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Crankshaft knowledge
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Crankshaft: One of the most important parts of the engine. Crankshaft for general use in carbon steel or medium carbon alloy steel forging made. To improve the abrasion resistance and fatigue strength, the the journal surface via the high-frequency quenching or nitriding treatment, and after the grinding process, in order to achieve a higher surface hardness and surface roughness requirements. It is in conjunction with the link will be the gas pressure acting on the piston into a rotation of the driving force passed to the chassis of the drive mechanism. The same time, the driver gas distribution agencies and other assistive devices, such as fans, pumps, generators.
In operation, the crankshaft is exposed to gas pressure, the role of the inertial force and the moment of inertia, the force is large and complicated forces, and to withstand the impact action of the alternating load. Meanwhile, the crankshaft is the high-speed rotary member and, therefore, requires the crankshaft has sufficient rigidity and strength, with a good ability to withstand the impact load, resistance to wear and good lubrication.
The crankshaft general main journal rod journal crank, balance weights, front-end and back-end, etc.. A main journal, a rod journal and a crank a crank, crank the crank number equal to the number of cylinders (line engines); V-type engine crankshaft crank number is equal to half the number of cylinders.
The spindle neck of the supporting portion of the crankshaft through the main bearing support in the main bearing in the crankcase. The number of main bearings is not only concerned with the number of engine cylinders, and also depend on the support of the crankshaft. The crankshaft supporting Usually there are two (Figure 2-34), a wholly supporting the crankshaft, another is a non-fully supporting the crankshaft.
Full support crankshaft: the number of crankshaft main journal one more than the number of cylinders, each connecting rod journal on both sides of a main journal. Such as the six-cylinder engine is a wholly-bearing crankshaft with seven main journal. Full supporting the crankshaft of a four-cylinder engine with five main journal. Such supports, the strength and rigidity of the crankshaft are better, and reduce the main bearing loads, reducing wear. Diesel and most gasoline use of this form.
Non wholly-bearing crankshaft: the number of crankshaft main journal is less than the number of cylinders or equal to the number of cylinders. Such supports called non wholly supporting the crankshaft, Such supports a main bearing load is large, but shortening the total length of the crankshaft, so that the overall length of the engine has been reduced. Some gasoline withstand the load is smaller using this crank type.
Crankpin of the crankshaft is a connecting portion of the crankshaft and connecting rod, and connected through a crank with the spindle neck, at the connection arc transition, in order to reduce stress concentration. The inline engine crankpin number and is equal to the number of cylinders. The number of V-type engine, the connecting rod journal is equal to half the number of cylinders.
Crank spindle neck and the connecting portion of the connecting rod journal, the cross section is elliptical, in order to balance the inertial force, the crank is cast (or fastening) balance weight. The balance weight is used to balance the engine imbalance centrifugal torque, also sometimes used to balance a portion of the reciprocating inertia force, so that the crankshaft rotates smoothly.
The front of the crankshaft is equipped with timing gear-driven fan and water pump pulley and the start claws, etc.. In order to prevent the oil along the the crankshaft journal leakage, gear chamber cover fitted in the front of the crankshaft is equipped with a rejection oil pan, oil seal. The rear end of the crankshaft is used to install the flywheel, the profile of the oil flange and the oil return thread, is made in the rear journal and flywheel between the flanges to prevent the oil blow backwards.
Crank shape and the bellcrank relative position (i.e. bellcrank arrangement) depends on the number of cylinders, the cylinder arrangement and an engine ignition sequence. Arrangements multicylinder engine ignition sequence should note that for a continuous function of the two cylinders apart as far as possible to alleviate the load of the main bearing, while avoiding the overlap of the air intake may occur. For power intervals should be uniform, that is the engine in the crank angle to complete a cycle of work, each cylinder should be angry for power, and each cylinder firing interval, called the fire angle crank angle. Four-stroke engine to complete a cycle of work crankshaft two turns, angle 720 ° ignition in each cylinder of the engine crank angle of 720 ° should work. And the ignition interval angle is uniform, so the engine ignition interval of the four stroke angle of 720 ° / i (i is number of cylinders), the crankshaft every 720 ° / i, should a cylinder for power, in order to ensure the smooth running of the engine. |
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