Created on：2021-10-11 10:46

# Aircraft hydraulic failure_ Servo valve failure of anti-skid system

Aircraft hydraulic failure_ Servo valve failure of anti-skid system

(1) Anti hysteresis principle

When the aircraft lands, put down the landing gear. When the braking pressure acts on the wheel, the aircraft wheel has been in contact with the airport runway. At this time, the heat generated by the wheel friction will cause the rubber material on the tire circumference to be significantly greater than that without braking force. If two identical wheels are installed on the same shaft in parallel, one wheel has braking and the other has no braking, the above phenomenon will be obviously observed.

When the braking force acts on the wheel, the angular speed of the wheel decreases rapidly by several percentage points, and the wheel tire produces the sliding speed relative to the runway. If the braking force continues to increase, the real sliding will not occur until the friction between the airport runway and the tire can no longer increase. The relationship curve between the friction and sliding speed generated by the airport runway and the tire is shown in Figure a.

It can be seen that after the sliding speed is generated, if the sliding speed continues to increase, a peak coefficient will be generated at an appropriate speed. At this time, the braking efficiency is the highest. If the sliding speed continues to increase after the peak coefficient, it will lead to increased tire wear. Under the braking force without monitoring and control, if the braking force is too large, it will lead to wheel locking and tire burst.

The anti-skid control system measures the sliding speed by detecting the wheel speed, so as to adjust the brake pressure and ensure that the sliding friction coefficient between the wheel and the ground is at the peak. The executive control element of the brake pressure is the anti-skid servo valve, so the repair of the servo valve in the Boeing aircraft brake system is very critical. The position of the servo valve in the system is shown in Figure B.

When the aircraft lands, the pilot controls the metering valve or automatic brake valve to provide pressure to the anti-skid valve. The valve automatically adjusts the brake pressure according to the output signal of the brake control unit (electrical control part), so as to ensure that the brake system works at the peak coefficient during sliding and provide the best braking effect for the aircraft, And significantly improve the stability when turning.

(2) Anti stagnation servo valve and maintenance

The working principle of the servo valve is shown in Figure C. It belongs to a two-stage valve. The first stage is the nozzle baffle type, and its outlet pressure is controlled by the control signal. The second stage is the slide valve type, which executes the pressure from the control stage to the brake cylinder. When there is no signal, the servo valve baffle rests against the oil return nozzle due to the oil pressure at the outlet of the pressure nozzle. At this time, the oil pressure at the pressure port acts on the spool valve, so that the brake port is directly connected with the metering oil port, and the pressure at the brake port is consistent with the metering oil pressure controlled by the pilot. When the wheel angular speed detects a deviation between the taxiing speed and the reference taxiing speed, The torque motor receives the deviation electrical signal. At this time, the torque motor drives the baffle to deflect towards the pressure nozzle to reduce the oil pressure acting on the upper end of the valve core. Under the action of the oil pressure at the lower end of the valve core, the valve core moves up and the metering pressure port is closed down, which will lead to the reduction of the pressure at the control port. When the pressure at the control port is reduced to a certain value, there will be the corresponding braking pressure, which is within the control range, The relationship between the control port pressure and the corresponding control current is shown in Figure D.

The common faults of this kind of servo valve are: ① oil leakage, that is, the 0-shaped seal ring at the torque rod of the servo valve is damaged. The seal at this position needs to be replaced and the spacing of the nozzle baffle needs to be readjusted; ② The nozzle is blocked, and the control of the nozzle is misadjusted due to pollution.

Due to the high precision of the servo valve, when the control current is adjusted from 0 ~ 50mA, the pressure can be adjusted from 2lmpa to 0.5MPa, and the flow change corresponding to the current change per Ma shall not be less than 0.01l/min.

After removing the permanent magnet, demagnetize the magnet. When assembling the permanent magnet again, magnetize it again to make the permanent magnet fill to the saturation state. The magnetization intensity of the permanent magnet determines the relationship curve between the working pressure and the control current.

The interference fit is adopted between the nozzle and the shell. During assembly, the nozzle needs to be put into dry ice for cooling and then pressed into the shell. The assembly and adjustment of the torque motor must be very careful, and the cleanliness of the oil must be guaranteed. If the oil pollution exceeds the standard, the nozzle oil port will be blocked, resulting in the imbalance of the servo valve.

The adjustment of the servo valve starts from the pressure nozzle. First, the position of the pressure nozzle is adjusted to the corresponding current and pressure values, then the position of the return nozzle is adjusted, so that the control pressure and current are adjusted to the corresponding value. Finally, by adjusting the magnetic coupling bolt on the permanent magnet (see Fig. E), the relationship between the control pressure and the current of the valve is shown in figure D.

The main test curves of the anti hysteresis servo valve are static and dynamic characteristics. The dynamic characteristics include frequency response test and step response test. The frequency response test is to detect the relationship between the output flow and the amplitude and phase difference of the sine wave signal when a sine wave control signal of a certain frequency is input, and take the 3dB amplitude attenuation frequency as the frequency band width of the valve respectively, The frequency when the output flow lags behind the input control current by 90 ° is taken as the limit frequency.

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