Document ByraeyO0y2z959R3Nj5rdqgwo

typical bear ING AND BUSHING APPLICATIONS IN A DIE$e|| ICE CONDITIONS AND MATERIALS OF DIESEL BEARINGS Aj this illustration shows, many, types of bearings find use in modern engines ' `'TV following Inforrastlon regarding hearing oppllciflom copper, 10% lead and 10% lio, with shafts el about 80 bi'die**! will tent a* a tulde to selecting Rockwell C hardness. 'unierisli*' PISTON-PIN BUSHINGS : MAIN AND CONNICTIHG-ROO BEARINGS High impact loading and, )n many eases, limited lubrica tion have made it necessary to incorporate extremely bard main and connoel<a|-rnd bearing*, the journal la rout- piston pins as well os hard and bigh-losd-corrying bearing iot and in almost all cases lubrication U sufficient 10 material material. Split sicel-backcd bushings lined with 80% copper, jalrction depends largely on bearing loads. Within their 10% lead, and 10% tin have almost entirely eliminated (Md-carrying capacity, lead- or tfe-boic babbitts ere pre> difficulty lu these applications. In some cases, an alloy of (erred. As loads /ncrcssv- highrr-dffty materials come into copper, with 4% each of lead, tin, aod sine. Is used as an die picture. Higb-leoil .copper alloys bovo been used, but in alternate, particularly in smaller diesels. rodi esses it is necessary to beep shaft hardnoss up to about god' brine/J. Tbit reduces shaft wear subitantially, portico- STARTER-MOTOR BUSHINGS brly under abrasive conditions. With tHmalol bearings-- consit^ng of on intermedioie Joyer of copper-lead, and e anrUte foyer of preeiston-piated white nicta^--utl alutninmn . benrjflgs,. It Is found that such shaft hardness Is not needed. Because of intermittent operations and tha possibility of improper lubrication, 25% copper-lead graphltcd split bush ings have feund wide use in starterdtoiblng applications. This makes (t possible to use hlgty-duty bearing materials OIL-PUMP BUSHINGS od large diesal shafts where heat treatment is not always economies!. Lnbriention usaally proves satisfactory in oil-pump body I ' ROCKER-ARM bushings and driven-gear applications, and tin- or lead-base babbitts, along with aluminum bearing alloys. And use. a(though oscillating motion is inherent In. rocker-arm bushings it hsi been found that lin* or lead-bue babbitts are satisfactory' for light foods. For medium loads, a 25% leaded bronto is often used, requiring a hard shaft, in highly loaded applications ami in large diesels, it becomes necessary to revert lo higher duly materials, such os 80% CAMSHAFT AND GENERATOR BUSHINGS Deeauso of rotating motion, light loads and omple lubrica tion, It is ndvnntagcoos to employ Use tin- or lead-base bob bins and aluminum alloys, although copper-fesd alloys are sometimes being rued. Modern Bearing Materials Meet Tough [ Demands of Diesel Service Conditions At new alloy, and combinations of metals come into use, se lection requires better knowledge of bearing requirements and properties of materials. Here are practical pointers on both* As skcins speeds ittcntAgs and bear ing loads grow heavier, lining materials face ever more severe requirement*. To meet these demands, original babbitts have been improved and new alloys de veloped. While the now materials are, in general, superior to babbitts in load carrying ability, they are apt to be less tolerant of departures from sound de sign, accurate construction and good operation. Thus selection of lining ma terials Is today an exacting job, requir ing thorough understanding of what Is required of a bearing In a given ap plication and of the properties of bear ing alloys. Beech's Requirements. To prove suit able for bearing service an alloy must display: (1) adequate fatigue strength (2) good embedability and conform* ability (3) resistance to leisure (4) re sistance to corrosion, and (5). If used as a lining,.bondability.. Let's look at each of these characteristics to grasp its meaning and assets its importance. Ferigus Strength. Many rate fatigue strength as the most important of bear ing-alloy characteristics. "Failure from fatigue usually starts with cracks at the surface, which work downward well into the metal and frequently turn side wards before reaching the bond. Such 74 |7I) [S cracking finally results in detachment of pieces of the lining, known as spall i ( ing or "shelling-out." This in tun causes progressive failure of the bear* j ing due to increased friction and over \ heating, resulting from disruption ol j y the surface end consequent faulty How of lubricating oU. The photos of Is* ligue failure on p 76 illustrate this. It ts hard lo obtain a true picture ol I : {. tha relative fatigue strength of alloyi because of the difficulty of duplicating service conditions. There is little sim ilarity between the ordinary (align# tests and actual bearing operating con ditions. The closest approach to actu ality would he a tot by flexing--open* j. \ * ; , * ^ lg and closing the besring--causing slieraating tension and compression stress In the metal. Unfortunately for cleir understanding, flexing of the beartog shell l* not always the major cause of failure. It is, however, a contributing saute and sometimes the sole cause. The tension stresses required, in addi- don to the normal compressive stresses, tniy result from the difference in rate of thermal expansion between the linbg snd the back. In a steel-backed tin-base babbitt-lined bearing o rise of 130 F can produce over 13,000-psi obess, higher than the yield point of say ordinary babbitt. Such tension 'tresses, ply* the normal compression, sy open cracks in the surface. Stress in lining material after casting * At tbs ISIS Nul*n*l Ofl nS Om tovr Oral*, of lh Oil and Uu povw Dlrldca. AULT- tor a OrtVod** Mm ts Vwctftti *** ptixsto V * OtfitOt* *ad O VT LsPlff, Cbritsftit OrivAfU Oss H I E*iry, NsAlansI liartag Dlflsto. Ajavtc** Orvka |M b!*l niU. tos**l-Mau! Car*; ** D B Waed. Abmlmts CD ol AwFtin. WU anlO* and a praeadiac on* Uu. VP S4-SM U mMOmO* aod fnv tdayraUpa af mms. i 00 (he backing is high and of the order \ [*B*gnlt\jde ol the lining's yield point, j when the bearing is subsequently p heated to ISO F, stress drops to a small * *nlue because the high rate af creep j. ' r^ER fobnury FOWER February 194$ II relieves it Cooling down lo room tem perature causes stress to return to a high value. Examination of operating conditions shows that during starting and initial running before the bearing warms up, tensile stresses will be high but they become smoil when the bear ing runs warm. It has also beon shown that craeks Indicating incipient fatigue failure can be produced by alternate heating and cooling of a portion of the lining surface through relatively few cycles. In early fatigue failure, the bearings often show signs ol excessive localised heat ing of the surface. Today's clearer picture of fatiguefailure causes has Influenced the trend to thinner linings. Reducing thickness of the liking does not prevent the stresses but it does decrease their site. Embedability. Inevitable presence of dirt in diesel engines demands that bearings display embedability. Require ments vary greatly from engine to en gine, and with geographical and cli matic conditions. Where oil and air filtration are employed, the variation, is less and tends to depend mostly on filtration effectiveness. When dirt enters the bearing clear ance space, it must embed in the bear ing material or cut o groove around shaft or bearing. This should not be confused with the scoring resulting from- insufficient anti-seizure character istics. Confusion may be avoided by reserving the term "scoring" (or the ease just mentioned and using the term "cut" or "dirt-cut" to refer to bearings grooved by dirt and foreign matiefT Conformobility. If it weTe possible to construct engines with complete ac curacy and alignment, and if no dis tortions occurred in operation, the eonformability of a bearing material would have littlo or no Importance. Actually, of course, firing loads and other oper- (79) 78