Microstructure and Fracture Surface Analysis
Part 1
Failure and Possible Causes
A physical examination of the car reveals the failure to originate from a faulty gearbox, particularly the gears. Inspection of the gears identifies two types of faults, involving breakage and surface spitting. The gears had teeth breakage often associated with the bending stress at the root area. Probable causes of this are fatigue or overload on the gears, leading to serious and hazardous failures in the vehicle. The common practice is to design the gears such that their bending rating or the resistance against tooth breakage is higher that the operational load. Therefore, the site of broken teeth implies that the bending rating was higher than the actual operating load. The second problem was the presence of surface pitting indicated by the production of small holes on the gears. Typically, gear meshes create holes at the beginning but with time, the holes disappear. The presence of surface pitting on the old gears implies that the surface was loaded more than its rated capacity, making the holes to grow with each use. This could also be a cause of the breakage of the teeth.
Analysis of the gears identifies that the mesh gears were in the sliding mesh gearbox design. In this design, the gears are meshed in a sliding pattern. In this type of gearbox, the four gears are put in a lay shaft, which slides axially on the vehicle’s main shaft. The need for inspecting the method of mounting of the gears on the vehicle’s shafts arises from the fact that the method can cause the faults on the gears. This is because the fitting of the gears on the shaft requires the measurement of the torque setting, which consequently determines the choice of the mounting method. Therefore, from this point of view, it is possible that the breaking of the gear teeth is the result of mounting of the gears there was little tolerance by the use of the wrong hub diameters of the shaft.
Microstructure and Mechanical Properties
A third factor to consider in the cause of the breakage of the gear teeth is the microstructure and mechanical properties of the gears. A close inspection of the surface of the gears reveals that the hardness of the gear is not evenly distributed throughout the gear teeth. However, a possible explanation for this is the fact that in manufacturing the outside surface of the gear cools faster than the inside, creating a harness gradient. On the other hand, since the gear teeth are made from carbon and low alloy steel, it is possible that the materials used were no adequate to offer a higher rate of hardness. Analysis indicates that the carbon used in the hardening process had pearlite and ferrite phases, while the low alloy steel has bainite structure. Since the test of the materials, indicate that their impact energy, crack initiation, crack propagation energy, brittleness, and ductility were low for the gear system. This is because, the use of carbon as a hardener for gears depends on the amount of carbon used in the alloy steel. This causes the hardness of the gears to depend on the harden ability of steel and the quench severity of the metals. The fact that these materials are indicated on the surface of the gears implies a need for increased hardness of the gears to prevent breakage and the formation of holes. This is important if the hardening of the gears with these materials occurred after the gears were cut. This is because in theory hardness and machinability must be factored in during the hardness of the teeth with carbon steel alloy after the teeth have been cut. The reason is that machinability tends to remove some or almost all of the hardening material on the surface of the gears.
The analysis of the microstructure of the gears reveals that the carbon and low steel allow were hardened through the case hardening technique. This is because, the gears indicate a hard, wear resistant layer or case on top of a more ductile and shock resistant interior. The goal of this technique is to keep the interior of the gear teeth at a range between 30 and 40 HRC to prevent the breakage of the teeth and increasing the pitting resistance of the outer surface. Given the miles on the vehicle and its history, it is likely that during the case hardening, the right amounts of carbon and low allow steel were not used to case the teeth. Implying that from the manufacturer of the gears, the surface hardness was low making the pitting resistance of the outer surface weak. With a low pitting resistance, it is easy for the holes to appear on the surface of the gears, and with time, these holes increase to cause the teeth to break. Another dimension to the microstructure of the carbon and low steel alloy is the bending strength. Inspection of the bending strength of the materials indicates that they were not adequate to give the gears strength of at least 50 HRC. In the manufacture of gears, the bending strength must be above 50 HRC in the case hardening method to have the strength or hardness of the gears increase with each increase in notch sensitivity. The analysis also finds that the use of the low alloy steel, in this case the 20MnCr5, is not applicable for hardening the gears of this vehicle since it needs high toughness and long fatigue life. This is because ration of low alloy steel does not resist abrasion of the gear teeth surfaces, as was indicated by the constant noise made by the gears. Given the need for increased core toughness and tensile strength, the carbon and low alloy steel rations need to be higher for the gears. Overall, analysis of the gears reveals that the gear system could not prevent inter-granular oxidation, had low hardenability for the gears, low toughness, and therefore weak grain boundaries. Therefore, these are the causes for the presence of holes on the surface of the gears as well as the broken teeth.
Functioning
The function of the gears and the gearbox is to control the vehicle’s transmission system. This is by assisting the engine drive the vehicle; allow the car to be reversed, and the connection of the transmission system to the engine. The gears in this analysis are from the sliding mesh gearbox, which has four gears on the shaft. The gears are three direct-speed and a reversed-speed gear. In this system, driving the main shaft from the lay shaft, leads to a reduction of speed with the first gear that is always in mesh. The gears system has a static stress of 500MPa, dynamic stress of 1010MPa, and dynamic amplification of 2.02. Often, the gearwheels must be run within the mixed friction range, with a high hydrodynamic lubrication along the path of contact. For this gear system, the circumferential speeds are less than 4m/s, more than 60% with high stress levels.
The alloying of carbon and steel for gears enhances properties like hardenability, which is the ability to become strong through heat treatment. Microstructure properties of carbon and steel alloys increase the toughness or the ability to withstand loads by the gears. In addition, it increases the environmental resistance of the gears, by withstanding weathering and corrosive environments. Lastly, the alloying of the materials increases the ability of the gears to resist elevated temperatures.
Part 2
Broken Shaft
The broken shaft manifested itself as a failure of the vehicle to move even when put in gear. There are many possible causes for breakage of this shaft above, one of these is the stripping or wearing out of the spline when the shaft is put into transmission. The second cause is overloading of the drive shaft and starting the car abruptly, this is more so given the shaft above is for a race vehicle. The other cause is that the pinion shaft broke by overload or it had too much torque like that of the drag racing. It is likely that the diameter of the shaft did not much the required diameter for the maximum horsepower given to the transmission system. The third cause is that the welds at each end of the shaft cracked or had fractures that broke under overload. The other possible cause is the bending of the shaft until it breaks. This arises from the fact that there is no adequate room for the shaft to bend inside the pump. This occurs when there is a misalignment of the pump and the driver, causing the shaft to bend and break on the outboard bearing.
Several measures can prevent the breakage of the shaft, including the prevention of liquid from reaching shaft’s point of stress. Proper lubrication of all the shaft parts and transmission system, can prevent parts from drying up and thereby failing. The driver and owner also need to prevent the jumping of the car, replace broken bolts. Lastly, ensuring the correct load and stress of the shaft system to prevent overload.
Transmission Line
The transmission lines transport the transmission fluid from the radiator, through the radiator core and back to transmission. The transmissions lines can be broken or damage by impact to the underside of the vehicle. Transmission lines also leak either from a crack or from weak sealing. The breakage or leaking of the transmission line is indicated by a low coolant level, which requires the regular inspection of the transmission and input gasket seals. Transmission lines that slip can break, where slipping is caused by faulty solenoid or by a fluid leak. Slipping of the transmission line is indicated by very slow shifts in the gear and the slipping of gears. Another problem with the transmission line overheating, caused by the car pulling too much load. This mostly occurs when the transmission fluid is dirty and old, causing a clog in the cooler lines. Overloading and stress on the transmission system requires the constant cooling of the parts, hence overheating will occur where there is a lack of enough transmission coolant.
If transmission slipping is caused by the leakage of the transmission fluid, this can be controlled by a replacement of the fluid. Transmission problems caused by faulty solenoids requires for the maintenance and removal of debris or any item that is obstructing the solenoid. Persistent problems with the transmission line, like breakage and faulty solenoids requires the replacement of these parts. The regular cleaning of the transmission fluid, by drainage and replacement, prevents transmission overheating. This also calls for the regular cleaning of the transmission filter to remove dirt and sediments that will make the transmission fluid dirty and old. The transmission line also needs to be checked regularly for every few thousand miles to identify any leaks, slippage, or breaks. The transmission fluid needs replacement when it turns brown from the original red color. When hauling heavy loads, the vehicle will need auxiliary oil coolant to assist the transmission system maintain optimal temperatures. Lastly, avoid crushing the bottom of the vehicle especially on rough roads.
Axle Break
The axle of the vehicle is broken if there is a sudden sputtering and clunking sound when the car is put into gear. Uncommon vibration and rumbling, when the car accelerates or turns is another sign that the break axle is broken or nearly broken. The axle breaks due to overloading of the vehicle creating stress on the shaft, breaking system, and transmission system. Stress from increased load on these parts can cause faster wear and tear as the load exceeds the maximum or the resistance limit of the car. The second cause of the broken axle is a weak car bearing that breaks when the car jumps or hits a severe bump on the road.
A vehicle with a broken or cracked rear axle as seen in the photograph requires driving in moderate speeds of 10-15 miles. The car is then parked for 30 minutes to allow for cooling of the axle and other transmission lines prior to fixing. To prevent frequent breakage or cracking of the break axle, it is vital to stop overloading of the transmission system, the shaft, and breaking system to reduce the strain on the axle. Regular lubrication of the breaking system will prevent the breaks and axle from drying up, and therefore, reduce the torque on the system. Increased torque on the breaking system increases the stress on the axle, thereby causing faster wear and tear and the eventual breakage of the axle. Rear axles also break under heavy loads at high speeds, therefore, calling for drivers to drive under moderate speeds not exceeding 60 mph and a lighter load on the rear axle.
