Honey Narang 626/MP/10 Keshav Mohan Katarya 629/MP/10 Mohammad Saad Khan 630/MP/10 Rachit Goel 646/MP/10

CRANKSHAFTS CASTING DESIGN ......A CASE STUDY

MA 207 Science Of Materials/ Manufacturing Processes I LAB

INDEX

Introduction

History

Crankshafts in action

Working of a crankshaft

Designing a Crankshaft

Manufacturing a Crankshaft

Selection of materials

Making of prototypes

Selection of casting process

Solidification of metal

Heat treatment of crankshaft

Machining

Hardening

Finishing

Bibliography

Introduction

The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation. To convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach.

It typically connects to a flywheel, to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a torsional or vibrational damper at the opposite end, to reduce the torsion vibrations often caused along the length of the crankshaft by the cylinders farthest from the output end acting on the torsional elasticity of the metal.

History

Classical Antiquity

A Roman iron crankshaft of yet unknown purpose dating to the 2nd century AD was

excavated in Augusta Raurica, Switzerland. The 82.5 cm long piece has fitted to one

end a 15 cm long bronze handle, the other handle being lost.[2][1]

The earliest evidence, anywhere in the world, for a crank and connecting rod in a

machine appears in the late Roman Hierapolis sawmill from the 3rd century AD and

two Roman stone sawmills at Gerasa, Roman Syria, and Ephesus, Asia Minor (both

6th century AD). On the pediment of the Hierapolis mill, a waterwheel fed by a mill

race is shown powering via a gear train two frame saws which cut rectangular blocks

by the way of some kind of connecting rods and, through mechanical necessity,

cranks. The accompanying inscription is in Greek.

The crank and connecting rod mechanisms of the other two archaeologically attested

sawmills worked without a gear train. In ancient literature, we find a reference to the

workings of water-powered marble saws close to Trier, now Germany, by the late 4th

century poet Ausonius; about the same time, these mill types seem also to be

indicated by the Christian saint Gregory of Nyssa from Anatolia, demonstrating a

diversified use of water-power in many parts of the Roman Empire. The three finds

push back the date of the invention of the crank and connecting rod back by a full

millennium; for the first time, all essential components of the much later steam engine

were assembled by one technological culture:

With the crank and connecting rod system, all elements for constructing a steam

engine (invented in 1712) Hero's aeolipile (generating steam power), the cylinder

and piston (in metal force pumps), non-return valves (in water pumps), gearing (in

water mills and clocks) were known in Roman times.

Middle Ages

The Italian physician Guido da Vigevano (c. 12801349), planning for a new crusade,

made illustrations for a paddle boat and war carriages that were propelled by

manually turned compound cranks and gear wheels (center of image). The Luttrell

Psalter, dating to around 1340, describes a grindstone which was rotated by two

cranks, one at each end of its axle; the geared hand-mill, operated either with one or

two cranks, appeared later in the 15th century;

Renaissance

The first depictions of the compound crank in the carpenter's brace appear between

1420 and 1430 in various northern European artwork. The rapid adoption of the

compound crank can be traced in the works of the Anonymous of the Hussite Wars,

an unknown German engineer writing on the state of the military technology of his

day: first, the connecting-rod, applied to cranks, reappeared, second, double

compound cranks also began to be equipped with connecting-rods and third, the

flywheel was employed for these cranks to get them over the 'dead-spot'.

In Renaissance Italy, the earliest evidence of a compound crank and connecting-rod

is found in the sketch books of Taccola, but the device is still mechanically

misunderstood. A sound grasp of the crank motion involved demonstrates a little later

Pisanello who painted a piston-pump driven by a water-wheel and operated by two

simple cranks and two connecting-rods.

Water-raising pump powered by crank and connecting rod mechanism (Georg Andreas

Bckler, 1661)

One of the drawings of the Anonymous of the Hussite Wars shows a boat with a pair

of paddle-wheels at each end turned by men operating compound cranks (see

above). The concept was much improved by the Italian Roberto Valturio in 1463, who

devised a boat with five sets, where the parallel cranks are all joined to a single power

source by one connecting-rod, an idea also taken up by his compatriot Francesco di

Giorgio.

Crankshafts were also described by Konrad Kyeser (d. 1405), Leonardo da Vinci

(14521519) and a Dutch "farmer" by the name Cornelis Corneliszoon van Uitgeest in

1592. His wind-powered sawmill used a crankshaft to convert a windmill's circular

motion into a back-and-forward motion powering the saw. Corneliszoon was granted

a patent for his crankshaft in 1597.

From the 16th century onwards, evidence of cranks and connecting rods integrated

into machine design becomes abundant in the technological treatises of the period:

Agostino Ramelli's The Diverse and Artifactitious Machines of 1588 alone depicts

eighteen examples, a number which rises in the Theatrum Machinarum Novum by

Georg Andreas Bckler to 45 different machines, one third of the total.

Middle and Far East Al-Jazari (11361206) described a crank and connecting rod system in a rotating

machine in two of his water-raising machines. His twin-cylinder pump incorporated a

crankshaft, but the device was unnecessarily complex indicating that he still did not

fully understand the concept of power conversion. In China, the potential of the crank

of converting circular motion into reciprocal one never seems to have been fully

realized, and the crank was typically absent from such machines until the turn of the

20th century.

Crankshafts in Action

As automakers have reduced the overall size and weight of their cars to meet fuel

economy and emission standards, their focus often has been on the engine. But as

engine size has shrunk and their components converted to lighter weight materials,

the stress on each of the structural and moving components has increased.

One of the components most affected by increased engine stress is the crankshaft,

as it serves as the torque transmitter for the entire automobile drive-train. Simply

described, a fuel/air mixture is pushed into the combustion chamber in an engine

where it is ignited, forcing the pistons, which are located within each of the engine

block cylinders, up and down. These pistons, which are attached to the crankshaft by

connecting rods, force the crankshaft to spin. This spinning then is passed to the

flywheel, which transfers the energy to the transmission and drive-train to turn the

wheels of the automobile. This spinning also is passed to the camshaft, which opens

and closes the intake and exhaust manifolds to let the air/fuel mixture in and out of

the combustion chamber.

The problem for automotive engineers is that as the engine has been forced to shrink,

the combustion chamber shrinks. To compensate, engineers have decreased the

diameter of the engine block cylinder and increased the distance the piston travels.

The result is an increase in the stress on the crankshaft. On average, the crankshaft

spins 30 times/sec. in operation. Throughout this spin, the crankshaft has

pistons/connecting rods exerting pressure in different directions, depending on the

engine design. The goal of automotive engineers is to ensure the integrity of the

crankshaft through design, material selection and production.

Working of a Crankshaft

Power from the burnt gases in the combustion chamber is delivered to the crankshaft

through the piston,piston pin and connecting rod.The crankshaft (fig.1) changes

reciprocating motion of the piston in cylinder to the rotary motion of the

flywheel.Conversion of motion is executed by use of the offset in

the crankshaft.Each offset part of the crankshaft has a bearing surface known as a

crank pin to which the connecting rod is attached.Crank-through is the offset from the

crankshaft centre line. The stroke of the piston is controlled by the throw of the

crankshaft. The combustion force is transferred to the crank-throw after the

crankshaft has moved past top dead centre to produce turning effort or torque, which

rotates the crankshaft. Thus all the engine power is delivered through the crankshaft.

The cam-shaft is rotated by the crankshaft through gears using chain driven or belt

driven sprockets. The cam-shaft drive is timed for opening of the valves in relation to

the piston position. The crankshaft rotates in main bearings, which are split in half for

assembly around the crankshaft main bearing journals.

Both the crankshaft and camshaft must be capable of withstanding the intermittent

variable loads impressed on them. During transfer of torque to the output shaft, the

force deflects the crankshaft. This deflection occurs due to bending and twisting of the

crankshaft. Crankshaft deflections are directly related to engine roughness. When

deflections of the crankshaft occur at same vibrational or resonant frequency as

another engine part, the parts vibrate together. These vibrations may reach the

audible level producing a thumping sound. The part may fail if this type of vibration

is allowed to continue. Harmful resonant frequencies of the crankshaft are damped

using a torsional vibration damper. Torsional stiffness is one of the most important

crankshaft design requirements. This can be achieved by using material with the

correct physical properties and by minimizing stress concentration.

The crankshaft is located in the crankcase and is supported by main bearings. Figure

3.62 represents schematic view of a typical crankshaft. The angle of the crankshaft

throws in relation to each other is selected to provide a smooth power output. V-8

engines use 90 degree and 6 cylinder engines use 120 degree crank throws. The

engine firing order is determined from the angles selected. A crankshaft for a four

cylinder engine is referred to a five bearing shaft. This means that the shaft has five

main bearings, one on each side of every big end which makes the crankshaft very

stiff and supports it well. As a result the engine is normally very smooth and long

lasting.

This Figure describes that how a crankshaft is fitted in a car.Its joining from Cylinders

to piston to crankshaft to itself and then to the wheels of the car.

Because of the additional internal webs required to support the main bearings, the

crank case itself is very stiff. The disadvantages of this type of bearing arrangement

are that it is more expensive and engine may have to be slightly longer to

accommodate the extra main bearings. Counter weights are used to balance static

and dynamic forces that occur during engine operation. Main and rod bearing journal

overlap increases crankshaft strength because more of the load is carried through the

overlap area rather than through the fillet and crankshaft web. Since the stress

concentration takes place at oil holes drilled through the crankshaft journals, these

are usually located where the crankshaft loads and stresses are minimal. Lightening

holes in the crank throws do not reduce their strength if the hole size is less than half

of the bearing journal diameter, rather these holes often increase crankshaft strength

by relieving some of the crankshafts natural stress. Automatic transmission pressure

and clutch release forces tend to push the crankshaft towards the front of the engine.

Fig. 1 Crankshaft

Thrust bearings in the engine support this thrust load as well maintain the crankshaft

position. Thrust bearings may be located on any one of the main bearing journals.

Experience shows that the bearing lasts much longer when the journal is polished

against the direction of normal rotation than if polished in the direction of normal

rotation. Most crankshaft balancing is done during manufacture by drilling holes in the

counterweight to lighten them. Sometimes these holes are drilled after the crankshaft

is installed in the engine.

Designing a Crankshaft

In the world of component design, there are competing criteria, which require the

engineers to achieve a perceived optimal compromise to satisfy the requirements of

their particular efforts. Discussions with various recognized experts in the crankshaft

field make it abundantly clear that there is no right answer, and opinions about the

priorities of design criteria vary considerably. In contemporary racing crankshaft

design, the requirements for bending and torsional stiffness (see the Stiffness vs.

Strength sidebar) compete with the need for low mass moment of inertia (MMOI).

Several crankshaft experts emphasized the fact that exotic metallurgy is no substitute

for proper design, and there's little point in switching to exotics if there is no fatigue

problem to be solved.

High stiffness is a benefit because it increases the torsional resonant frequency of the

crankshaft, and because it reduces bending deflection of the bearing journals. Journal

deflection can cause increased friction by disturbing the hydrodynamic film at critical

points, and can cause loss of lubrication because of increased leakage through the

greater radial clearances that occur when a journal's axis is not parallel to the bearing

axis.

At this point, it is important to digress and emphasize the often-misunderstood

difference between STIFFNESS and STRENGTH.

Metal parts are not rigid. When a load is applied to a metal part, the part deflects in

response to the load. The deflection can be very small (crankshaft, conrod, etc.) or it

can be quite large (valvesprings, etc). But to one degree or another, all parts behave

like springs in response to a load. The ultimate strength of a material is a measure of

the stress level which can be applied to a lab sample of the material before it

fractures. The degree to which a given part resists deflection in response to a given

loading is called stiffness. It is important to understand that the ultimate strength of a

material has nothing whatever to do with stiffness. Stiffness is the result of two

properties of a part: (1) the Young's Modulus of the material (sometimes called

Modulus of Elasticity, but more appropriately named Modulus of Rigidity) and the

cross-sectional properties of the part to which the load is applied.

For example, suppose you have two components which are identical in all respects

(same material, same dimensions) except the tensile strength to which those

components have been heat-treated. If you apply an increasing load to each

component, both will deflect the same amount for each load value, until the

component with the lower strength permanently deforms (and breaks if it is loaded

and constrained in a certain way) at a relatively low stress level. The component with

the higher strength will continue to deform with increasing load until its yield stress is

reached, at which point it too will permanently deform.

Since the current crankshaft materials are alloy steels, the Young's Modulus is fairly

constant. That means that altering the section properties of the highly-stressed

portions of the crankshaft is the only way to increase stiffness. Of course, adding

material works at cross-purposes to maintaining low MMOI.

Three major parameters which affect crank stiffness are length, journal diameter and

crankpin overlap. The torsional rate of a cylindrical section varies directly with length

and with the fourth power of diameter. Crankpin overlap is a measurement of how

much crankpin material is horizontally aligned with the material of the adjacent main

journals, as illustrated in Figure 2, showing a CPO of 0.225 with a 4.250" stroke

crank having 2.100 rod journals and 2.600 main journals.

CPO = (main diameter + crankpin diameter - stroke) / 2

Figure 2

There is a continuing emphasis on research and design among F1 and Cup teams to

increase stiffness with minimal impact on MMOI. However, all the experts I spoke with

were understandably reluctant to discuss the specifics of where and how they are

adding material and how effective their changes are. From examining some available

pictures and gathering data on other engine parameters, I would hazard a guess that

the conrod bearing widths are being reduced to make room for thicker webs. It is also

possible that main bearing journals are being undercut to produce the required fillet

radii at the intersection with the web, again making more room for thicker webs.

Undercutting the journals increases the stress levels and locally reduces the section

properties. However, the immense fatigue strengths of the contemporary materials

and the relative lack of crank failure at the highest levels of racing suggest that the

endurance limits can be pushed a bit further. It is apparent that a great deal of FEA

work is essential at the top.

One stiffness area where most two-plane V8 engine people agree is the use of center

counterweights. It has been known for some time that there are significant power

gains available in two-plane crank V8s from the use of counterweights around the

center main bearing (Figure 3).

Figure3

8-Counterweight V8 Crank (Courtesy of Bryant Racing)

Traditionally, many two-plane V8 crankshafts had been produced without center

counterweights because of economies and difficulties forging the blanks, because the

six-counterweight crank typically has a slightly lower MMOI, and because the benefits

of an eight-counterweight crank in a short-stroke application were not fully

appreciated. However, the bending deflection across the center main at high loadings

and high speeds causes measurable losses, so many areas of racing which use two-

plane V8 cranks are moving (or have already moved) to eight-counterweight cranks.

From an overall engine design perspective, the relocation of the thrust bearing from

the rear main to the center main also helps reduce center-main bending deflection.

There are varying opinions about whether high stiffness or low MMOI is more

important. Low MMOI is most important at high engine acceleration rates. Road-

course racing typically involves greater vehicle speed variation per lap, which implies

greater requirements for quick acceleration through several gear ratios. In certain

classes, the low weight of the vehicle and the high power of the engine can yield very

high engine acceleration rates. At the higher-speed Cup racing circuits, the engine

acceleration rates at speed are often less than 100 RPM per second, while at some of

the shorter tracks, they can exceed 500 RPM per second. Of course, there are

restarts and pit stops to be dealt with at all tracks, so it is easy to see how there can

be varying approaches to this issue.

Reducing MMOI involves removing material, especially from places which are a long

distance from the main bearing axis. However, these are also some of the most highly

loaded areas as well, so reducing cross sectional properties necessarily increases the

cyclic stress levels. Pushing the cyclic stress levels up impinges on the fatigue life of

the component, which is especially important in classes where an engine must, by

regulation, survive more than one meeting. Determining acceptable levels of cyclic

stresses vs. expected life is not an exact science. Endurance limit testing of materials

produces a highly statistical array of results data .

There has been quite a bit of discussion about the use of bolt-on counterweights in an

attempt to reduce MMOI values. An example of this technology is shown in Figure 4.

These detachable counterweights are made from variants of the heavy metal used to

balance crankshafts. This heavy metal is a tungsten-based alloy with several different

chemistries (W-Ni-Cu; W-Ni-Fe; W-Ni-C) depending on the required properties. These

alloys have nearly 2.5 times the density of steel, and are extremely expensive.

Figure4

Bolt-On Counterweights

Another benefit of bolt-on counterweights is that several of the machining operations

are much simpler to accomplish without having to deal with the integral

counterweights getting in the way. If journal coatings are used, the more complete

access to the journals provided by the absence of integral counterweights could also

be a benefit.

There were some initial problems with bolt-on counterweights, which resulted (as one

Formula One designer told me) in "several deep holes being dug in the surface of a

few racetracks". There are tensile and fatigue stress issues, as well as the inevitable

fretting between contact surfaces and the requirement for highly developed fastener

technology. Usage in Formula One suggests that those issues have been resolved.

There is a variance of opinion as to whether bolt on counterweights are being

investigated in Cup. One person told me they are explicitly illegal, while two others

told me they know of a certain amount of investigation and development going on in

that regard.

In the world of two-plane V8 cranks, the traditional calculation for the balance-

bobweight value is 100% of the rotating weight (big end, inserts and oil) plus 50% of

the reciprocating weight (small end, wristpin, retainers, piston, rings and oil).

However, there are differing approaches to the question of overbalance or

underbalance. . Some experts stick with the 100% + 50% distribution, while others

opt for a 46-47% underbalance (100% + 47%). Others prefer a 52-53% overbalance,

while others add an arbitrary 100 grams to the 50% reciprocating calculation. There

was a general reluctance to discuss the expected or observed effects of these

strategies.

There has been an interesting development regarding two-plane V8 crankshaft

lubrication drillings. Traditionally, each rod bearing was fed oil by a single angled hole

from the loaded-during-compression side of the rod journal to the less-loaded side of

the adjacent main journal, sometimes called straight-shot oiling, shown in Figure 5.

That strategy reduced the effect of centrifugal-force starvation at high RPM and

assured the availability of sufficient oil to provide the dynamic film strength for the

combustion loading.

Figure5

"Straight-Shot" Oiling

The problem with this scheme is that the intersection of the angled hole with the rod

journal produces a large elliptical interruption in the journal surface. Add the

chamfering usually done around that hole, and what results is a significant

interruption of the hydrodynamic surface area. Coupled with the reduced bearing

widths, that divot creates a substantial leakage path for the oil to escape.

The new approach rearranges the drillings so the holes in the rod journal can be

perpendicular to the surface. One method is to drill a perpendicular oil hole into the

rod journal, and drill an intersecting parallel hole partially through the rod journal and

plug the open end. Next, an angled drilling from an adjacent main journal is made to

intersect the parallel drilling. Another method involves horizontal drillings through the

main journal, through the CPO into the rod journal, with perpendicular feeds into both

journals. This rearrangement enables the lubrication of both rods on the same

crankpin from a single main journal. That can be an advantage in view of data

showing that two-plane V8 main journals numbers two and four are the most highly

loaded, so the rods can be oiled from one, three and five while the oil delivered to

mains two and four can do a better job because of reduced leakage and no surface

interruptions. Figures 3 and 4 show examples of this approach.

An interesting byproduct of this new drilling strategy is the creation of internal sharp

corners and edges where the drillings intersect. These sharp corners introduce the

flow-restricting effect of sharp-edged orifices into the lube system at a critical point.

Further, sharp corners and machining marks introduce stress concentrations due to of

the surface discontinuities.

One major crank manufacturer (Bryant Racing) has developed a proprietary extrude-

honing system in which an abrasive slurry is pumped through these drillings at high

pressure. This abrasive treatment removes the sharp edges and surface flaws which

cause flow restrictions and stress concentrations, leaving the inside surfaces of the

holes with a mirror finish and nicely rounded intersections, which adds substantially to

the fatigue life of the part.

Manufacturing a Crankshaft

Crankshafts can be monolithic (made in a single piece) or assembled from several

pieces. Monolithic crankshafts are most common, but some smaller and larger

engines use assembled crankshafts. They can be produced either by casting or

forging but today they are mostly produced by casting . The entire manufacturing

process involved is as shown in the flow diagram below :

Figure 6

Basic Manufacturing Processes for a Crankshaft

Selection of Material

The design engineer must determine the material for the crankshaft. Historically,

crankshafts have either been made of gray or ductile iron (ductile iron since the

1970s) or steel, depending on the stress that it must endure. The decision on the

material also relates to the decision on the manufacturing method (historically either

metal casting or forging) to produce the component. In addition, as with any

component being manufactured today, the largest determining factor is cost.

In their design, engineers must weigh the cost benefits (of one manufacturing process

or material) against the mechanical property benefits (of another process or material)

to determine which ones they should use to manufacture the crankshaft. The three

key considerations for a crankshaft in terms of mechanical properties are modulus of

elasticity (the amount of stress it can take before failure), hardness (how rapidly it will

wear) and noise dampening capabilities. Following is a look at the four material

options for crankshafts:

Cast ductile ironGM casts 99% of its crankshafts in ductile iron (which has

a higher modulus than gray iron). The main reason for this is cost. The

finished, cast ductile iron crankshaft for a 4-cylinder engine costs $25 less than

a forged steel one (the only other process/material combination used to

manufacture crankshafts); for a V6, it costs $35 less; and for a V8, it costs $50

less. This cost savings is due primarily to reduced machining and material

costs. In general, steel is more difficult to machine than iron, but also,

according to GM engineers, cast crankshafts hold tighter tolerances with less

finish stock than forged ones. In the future, however, as engines are further

compacted, the stresses on the crankshaft will grow beyond what can be

handled by ductile iron. As a result, there may be a shift to more forged steel

crankshafts.

Cast austempered ductile iron (ADI)Although this option has only been

tried in low volume applications, the austempering process (a form of heat

treating) increases the mechanical properties of ductile iron to that of cast or

forged steel. Although the modulus of ADI is the same as ductile iron, the

increased mechanical properties may allow casting to remain a viable option

for crankshafts at higher stress levels than available with just ductile iron.

Cast steelIn the case of crankshafts, cast steel isnt an option for high

volume due to the manufacturing issues with the components design. To

ensure the directional solidification of the molten steel that results in a defect-

free casting with the necessary mechanical properties, extra gating and risers

(for feeding of the molten metal into the mold) are required beyond that used

for cast iron. As a result, the machining cost (which is already higher for steel

due to the materials modulus) for the cast steel component will be higher than

a forged steel component.

Forged steelThis is the default option for automobile designers when the

stress levels are too high for the cast ductile iron crankshaft. However, this is

becoming an increasing popular option for automobile engineers because the

stress on crankshafts is increasing with every new design. With a modulus

20% higher than that of iron, as well as increased hardness and noise

dampening, steel is often a better option for mechanical property regulations.

As a high-enough level, certain iron will not pass the safety-critical tests related

to engine stress, so the jump has to be made to forged steel regardless of cost

concerns.

Making of Prototypes

Once the material has been chosen, the solid model is ready to undergo finite

element, structure, and thermal analysis. In addition, a casting and gating system

model can be created (by the design engineer or foundry) for casting process

modeling (mold filling solidification). Then, a pattern and tooling model is created (by

the design engineer, foundry or tooling shop) to generate rapid tooling and rapid

prototypes.

Each of these models and analyses are vital to the successful design of the cast

component because they predict with very high confidence that the proper physical

and mechanical properties will be achieved. Once production-intent tooling is

produced, it is costly to return to the design board for changes.

Before material reaches the manufacture of hard tooling stage, it often has

manufactured several rapid prototypes of the crankshaft (in metal, plastic and wood-

like materials) and tooling for test runs. Due to the size of its production runs, the

speed and accuracy at which prototypes can be produced (within days) makes them

a critical step to casting design. In addition, many rapid prototype techniques for

casting can provide soft tooling for small production runs (in the hundreds) while hard

tooling for high-production sand casting is made. In terms of the production of hard

tooling for a cast component, GM typically builds in a 12-week lead time.

Selection of Casting Process

Generally crankshafts are being cast in three moulding processesgreen sand, shell

and lost foam. But how does an end-user choose the process or supplier?

At this point, the decision must be based on discussions with various foundries that

determine which plant can provide the most optimized cast component (including all

post-casting processing) at the lowest system cost. If the sourcing decisions were

based only on the cost of producing the casting itself, then the process decision

would be: 1. green sand; 2. shell; and 3. lost foam. This decision, however, would

only focus on the unit cost.

The molten metal that solidifies in the mould at a foundry is a casting; however, in

most cases, this alone is not what is being supplied to the customer. Many castings

require heat treatment, grinding, machining, polishing, painting, assembly and other

value-added services after they leave the mold. It is vital for casting designers and

buyers to incorporate all of these services (and their costs) into their final decision on

part design and where to source a component (whether the foundry performs all

these operations or not) because this is the only accurate method to determining if

one manufacturing method (green sand casting, lost foam casting, forging, welding,

etc.) is more cost-effective than another.

Green sandThe majority of ductile iron crankshafts are cast in green

sand molds. In this process, sand, clay, water and other stabilizing materials

are compacted around two halves of a pattern to form a mold for pouring. Due

to the high-production nature of the process , it is the most economical casting

process to produce the component. However, according to engineers, in

comparison to the identified two competitive techniques of shell and lost foam,

green sand process capability may require more finish stock on the casting for

machining later in production. Because no as-cast internal cavities are required

in most crankshafts, the production and insertion of sand cores in the mold is

generally avoided on each of the processes described. There are six steps in

this process:

1. Place a pattern in sand to create a mold.

2. Incorporate the pattern and sand in a gating system.

3. Remove the pattern.

4. Fill the mold cavity with molten metal.

5. Allow the metal to cool.

6. Break away the sand mold and remove the casting.

ShellShell mold casting holds tighter tolerances and tooling draft angles

than green sand by allowing the production of a mold that is narrower with

deeper pockets, reducing the machining cost and increasing dimensional

accuracy. The reason is that this process cures the sand around the pattern

with heat to glue the grains together. In addition, its surface finish is superior

to green sand molding. However, its unit cost is higher because it uses resin-

coated sand that is then heated to form the molds.The process of creating a

shell mold consists of six steps:

1. Fine silica sand that is covered in a thin (36%)thermosetting phenolic

resin and liquid catalyst is dumped, blown, or shot onto a hot pattern. The

pattern is usually made from cast iron and is heated to 230 to 315 C (450 to

600 F). The sand is allowed to sit on the pattern for a few minutes to allow the

sand to partially cure.

2. The pattern and sand are then inverted so the excess sand drops free of the

pattern, leaving just the "shell". Depending on the time and temperature of the

pattern the thickness of the shell is 10 to 20 mm (0.4 to 0.8 in).

3. The pattern and shell together are placed in an oven to finish curing the sand.

The shell now has a tensile strength of 350 to 450 psi (2.4 to 3.1 MPa).

4. The hardened shell is then stripped from the pattern.

5. Two or more shells are then combined, via clamping or gluing using a

thermoset adhesive, to form a mold. This finished mold can then be used

immediately or stored almost indefinitely.

6. For casting the shell mold is placed inside a flask and surrounded with shot,

sand, or gravel to reinforce the shell.[4]

The machine that is used for this process is called a shell molding machine. It heats

the pattern, applies the sand mixture, and bakes the shell.

Lost foamThe third process is lost foam casting. Although it has the

highest unit cost of the three processes, lost foams advantages are

recognized after the component has been cast. This loose-sand process,

which replaces polystyrene patterns with molten metal, has the best

dimensional repeatability (in terms of tolerances) of the three processes,

reducing machining time. It also alloys designers to cast-in holes and

passageways for improved functionality or the reduction of mass that otherwise

would require machining or additional cores. In the case of the lost foam

crankshaft, holes can be cast-in at the bearings to reduce mass. The basic

processes involved in this process are :

1. Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The

gating system forms the channels through which the molten metal will flow to the mold cavity.

2. Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting.

3. Pouring - The mold is preheated in a furnace to approximately 1000C (1832F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.

4. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.

5. Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. The ceramic mold is typically broken using water jets, but several other methods exist. Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen).

6. Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part.

foam

Foam pattern

PROCESSES IN LOST FOAM MOULDING

Solidification of Metal

With all of these processes, metal solidification time is another factor to consider. A

foundry that is able to control the solidification and cooling times of its castings

through its molding and shakeout (separation of the solidified casting from the mold)

processes can aid in the development of the specified components mechanical

properties and eliminate the need for subsequent heat treatment. Casting is

a solidification process, which means the solidification phenomenon controls most of

the properties of the casting. Moreover, most of the casting defects occur during

solidification, such as gas porosity and solidification shrinkage. Once shaken out, the

cast components undergo rough finishing and grinding in anticipation for any value-

added services or operations.

If a cast component is to undergo heat treatment, machining, painting, etc., this

should be specified to the foundry up front. The old days of foundries being the

producers of just castings have disappeared as producers of cast components offer

many of these value-added services in-house or through sub-contracts. In regard to

the crankshafts, some undergo heat treatment to improve mechanical properties or

ensure property consistency (especially hardness and noise and vibration

dampening) throughout the component. Every crankshaft undergoes machining to

achieve the required tolerances and features that are cost prohibitive to achieve in

casting.

Heat Treatment of Crankshaft

Regarding the steel alloys typically used in high-grade crankshafts, the desired ultimate (and hence yield and fatigue) strength of the material is produced by a series of processes, known in aggregate as heat treatment. The typical heat-treating process for carbon-steel alloys is first to transform the structure of the rough-machined part into the face-centered-cubic austenite crystalline structure (austenitize) by heating the part in an oven until the temperature throughout the part stabilizes in the neighbourhood of 1550F to 1650F (depending on the specific material). Next, the part is removed from the heating oven and rapidly cooled ("quenched") to extract heat from the part at a rate sufficient to transform a large percentage of the austenitic structure into fine-grained martensite. The desired martensitic post-quench crystalline structure of the steel is the high-strength, high-hardness, form of the iron-carbon solution. The rate of cooling required to achieve

maximum transformation varies with the hardenability of the material, determined by the combination of alloying elements. Distortion and induced residual stress are two of the biggest problems involved in heat-treating. Less severe quenching methods tend to reduce residual stresses and distortion. Some alloys (EN-30B and certain tool steels, for example) can reach full hardness by quenching in air. Other alloys having less hardenability can be quenched in a bath of 400F molten salt. Still others require quenching in a polymer-based oil, and the least hardenable alloys need to be quenched in water. The shock of water-quenching is often severe enough to crack the part or induce severe residual stresses and distortions. As the hardenability of a material decreases, the hardness (thus strength) varies more drastically from the surface to the core of the material. High hardenability materials can reach much more homogeneous post-quench hardness. Cryogenic treatment, if used, directly follows quenching. The body of belief-based and empirical evidence supporting cryo is now supported by scientific data from a recent NASA study confirming that a properly-done cryo process does transform most of the retained austenite to martensite, relaxes the crystalline distortions, and produces helpful ("eta") particles at the grain boundaries. The resulting material is almost fully martensitic, has reduced residual stress, more homogeneous structure and therefore greater fatigue strength. After quenching (and cryo if used), the alloy steel material has reached a very high strength and hardness, but at that hardness level, it lacks sufficient ductility and impact resistance for most applications. In order to produce the combination of material properties deemed suitable for a given application, the part is placed in a tempering oven and soaked for a specific amount of time at a specific temperature (for that alloy) in order to reduce the hardness to the desired level, hence producing

the desired strength, ductility, impact resistance and other desired mechanical properties. In the case of certain alloys, a double-tempering process can further improve fatigue strength and notch toughness. The tempering temperature and time must be carefully determined for each specific steel alloy, because in mid-range temperature bands, martensitic steels exhibit a property known as temper embrittlement, in which the steel, while having high strength, loses a great deal of its toughness and impact resistance. Typically, the post-temper hardness which results in the best ductility and impact properties is not sufficient for the wear surfaces of the crank journals. In addition, the fatigue strength of the material at that hardness is insufficient for suitable life. The currently-favoured process which provides both the hard journal surfaces and dramatic improvements in fatigue life is nitriding (not nitrating - nitrates are oxygen-bearing compounds of nitrogen). Nitriding is the process of diffusing elemental nitrogen into the surface of a steel, producing iron nitrides (FeNx). The result is a hard, high strength case along with residual surface compressive stresses. The part gains a

high-strength, high hardness surface with high wear resistance, and greatly improved fatigue performance due to both the high strength of the case and the residual compressive stress. These effects occur without the need for quenching from the nitriding temperature. The case thickness is usually quite thin (0.10 to 0.20 mm), although at least one crankshaft manufacturer has developed a way to achieve nitride layer thickness approaching 1.0 mm. There are three common nitriding processes: gas nitriding (typically ammonia), molten salt-bath nitriding (cyanide salts) and the more precise plasma-ion nitriding. All three occur at approximately the same temperatures (925 - 1050F) which, of course, becomes the ultimate tempering temperature of the part. The effectiveness of nitriding varies with the chemistry of the steel alloy. The best results occur when the alloy contains one of more of the nitride-forming elements, including chromium, molybdenum and vanadium. Older crankshaft technology involved heat-treating to a higher core hardness and shotpeening the fillet radii for fatigue improvement. Figure 6 shows the relative fatigue strength of 4340 material from heat treating alone, heat-treating plus shotpeening, and heat treating plus nitriding.

FIG.6

Machining

Crankshafts can also be machined out of a billet, often using a bar of high quality vacuum remelted steel. Even though the fiber flow (local inhomogeneities of the material's chemical composition generated during casting) doesnt follow the shape of the crankshaft (which is undesirable), this is usually not a problem since higher quality steels which normally are difficult to forge can be used. These crankshafts tend to be very expensive due to the large amount of material removal which needs to be done by using lathes and milling machines, the high material cost and the additional heat treatment required. However, since no expensive tooling is required, this production method allows small production runs of crankshafts to be made without high costs.

Billet Crankshaft Machining (Courtesy of Bryant Racing)

Machining operations on crankshaft

Before machining operations tolerances are needed to be provided.

Although the tolerances that customers supply to foundries differ from component to

component, it is important that end-users do not over-

specify the tolerances required. On crankshafts,

general profile tolerance for the casting is measured to

millimeters. For machining crankshafts, it will specify

tolerances one-tenth of the casting tolerances. For

polishing it will specify tolerances at a thousandth of

the casting tolerances. Components must be designed

and toleranced for the process in which they will be

manufactured. As a result, engineers must know what

a process can achieve. Too strict a tolerance at any step of the manufacturing

process will increase the overall costs of the component.

Hardening

Most production crankshafts use induction hardened bearing surfaces since that

method gives good results with low costs. It also allows the crankshaft to be reground

without having to redo the hardening. But high performance crankshafts, billet

crankshafts in particular, tend to use nitridization instead. Nitridization is slower and

thereby more costly, and in addition it puts certain demands on the alloying metals in

the steel, in order to be able to create stable nitrides. The advantage with nitridization

is that it can be done at low temperatures, it produces a very hard surface and the

process will leave some compressive residual stress in the surface which is good for

the fatigue properties of the crankshaft. The low temperature during treatment is

advantageous in that it doesnt have any negative effects on the steel, such

as annealing. With crankshafts that operate on roller bearings, the use

of carburization tends to be favored due to the high Hertzian contact stresses in such

an application. Like nitriding, carburization also leaves some compressive residual

stresses in the surface.

The final operation is finishing.

A Crankshaft weighs about 26

kg before machining and

about 18 kg after machining

Finishing

In the gasoline or diesel internal combustion engine the surfaces which the crankshaft

presents to the connecting rod and the main bearings are very important in

determining the wear rate of the bearings. Nodular iron crankshafts, which are now

used almost exclusively in automotive gasoline and diesel engines, are particularly

sensitive to variations in finishing technique. The microstructure of the nodular iron

crankshaft contains spherical graphite nodules surrounded by ferrite. When

improperly finished, the ferrite can form a burr which protrudes above the surrounding

surface. The prevalence, size and directionality of these ferrite burrs have a marked

influence on the wear rate of the bearings. A large burr lying in the unfavorable

direction can act as a file on the soft bearing material. Burrs can also impede

formation of a suitable oil film, causing high bearing wear rates and reduced service

life.

Journals on highly loaded crankshafts such as diesel engines or high performance racing engines require a finish of 10 micro inches Ra or better. The above is a simple straight forward specification which can be measured with special equipment. However, there is more to generating a ground and polished surface than just meeting the roughness specification. To prevent rapid, premature wear of the crankshaft bearings and to aid in the formation of an oil film, journal surfaces must be ground opposite to engine rotation and polished in the direction of rotation. Metal removal tends to raise burrs. Grinding and polishing produces burrs that are so small that we cant see or feel them but they are there and can damage bearings if the shaft surface is not generated in the proper way. This microscopic fuzz has a grain or lay to it like the hair on a dogs back. Figure 1 is an illustration depicting the lay of this fuzz on a journal. (Note: All figures are viewed from nose end of crankshaft.) The direction in which a grinding wheel or polishing belt passes over the journal surface will determine the lay direction of the so-called, micro fuzz. In order to remove this fuzz from the surface, each successive operation should pass over the journal in the opposite direction so that the fuzz will be bent over backward and removed. Polishing in the same direction as grinding would not effectively remove this fuzz because it would merely lay down and then spring up again. Polishing must, therefore, be done opposite to grinding in order to improve the surface. In order to arrive at how a shaft should be ground and polished, we must first determine the desired end result and then work backwards to establish how to achieve it.

BIBLIOGRAPHY

www.google.com

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