In addition, the crankshaft, if the crank arm and the crank radius are offset, will cause the imbalance to exceed the allowable value greatly, thereby causing the crankshaft to exceed the allowable value. The unbalanced centrifugal force will cause overload of the bearing and cause strong vibration of the diesel engine, which will cause fatigue fracture of the crankshaft for a long time.
The most important factor affecting the bending fatigue strength of the crankshaft is the transition round angle between the connecting rod journal and the crank arm, and the transition round angle between the spindle neck and the crank arm. So, when the crankshaft is mended, the coaxiality, parallelism, crank radius, neck shoulder and other repairs of the crankshaft's spindle neck and the connecting rod journal are only paid attention to.
The roughness of the surface, as well as the angle between the various connecting rod necks and the angle between the neck of the connecting rod, and neglecting the size and geometry of the transition angle, so that the grinding wheel is unproperly trimmed when grinding the crankshaft neck.
Crack now. Take the Diesel engine as an example, the piston is divided into three groups according to the mass, and the piston installed on the same diesel engine should be divided into four groups A, B, C, and the difference of the same group quality not exceeding the 8g.
The connecting rod on the same diesel engine should also be divided into the same quality group red, blue, white and yellow and installed in the same one. The total mass of the piston rod assembly on the diesel engine can not exceed 30g. Thus, the force of the crankshaft is not equal. When the crankshaft is struck by vibration, it may cause the fracture of the crankshaft. When the crankshaft is found to be bent, when the cold straightening method is used to repair the crankshaft, the straightening amount can often be from several um to several mm.
So, you have a broken crankshaft and it needs replacing. What now? Well you could just buy a new or remanfactured crank and install it, but it's not that simple. Well that not only causes damage to the crank but also causes damage to the housing bores of the connecting rods and mains in the engine block. Because the crank jumps around in the engine, the crankshaft can pull on the housing bores of the connecting rods and engine block.
The massive amount of torque from the jumping crank can also stretch the bolts and distort the bores. Main caps can become loose in the registers causing misalignment. It is never a good idea to just swap the crankshaft out and put a new one back in without properly checking these housing bores.
The most accurate way to check the housing bores is by using a dial bore gauge. In many case the engine must be pulled out of the machine in order to get an accurate measurement. If you do not have a dial bore gauge then you should strip the block and bring it with the connecting rods to your local machine shop.
They can check all of the housing bores to see if they are out of round. This area is very thin due to the angle of the oil drilling and will crack if not reshaped correctly after grinding. The reduction of vibration by balancing engine components will reduce the likelihood of crankshaft cracks.
Shot peening and rolling of the radius area will also strengthen these areas against increased stress produced in high performance diesel's and racing engines. When crankshaft and bearings seize it is the end result of major damage caused by a breakdown in lubrication or assembly bearing clearances being under minimum specifications. The clearance is referred to as " vertical oil clearance ". As the lubrication breaks down and allows metal to metal contact between the shaft and the soft bearing material, heat created by friction melts and welds bearing material on to the shaft.
In a very short period the bearing and shaft can be destroyed. This same scenario takes place during a dry start after re-assembly. Engines have to be primed to obtain oil pressure before the initial start after a rebuild.
This involves cranking the engine until oil pressure is obtained. Removal of spark plugs in a petrol or removal of injectors or glow plugs in a diesel allow oil pressure to be obtained at starter cranking speed. The results of a dry start may not always destroy the bearings immediately but even minor bearing contact with the shaft will damage the bearing enough to result in premature failure. Note well, if an engine will not turn freely by hand or under normal starter motor action the engine should be dismantled until the source of incorrect clearances is identified and repaired.
Common causes are incorrectly fitted wick rear mains, misaligned con rods, dirty assembly, overlapped oil rings, insufficient ring end or back clearance etc, etc. If crankshaft-bearing seizure occurs many thousands of miles after initial engine assembly the failure is usually related to a breakdown in lubrication. Pre maturely worn out crankshaft and bearings can be the result of dirty or diluted engine oil.
When unsure and a cause must be established take a sample of the used oil from the engine in a clean glass container and have it analysed. The result of the oil analysis will give definite answers to the cause of the failure.
Most new crankshafts will attain some bend during normal operation. Up to. This amount of bend is acceptable and can mostly be removed by regrinding. Crankshafts with more bend should be straightened before regrinding. Crankshafts with sever bend should be replaced.
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