Methods to reduce crankshaft deformation
The crankshaft is a core component of the engine, responsible for converting reciprocating motion into rotational motion. Deformation can lead to increased engine vibration, reduced power, and even premature component failure. Crankshaft deformation primarily manifests as bending (straightness error), torsional deformation, and journal roundness error. Causes include manufacturing defects, heat treatment stress, improper machining processes, and assembly stress. Reducing crankshaft deformation requires systematic measures encompassing blank selection, heat treatment, machining methods, and clamping strategies. Through comprehensive process control, the crankshaft’s dimensional accuracy and form and position tolerances are guaranteed to meet the engine’s high-performance requirements.
Quality control during the blank manufacturing stage is fundamental to minimizing crankshaft deformation. Proper selection of blank type and forging process can significantly reduce the risk of deformation during subsequent processing. Solid-piece forging crankshafts (such as those made from 42CrMo steel) reduce internal structural stresses through optimal forging streamline distribution (evenly distributed along the crankshaft axis and crank arms), resulting in deformation that is over 50% less than that of cast crankshafts. During the forging process, the final forging temperature (850-900°C) and cooling rate (air or mist cooling) must be controlled to avoid thermal stress caused by uneven cooling. Post-forging, isothermal annealing (at 700-720°C for 4-6 hours) is required to achieve uniform hardness (HB180-200) and eliminate forging stresses. For large crankshafts (such as those for marine engines), using die forging instead of open die forging can improve blank dimensional accuracy by 20%, reduce stock clearance to 3-5mm, and minimize deformation caused by stress release during processing. In addition, flaw detection of the blank (such as ultrasonic testing) is essential to ensure that there are no defects such as cracks and looseness inside. These defects will become sources of stress concentration in subsequent processing, leading to increased deformation.
Optimizing the heat treatment process is crucial for controlling crankshaft deformation. By optimizing heating, holding, and cooling parameters, stresses caused by microstructural transformation can be reduced. Quenching and tempering is a common heat treatment method for crankshafts. The quenching and tempering process for 42CrMo steel crankshafts includes oil quenching at 860±10°C (oil temperature 60-80°C), followed by tempering at 580±10°C (holding for 3-4 hours), with a hardness control range of HB240-280. Quenching utilizes a stepwise temperature ramp (200°C → 500°C → 860°C), with each stage holding for 1-2 hours to avoid thermal stress caused by rapid temperature increases. A circulating oil bath is used as the quenching medium to ensure uniform cooling rates across all crankshaft components (30-40°C/s), minimizing quenching distortion. Tempering utilizes a slow ramp (50°C/h) to the tempering temperature. After holding, the material is cooled in the furnace to below 200°C, followed by air cooling to further eliminate quenching stresses. Ductile iron crankshafts are subjected to an austempering process (880°C austenitizing followed by a 350°C salt bath isothermal hardening) to achieve a bainitic structure with a hardness of HB300-350. This reduces deformation to just one-third of that achieved with conventional quenching, while also increasing fatigue strength by 20%. Straightening is required after heat treatment, and a press straightening machine is used to correct excessive bending deformation, with the correction controlled to 0.1-0.3 mm/m to avoid overcorrection and the generation of new internal stresses.
Improved machining techniques can effectively reduce deformation during crankshaft machining. A step-by-step machining strategy of “roughing-semi-finishing-finishing” is employed to gradually release internal stresses. During the roughing stage (milling the main and connecting rod journals), large stock removal (3-5mm per side) is employed, but cutting parameters must be moderate (cutting speed 80-100m/min, feed 0.2-0.3mm/r) to avoid excessive cutting forces that could cause elastic deformation in the workpiece. After roughing, stress relief annealing (at 600-650°C for 2-3 hours) is performed to eliminate machining stresses and keep residual stresses below 100MPa. Semi-finishing is performed on a CNC lathe or a dedicated crankshaft lathe. For the main and connecting rod journals, a stock removal of 1-2mm is used, at a cutting speed of 100-150m/min and a feed of 0.15-0.2mm/r. Simultaneously, the journals are chamfered and rounded (corner radius 2-3mm) to reduce stress concentrations. During the finishing phase, grinding (external cylindrical grinding and crankshaft grinding) is performed with a grinding allowance of 0.1-0.3mm, a grinding wheel linear speed of 30-40m/s, and a feed rate of 0.01-0.02mm/r. This low-allowance, high-precision grinding process ensures dimensional accuracy (journal diameter tolerance IT6) and surface quality (Ra 0.4μm). For high-precision crankshafts (such as those for high-speed engines), an aging treatment (120-150°C for 8-12 hours) is performed after finishing to further stabilize dimensions and reduce deformation during use.
Innovations in clamping methods reduce clamping deformation during machining. Specialized fixtures and support devices are designed for the unusual crankshaft structure. A “center hole positioning at both ends + intermediate support” method is used for machining the main journal. The center hole is ground to IT5 precision. Adjustable V-blocks are used for intermediate supports, positioned away from the connecting rod journal. The support force is hydraulically controlled (5-10kN), ensuring rigidity while preventing over-positioning. An eccentric clamp is used for machining the connecting rod journal. Its eccentricity matches the eccentricity of the connecting rod journal and is automatically indexed by a servo motor. A combination of axial positioning and radial preload is used during clamping to minimize vibration and deformation caused by centrifugal force. For slender crankshafts (aspect ratio > 10), a toolholder is added during machining. The toolholder’s support blocks are made of wear-resistant bronze, maintaining a contact pressure of 1-2kN with the workpiece to minimize radial runout. The crankshaft must be balanced before clamping, and the imbalance must be controlled within 5g · mm to avoid bending deformation caused by centrifugal force generated during high-speed rotation.
Precautions during assembly and operation can reduce final crankshaft deformation and ensure engine assembly accuracy. The crankshaft and cylinder block assembly requires locating pins and thrust washers to control axial position. Axial clearance should be maintained within 0.05-0.1mm to prevent axial play caused by excessive clearance or additional stress caused by insufficient clearance. The assembly clearance between the main bearing and connecting rod bearings must be strictly controlled (0.03-0.05mm). Excessive clearance can easily generate impact loads, while insufficient clearance can increase friction due to thermal expansion, both of which can cause crankshaft deformation. During engine commissioning, cold and hot tests should be performed according to regulations. During the cold test, the crankshaft’s rotational flexibility should be checked. During the hot test, the oil temperature (80-90°C) and vibration level (≤0.1mm/s) should be monitored to prevent deformation caused by overheating or severe vibration. Regular maintenance should be performed during operation to keep the lubricating oil clean (particle size ≤NAS grade 8) to prevent impurities from entering the friction pair, causing abnormal wear and stress concentration, thereby reducing the risk of crankshaft deformation. By controlling deformation throughout the entire life cycle, the crankshaft’s service life can be guaranteed to be more than 10,000 hours, meeting the engine’s reliability requirements.
