Auto Material Connecting Rod

8/12/2019 Auto Material Connecting Rod

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CONNECTING ROD


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Contents

Functions

The monolithic con-rod

The assembled con-rod

The plain bearing

Fracture splitting


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Functions

The crankshaft con-rodmechanism transformsreciprocative motion torotational motion.

The con-rod connects the

piston to the crankshaft totransfer combustionpressure to the crankpin.

There are bearing portionsat both ends, the piston

side is called the small end,and the crankshaft side, thebig end.


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Stresses on Connecting rod

The con-rod must withstandvery high forces as the pistonmoves within the cylinder bore.

The shaft portion of the con-rodis subjected to bending as well

as tension and compression. The bearing portions receive

load from the weight of thepiston and the con-rod. To avoidfailure of the bearings, the con-

rod should be made as light aspossible. To avoid buckling, therod portion usually has an I-beam shape because of the highrigidity-to-weight ratio of this

shape.


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Although con-rods for both four-stroke and two-stroke engines have an I beam shape, the thicknessdistribution is slightly different in the two engines.

The four-stroke con-rod receives a large tensile loadduring the exhaust stroke as well as a compressive

load during the combustion stroke. The inertial force of the reciprocating mass generates

a tensile load which is proportional to the product ofthe piston assembly weight, reciprocating mass of thecon-rod and square of the rotational velocity.

It is bigger than the compressive load above a certainrotational speed.

Stresses on Connecting rod


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Types of connecting rod

1. The monolithic con-rod

2. The assembled con-rod


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Figure shows a monolithic con-rod. The monolithic con-rodhas a needle roller bearing at the big end, which is illustratedin Fig. 9.5

The two-stroke engine requires a needle roller bearingbecause the big end has less lubricating oil due to thestructure. In four-stroke engines, lubricating oil is abundant inthe crankcase, and the assembly type of con-rod is used

because of the lower cost of this simpler structure.


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Figure shows a needle roller bearing for the bigend. The needle rollers held in the retainer areinserted into the big end and run on the outerraceway of the big end and the inner raceway of

the crankpin. The roller itself receives high stress and also

exerts high Hertzian stress on the rolling surfaces.

The retainer(cage) separates the rollers,

maintaining an even and consistent spacing duringrotation, and also guides the rollers accurately inthe raceways to prevent the rollers from fallingout.


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The big end is carburized to increase rollingcontact fatigue strength, and honing finishesthe surface accurately.

Carburizing is required only at the rollingsurface and copper plating is used as acoating to prevent other portions fromcarburizing.

If carburizing hardens the entire con-rod, thesubsequent straightening tends to causecracking



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Structure and material:

The big end consists of two parts. The bottom partis called the bearing cap, and this is bolted to thecon-rod body.

Honing finishes the assembled big end boss to anaccurate circular shape. The mating planes of the

cap and rod body should be finished accurately inadvance because this influences the accuracy ofthe boss. The plain bearing is sandwiched betweenthe crankpin and big end.

Hot forging shapes the assembled con-rod.

Free-cutting steels are frequently used when highmachinability is required.

Toughening is a typical heat treatment for carbonsteel.


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The con-rod bolt Con-rod bolts and nuts clamp the bearing cap to the con-rod

body, sandwiching the plain bearing. The bolt is tightenedwith an appropriate load to prevent separation of the jointduring operation, and so the bolt must be able to withstandthe tightening load and the maximum inertial force.



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To reduce the weight of the big end, the bolt holeshould be positioned close to the big end boss.

Some bolt heads have elliptical shapes to preventthem from coming loose.

To prevent the joint between the cap and body fromshifting, the intermediate shaft shape of the close-tolerance bolt should be finished accurately.

The pitch of the screw portion must also be narrow.

Some con-rods do not use a nut because the cap screw

threads into the con-rod body itself. This type can lighten the big end, but is likely to

cause stress concentration on the screw thread. Usinga nut can help to prevent fatigue failure in bolts.



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The inertial forces from the piston, piston pin andcon-rod body tend to separate the joint between thebody and cap.

Even a slight separation increases friction loss at thebig-end boss, and shortens the life of the plain

bearing. The stress on the con-rod bolt relates not only to the

shape of the big-end boss but also to the rigidity of thebolt itself.

The big-end boss should remain circular when the

connecting rod bolts are tightened. The mating planes in the joint should lock the con-rod

body and cap in perfect alignment, hence smoothmating surfaces are required. Stepped mating planescan prevent the joint from shifting.



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Figure 9.18 shows distortions in the big-endbore under load.

The conrods under comparison have thesame shape but are made of differentmaterials; titanium (Ti-6Al-4V, indicated asTS) and Cr-Mo steel SCM435 (SS).

Both circles show upward elongation, while

the titanium con-rod, which has a lowerYoungs modulus, shows the largerdistortion.



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The plain bearing

An appropriate gap is necessary between theplain bearing and crankpin so that oilpenetrates the gap to lift up the crankpin,providing hydrodynamic lubrication during

rotation. The plain bearing must conform to the

irregularities of the journal surface of thecrankpin.

It should have adequate wear resistance atthe running-in stage, high fatigue strength athigh pressure and sufficient seizureresistance at boundary lubrication.


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The plain bearing should also have the ability to absorb dirt,metal or other hard particles that are sometimes carried intothe bearings.

The bearing should allow the particles to sink beneath thesurface and into the bearing material.

This will prevent them from scratching, wearing anddamaging the pin surface.

Corrosion resistance is also required because the bearing mustresist corrosion from acid, water and other impurities in theengine oil.

The allowable contact pressure was only 10 MPa.

Because of this low contact pressure, the crankpin diameterhad to be increased to decrease the contact pressure.

To overcome this, a Cu-Pb alloy bearing having a higherallowable contact pressure was invented.

The plain bearing


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Figure 9.19 schematically illustrates the cross-sectional view of aplain bearing.

It comprises three layers; the backing metal, which is a steel platefacing the con-rod, an intermediate aluminum alloy layer (Al-Sn-Sialloy) that has particulate Sn dispersed in the aluminum-siliconmatrix, and a soft layer (Sn plating), called overlay, on the inside.

The steel backing plate supports the soft aluminum alloy and theadditional soft overlay gives wear resistance during running-in.

The plain bearing


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The Sn overlay has a low melting temperature of 232 C. Friction heat is likely to accelerate the diffusion of Sn into the

bearing layer and cause a loss of Sn from the overlay.

To prevent this, a thin layer of Ni is inserted between thebearing metal and overlay. This is referred to as the Ni dam.

The steel backing plate is laminated to the aluminum alloysheet by cold rolling.

Figure 9.20 illustrates schematically the rolling process.

The plain bearing


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The high pressure between the rollers causesplastic deformation at the interface between themetals, resulting in strong metallic bonding.

This two-metal structure is called clad metal.

The plain bearing is shaped from the clad metalby a press machine.

The Cu-Pb plain bearing also has a bimetalstructure, where sintering laminates the Cu-Pblayer to the steel backing plate.

In this process, a Cu-Pb alloy powder is spreadonto the Cu-plated steel plate. The powder layer issintered and diffusion-bonded to the steel plate athigh temperatures.

The plain bearing


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Fracture splitting

The mating planes for the joint should besmoothly machined to lock the con-rod bodyand cap in perfect alignment.

Positioning using a step mating plane or aknock pin, which prevents the joint from

shifting, is sometimes used. These joint structures give good accuracy forplain bearings, but the machining requiredraises the cost of production.

To address this increase in cost, analternative method using a broken jaggedsurface at the joint plane has beenintroduced.


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Fracture splitting The con-rod in this instance is referred to as a fracture

split con-rod. The cap is cracked off to produce a

rough mating surface as shown in Fig. 9.22 This surface helps lock the con-rod body and cap in

perfect alignment and prevents the cap from shifting.

The manufacturing process is as follows: first, forgingand machining shape the big end monolithically.


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After completion, the monolithic big end is brokeninto two pieces (the bearing cap and the rod body), byintroducing a crack at the joint surface.

Special splitting tools have been developed in order tosplit the big end with minimal plastic deformation.

To generate cracking at the correct position, notchesare carved in the internal surface of the big end bylaser or mechanical broaching.

The fractured surfaces should fit exactly into positionwhen both portions are overlapped and fastened by

bolts. Any plastic deformation during splitting should be

avoided, because if plastic deformation takes place,the broken surfaces cannot fit together.

Fracture splitting


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The crack should not cause branching, otherwise it isdifficult to reassemble.

The fracture split con-rod is made from sintered steel,hot forged high carbon steel or hot forged micro-alloyed steel.

Good mold yield of the sintering (powder metallurgy)method lowers costs.

The manufacturing process for sintered steel has twosteps, first, cold compaction of a powder in the moldand secondly, sintering the pre-form in a furnace to

give enough bonding between powder particles. The added Cu increases the density of the sintered

part through liquid phase sintering.

Fracture splitting


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Additional hot forging (called powder forging)increases strength by removing small pores in thesintered steel.

Splitting should take place in a brittle manner withoutplastic deformation, and the sintered con-rod is

suitably brittle. High carbon steel, around 0.7% C, is particularly

good for this type of con-rod because it can be brokeneasily and the microstructure is pearlitic.

Micro-alloyed steel with a typical chemical

composition of Fe-0.7%C-0.2Si-0.5Mn-0.15Cr-0.04Vhas also been tested for fracture splitting.

The vanadium is alloyed to give precipitationhardening properties, and the cooling process after hotforging is controlled so that precipitation guarantees

strength

Fracture splitting


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The use of fracture split con-rods is increasing because of lowcosts and high dimensional accuracy at the big end.

To increase fatigue strength against bending, a case-hardenedfracture split con-rod has also been developed.

The fracture split con-rod was originally developed foroutboard marine engines, which are placed at the stern of a

boat.

Two-stroke multi-cylinder engines are widely used because ofgood acceleration performance and high durability.

The exhaust displacement measures about 3,000 cm3at itsmaximum.

The multi-cylinder engine employs a monolithic crankshaftbecause of the need for dimensional accuracy.

Fracture splitting


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However, a needle roller bearing is indispensableat the big end of the two-stroke engine, and so thecon-rod should be of the assembled type.

Since it is difficult to keep dimensional accuracy

in the machined con-rod, fracture split technologyhas been introduced.

The needle-bearing retainer also has to be split.Both ends require high hardness for needle rollers,

so carburizing is used to give sufficient hardnessto the rolling surface.

The monolithic crankshaft is also carburized toimprove wear resistance to needle rollers.

Fracture splitting


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The science and

technology of materials inautomotive engines

by Hiroshi Yamagata

Automotive Engineering

Lightweight, Functional,and Novel Materials

by Brian Cantor.

Advance composite

materials for automotiveapplications

by Ahmed Elmarakbi

Reference Books

Documents

Auto Material Connecting Rod