The setting pressure value of the make-up relief valve is too high, resulting in fracture

补油溢流阀的作用是调节进入补油液压系统的油量，并在不断溢流的过程中保持控制系统压力基本稳定。当系统中伺服液压缸突然停止运动，而不需要油液时，补油溢流阀在标准设定压力值时要能迅速地被打开，这就要求补油溢流阀要具有高的灵敏度，并且流量变化对压力的影响要小。

The function of oil filling relief valve is to adjust the oil quantity into the oil filling hydraulic system, and keep the pressure of control system basically stable in the process of continuous overflow. When the servo hydraulic cylinder in the system suddenly stops moving and no oil is needed, the oil filling relief valve should be opened quickly at the standard set pressure value, which requires that the oil filling relief valve should have high sensitivity and the influence of flow change on pressure should be small.

因为三轮底盘搅拌车液压泵在一直运转供油，如果补油溢流阀灵敏度低而不能迅速地被打开，则控制系统中的油液就不能及时排出，压力就会急剧升高，超过补油溢流阀的调整压力，便会使补油系统各元件受力增加。在超调压力作用下，伺服液压缸活塞位移增加促使反馈杆转动角加大。结果使反馈杆缩颈处与反馈阀套的尖边相接触，在超调压力达到特定值时而导致反馈杆断裂。

Because the hydraulic pump of the three wheel chassis mixer has been running to supply oil, if the sensitivity of the oil filling relief valve is low and cannot be opened quickly, the oil in the control system will not be discharged in time, and the pressure will rise sharply. If it exceeds the adjustment pressure of the oil filling relief valve, the stress on the components of the oil filling system will increase. Under the action of overshoot pressure, the increase of piston displacement of servo hydraulic cylinder makes the rotation angle of feedback rod increase. As a result, the necking part of the feedback rod contacts with the sharp edge of the feedback valve sleeve, and the feedback rod breaks when the overshoot pressure reaches a certain value.

双向弯曲疲劳导致断裂

Fracture due to biaxial bending fatigue

金属要断裂，要形成微裂纹。三轮底盘搅拌车反馈杆实际上相当于一根摆杆，围绕固定偏心转轴瞬时，由伺服液压缸活塞往复运动带动反馈杆双向摆动。反馈杆带动反馈阀套动作而关闭控制油路并达到控制液压泵斜盘角度，从而控制液压泵排量的目的。

To break a metal, microcracks must be formed first. The feedback rod of three wheel chassis mixer is actually equivalent to a swing rod. Around the instantaneous center of the fixed eccentric shaft, the feedback rod is driven to swing in two directions by the reciprocating motion of the piston of the servo hydraulic cylinder. The feedback rod drives the feedback valve sleeve to close the control oil circuit and control the swash plate angle of the hydraulic pump, so as to control the displacement of the hydraulic pump.

反馈杆材料较薄弱的缩颈与杆身的交接处，由于尖角过渡在其杆身通向缩颈时产生应力集中。反馈杆缩颈与杆身的交接处在伺服液压缸活塞和反馈阀套的作用力的反复作用以及与反馈阀套尖边反复接触下，疲劳微裂纹开始萌生并不断扩展，之后导致双向弯曲疲劳断裂。

At the junction of the necking and the body of the feedback rod with weak material, due to the sharp angle transition, the stress concentration occurs when the body of the feedback rod leads to the necking. The interface between the necking of the feedback rod and the rod body is under the repeated action of the force between the piston of the servo hydraulic cylinder and the feedback valve sleeve, and under the repeated contact with the tip of the feedback valve sleeve, the fatigue microcracks begin to initiate and continue to expand, and then lead to bi-directional bending fatigue fracture.

冲击作用加速疲劳断裂

Impact accelerated fatigue fracture

液压系统实际工作中，由于液压泵容积的变化或者元件的某些缺陷，系统在输出压力油液时总是产生特定的振荡。

In the actual work of the hydraulic system, due to the change of the volume of the hydraulic pump or some defects of the components, the system always produces a specific oscillation when it outputs the pressure oil.

再者，三轮底盘搅拌车进出料操纵换向装置由于换向手柄推拉的一步到位，在突然换向或制动的时候，控制阀开关控制油路而对伺服液压缸突然地供油或排油，但由于流动液体和运动部件具有惯性作用，突如其来的换向变化必然引起液压系统瞬时压力急剧变化而产生液压冲击。液压振荡或冲击的结果使反馈杆和控制液压缸活塞、反馈阀套尖边发生反复碰撞而使反馈杆加速疲劳断裂。

In addition, because the reversing handle is pushed and pulled in place, the control valve switch controls the oil circuit to supply oil or drain oil to the servo hydraulic cylinder suddenly when the reversing handle is suddenly changed or braked. However, the flowing liquid and moving parts have inertial effect, The sudden change of direction change will inevitably lead to the rapid change of instantaneous pressure of hydraulic system and produce hydraulic shock. As a result of hydraulic oscillation or impact, the feedback rod repeatedly collides with the piston of the control cylinder and the tip of the feedback valve sleeve, resulting in accelerated fatigue fracture of the feedback rod.