According to the working principle and structural characteristics of the single hydraulic prop, the following conditions are analyzed for the conditions for forming the expansion cylinder:
1. When the single hydraulic prop is lifted to the tensioning top plate to reach the initial supporting force, the liquid supply pressure continues to rise due to the failure of the unloading valve of the liquid supply pumping station system, resulting in the expansion cylinder.
2. As mentioned above, the upper chamber air pressure cannot be released under the reliable sealing and closing when the column is lifted, and the ratio of the upper and lower chamber areas is 10:1. If the rated working pressure of the liquid pumping station is 15-20 MPa (or 31.9 MPa) and the flow rate is 125 L/min (or 200 L/min), then the pressure in the upper chamber will reach 150-200 MPa (or 319 MPa) and the column-up speed will be 0.27 m/ s (or 0.4 m/s). This achieves the conditions for forming the expansion cylinder.
3.The original design cylinder body thickness strength is insufficient, the working stress is greater than the allowable stress, or there is a substitute phenomenon in the manufacturing process to reduce the design strength.
4.The three-way valve fails. When the pressure of the top plate exceeds its rated working pressure, the safety valve cannot open the unloading, causing the cylinder piston cavity pressure to exceed its allowable stress and cause the cylinder to deform.
