The setting pressure value of the make-up relief valve is too high, resulting in fracture
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.
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.
Fracture due to biaxial bending fatigue
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.
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.
Impact accelerated fatigue fracture
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.
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.