a msterli] fal bearing service may be summed op 4 wear resistance, defined by Gillen, fiej sell and Dayton as "the resultant elU and finish, seizure propensities, behstiej with grit, corrosion resistance ((ere tion of corrosion products that t wiped ofi shows up as wear) n strength end hardness. It is vastly i fected by how tho bearing is operstdjl as to temperature, nature of loading twl cleanliness of oil, and especially tk|| relative number of stops end sunt that is the opportunity for sebum* . Bobbins. Invention of babbitt osetd as a lining for bearings represented t tremendous advance. In general, we e* still say that within their strength its, babbitts ere acknowledged the bt# bearing materials. They are tolerui of dirt and work well with all oonxdJ types of shafting materia). They **j| dure Insufficient or Intermittent luhdj cation-ond considerable distortion base babbitts ace well known, and hz*j enjoyed general usage In perhaps doHi varied applications than ony other lb; ing material. High-lead babbitu, general substitute for tin babbitts cause of tin shortages, comprise sewni^ general groups differing mainly ia ^ minor constituents added for horded; ing. Tin-antimony, silver, arsenic ttfj alkali earths are used. When babbitts are stressed pounded too heavily, they fail in (atigo* ;l Thus (t has proved necessary to d*j| relop stronger materials to meet ax^f severe demands. In general, th**1-* kbci of bonded bearing and lead* b,,iua seemed suited for the job, Good nsaltt were obtained. To extend the life in heavy-duty serv. GRfDOEO BEARING is mode of leod-branae. In one place. Surfoca Indentations ore filled With o *llver-fod babbitt. This construction Improves the embedabHry ond contormaWUty. and ollmlnotes the necessity for hordenlng the shaft, {oumols lee, lead-bronze bearings were grfddod, that U, the bearing surface was indated sad filled with sifver-lead bab* bin. It *** found that shape and depth of indentations, and relative pro. (see layer of white metal is precision plated so no further machining is re quired. In the early days of such bearings ment of bearings made of aluminum alloyed with tin and other minor addi tions. Most builders have tested or ore testing such materials and at least one tW`portion oi gridded area to parent octal are not critical. Economical manofscluring methods were developed for (ridding It improves embedability, provides conlormsbiUty ond removes seed for hardening the shaft. Tn periods of.grest overstress the babbitt melts out of the grooves to form a running sur face snd prevent seizure. Addition ol sa QjOOZ-tn. run-in surface of electro plated feid-tin further improves seizure rnifUncs and increases corrosion re- alliance. Coppt-Lad Alloy*. Some mixtures of copper sod lend have been used as hearing materials for Soma years and an by the nature of their structure tilted (or high loadings. A sacrifice In coafortnsbiiity and the need for a hard ened or treated shaft o( 300 brinell or hsrder ore characteristics of these bearings. Where soft shafts must be used, or bars citremriy high speeds are met, eelj the white metals will serve. It is tlio known that load-carrying ability ri white metals, their weak point, can Increased by decreasing fining thick* om drastically. This led to the so* <*Ued micro bearing, with a few thouHoothj of babbitt on a steel back, and > trimetal bearing, with a steel back, intermediate layer of leaded bronze d a surface layer ol babbitt. n l^e babbitt thickness to about 1 in. in thickness and using a eop- there was some concern about operat ing on rite Intermediate copper-lead layer after the plated white metal had disappeared. Considerable experience proves that the soft overlay adjusts it self to off-conditions ol assembly ond distortion so that when the overlay goes the load is carried over the entire length of tbe bearing with resulting satisfactory performance. Other Lining Material*. Cadmium linings, oi which the popular alloys ore cadratum-*Hver copper and cadmiumnickel, are capable of high loading*. They require o minimum shaft hard ness ol about 2S0 brinell. In this con nection ahofl surfaces should, in gen eral, be os hard as possible, regardless oi the lining material. Since hard nesses above those normal to customary shafting materials involve hear treat ment or other special processing, they present problems to the engine manu facturer. Jt is thus desirable to held shaft hardness to the minimum con sistent with the all-round requirements of the engine application. Wartime requirements popularized silver, bonded to a steel back, for highload-carrying bearings in aircraft en gines. A plated run-in surface of leadtin or lead-indium is required to pro vide satisfactory performance. Since dirt is an enemy of such a hard bearing material, even with an overlay, this ma manufacturer ia using^Jumlnutn^lloy bearings in commercial engine^' alloy on which most work has been done consists of commercially pure alu minum with a 63% tin, 1% copper and 1% niekel. Another alloy, having the same composition except lor the addition of silicon, shows improved re sistance to scuffing and has been re leased on sn experimental basis. Doth alloys may be given a stabilizing heat treatment after costing, or may be coldworked. The cold-wotldng improves load-carrying ability.The alloy with sili con is recommended only where shall hardness Is ever 300 brinell and esn be used to advantage where doubtful lubri cation conditions exist. Aluminum-alloy bearings era nor mally fabricated from a single piece ol material--the mono-alloy construction pictured in last month's article. Such bearings seem to have high load-carry ing ability, good embedability and conformability, high resistance to seizure and corrosion. The inherent good beattransmuting ability ol aluminum should result in lower temperatures. On the other hand, aluminum haj a relatively high coefficient of thermal ex pansion, and ample room-temperature oil clearauce must be provided. Clear-^ once should be at least 0,001 in. per in. oi journal diameter and preferably 0.00125 in. per in. Clean and positive jJui i intermediate layer petrous ... artb" increase in load-carrying Tbe steel back and Interne*** "? are formed and machined, eoPlete to precision limits. The sur- terial is not normally recommended for diesels, where service conditions are opt to bo severe. Aluminum Storing!, Considerable interest attaches to the recent develop lubrication are desirable in any bearing operation, but for best results with alu minum alloys special care should be taken to insure a dean engine at the start and positive fc/bricei/on. POWER Febnsrv 1611 11