The key control parameters to be optimized in the manufacturing of oil-impregnated bearings cover four major sections: material formula, compaction process, sintering process and post-treatment. Details are as follows:
I. Material Formula Optimization
Powder Particle Size and Distribution
Impact: Powder particle size directly affects porosity and material strength. For instance, mixing bronze alloy powder with particle size below 175μm and iron powder below 85μm can improve the load capacity of bearings. Excessively fine particles (below 50μm) may result in poor pore connectivity and lower oil impregnation efficiency.
Optimization: Adjust powder particle size combination according to operating conditions. Coarse powder (100-200μm) is adopted for high-speed and light-load scenarios such as home appliance motors to increase porosity; fine powder (50-100μm) is used for low-speed and heavy-load scenarios such as automotive motors to enhance material density.
Additive Ratio
Graphite and Solid Lubricants: Flaky graphite powder (particle size below 75μm) can enhance the self-lubricating performance of bearings, while excessive addition (over 10%) will reduce material strength. For bronze-graphite bearings, when graphite content is controlled at 6%-8%, the friction coefficient decreases by 30% and the radial fracture strength remains no less than 2.5N.
Metal Reinforcing Elements: Adding 5%-10% copper to iron-based bearings can improve boundary lubrication performance with a balance between cost and performance. One enterprise added 3% nickel to raise the creep resistance of bearings by 50% at 150℃.
II. Compaction Process Optimization
Compaction Pressure
Impact: Too low pressure (below 300MPa) leads to insufficient green compact density (below 6.5g/cm³) and large fluctuation of shrinkage after sintering. Excessively high pressure (over 600MPa) may deform powder particles and block pores.
Optimization: Adopt staged compaction. For example, pre-compact at 400MPa followed by final compaction at 500MPa improves the density uniformity of green compacts by 15%, and keeps the dimensional tolerance within ±0.02mm after sintering.
Compaction Temperature
Impact: Compaction at low temperature (below 100℃) results in weak bonding between powder particles. Compaction at high temperature (over 200℃) may volatilize lubricants and impair demolding performance.
Optimization: Warm compaction at 150℃ for iron-based bearings increases green compact strength by 40% and reduces the risk of cracks during subsequent sintering.
III. Sintering Process Optimization
Sintering Temperature and Holding Time
Impact: Sintering at too low temperature (below 750℃) causes poor pore connectivity and an oil impregnation rate lower than 10%. Excessively high temperature (over 950℃) leads to grain coarsening and reduced material strength. For copper-based bearings, the radial fracture strength reaches 3.0N after sintering at 850℃ for 2 hours, while it drops to 2.5N at 900℃.
Optimization: Apply staged sintering. For example, hold at 780℃ for 1 hour to relieve compaction stress, then heat up to 895℃ and hold for another 2 hours. This method increases the uniformity of bearing porosity by 20% and lowers deformation.
Sintering Atmosphere
Impact: Oxidizing atmosphere such as air causes surface oxidation of bearings and reduces oil impregnation efficiency. Reducing atmosphere such as hydrogen can prevent oxidation but comes with higher costs.
Optimization: A mixed atmosphere of nitrogen plus 5% hydrogen is used for sintering iron-based bearings. It controls the surface oxide layer thickness below 1μm and raises the oil impregnation rate by 15% with controllable cost.
IV. Post-treatment Process Optimization
Oil Impregnation Process
Impact: Vacuum pressure below 0.01MPa results in inadequate oil impregnation, while pressure above 0.1MPa may damage the pore structure. By optimizing the vacuum oil impregnation parameters (0.05MPa, 80℃, 2 hours), an enterprise stabilized the oil content of bearings at 25%-30% and extended service life by 50%.
Optimization: Introduce ultrasonic-assisted oil impregnation. Under 40kHz ultrasonic vibration, the penetration speed of lubricating oil triples and oil distribution uniformity improves by 20%.
Sizing and Machining
Impact: Excessively high sizing pressure (over 100MPa) may close surface pores and degrade self-lubricating performance. Excessive machining allowance (over 0.1mm) will destroy the sintered structure and reduce strength.
Optimization: Adopt CNC sizing with pressure controlled at 50-80MPa and machining allowance within 0.05mm. The surface roughness of bearings can reach Ra≤0.4μm and the operating noise decreases by 5dB.
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