Abstract: The bending deformation of parts is the main form of failure of agricultural machinery parts, which seriously affects the normal use of agricultural machinery. An analysis was conducted on the hazards and causes of component deformation, and the correction methods for deformed parts were introduced to improve the repair quality of the parts.
Many parts of agricultural machinery will undergo bending, twisting, and warping deformation during use. When reaching the limit value, it will lead to component failure or fracture, which is the main form of mechanical component failure. The frame of a tractor is prone to twisting and deformation, the engine crankshaft is prone to bending and twisting deformation, and shell parts are prone to warping and deformation. After the connecting rod parts bend or twist, it will cause the piston to not run in the correct trajectory, but to lean towards one side of the cylinder barrel, causing cylinder wear, poor sealing of the piston ring, gas flowing down the crankcase, and oil flowing up the combustion chamber to participate in combustion; The deformation of the connecting rod can also cause wear and loss of roundness in the large and small end holes of the connecting rod, wear of the connecting rod bushing, abnormal noise during operation, failure of the piston to reach the required top dead center, decreased compression ratio, and difficulty in starting. When the stiffness of the transmission gear shaft is insufficient, it will also undergo bending deformation under large loads, causing changes in the position of gear meshing marks and tooth side clearances, resulting in unstable and noisy gear transmission. The crankshaft deforms due to periodic gas pressure, reciprocating inertial force, rotational centrifugal force, and mechanical braking force. The deformation of the crankshaft is usually bending deformation and twisting deformation. Bending and deformation of the crankshaft can lead to increased wear and tear on itself and connected parts, which can disrupt the balance of the engine. In mild cases, the engine may experience high vibration and accelerated fatigue, and over time, it may result in component breakage. The deformation of the basic components of the shell type will damage the interrelationships of all components assembled on this basic component, exacerbate the wear and impact of these components, and accelerate the damage of the components. Therefore, sufficient attention should be paid to the deformation of agricultural machinery parts.
1 Reasons for Part Deformation
The reasons for the deformation of parts are multifaceted, such as additional loads caused by unreasonable use and assembly, insufficient stiffness of parts, and residual stresses that have not been eliminated in parts, all of which are factors that cause part deformation.
(1) In terms of repair. ① The assembly accuracy is too low, and the clearance between the crankshaft bearings and connecting rod bearings is too large. During operation, the crankshaft will be subjected to excessive impact, causing deformation of the crankshaft or connecting rod. When repairing the engine, failure to conduct dynamic balance experiments seriously resulted in additional inertial forces and moments generated by crankshaft rotation, causing increased vibration of the unit. The clearance between the piston and connecting rod components is too large, and the connecting rod undergoes significant lateral bending stress during operation, which leads to deformation of the connecting rod The torque of the connecting bolts is uneven and not tightened according to the required torque, which can cause excessive shape and position errors of the connected parts. For example, when disassembling and assembling cylinder head bolts, a torque wrench or special socket should be used to loosen or tighten each bolt in diagonal order. The specified torque should be achieved several times to ensure that the cylinder head and gasket are evenly stressed, correctly positioned, and avoid deformation The welding or pressure processing methods used in agricultural machinery repair can easily cause new stresses and deformations in the parts.
(2) Improper use. The load that agricultural machinery can bear is carefully calculated during design. If the machine frequently works under overload and overheating conditions, it will inevitably increase the load on the parts, causing deformation or even fracture of the parts. Therefore, agricultural machinery should be strictly operated according to the operating procedures, and overloading and overspeed operation should be avoided as much as possible. If the engine overheats or other parts overheat, it should be stopped for maintenance to reduce the deformation of the parts.
2 Correction of deformed parts
The basic principle of component correction is to use external forces or flames to cause new plastic deformation of the component, in order to eliminate the original deformation. The calibration methods for deformed parts are divided into cold calibration method and hot calibration method. The cold calibration method is divided into pressure calibration and cold work calibration.
2.1 Static pressure correction
Place the part (shaft) on the V-shaped block of the press, with the convex surface facing upwards, and use pressure to bend the part. The bending deformation should be 10-15 times the original deformation. After maintaining it for 1-2 minutes, remove the pressure and check the deformation. If it is not straightened at once, it can be done multiple times until it is straightened.
In order to stabilize the deformation of the parts after cold calibration and improve their rigidity, heat treatment is carried out to eliminate stress and stabilize deformation after the parts are calibrated. For surface quenched parts (such as crankshafts), they can be heated to 200-250 ℃ for 5-6 hours.
The advantage of static pressure correction is that it is simple and easy to implement, but the disadvantage is that the correction accuracy is difficult to control, leaving a large residual stress in the parts, the correction effect is unstable, and the fatigue strength of the parts decreases.
2.2 Cold work correction
Cold work correction is the use of a round or pointed hammer to strike the concave surface of the shaft, causing plastic deformation and straightening the part due to residual stress in the deformation layer. When hammering, the metal in the plastic deformation part is extruded and stretched, generating compressive stress in this plastic deformation layer (cold working layer), causing the deformed part to be straightened. If metal is used to strike the surface of the crank arm of the crankshaft, it can cause bending deformation of the crank arm, thereby obtaining the ideal centerline displacement of the crankshaft. When straightening, an appropriate surface of the crankshaft arm can be selected for cold work tapping based on the bending state of the crankshaft, so that various deformations of the crankshaft can be straightened.
2.3 Flame correction
For parts with large shaft diameter and high stiffness, if manual cold correction is used, a large correction force needs to be applied, so flame correction method is usually used. The flame correction method is to rapidly heat the Z-high point of the bent part of the shaft to above 450 ℃ using a neutral flame of gas welding, and then rapidly cool it. After the heating starts, due to the expansion of the metal in the heated part, the curvature of the shaft actually increases. However, after the metal in the heated part cools and contracts, the shaft is straightened. After metal is heated, its plasticity increases with temperature. Generally, carbon steel has good plasticity when the temperature reaches 600 ℃. Due to the obstruction of surrounding cold metals, it cannot stretch with temperature increase.
(1) Heating temperature and heating area. The heating temperature depends on the degree of bending of the part, and the larger the bending, the higher the heating temperature. If the curvature of the rear axle of the tractor is less than 0.5mm and the heating temperature is 600 ℃; When the curvature of the rear axle is between 0.5-1.0mm, the heating temperature is between 650-700 ℃; When the curvature of the rear axle is greater than 1.0mm, the heating temperature is 700-720 ℃. The ability to correct bending deformation increases with the increase of heating area. During calibration, the heating area can be determined based on the deformation situation.
(2) Heating depth. As the heating depth increases, the ability to correct deformation increases. When the heating depth increases to one-third of the thickness of the part (diameter of a cylindrical part), the straightening effect is Z good. However, as the heating depth continues to increase, the straightening effect actually decreases, and the individual parts of the part do not have a straightening effect due to heat penetration. During the straightening process, it is mainly important to rely on experience to grasp the heating depth. It is not advisable to heat the parts thoroughly, and cooling measures should be taken if necessary.
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2024-05-27