Progress in gear transmission technology of the ho

Advances in gear transmission technology of large-scale wind turbines

large-scale wind turbines are mainly composed of wind turbines, mechanical transmission systems, power generation equipment and control systems. The mechanical transmission system is an intermediate device that mechanically transmits the wind energy absorbed by the wind turbines to the generator, including transmission shafting, couplings, gear boxes, clutches and brakes. In order to facilitate the capture of wind energy and meet the needs of unit performance control, The unit must also be equipped with yaw drive, variable pitch drive, damping, braking and other auxiliary devices. Figure 1 shows a typical large wind turbine. The left wind turbine transmits power to the right generator through the gearbox through the main shaft. The equipment in the engine room is installed on the base and supported on the tower through the yaw bearing

the special environment and operating conditions of large wind turbines put forward unusual requirements for the transmission device, and a large number of uncertain factors, such as the external dynamic load and variable wind turbine shown in Figure 2, the role of abnormal electrical load, strong vibration caused by insufficient rigidity of the engine room, load spectrum and limit load distribution that can only be estimated and simulated, are all major issues that must be considered by the transmission device

the main transmission gearbox of large-scale wind turbine is located between the wind wheel and the generator. It is a heavy-duty gear speed-up transmission device that works under the action of irregular directional load and instantaneous strong impact load. Gearbox is one of the most important and fragile components in the transmission shaft system of wind turbine

the gearbox cannot have a solid base in the engine room as on the ground. The factors of power matching and torsional vibration of the whole transmission system are always concentrated in a weak link, which is often the gearbox in the unit. Of course, the most ideal situation is to let the gearbox complete the task of transmitting torque and increasing speed without bearing other additional loads. In fact, this is not only impossible, but also because the variable wind conditions and the complex deformation of the unit can not avoid the role of many additional loads, which adds many uncertain factors to the design of the gearbox

obviously, it is very important to reduce the overall size and weight of components in the narrow engine room space. Therefore, the gearbox design must ensure that on the premise of meeting the reliability and expected life, the structure is simplified and the weight is the lightest, and the requirements of easy maintenance should also be considered. According to the parameters provided by the unit, using CAD Optimization design, according to the scheduled best transmission scheme, selecting stable and reliable structure and materials with good mechanical properties and stable under the extreme temperature difference of the environment, and equipped with perfect lubrication, cooling and monitoring system are the necessary prerequisites for the design of the gearbox

therefore, the design and component selection of the transmission device must be selected according to the requirements of the host machine and different service conditions after analysis and comparison. The main factors to be considered are:

1) operating conditions and performance parameters of the main engine, dynamic analysis results

2) load distribution and structural form of transmission system; Wind power material equipment

3) requirements for transmission device and its connection

4) safety and environmental protection requirements

5) life requirements

6) economic and benefit analysis

7) operation and maintenance conditions

the main gearbox of the traditional unit is used to change speed and torque, so that the compact standard generator can be applied to the unit. Gear boxes with different power levels adopt different transmission forms (see Figure 3)

in the 1980s, parallel shaft cylindrical gear drives were applied to 100 to 500kW standard wind turbines. In the 1990s, the average power of wind turbines increased to 600 to 800KW. In order to save space and obtain a larger speed ratio, the structure of cylindrical planetary gear transmission or combined planetary and parallel shaft gear transmission was introduced, and good results were achieved

the primary planetary two-stage parallel shaft gearbox shown in Figure 4 is a widely used unit transmission structure at present. In order to obtain high power density and high speed ratio, the planetary carrier of the planetary level divides the power into multiple planetary gears, then combines it to the sun gear and uploads it to the parallel shaft gear. The common power is less than 2MW. The design structure of the gearbox depends on the arrangement of the transmission shafting, and it is suitable for "two-point" or "three-point" support together with the main shaft. The expansion sleeve connects the main shaft and the gearbox input shaft (planet carrier), the fixed end is set on the main shaft, and the planet carrier bearing (or box) should be able to float axially. The planet carrier adopts double supports to improve the structural stiffness. Three planet gears are commonly used, and the sun gear is floating and loaded evenly; Helical gears are used to ensure smooth transmission and reduce noise

there are also many application examples of two-stage planetary and one-stage parallel shaft gear transmission, with power up to 3 ~ 3.5MW

for higher power units, in order to reduce the overall size and save engine room space, the gearbox tends to use the combined transmission of planetary, differential and parallel shaft gears. There are often more than three planetary gears to reduce the volume and obtain higher power density

the planetary differential and fixed shaft gear combined transmission structure shown in Figure 5 has been applied in domestic large-scale wind turbines. The structure adopts three-stage gear transmission: the first stage is planetary differential gear transmission; The second stage is fixed shaft gear split transmission; The third stage is parallel shaft gear transmission

the power transmitted from the main shaft is transmitted in two ways: the one marked with the red line of the arrow is transmitted from the second stage internal gear ring directly connected to the planet carrier to the second stage central gear through a set of fixed shaft gears distributed circumferentially, and then transmitted back to the planet carrier through the first stage internal gear ring connected to the central gear; The other way (indicated by the green arrow line) is directly transmitted by the planet carrier and combines with the power of the previous way on the first stage planetary gear, and is transmitted to the third stage parallel shaft gear pair through the first stage central gear (sun gear)

the total speed ratio of this transmission mode can reach 200:1 (planetary/differential stage: 35:1; parallel axis: 6:1 to encourage vehicle enterprises to adopt mixed materials in DT Reading Guide: with the pursuit of material lightweight and excellent performance in various industries, vehicle design adopts mixed materials 1). Proportion of power diversion: 72.1% from differential of inner gear ring to final driving wheel; 27.9% from planetary gear, sun gear to final drive gear. Due to the multi row star wheel and flexible planet pin structure, its volume and weight are reduced by more than 25% compared with the traditional structure

the planet carrier of the low-speed and heavy-duty planetary gear transmission device usually adopts the double support structure, as shown in Figure 6 (a). The uneven load-bearing problem of the gear is affected by manufacturing and installation errors, tooth deformation, temperature changes, non-human factors warranty within 2.1 years and other factors. In addition to being partially relieved by using a reasonable load sharing mechanism (such as the floating of the sun gear), the planet gear is still affected by the uneven load, The above is the matters needing attention in the handling process of hydraulic material testing machine, which affects the transmission quality. Therefore, the load distribution can be further improved by adding an elastic element load sharing mechanism. The use of flexible planetary axles is a simple and effective form

as shown in Figure 6 (b), the planet carrier adopts a single support, the elastic spindle is interference connected with the elastic shaft sleeve outside the planet carrier and the spindle, the cantilever of the elastic sleeve is fixed on the free end of the spindle, the shaft sleeve is installed with rolling bearings, and the planet gear rotates on the bearings. When the load acts on the middle or the left end of the planetary gear, the spindle and shaft sleeve are deformed, and the spindle and planetary gear axes are inclined, so the load is transferred in the direction of extending the tooth width, so that the load distribution of the gear teeth tends to be uniform. Figure 6 (c) shows the structure of the flexible planetary shaft actually used in the above domestic large-scale wind power gearbox. Fig. 7 is the measured relative strength distribution of the planetary gear root stress during the prototype test. The load distribution of several groups of curves under different loads in the figure is basically the same, and the load nonuniformity coefficient (the ratio of maximum load to average load) is close to 1.1, which has a good load sharing effect

the structure of four-stage planetary differential combined transmission is shown in Figure 8, and the power is transmitted in two ways:

the main transmission input stage of the gearbox is composed of a primary planetary gear and a ring gear fixed on the box. Contrary to the traditional gearbox power transmission, the power is not completely combined to the sun gear, but partially transmitted to the second stage rotating inner gear ring through the planet carrier. In the second stage transmission, a group of gears are supported on the box, together with the meshing inner ring gear and sun gear, to be used for speed diversion and rotation direction change. The torque changes through the sun wheel

on the third stage differential planetary gear stage, the power flow from the first stage sun gear and the second stage sun gear converge. The first stage sun gear drives the planet carrier, while the second stage sun gear drives the inner ring gear. This third stage is called three-axis planetary differential transmission, where the two power flows converge to the sun gear and then transmitted to the fourth stage parallel shaft driving gear. The total speed increase ratio can reach more than 200 ∶ 1, of which the first to third stage is ~ 40 ∶ 1, and the parallel axis stage is ~ 5 ∶ 1

power split ratio: 61.8% from the first stage solar wheel; 38.2% from planet carrier and inner ring gear. The volume and weight are reduced by about 20% compared with the traditional structure. The weight reduction requirement can be achieved because of the ingenious power split transmission path. The space and weight of the first stage planetary gear are basically reduced. By using the multi-channel power transmission and diversion and the different functions of each gear stage, the power converges and balances the changing speed and steering at the same time, so as to meet the specific requirements of the transmission chain of the unit

the final drive of the gearbox adopts a fixed shaft gear pair, which follows the rules of the non coaxial design of the gearbox of the wind turbine. This is to arrange pipelines or cables in the center hole in order to control the pitch variation of impeller blades. In addition, generating the necessary center offset can easily adjust the different generator speed output

as shown in the left figure of Figure 9, the traditional unit adopts the main shaft to transmit power between the wind wheel and the gearbox and bears most of the abnormal loads from the wind wheel, reducing the risk of gear damage. However, this will prolong the length of the engine room and increase the volume of the engine room. This effect is not obvious on smaller power units

with the increase of power, the diameter and weight of the main shaft also increase with it. When the transmission shafting is arranged for units above 3MW, the large and heavy main shaft becomes the target of engine room weight reduction, and the direct connection method is preferred in the design. As the structure shown in the right figure of Figure 9, the wind turbine is hung on the base through a tapered roller bearing that bears huge loads in three directions, and the power is directly transmitted to the gear unit. The following problems are the development of super large double row tapered roller bearings, the design and manufacture of high strength and high power density of gear transmission devices, the setting of dynamic edge conditions of shafting, etc. all these should find effective solutions before deciding to adopt the "direct connection" scheme

although direct drive wind turbines have the characteristics of simplifying the transmission structure, today, when the capacity of wind turbines is becoming larger and larger, the transportation and hoisting problems caused by too large low-speed generators, coupled with the constraints of high manufacturing costs, have to turn around and think about how to reduce the volume and weight of the mechanism and the ways to reduce costs. One of the ways to solve the problem is to appropriately use the method of gear speed increase or power diversion

there are so-called "semi direct drive" or "hybrid drive" units that use gear transmission with small speed increase ratio to reduce the structural size of the motor between the wind wheel and the low-speed motor