Crankshaft


1. 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.3.62) 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.


Fig. 3.62.

Crankshaft.


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 crankshaft’s natural stress. Automatic transmission pressure and clutch release forces tend to push the crankshaft towards the front of the engine. 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.

2. Crankshaft Nomenclature


Crank-throw.

This is the distance from the main-journal centers to the big-end-journal centers. It is the amount the cranked arms are offset from the center of rotation of the crankshaft. A small crank-throw reduces both the crankshaft turning-effort and the distance the piston moves between the dead centers. A large crank-throw increases both the leverage applied to the crankshaft and stroke of the piston.

Crank-webs.

These are the cranked arms of the shaft, which provide the throws of the crankshaft. They support the big-end crankpin. They must have adequate thickness and width to withstand both the twisting and the bending effort, created within these webs. But their excessive mass causes inertial effect, which tends to wind and unwide the shaft during operation.

Main-bearing Journal.

Main-journal is the parallel cylindrical portions of the crankshaft, supported rigidly by the plain bearings mounted in the crankcase. The journals diameter must be proper to provide torsional strength. The diameter and width of the journal should have sufficient projected area to avoid overloading of the plain bearing.

Connecting-rod Big-end (Crankpin) Journals.

These journals have cylindrical smooth surfaces for the connecting-rod big-end bearings to rub against.

3. Crankshaft Design Considerations and Proportional Dimensions

The present design consideration is to increase the stiffness of the crankshaft and reduce its overall length by incorporating narrow journals of large diameter. For the required wall thickness and coolant passages, the minimum cylinder centers can be around 1.2 times the cylinder bore diameter for an engine having its stroke equal to the bore. The maximum diameter of the big-end for the connecting-rod assembly that can pass through the cylinder is 0.65 times
of the bore. The proportions of the crankshaft are as follows :

Cylinder bore diameter = D

Cylinder centre distance = 1.20 D

Big-end journals diameter = 0.65 D

Main-end journal diameter = 0.75 D

Big-end journal width = 0.35 D

Main-end journal width = 0.40 D

Web thickness = 0.25 D

Fillet radius of journal and webs = 0.04 D

To increase the fatigue life of the shaft, the fillet radius between journals and webs should be as large as possible but not less than 5% of the journal diameter. The overlap between the diameters of the big-end crankpin and the main-end journal depends on the length of the stroke i.e. the crank-throw. A long-stroke engine has very little overlap, requiring thicker web sections, and a short-stroke engine has considerable overlap which strengthens the shaft.

Collars are machined on the webs adjacent to the journals to accurately align the crankshaft and the bearings with the correct amount of side-float and, if necessary, to absorb the crankshaft end-float. Most crankshafts dimensions are such that the nominal stresses in the material under operating conditions do not exceed 20% of the tensile strength in bending and 15% in torsion. Crankshaft journals are ground to provide a surface finish better than 0.5 urn, to minimize bearing wear.


Fig. 3.63.

Integral crankshaft.




Fig. 3.64.

Attached crankshaft.


4. Crankshaft Counterbalance Weights :

Crankshafts normally have either integral (Fig. 3.63) or attachable (Fig. 3.64) counterweights. These counterweights counteract the centrifugal force created by each individual crankpin and its webs as the whole crankshaft is rotated about the main-journal axis. In absence of the counterweights, the crankpin masses tend to bend and distort the crankshaft causing excessive edge-loading in the main bearings. Therefore, each half crank-web is generally extended in the opposite direction to that of the crankpin, to counterbalance the effects of the crankpin.


Fig. 3.65.

Crankshaft with single diagonal oil drillings.



Bolt-on counterweights are, sometimes, used for large in-line and *V engines (Fig. 3.64) because of the simplicity in casting or forging the crankshaft. The use of detachable weights allows their slight overlap on the webs and this increase in web width permits concentration of more mass at a smaller radius from the axis of rotation. The attaching weights to the web is to be located and attached very accurately, otherwise any error in assembly results in an unbalanced crankshaft.

5. Crankshaft Oil-hole Drillings :

Oil from the main oil gallery reaches each individual main-journal and bearing. Oil is fed through a central circumferential groove in the bearing and it completely surrounds the central region of the journal surface. Diagonal oil hole drills are provided in the crankshaft which pass through the webs between the main and big-end journals (Figs. 3.62 and 3.65) for lubrication of the big-end journal. For effective lubrication of the big-end, these oil holes emerge from the crankpin at about 30 degrees on the leading side of the crank’s TDC position. The drilled oil passages should not be close to the side walls of the webs or near the fillet junction between the journal and the webs to avoid high stress concentration, which may cause fatigue failure. Also the oil holes on the journal surfaces must be chamfered to reduce stress concentration, but excessive chamfering can destroy the oil film.

6. Crankshaft Materials

Crankshafts materials should be readily shaped, machined and heat-treated, and have adequate strength, toughness, hardness, and high fatigue strength. The crankshaft are manufactured from steel either by forging or casting. The main bearing and connecting rod bearing liners are made of babbitt, a tin and lead alloy. Forged crankshafts are stronger than the cast crankshafts, but are more expensive. Forged crankshafts are made from SAE 1045 or similar type steel. Forging makes a very dense, tough shaft with a grain running parallel to the principal stress direction. Crankshafts are cast in steel, modular iron or malleable iron. The major advantage of the casting process is that crankshaft material and machining costs are reduced because the crankshaft may be made close to the required shape and size including counterweights. Cast crankshafts can handle loads from all directions as the metal grain structure is uniform and random throughout. Counterweights on cast crankshafts are slightly larger than counterweights on a forged crankshafts because the cast metal is less dense and therefore somewhat lighter.

Generally automobile crankshafts were forged in past to have all the desirable properties. However, with the evolution of the nodular cast irons and improvements in foundry techniques, cast crankshafts are now preferred for moderate loads. Only for heavy duty applications forged shafts are favoured. The selection of crankshaft materials and heat treatments for various applications are as follows.

(i) Manganese-molybdenum Steel.

This is a relatively cheap forging steel and is used for moderate-duty petrol-engine crankshafts. This alloy has the composition of 0.38% carbon, 1.5% manganese, 0.3% molybdenum, and rest iron. The steel is heat-treated by quenching in oil from a temperature of 1123 K, followed by tempering at 973 K, which produces a surface hardness of about 250 Brinell number. With this surface hardness the shaft is suitable for both tin-aluminium and lead-copper plated bearings.

(ii) 1%-Chromium-molybdenum Steel.

This forging steel is used for medium-to heavy-duty petrol- and diesel-engine crankshafts. The composition of this alloy is 0.4% carbon, 1.2% chromium, 0.3% molybdenum, and rest iron. The steel is heat-treated by quenching in oil from a temperature of 1123 K and then tempering at 953 K. This produces a surface hardness of about 280 Brinell number. For the use of harder bearings, the journals can be flame or induction surface-hardened to 480 Brinell number. For very heavy duty applications, a nitriding process can produce the surface to 700 diamond pyramid number (DPN). These journal surfaces are suitable for all tin-aluminium and bronze plated bearings.

(iii) 2.5%-Nickel-chromium-molybdenum Steel.

This steel is opted for heavy-duty diesel-engine applications. The composition of this alloy is 0.31% carbon, 2.5% nickel, 0.65% chromium, 0.55% molybdenum, and rest iron. The steel is initially heat-treated by quenching in oil from a temperature of 1003 K and then tempered at a suitable temperature not exceeding 933 K. This produces a surface hardness in the region of 300 Brinell number. This steel is slightly more expensive than manganese-molybdenum and chromium-molybdenum steels, but has improved mechanical properties.

(iv) 3%-Chromium-molybdenum or 1.5%-Chromium-aluminium-modybdenum Steel.

These forged steels are used for diesel-engine crankshafts suitable for bearing of hard high fatigue-strength materials. The alloying compositions are 0.15% carbon, 3% chromium, and 0.5% molybdenum or 0.3% carbon, 1.5% chromium, 1.1% aluminium, and 0.2% molybdenum. Initial heat treatment for both steels is oil quenching and tempering at 1193 K and 883 K or 1163 K and 963 K respectively for the two steels. The shafts are case-hardened by nitriding, so that nitrogen is absorbed into their surface layers. If the nitriding is carried out well in the journal fillets, the fatigue strength of these shafts is increased by at least 30% compared to induction and flame-surface-hardened shafts. The 3%-chromium steel has a relatively tough surface and hardness of 800 to 900 DPN. On the other hand the 1.5%-chromium steel casing tends to be slightly more brittle but has an increased hardness, of the order of 1050 to 1100 DPN.

(v) Nodular Cast Irons.

These cast irons are also known as speroidal-graphite irons or ductile irons. These grey cast irons have 3 to 4% carbon and 1.8 to 2.8% silicon, and graphite nodules are dispersed in a pearlite matrix instead of the formation off fake graphite. To achieve this structure about 0.02% residual cerium or 0.05% residual niagnesium or even both is added to the melt due to which the sulphur is removed and many small spheroids in the as-cast material are formed. The surface hardness of as-cast nodular iron is greater than for steel of similar strength, their respective hardnesses being 250 to 300 and 200 to 250 Brinell number. The flame or induction hardening can produce a surface with Brinell numbers of 550 to 580, and also a form of nitriding can be applied if necessary.

Nodular cast iron has the advantageous properties of grey cast iron (that is, low melting point, good fluidity and castability, excellent machinability, and wear resistance) as well as the mechanical properties of steel (that is relatively high strength, hardness, toughness, workability, and harden ability). Now-a-days a large number of crankshafts for both petrol and diesel engines are made from nodular cast iron in preference to the more expensive forged expensive forged steel. To support the slightly inferior toughness and fatigue strength of these cast irons, larger sections and the maximum number of main journals are used.

Heat Treatment.


(a) Flame and Induction Surface-hardening.

These are the surface hardening methods for steel having 0.3 to 0.5% carbons without the use of special compounds or gases. The basic principle is to rapidly apply heat to the surface followed by only water quenching. As it is heated locally instead of heating the entire mass, the hardening is greatly reduced and distortion of the journal is avoided.

Flame hardening is carried out by oxyacetylene flame at the surface layer temperature between 993 and 1173 K. The surface temperature depends on the carbon content equivalent of the different alloying elements in the steel. The heating process is followed by a water-jet quenching operation. Since the actual period for heating and cooling is critical, it is predetermined and is mostly automatically controlled.

Induction hardening is carried out by inducting heat electrically into the surface to be hardened. This case eliminates the danger of either overheating or burning the surface of the metal as with a flame hardening. An induction coil surrounds the journal and carries a high-frequency current. This induces circulating eddy currents in the journal surface thereby raising of its temperature and heat is mostly confined to the outer surface of the journals. In this process the higher the frequency of the current, the closer the heat is to the skin. The current is automatically switched off when the required temperature is attained and the surface is simultaneously quenched by water jet, which passes through holes in the induction block.

(b) Nitriding Surface-hardening Process.

In this process the journals are heated to 773 K for a predetermined time in an ammonia gas atmosphere, so that the nitrogen in the gas is absorbed into the surface layer. The alloying elements such as chromium, aluminium, and molybdenum, present in the steel, from hard nitrides. Aluminium nitrides form an intensely hard shallow case. Chromium nitrides diffuse to a greater depth than aluminium nitrides. The molybdenum increases hardenability, gives grain refinement, and improves the toughness of the core.

This process can use directly the journals ground to their final size as there is no quenching after nitriding thereby avoiding distortion unlike other surface-hardening processes. The slow rate of penetration of the surface makes the cost of the process high for example, it takes 20 hours to produce a case depth of about 0.2 mm.

(c) Carbonitriding Surface-hardening Process.

Tufftride’ is the best-know salt-bath carbonitriding process. The crankshaft is immersed in a bath of molten salts at a temperature of about 853 Kfor a relatively short cycle time of two to three hours. In the process both carbon and nitrogen dissociate from the salts and diffuse into the surface. Since nitrogen is more soluble than carbon in iron, it diffuses further into the material. Hard iron carbides and tough iron nitrides are formed on the surface thereby resistance to wear, galling (surface peeling), seizure, and corrosion are greatly increased.

Depending on the steel used, this outer layer is 6 to 16 jam deep with hardness varying from 400 to 1200 DPN. Underneath this outer layer, the excess nitrogen goes into solid solution with the iron due to which it is strengthened. This inner diffusion zone forms a barrier which prevents spreading of cracks leading to fatigue failure.

This surface-hardening treatment, also known as soft FLYWHEEL nitriding, is becoming increasingly popular for both steels the cast irons, and is expected to replace other more expensive processes for the components using plain carbon steels requiring surface hardness and corrosion resistance. This process is much quicker and cheaper and produces similar properties to nitriding, but the depth of hardness is normally less, which can be a problem if the shaft is to be reground.

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