Document GDEKMno1K5kpk5B64MoMYED4

CONNECTING RODS PIFFER IN DETAILS OF CRANffl Hi |ND ARRANGi^ .............................. " '' ba|1nG^ ' ` #''" W\^/W 21 24, jfcjfci`abovo riiow wrlstplw eon*. ' " j5!in^ Ufr to right! fulMJoatlnp do* bearing In' bolti rod and piston)' fltfrod, bearing In piston;, pin.tight In rad; and pin boltod r'at tap at rad and boarlng in lha. "H , piston;' Cutaway' vlow at far right shows' a ' (oftiirurtton whlrh ovareomai usual ob|a<tlon to fastening: pin In piston. Pin baits to In* . farriol' supporting structure Ip piston. There is no distortion of piston such os results from _ o. press fit, and' disassembly Is focllltotod 22; ^ W;.o a '.Tjrplcol (nn*<tlfle*rod.;dlgn*i. tii*4 ln pla< gt.blt for ca(npat(iii-12l)r .rod with loot and separata crank* " anglo-tpllt crenkpin'boi mokoi Itaaitor .to,:-', ; fy. MtUd.VmeHna"b|g and(19); . ''withdraw'ilirowgh''Vyiliidor (221 Mngod*' *common 'doilgn ' has upper half of crankpin '' itrap.constrwctlbh 'is`also compact (29) fide* ' boaring ..Integral with rod -.(20)) studs era tlonol.typicat rodthowstubrUallonpath.(t4)'..: V'-'J.Vtf-i . !(>; -Oj>-bearing do** Bteriq $/*/> touring domf onglno'llto*-:''. ot '(trank;\ floating' wrlstr;,'**-.*;'arlngx.' Skirt length in a trunk piston acta as a crosshead; it also carries one or more rings. Below the upper ring band, where most heat Rows, well thickness falls off rapidly. Clearance between skirt and wall must be small, hence skirt must be truly round. Interior ribs stiffen skirt. FUfoa Cooling. Large engines, and units operated at high ratings, usually require piston cooling. Both water and oil have been used but although water has greater heat-absorbing ability, oil ` is almost universally used because of convenience and freedom from con tamination hasard. In one simple system, Pig. 14, part of wristpin lubricating oil jets out through nosxles in the rod end and sprays against underside of piston crown. The oil drips down the piston and into the erankcese. In many cooled pistons, the oil cir culates in a space under the crown. Pump forces an excess of oil to the wristpin and thence through passages, Pig. IS, to the crown. Oil returns to the sump by gravity. In the piston of Pig. 16, circulation is intermittent. Oil en ters the crown space from the hottow wristpin and is retained there until the piston descends. At stroke end, a re lease passage in the piston skirt mates with a slot in the liner and oil dis charges into the crankcase, In the large piston of Pig. 17. oil from wristpin bearing is forced through labyrinth passages In crown space, returns from the crown through a telescopic tube. Pliraa Rings. The typical piston car ries a wide band of rings at the top (usually four to six) and a few rings on the skirt. Rings have three basic jobs: (I) to seal gas pressure (2) to provide path for heat flow from piston to cylin. der wails, and-(3) to control cylinderwall lubrication. Commonest are snap rings with angle or step joints. To reduce compression loss by leakage through the gop in plain rings, special aaa/ rings may be used. Oil'Control rings are designed to scrape excess oil from cylinder wells on the down stroke; ride over the film on the way up. Ring groove may be con nected to inside of piston by channels or ports to carry away excess oil. Most rings are made of cast iron; some bi metallic constructions (phosphorbronze wear strips, for example) have been used. Chromium plating and other surface treatments have also been employed. Connecting Rod. In essence a strut with a bearing at each end, the con necting rod transmits piston thrust to the crankshaft. It must be strong, yet because of inertia forces must be with 71 (208) out excessive weight, especially in high3 speed engines. The rod is usually torg from alloy steel, frequently with a or channel section. In large engines, the cronkpin e may consist of a separate bearing fa (in.two parts) bolted to a "foot'1 oa tl rod. Shims between-foot and boxpe mit adjusting compression. This ,'*o celled `'marine" design has latg* given way to the construction of f 20, in which upper half of crank] box is integral with the rod. i Need for large crankpin bearingsjk highly loaded engines makes it d'" to keep crankpin end small enoughjkl withdrew through the cylinder. Moitl compact construction results front: (OJ use of four bolts instead of two C use of studs instead of bolts, Ktg. 2 -(3) splitting box at an angle. Fig' * and (4) employing hinged-strap r traction, Fig. 33. Rod Bolts. Connecting-rod bolts _ made from heat-treated carbon steel cj alloy steel in some cases; the 6^ threads used for strength are carefuj cut and finished to ovoid fecta which might start fatigue cr** To assist bolts in resisting side-in* forces many designers put tongue-and-groove at the joint b'"* rod foot and upper bearing half, j tween bearing halves. sown . Apri'*! 0: (&Vfr(ttpl CoMtructiea. The wristpin 'uiktfconnecting rod and piston. Of possible arrangements - (I) -Iwrulpin secured in piston', bearing in ii(|2) wristpin fastened to rod. beartin piston or (3) wristpin bearing 'a'.boih piston and rod-- the latter, or 'tolMtottint, design is now most comoim>Piston bosses usually carry bush"for the pin; either snap rings or wjjtapi hold the pin against longituj&ialimov'ment. End caps keep wrist^`bearing lube oil from leaking into fcrjwtier. Tell.floating design makes almost ^^ttaof wristpin available at bear* jWfijrfiee. The "fastened in the rod" HTMtttruction may be used in such a *^?*`If* to secure maximum bearing ^uifiee on top of pin, where it Is most The pin bolts to a saddle or j?1it* *** rd end, freeing entire top 55? wrfset for bearing, ^$P.flftcipal'objection to fastening the . '5v^ the piston conies from use of ? * hold pin in bosses. This ,5wy* resulted in distortion of -jgv. Fiaton. and made disassembly dif* jUy **rge pistons, the wristpin can t0 an internal supporting Fig. 35, avoiding these draw- r>^ connecting-rod wristpin bearing &fly a solid bushing. It receives lubrication from the crankpin bearing Induction hardening b often applied to through rifle-drilled passages in the give journals a hard skin over a tough rod, Fig. 24. The wristpin itself is usu- . inner core. In addition, journal surfaces ally steel and hollow. Fig. 26 shows a may be uuptrBniahed to make them ex wristpin fitted with small-diameter tremely smooth. These steps greatly cageless rollers In what b called a increase life. needle bearing. This design permits ex Couattrwolgtiling. Crankshafts are tremely smoll clearance and hence less sometimes counterwelghted to relieve tendency to "pound". It also offers high loads on main bearings by offsetting load-carrying capacity. inertia forces. It may abo improve bal Crankshafts. The principal force a ance, especially in 1 2-, and 3-cylinder diesel crankshaft must resist is the units. In crankcase-scavenging engines, bending action of connecting-rod thrust counterweights help AU crankcase space when piston Is at top center. Then and hence improve pumping action. maximum gas pressure acts straight Major disadvantage lies In fact that . .down on the crankpin, tending to bend counterweights lower crankshaft criti the shaft between supporting bearings. cal speed end may bring it into the The crankshaft must also withstand engine's operating range. twisting or tortionol forces from con Baariagt. With the exception of necting-rod angular thrust. needle-bearing wristpins previously Moat modem engines have one-piece mentioned and occasional use of ball crankshafts forged from carbon steel and roller bearings, diesels use sleeve or steel alloyed'with nickel, chromium bearings almost exclusively. Main and and molybdenum. In extremely large erankpln bearings are split and, where engines, crankshaft may be two forg load conditions permit, lined with tin- ings, bolted together. In some engines, or lead-bate babbitts. Current practice - crankshafts too large to be mode as leans heavily to precision. bearings, single forgings are built up. Webs are consisting of a thin back or shell and forged and joumnb turned; journals an extremely thin lining, machined to are shrunk and keyed into web forgings. be Interchangeable without hand fitting. Some modern engines have crankshafts Por a comprehensive dbcussion of cast from ehrome-nickel-moly iron. bearing construction, lining materials, It b customary to heat-treat forged and applications In diesels, see Powxa, shafts to relieve any Internal stresses. Jan 1948, pp 84-87; Feb, pp 74-77. SSSJ- * *oril iv8 n(209)