With the progress of China's wind power industry, domestic enterprises have also achieved rapid development in the matching of wind power gearboxes. Wind power gearboxes are important components of several types of wind turbines. Their main function is to transmit the power generated by the blades under the action of wind to the generator and get the corresponding speed. The data shows that more than 85% of the wind turbines in the world are gearboxes. Moreover, as the price of rare earths brings pressure on the permanent magnet direct drive unit, it is expected that the proportion of units with gearbox models will increase in the future. This brings opportunities to domestic wind power main gearbox manufacturers, but it also brings new challenges. The wind power gearbox is a high-reliability industrial gearbox with high gear modulus, high precision parts, high weight, high value, high production cost, long production cycle and expensive gearbox maintenance. Domestic wind turbine gearbox manufacturers are generally developed from single-piece, small-batch industrial gearbox manufacturers. Faced with the demand for mass production of wind turbine main gearboxes brought by wind power market demand, many manufacturers still use traditional production methods. As a result, the opportunity is lost in front of the market, or the customer is lost due to the quality of the product, or the initiative is stopped due to high production costs.
This paper starts with the mass production of wind turbine gearbox gears. With the quick market response, the product quality to meet customer needs and the lower production cost as the fundamental principle of discussion, a typical process in gear processing, hobbing, is discussed. The contents of machine tool selection, tool selection, fixture design and precision system involved in hobbing are described.
Selection of hobbing machine
The choice of the hobbing machine is closely related to the characteristics of the part to be machined. The wind power gearbox is generally a three-stage transmission, which is divided into an input stage, an intermediate stage and an output stage. Each level of parts differs greatly depending on the design. Therefore, when selecting a machine tool, it is necessary to select a gear hobbing machine according to the characteristics of different parts.
The input stage is generally a planetary transmission, and the modulus of the gear is large due to the large load. For example, for a 3MW wind turbine gearbox, the input stage gear modulus is generally around 20mm. Therefore, the rigidity of the hobbing machine is relatively high, the power of the main drive motor is large, the chip discharge performance of the worktable is good, the length of the Y-axis of the machine tool is long, and the machine tool needs to have the rolling and milling compound function.
The intermediate stage may be designed as a planetary or parallel drive depending on the design requirements. If it is a planetary transmission, the gears, the pinion and other parts have no special requirements for the machine tool. If it is a parallel-stage transmission, because the gear member is generally large in diameter, in order to improve the gear hobbing efficiency, the rotation speed of the hob and the Z-axis travel speed will have higher requirements, and the Z-axis drive motor power is also required.
The output stage is generally parallel, with small load, but high speed, and the gear member has a small modulus, generally about 7mm. Since the number of teeth is small and the number of teeth of the pinion is small, the rotational speed of the table is relatively high at the time of high-speed hobbing. Therefore, when machining such gear shaft parts, it is necessary to pay attention to whether the rotation speed of the hobbing machine table can meet the processing requirements.
Selection of gear cutters
The selection of gear cutters is a key part of the hobbing process. Reasonable selection of gear cutters not only can balance the quality and efficiency of gear machining, but also improve the service life of gear cutters and save costs.
The machining method that can be selected for the hobbing process can be divided into two methods: “forming milling†and “forming hobbingâ€. The profile milling cutters used in wind power gears are mainly carbide insert-molding cutters. The tools for hobbing are mainly carbide insert hobs, high-speed steel double-cut hobs, high-speed steel single-blade overall hobs. . Because the input, intermediate and output stage gear parts of the wind turbine gearbox are very different, depending on the modulus of the machined gear (pinion), the number of teeth and the design structure, or the application of the milling method, or the application of hobbing Method, or integrated application of two processing methods (rolling, milling composite machining).
In the following, according to the different characteristics of the input stage, intermediate stage and output stage of the wind turbine gearbox, the middle stage large wheel is selected for the machining method and tool selection analysis.
The intermediate stage large wheel has a modulus of 10 to 12 mm, a number of teeth of about 100, and a tooth width of 350 mm. According to the above data, from the modulus and the number of teeth, since the number of teeth is large and the modulus is moderate, the method of forming milling is eliminated (low cutting efficiency); the length of the gear is cut from the number of teeth and the width of the tooth (tooth width and The product of the number of teeth exceeds the cutting life of the general single-edged hob (single-blade hob can increase the outer diameter and increase the number of cutting edges, but it causes waste of the material of the cutter body. As for how to optimize the design hob, it is beyond the scope of this article. Therefore, the ordinary single-blade hob is excluded; from the analysis of the tool cost, since there is only one gearbox per intermediate wheel, the quantity demand is not enough, so it is not necessary to choose a high-cost carbide insert hob. And the double-cut hob just meets the requirements of efficiency, life, cost and so on.
Fixture design
Fixture design is another key aspect of the hobbing process. A good hobbing fixture should have at least the following characteristics: stable clamping, quick loading and unloading, convenient calibration reference, easy to disassemble the fixture, and strong versatility, and can seamlessly interface with the hobbing machine. As mentioned above, the gear stage of the input, intermediate and output stages of the wind turbine gearbox has large changes and different structures. Therefore, it is necessary to batch and stabilize the production. It is necessary to combine the specific characteristics of each part with the materials of the specific gear hobbing machine, tool and parts. Formulations are required for process requirements such as hardness and cutting parameters.
The sun wheel spline is a special gear part in the input stage of the wind power gear box. When the tooth is hobbed, the material has been carburized and quenched and heat treated. Despite the hardness protection measures, its hardness has exceeded the hobbing requirements of medium hard toothed gears, and the machining accuracy is very high. For this type of part, an over-positioning fixture design combining a chuck and a tip can be used. When the jig is used, not only the accuracy of the clamping can be ensured, but also the rigidity is good, and the contradiction between the material hardness, the tool life, the spline precision and the rigidity of the machine tool is well solved, and the cutting process is stabilized.
Hobbing accuracy system
The hobbing accuracy system is a system guarantee for batch processing of wind power gears. Individual parts of wind power gears are generally heavy and expensive. For example, the individual weight of the 3MW wind turbine gearbox input stage planetary gear is about 1t, and the light material is worth a lot. In the production process, the quality of the parts should be eliminated, and the quality of the batch parts should not be qualified. This requires high accuracy of the hobbing system. By analyzing the operation of the hobbing process, the tool accuracy system, the machine tool precision system, the fixture accuracy system, and other precision assurance systems are separated. Under the premise that the accuracy of each subsystem meets the processing requirements, the precision guarantee of the whole system is obtained, so that qualified rolling (milling) tooth parts can be produced stably and orderly.
Conclusion
The hobbing process is the most basic and simple process in the wind turbine main gearbox gear processing process. Proper selection of machine tools and accuracy systems is the basis for mass production of wind power gears. It is with these process guarantees that we can achieve rapid market response, meet customer quality requirements, and have a competitive advantage in the market.
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