Material manufacturing and analysis of engine crankshaft
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The report outlines the detail of material and manufacturing process selection exercise carried out on a typical car engine camshaft. Camshaft being one of the most important engine components requires careful selection of material. Moreover, owing to its specialized operation, the required geometry is relatively complex and certain improvements in mechanical properties are required to be introduced during manufacturing. This calls for and rather complex manufacturing route to be followed. The report initially mentions the operation of the component and describes its required characteristics. The material and manufacturing process selection is then carried out based on the intended use. A detailed description is provided in the end about the selected manufacturing route.
The crankshaft is the part of an engine which converts reciprocating motion of the pistons into a rotary motion. The rotary motion has the advantage that it can be used rotate the wheel of the car. Crankshaft is an essential component in reciprocating engines because, rotary motion is simpler to mobilize the vehicle in which reciprocating engines is installed. Crankshafts are equipped with crankpins and additional bearing surfaces. The axis of bearing surfaces is offset from the crank.
During operation, crankshafts are subjected to following mechanical stresses:
Bending stresses due to up and down movement of pistons.
Torsion stresses due to rotation of crankshaft and transmission of force to drive train, which subsequently transmit motion to various services.
Owing to reciprocating motion of the pistons, bending forces on crankshaft are always cyclic in nature. Therefore, the stress behavior is further complicated due to constant fatigue factor hence necessitating higher fatigue resistance in the component.
Friction of bearing surfaces is also important during the operation. The piston arms have to slide past the crankshaft surface. Therefore roller bearings are equipped between the sliding surfaces. However, as rpm of a typical engine reaches 4000-5000 during normal operation, an efficient lubrication is extremely essential for bearing. The crankshaft therefore contains holes for lubrication system.
Operating temperature inside the engine is extremely high. Therefore, the material should be such that it retains required mechanical properties at elevated temperature.
In order to suppress pulsating behavior of reciprocating engines, crankshafts generally connect to flywheel. In certain cases, a vibration damper is also installed at the opposite end to reduce vibration.
Figure No 1: Engine Crankshaft Along with Connected Parts
The analysis presented in this report focuses on crankshaft manufacturing which is feasible for large scale manufacturing.
Figure No 2: CAD Drawing of the Camshaft
Based on the stress imposed on the component during the operation, operating temperatures and intended operation, the material to be selected for this component should posses following characteristics:
The material should be strong in bending
It must have excellent fatigue resistance.
The material should be light weight so that it has small value of moment of inertia and transmit motion more efficiently.
It must have lesser coefficient of thermal expansion so that the component can retain its original dimension at varying temperatures.
The material should be easily machineable so that it can take complex shape (as required for the geometry of crankshaft) easily and without developing unnecessary stresses.
After carrying out an extensive research of materials, following materials were short listed for crankshaft:
Aluminum is an excellent machineable metal. It is light weight and can take complex shapes easily. Moreover, it can absorb vibration very efficiently. However, the metal has lower modulus of elasticity and higher coefficient of thermal expansion. Therefore, it will be subjected to larger strains at higher stresses and high temperatures. Moreover, it does not possess good resistance to fatigue loads and corrosion.
Copper possess very good corrosion resistant properties. It is easily machineable and has high strength. Moreover, it can possess good surface finish which proves helpful in achieving reduced friction properties. However, the biggest disadvantage of copper is that it is no corrosion resistant. Corrosion rate is significantly higher at higher temperature. It is therefore, not considered suitable for the intended use.
Steel is another option to be used as a material for camshaft. Steel is a better choice because, it possess excellent mechanical characteristics which suits best with the operational requirements of engine camshaft. It has the highest modulus of elasticity. It is tough, strong, easily available, cheap and it has very less coefficient of thermal expansion which makes it best suited for high temperature operation. However, steel itself has a large number of variants which posses large range of properties. Therefore, selection of most appropriate type of steel is also very important.
An option for crankshaft material is carbon steel. However, these steels require additional heat treatments to acquire required level of strength.
Iron crankshaft is also an option. However, iron cannot take higher loads therefore, iron crankshafts are suitable for low output engines where stresses are lower. They have the advantage of being low cost.
In fact, the most widely used material for crankshaft worldwide is Vanadium Microalloyed steel. It has following advantages:
Vanadium Microalloyed steel can be air cooled after reaching high strengths without further heat treatment. However, surface hardening is required for the bearing surfaces.
Low alloy content also makes the material cheaper than high alloy steels.
MANUFACTURING ROUTE SELECTION
Crankshafts can be manufactured using following methods:
Forging and casting
These processes are discussed separately in the lines below.
Machining is yet another process which can be used to manufacture crankshafts. Crankshafts can be machined out of a billet, often using a bar of high quality vacuum re-melted steel. Machining process has following advantages:
Higher quality of steels, which cannot be forged can be used through machining process.
No expensive tooling is required for machining process.
Extremely high quality crankshafts can be manufactured.
However, machining process also has following disadvantages:
It is a highly expensive process because; it generally uses high quality material. Moreover, a significant quantity of material is also wasted during machining process.
Additional heat treatments are required to get required material properties.
Forging and Casting Process
Forging is the most widely used process for manufacturing crankshafts today. It is due to the following reasons:
The component has lighter weight
The forged shafts have better damping characteristics
More compact dimensions can be achieved
The camshaft is a complex component. Therefore, it is not feasible to manufacture the complete component using a single process. Instead, the manufacturing process comprises of multiple steps encompassing various machining, milling, forging and heat treatment operations. The details are outlined in the next section.
DESCRIPTION OF SELECTED MANUFACTURING ROUTE
Following is the detailed outline of manufacturing process selected for the camshaft:
The raw steel are required for a diameter range from 2.125" to 2.5" and 20ft in length (Figure No 1 in Appendix 'A').
The raw steel bars are then turned on a lathe to remove the rough surface and then cut into the proper lengths depending on the engine size (Figure No 2 in Appendix 'A').
Subsequently, the steel begins the process of becoming a camshaft as both the journals and lobes are cut and evenly spaced out.
The Camshafts are then stack up and are prepared for the copper plating (Figure No 3 in Appendix 'A'). The copper plating is done to keep the steel from becoming brittle and helps straighten the camshaft after the heat treating process.
Once the copper is applied, the journals are lathe down to the thickness that is needed (Figure No 4 in Appendix 'A'). The surfaces are then finished.
Next, a special lobe milling machine is used to create the lift for the cam. These CNC machines use computerize programs that allow the manufacture to create the desired lobe and are precise at 0.0001 of an inch (Figure No 5 in Appendix 'A').
Once the cams are roughly cut, they are placed in a furnace for heat treating. This process hardens the steel making the cam less likely to warp or snap when put under the stress of an engine.
After the heat treating, the journals go through their final grounding stage and are milled to the correct dimensions (Figure No 6 in Appendix 'A').
Following the journals are the lobes. They are grounded and polished to provide a smooth surface which reduces friction in the engine, freeing up horsepower.
The last process uses a computerized scanner that checks the tolerances on the cams. Making sure that every lobe and journal are exactly the same.
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