Testing the effectiveness of release valves under intensive dynamic load to the leg

Release valves are commonly used to protect hydraulic legs against overload caused by rock bursts or bumps. Due to an essential role in ensuring safety in the working, an application of a release valve is conditioned by a positive yield test results of a leg equipped with such a valve. A method of leg yield testing, used in Poland, enables a complex determination of an impact of not only a release valve but also of the parameters of the hydraulic leg, determining its stiffness such as for example a volume of the under-the-piston space, which has an impact on an observed pressure increase. The subject of this publication covers cognitive tests oriented onto a determination of an impact of a release valve exclusively on the pressure changes observed in the leg. The results of the efficiency tests of spring valves (Stoinski, 2018) on a rammer are discussed. The difference between the maximum pressure in the under-piston area of the leg with the release valve and the maximum pressure generated by the same dynamic load in the leg without this valve was the measure of the valve operation's effectiveness. Dynamic load, realized on a rammer, is characterized by a longer increase time than in the case of dynamic load acting on a powered roof support unit from the floor. The time process of the force in the leg is then characterized by a short load rising time - t(n), large load increment factor - K-d and the average load growth rate -w(p,n). Referring to that aspect, the features of a release valve were analyzed in relation to the parameters characterizing dynamic load acting on a powered roof support unit from the floor. Parameters characterizing the effectiveness of the release valve, i.e. change in the leg load increment index -Delta K-d and change in the rate of load increase -Delta w(pn) were defined. The test stand for generating the load of such parameters using the explosive method is described. Comparison of effects of the dynamic load generated by firing the same mass and the same type of explosive on a hydraulic leg with a release valve and the leg without this valve was the test objective. The effectiveness of the spring valve and two gas valves are analyzed. It was found that despite a very short load rising time, the release valve reduces the load acting on the leg. The positive values of the Delta K-d and Delta w(p,n) indices are the evidence.