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The main gearbox of the helicopter

# The main gearbox of the helicopter

Rotational speed of modern gas turbine HPT ranges from 6000 17000 to rev / min (low power engines and higher). For best efficiency in the current HB helicopter flight mode HB speed must be considerably lower than the frequency of rotation of the gas turbine is achieved by GH.

The gearbox can be source of a shaft torsional oscillation, as in the wheels are always errors in Step teeth, as well as deformation of the teeth under load, causing changes angular velocities of shafts. Reduce the excitation of these oscillations can increase the coefficient of overlapping meshes, increasing manufacturing precision gears and special tooth profile correction.

The size of the gears, bearings and shafts GR determined mainly by depending on the transmission gear torque. Therefore, the mass is calculated according to the formula GH

The coefficient k r can be considered comparable for reducers of the same size, similar circuits, and with close values ​​of the transfer ratios. It follows from the analysis that with a decrease in the torque transmitted by the reducer, the weight coefficient k increases. This is explained by the fact that the thicknesses of the walls of the main parts of small reducers are relatively greater, both because of technological difficulties in the manufacture of parts with very small wall thicknesses, and for reasons of providing the necessary rigidity and static stability of the walls. Because of this, the mass of parts of small reducers is obtained relatively higher. To somehow reduce this effect, such reducers are expedient to do by simpler schemes, in particular, with the transfer of the output torque to the smaller number of points of engagement.

In the kinematic scheme of the mechanism gear can be divided into three groups: simple peredachami- gear, planetary gears, which have single and double satellites; Mixed gears, mechanisms that are simple and planetary gears. To prevent large voltage in the teeth of a simple transfer, you must install several

bust, placing them evenly over the circumference. It is necessary to have an exhaustive search each clutch or elastic member, which allows to collect send a guaranteed clearance and ensure uniform loading of all bust.

In the case of combined gear rationally use the planetary gear in a second stage, which reduces the speed of the leash and centrifugal forces, which load bearing satellites.

Figure 4.3.1 shows the kinematic diagram of the GR of the Mi-26 helicopter. The creation of the GR for the transmission of power from two HPTs to NV, equal to 22000 hp, is associated with the solution of a number of complex technical and technological problems. This problem was successfully solved by G.P. Smirnov, an engineer at the Moscow Helicopter Plant (MVZ) named after V.I. M.L. Mile.

The design feature of GR-26 is a large gear ratio in the final reduction stage. For the first time in the practice of world helicopter industry, the usual involute gear train with a large gear ratio (i = 8,76) was used as the last stage of reduction. The reducer has a modular design. Its separate modules: the ball joint of engines, plate compensating couplings, freewheels, front and rear bevel gears, drive РВ, upper reducer (the last two stages of reduction of the main kinematic chain), oil separator and oil aggregate are made as independent units in their own buildings. They are connected with each other by flanges and splined shafts. In principle, each module can be manufactured, tested, modified structurally and used in other designs. The modularity of the construction with respect to the gear unit of such dimensions simplifies the manufacturing and debugging, reduces the weight.

The upper gearbox consists of a housing in which the HB shaft is mounted on two bearing supports. Directly on this shaft with the help of two hubs are fixed two driven helical gears, with each of which are engaged in eight driving wheels. The gears of the upper and lower rows have opposite tilting directions. Each drive wheel is mounted on two roller bearings that do not have thrust collars on the inner rings. Axial forces arising on the driving wheels of the last stage have the opposite direction and are perceived by tubular screeds.

The result is a kind of herringbone gearing, in which each half of the drive wheel is mounted in its bearings. The possibility of free axial movement of the groups of gears consisting of two drive wheels of the last stage and the second stage of the driven wheel, allows a uniform power division between upper and lower drive wheels of the last stage. HB shaft in its lower part is made thin-walled barrel-shaped, which allows to give it the required strength and stiffness with minimum weight.

Housing top gear takes all the loads coming from the NV, including torque, and transmits them to the fuselage of the helicopter through eight podreduktornuyu pivotal frame. In the middle part of the body has a belt attachment with six flanges, which are attached to the flanges of the frame.

Modular design makes it easy to create the required rigidity of the problem cases. All gear wheels are simple and technological forms. In order to simplify the manufacture of wheels, introduced the traditional flange connectors.

One of the main features of the main reducer BP-26 is to ensure uniform power distribution over the flows due to splined shafts (springs) with small torsional stiffness. The power division in the last stage of reduction is provided due to the opposite in the direction of inclination of the teeth in the upper and lower rows of gears. The power division in the first and second stages of reduction is due to the small torsional stiffness of the springs, mainly the springs of the last stage of reduction. The construction maintains the equality of torsional stiffness in parallel flows.

The required uniformity of load distribution based backlash in gears and splines, backlashes in bearings provided in the process of assembling the gear through the use of a number of structural and technological methods.

Torsional rigidity of the basic kinematic chain and drive RV, backlash in the teeth and splines its drive selected accordingly. As a result, when using one engine at maximum takeoff power of the RV through the drive goes to the opposite side of the bevel gearboxes, bevel gearboxes unloading on the side of the engine running.

Gears BP-26 made of steel 12H2N4A-III undergo carburizing and hardening. As used finishing grinding.

Body parts of the upper gear diameter 2000 mm, made by stamping of high strength aluminum alloy AKCH-1 followed by treatment in the milling machines. Body parts other nodes are made of cast alloy MJI-5. Hubs top gear driven gears are made of a titanium alloy forging VPP-1. Shafts and springs are made of steel 40H2N2MA nitride.

Many threading, modularity similar layout GR create certain advantages over planetary gears.

As a result of these structural and kinematic solutions specific gravity BP GR-26 unit takeoff torque is significantly smaller than the GH Mi-6, performed by a four kinematic scheme.

The loads are transferred from the body of the GR in the corresponding power elements of the fuselage usually with a rod system.

On 4.3.2 one of the variants of the KSS of the GR mounting frame is shown.

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