Crankshaft is one of the key components of automobile engine, and its performance directly affects the service life of automobile. When the crankshaft works, it bears heavy load and changing bending moment and torque. The common failure forms are bending fatigue fracture and journal wear. Therefore, the crankshaft material is required to have high rigidity, fatigue strength and good wear resistance.
1. Foundry Technology
(1) Smelting
For the melting of high-grade cast iron, large capacity medium frequency furnace or variable frequency medium frequency furnace will be used for melting, and direct reading spectrometer will be used to detect the composition of molten iron. Nodular cast iron is treated by subcontracting, new varieties of nodulizing agents are developed, and advanced inoculation methods such as flow inoculation, in mold inoculation and compound inoculation are adopted. The parameters of the melting process are controlled by microcomputer and displayed on the screen.
(2) Styling
EPC will be developed and popularized. In sand mold casting, non box injection molding and extrusion molding will be paid attention to and continue to be popularized and applied in new or rebuilt plants. The original high-voltage molding line will continue to be used, and some key components will be improved to realize automatic core assembly and core lowering.
2. Forging technology
Automatic lines with hot die forging press and electro-hydraulic hammer as the main engines are the development direction of forged crankshaft production. These production lines will generally adopt precision cutting, roll forging (wedge cross rolling) billet making, medium frequency induction heating, etc.
3. Machining technology
In the rough machining of crankshaft, advanced equipment such as CNC lathe, CNC internal milling machine and CNC lathe broaching machine will be widely used for CNC turning, internal milling and turning broaching of main journal and connecting rod journal, so as to effectively reduce the deformation of crankshaft machining. Crankshaft finishing will widely use CNC controlled crankshaft grinder to finish grind its journal.
This kind of grinder will be equipped with automatic dynamic balancing device of grinding wheel, automatic tracking device of center frame, automatic measurement, automatic compensation device, automatic dressing of grinding wheel, constant linear speed and other functional requirements to ensure the stability of grinding quality. It is estimated that the current situation of high-precision equipment relying on imports will not change in the short term.
4. Heat treatment technology and surface strengthening technology
(1) Crankshaft medium frequency induction hardening
The crankshaft medium frequency induction quenching will adopt the microcomputer monitoring closed-loop medium frequency induction heating device, which has the characteristics of high efficiency, stable quality and controllable operation.
(2) Crankshaft soft nitriding
For crankshafts produced in large quantities, in order to improve product quality, a microcomputer controlled nitrogen based atmosphere gas soft nitriding production line will be used in the future. Nitrogen based atmosphere gas soft nitriding production line is composed of front cleaner (cleaning and drying), preheating furnace, soft nitriding furnace, cooling oil tank, rear cleaner (cleaning and drying), control system and gas production and distribution system.
(3) Crankshaft surface strengthening technology
Spheroidal graphite cast iron crankshaft fillet rolling strengthening will be widely used in crankshaft processing. In addition, compound strengthening processes such as fillet rolling strengthening and journal surface quenching will also be widely used in crankshaft processing. Forged steel crankshaft strengthening methods will more adopt journal and fillet quenching.
Main causes of crankshaft fracture:
(1) The engine oil deteriorates after long-term use; The serious overload and over coupling cause the engine to run overloaded for a long time and cause the Bush burning accident. The crankshaft was severely worn due to the burning of engine pads.
(2) After the engine is repaired, the loading has not gone through the running in period, that is, the engine is overloaded for a long time, which makes the crankshaft load exceed the allowable limit.
(3) Surfacing welding was used in the repair of the crankshaft, which destroyed the dynamic balance of the crankshaft and did not do balance verification. The imbalance exceeded the standard, causing greater vibration of the engine and fracture of the crankshaft.
(4) Due to poor road conditions and serious overloading of vehicles, the engine often operates within the torsional vibration critical speed, and the failure of the shock absorber will also cause the torsional vibration fatigue damage and fracture of the crankshaft.
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