Crankshaft measurement method
The crankshaft is a core component of the engine. Its complex structure, consisting of the main journal, connecting rod journal, crank arm, and counterweight, plays the crucial role of converting the piston’s reciprocating motion into rotational motion. Its accuracy directly impacts the engine’s performance, lifespan, and reliability. Crankshaft measurements include dimensional accuracy (such as the diameters of the main and connecting rod journals), form and position tolerances (such as roundness, cylindricity, coaxiality, and parallelism), and surface quality (such as roughness). Due to the unique structure of the crankshaft, measurement requires specialized measuring tools and advanced equipment, combined with appropriate measurement methods, to accurately assess its machining quality and provide a reliable basis for subsequent assembly and operation.
Measuring the diameters of the main journal and connecting rod journal is the most basic crankshaft measurement item, and the diameter dimensions must be within the design tolerance range. The main journal is the part where the crankshaft mates with the engine bearing, and the connecting rod journal is the part where the connecting rod bearing mates. The diameter tolerance of both is generally IT6-IT7, with tolerance values usually ranging from 0.01-0.03mm. Commonly used tools for measuring diameters include outside micrometers, lever micrometers, and digital micrometers, with measurement accuracy up to 0.001mm. Measurement methods use either the two-point or three-point method: the two-point method measures the diameter in two mutually perpendicular directions on the same cross section of the journal, and the average value is taken as the diameter of that section; the three-point method measures the diameter at three points evenly distributed along the circumference of the same cross section of the journal, and the roundness error and average diameter are calculated. For multi-cylinder engine crankshafts, the diameter of each main journal and connecting rod journal must be measured to ensure dimensional consistency. The diameter difference between adjacent journals should be ≤ 0.01mm to ensure even bearing load distribution. For example, the main journal diameter of a four-cylinder engine crankshaft is designed to be 60mm with a tolerance of + 0.015/-0.01mm. During measurement, the actual diameter must be within the range of 59.99-60.015mm.
Measuring the geometric tolerances of crankshafts is crucial for ensuring their assembly performance and kinematic accuracy. These tolerances primarily include roundness, cylindricity, coaxiality, and parallelism. Roundness error refers to the deviation of the actual profile of the journal cross section from the ideal circle. Measurements are performed at multiple points on different journal cross sections (typically 3-5 sections). The roundness error is calculated using the least squares method or the minimum area method. The roundness error of main and connecting rod journals should be ≤0.005mm. Cylindricity error refers to the deviation of the actual cylindrical surface of the journal from the ideal cylindrical surface. Measurements are performed at multiple points on 3-5 cross sections evenly spaced along the journal axis. The cylindricity error is generally ≤0.008mm. Coaxiality error refers to the deviation of the axis of each main journal from a common reference axis. The center holes at both ends of the crankshaft are used as a reference. The radial runout of each main journal is measured using a dial indicator or a coordinate measuring machine to calculate the coaxiality error. The coaxiality error should be ≤0.01mm. Parallelism error refers to the deviation of the connecting rod journal axis from the main journal axis. The distance difference between the connecting rod journal and the main journal axis is measured at both ends of the connecting rod journal. The parallelism error should be ≤ 0.01mm/100mm. For example, when measuring the parallelism of the connecting rod journal of a crankshaft, the distance difference measured at both ends of the journal is 0.008mm. The journal length is 50mm, so the parallelism error is 0.016mm/100mm, which is outside the allowable range and requires correction.
The axial dimensions and positional tolerances of a crankshaft include parameters such as crank arm thickness, journal length, and counterweight position. Crank arm thickness affects crankshaft rigidity and strength. Measurements are made with a vernier caliper or micrometer at various locations on the crank arm, with a typical thickness tolerance of ±0.1mm. Journal length (such as the width of the main and connecting rod journals) affects the axial positioning of the bearings. Measurements are made with a depth vernier caliper or digital caliper, with a tolerance of ±0.15mm. The positional tolerance of the counterweight affects the dynamic balancing performance of the crankshaft. The center of gravity of the counterweight is determined by scribing or using a coordinate measuring machine. The deviation from the designed position should be ≤0.5mm. Furthermore, the crankshaft’s end face perpendicularity (such as the perpendicularity of the front face to the main journal axis) must be measured using a dial indicator at multiple points on the end face. The perpendicularity error should be ≤0.02mm/100mm to avoid axial forces during assembly. For example, when measuring the verticality of the front end face of a crankshaft, install the crankshaft on the yaw meter, rotate the crankshaft, the difference between the maximum reading and the minimum reading of the dial indicator is 0.015mm, and the end face diameter is 100mm. The verticality error is 0.015mm/100mm, which meets the requirements.
Measuring crankshaft surface quality and runout is crucial to ensuring wear resistance and smooth running. Surface roughness measures the microscopic unevenness of the journal surface. The surface roughness requirement for main and connecting rod journals is generally Ra ≤ 0.8μm. Measurements are made using a surface roughness tester at 3-5 different locations on the journal surface, and the average result is taken. Runout includes radial runout and axial runout. Radial runout refers to the change in radial position of the journal during rotation. To measure, mount the crankshaft on the center, touch the journal surface with a dial indicator, and slowly rotate the crankshaft. The difference between the maximum and minimum readings on the dial indicator is the radial runout. The radial runout for the main journal should be ≤ 0.01mm, and for the connecting rod journal, it should be ≤ 0.015mm. Axial runout refers to the change in axial position of the crankshaft during rotation. A dial indicator is placed on the front or rear end of the crankshaft and rotated. The change in the dial indicator reading is the axial runout and should be ≤ 0.02mm . For high-speed engine crankshafts, dynamic balancing is also required. The unbalance of the crankshaft is measured using a dynamic balancing machine. The unbalance is generally required to be ≤ 5g · cm to reduce vibration and noise during engine operation.
The application of advanced measurement technologies in crankshaft measurement can significantly improve measurement accuracy and efficiency. Coordinate measuring machines (CMMs) are currently the most commonly used high-precision measuring equipment. Using contact probes or non-contact laser probes, CMMs can comprehensively measure crankshaft dimensions, form and position tolerances, and surface profiles, achieving accuracy up to 0.001mm. They are suitable for measuring complex surfaces and high-precision parameters. In mass production, dedicated crankshaft measuring instruments enable automated measurement, resulting in high efficiency (measurement time per crankshaft ≤ 5 minutes). They also provide real-time feedback, facilitating timely adjustments to the machining process. Furthermore, optical measurement technologies (such as laser scanning) can detect surface defects (such as scratches and cracks) on crankshafts, ensuring crankshaft quality. The measurement environment significantly impacts measurement accuracy. The ambient temperature should be maintained at 20±2°C and the humidity between 40% and 60% to prevent thermal deformation of the workpiece and measuring equipment caused by temperature fluctuations and to minimize interference from vibration and dust. By comprehensively using traditional measuring tools and advanced measuring equipment to comprehensively and accurately measure various parameters of the crankshaft, the processing quality of the crankshaft can be guaranteed to meet the use requirements of the engine.
