Abstract: Bending deformation of parts is the main form of failure in agricultural machinery parts, which seriously affects the normal use of agricultural machinery. An analysis was conducted on the hazards and causes of part deformation, and the correction methods for deformed parts were introduced to improve the repair quality of the parts.

Many parts of agricultural machinery will experience bending, twisting, and warping deformation during use, which can lead to part failure or breakage when reaching the limit value, making it the main form of mechanical part failure. The frame of a tractor is prone to twisting and deformation, the engine crankshaft is prone to bending and twisting deformation, and the housing parts are prone to warping and deformation. After the connecting rod parts are bent or twisted, it will cause the piston to not move along the correct trajectory, but to lean towards one side of the cylinder, resulting in 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 roundness of the connecting rod end holes, wear of the connecting rod bushing, abnormal noise during operation, the piston not reaching the required top dead center, a decrease in compression ratio, and difficulty in starting. When the stiffness of the transmission gear shaft is insufficient, it will also undergo bending deformation under high loads, causing changes in the position of gear meshing marks and tooth clearance, resulting in unstable gear transmission and high noise. The crankshaft deforms due to periodic gas pressure, reciprocating inertia 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 its connected parts, which can disrupt the engine's dynamic balance. In mild cases, the engine may experience high vibration during operation, accelerate fatigue, and over time, result in mechanical failure. The deformation of the basic components of the shell type will destroy the interrelationships of all the components assembled on this basic component, intensify 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 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) Regarding repairs. ① The assembly accuracy is too low, and the clearance between the crankshaft bearing and the connecting rod bearing is too large. During operation, the crankshaft will be greatly impacted, causing deformation of the crankshaft or connecting rod. During engine repair, the lack of careful dynamic balance testing resulted in additional inertial forces and moments generated by crankshaft rotation, leading to increased vibration of the unit. The clearance between the parts of the piston connecting rod group is too large, and the connecting rod is subjected to excessive lateral bending stress during operation, which causes 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 a special socket should be used to loosen or tighten each bolt in diagonal order, and several times should be used to reach the specified torque, so 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 operates under overload or overheating conditions, it will increase the load on the parts, causing deformation or even fracture of the parts. Therefore, agricultural machinery should strictly follow the operating procedures when in use, and try to avoid overloading and overspeeding. If the engine overheats or other parts overheat, the machine should be stopped for maintenance to reduce the deformation of parts.

2 Correction of deformed parts

The basic principle of component calibration is to use external forces or flames to induce new plastic deformation in the component, in order to eliminate the original deformation. The correction methods for deformed parts are divided into cold correction method and hot correction 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 holding for 1-2 minutes, remove the pressure and check the deformation. Once the school is not straight, it can be repeated multiple times until it is straight.

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 part calibration. For surface quenched parts (such as crankshafts), they can be heated to 200-250 ℃ and maintained 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 large residual stresses inside 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 a shaft, causing it to undergo plastic deformation, and straightening the part due to residual stress in the deformation layer. During hammering, the metal in the plastic deformation part is squeezed and stretched, generating compressive stress in this plastic deformation layer (cold working layer), which straightens the deformed part. If metal is used to strike the surface of the crank arm of the crankshaft, it will cause bending deformation of the crank arm, thereby obtaining the ideal displacement of the crankshaft centerline. When straightening, appropriate crank arm surfaces can be selected for cold striking according to the bending state of the crankshaft, so that various deformations of the crankshaft can be straightened.

2.3 Flame calibration

For parts with larger shaft diameter and higher stiffness, if manual cold correction is used, a large correction force is required, so flame correction method is usually used. The flame calibration method is to quickly heat the Z-high point of the bent part of the shaft to above 450 ℃ using a neutral flame welded by gas, and then rapidly cool it down. After the heating process begins, the curvature of the shaft increases due to the expansion of the metal in the heated area. However, after the metal in the heated area cools and contracts, the shaft is straightened. After heating, the plasticity of metal 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 increasing temperature.

(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 tractor rear axle is less than 0.5mm and the heating temperature is 600 ℃; When the rear axle curvature is between 0.5-1.0mm, the heating temperature is 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. The heating area can be determined based on the deformation during calibration.

(2) Heating depth. As the heating depth increases, the ability to correct deformation increases. When the heating depth increases to 1/3 of the thickness of the part (diameter of the cylindrical part), the straightening effect is good. However, as the heating depth continues to increase, the straightening effect actually decreases, and the individual parts do not have a straightening effect due to heat penetration. During the straightening process, it is mainly necessary to rely on experience to master the heating depth. It is not advisable to heat through the parts, and cooling measures should be taken if necessary.

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2025 March 2nd Week FK Product Recommendation:

Pillow Block:

A pillow block bearing (or plummer block) is a mounting used to support a rotating shaft with the use of bearings and various accessories. The assembly consists of a mounting block which houses a bearing. The block is mounted to a foundation, and a shaft is inserted, allowing the inner part of the bearing/shaft to rotate. The inside of the bearing is typically 0.025 millimetres (0.001 in) larger diameter than the shaft to ensure a tight fit. Set screws, locking collars, or set collars are commonly used to secure the shaft. Housing material for a pillow block is typically made of cast iron or cast steel.

https://www.fk-bearing.com/products/Pillow-Block/698.html

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