Connecting Rod Application - Essay Example

Connecting Rod A connecting rod or con rod is an engineering component that is used to transform reciprocating motion into rotating motion (Windsor n.d.). They may also be used to transform rotating motion into reciprocating motion. Con connecting rods are commonly used in reciprocating piston engines to connect the crankshaft to the piston. One property that makes the connecting rod ideal for use in performing its function is its rigidity. Its rigid nature gives it the capacity to rotate the crankshaft through the two halves of a revolution as it pulls and pushes the piston. In this respect, the connecting rod works as a lever arm. This paper will discuss the connecting rod with a focus on its application, materials, and production methods. Connecting Rod Application Connecting rods are a common phenomenon in internal combustion engines (Windsor n.d.). The automotive engine almost always never lacks at least one connecting rod. Steam engines and some airplane engines also make use of connecting rods although the designs used thereof are distinctly different from those used in automotive engines. The connecting rod commonly features a big end and a small end. While the big end is connected to the crank shaft bearing journal, the small end commonly connects to the piston through a gudgeon pin (Windsor n.d.). In many cases, the gudgeon pin is press fitted into the rod so that it is fixed in the rod while allowing the piston to swivel. So as to avoid its damage, the con rod-crankshaft assembly is constantly lubricated with oil when the system is at work. Failure of Connecting Rods The connecting rod is subjected to enormous stresses as it works. Every rotation of the crank shaft means that the rod experiences tension and compression from the piston. The rod can fail when the eng8ine is at work causing irreparable damage to the crankcase or the engine as a whole. The connecting rod can also fail from fatigue especially where the rod has a physical defect. Other possible causes of con rod failure include inadequate lubrication of its bearing, improper positioning and tightening, subjecting the engine to extremely high speeds/revs, and failure of the bolts that are used to connect it to the crankshaft. To avoid connecting rod failures in high performance engines, their edges are often ground to form smooth radii. Another strategy applied in this respect is shot peening which serves to create compressive surface stresses which in turn help in preventing the formation of cracks. To avoid failures that may be caused by improper tightening of the bolts that connects the connecting rod to the crankshaft, the bolts are torque to the precise values specified by manufacturers. One other strategy that is applied in high performance engines to ensure that their connecting rods do not fail is precisely balancing the connecting rod-piston assemblies statically and dynamically. Materials Used in Manufacturing Connecting Rods The choice of materials used in making connecting rods is impacted by the environments under which the components work. The material should be one that can withstand the stretching and compression that it is subjected to as the engine rotates. The material should also be strong enough as to be able to accommodate and transmit the force that comes from the pistons to the crankshaft (Power Industries 2012). The material should also have the capacity to withstand relatively high temperatures so that they do not weaken or melt during use. The material shouls also be light in weight Different materials are used in the manufacturing of connecting rods, depending on the level of performance desired. Most of the connecting rods that are mass produced for use in automotive engines are made of steel or steel alloys like EN-8D, 42CrMo4, SAE1141, and C-70 (Power Industries 2012). Other materials that are used in their making include cast iron, and aluminium (Creason 2013). In motor scooters, it is not uncommon that cast iron is used in the making of connecting rods. When high performance is demanded of connecting rods, the choice material is often titanium (Creason 2013). Titanium is notably lighter in weight and more reliable than steel although it is more expensive. The Manufacture of Connecting Rods Connecting rods are often mass produced especially given that they have wide application in the automotive industry. The main methods used in their production include forging, billeting, and casting (Creason 2013). Powder metallurgy has also been applied in making high precision connecting rods for high performance needs. Each of the methods of manufacturing have their fair share of advantages and disadvantages. Forging Forging is the process of manufacturing components by using tooling dies, high pressure and heat. The die which has striking similarities with a mould is the negative that is used to produce the connecting rod. During forging, a blank metal piece is subjected to extreme heat to make it malleable (Creason 2013). Extreme pressure is then applied onto the piece to force it into the die. This is often done by hammering the piece against the die. Once the process of dyeing is complete, the piece that not takes the general shape of the final product is extracted and taking through machining processes. The machining processes generally serve to cut and size the rod so that the end cap can fit properly into it. The holes through which lubricants flow are also drilled. The hole through which the bushing will be pressed is also drilled. To make the material acquire the necessary properties such as strength and wear resistance, the materials is stress relieved and heat treated. To ensure that it has the right weight such as is necessary for balancing, the component may also be fine tuned (Creason 2013). One advantage of forging over casting is that it allows correct grain alignment which makes connecting rods have high strength. Hot forging is known to compress the grains properly align and compress the grains. One main disadvantage associated with forging is the high initial cost of production (Creason 2013). Producing dies that are used in the production of con rods is expensive and with continued use, they wear out. This means that they have to be replaced every so often. When a new con rod design is to be made through forging, a new die that accommodates the new design has to be made or the machining processes have to be changed. In as forging has the advantage of producing high strength connecting rods, the process can be extremely expensive especially where the numbers of connecting rods to be made are few. What this means is that forging as a production method is suited to large volume manufacturing. Billeting During the production of connecting rods by billeting, a flat piece of steel that has been forged is used. The connecting rod is first designed using a computer aided design program. Based on the design, the billet material is specifically cut to shape using a computer numerical control (CNC) machine or water jet machine (Creason 2013). One main advantage of this production method is that different connecting rods can be custom made to match the specific needs of every engine. Given that the process relies not on new dies and retooling, it is easy to change designs and accommodate variations in respect of weight, length, strength requirements, oiling, and gudgeon pin diameter (Creason 2013). The process generally offers a lot of flexibility for the design and production of the products. The same equipment can be used to manufacture rods for small and large equipment. One main shortfall of the billet production process is with respect to the grain structure of the rod produced. Given that the billet rod that is used in the production of the component is cut from a flat piece of steel, the gain flows not around the rod’s big end as is the case with forged con rods (Creason 2013). Instead, the grains remain straight across the component. One other disadvantage of the billet production method is that it is time consuming and can be more expensive compared to forged connecting rods . The longer time comes by the fact that production is always done in small batches which take time to design, in addition to the time taken for machine setup and finishing processes. Casting The production of casting involves melting material (mostly iron) and pouring it in a mold that has been designed to take the shape of the final product (Todd, Allen, Alting 1994). The molten metal is then left to cool and solidify. The unnecessary parts of the product are removed by cutting them of. The product is then subjected to machining and finishing processes. The casting process has a number of advantages and disadvantages. One of the disadvantages of producing the product using this method relates to its lower accuracy. Other disadvantages include higher labour cost, more waste, low rate of production, high chances of defects occurring in the product, and lower strength. On the other hand, casting is ideal for mass production as it tends to be cheaper than other production methods when the components are to be produced in large numbers (Todd, Allen, Alting 1994). Another advantage of casting is that if properly done, the grain structure of the components can be controlled to increase their strength. The fibrous structure that results from casting gives the connecting rod the capacity to handle extremely high compressive forces and stresses. Powder Metallurgy Powder metallurgy is a process that involves blending materials that have been finely ground into powder, compacting the materials into shape and heating them under controlled conditions so that they bond and set (Todd, Allen, Alting 1994). In other words, the process involves manufacturing powder, blending the powdered materials, compressing the powder into shape, and sintering. During the manufacture of connecting rods through powder metallurgy, the blended powder that may consist of finely ground alloy steel is compacted to take the shape of the final product. The material is then heated under controlled temperatures to allow the materials to bond and firmly solidify. The advantage of powder metallurgy is that it allows for the precision manufacturing of the connecting rods with respect to weight and size (Todd, Allen, Alting 1994). Furthermore, it may be cheaper than forging given that the product mad through the process does not require a lot of machining and balancing. Because the rod and its cap are produced in one piece, they are often separated finally by fracturing which produces an uneven mating part. This allows more accurate alignment of the mating parts and therefore avoids the misalignments that often result when flat surfaces are involved. References Don Creason (2013). ‘Connecting Rod Tech: Forged And Billet Steel Rods’, viewed 11 April, 2014 http://www.stangtv.com/tech-stories/engine/connecting-rod-tech-forged-and-billet-steel-rods/ Power Industries 2012, ‘Connecting Rod Manufacturing Process’, viewed 11 April, 2014 http://powerconnectingrods.com/process.php Sheasby, J. Oct 1979, ‘Powder Metallurgy of Iron-Aluminum’, Intern. J. Powder Metallurgy and Powder Tech. Vol. 15, no. 4, pp. 301–305. Todd, R., Allen, K., Alting, L. 1994, ‘Manufacturing Processes Reference Guide’, 1Industrial Press Inc., New York Windsor N.d., Connecting Rod, viewed 11 April, 2014 http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Connecting%20Rod.htm Read More