The pressure difference before and after the speed regulating valve in the speed regulating circuit is too small

The pressure difference before and after the speed regulating valve in the speed regulating circuit is too small

In the system shown in Figure 39, the hydraulic pump is a quantitative pump, the reversing valve is a three position four-way O-shaped electro-hydraulic reversing valve, and the speed regulating valve is installed on the oil return circuit of the hydraulic cylinder, so this circuit is the oil return throttle speed regulating circuit of the speed regulating valve.

The fault phenomenon of the system is that when the external load increases, the movement speed of the hydraulic cylinder shows an obvious downward trend. This phenomenon is obviously inconsistent with the speed regulation characteristics of the speed regulating valve.

Detection and commissioning found that the hydraulic components in the system worked normally. When the hydraulic cylinder moves at low load, the speed is basically stable, and when the load is increased, the speed decreases significantly. Adjust the pressure of the overflow valve: when the pressure of the overflow valve is increased, the fault phenomenon is basically eliminated; When the pressure of the overflow valve is reduced, the fault phenomenon is very obvious.

The speed regulating valve is used for system speed regulation. Its main principle is to use a variable hydraulic resistance that can be automatically adjusted (the differential pressure reducing valve in series in front of the throttle valve) to ensure that the pressure difference before and after the other fixed hydraulic resistance (the throttle valve in series behind the pressure reducing valve) is basically unchanged, so that the flow through the speed regulating valve remains constant when the pressure difference before and after the speed regulating valve changes, Therefore, the movement speed of the actuator can still maintain a constant speed under the working condition of changing external load.

In the speed regulating valve, because the two hydraulic resistances are connected in series, the pressure difference between the front and rear of the speed regulating valve should be higher than the pressure difference between the front and rear of the throttle valve when it is used for speed regulation. Generally, the pressure difference before and after the speed regulating valve should be kept within the range of 0.5 ~ 0.8MPa pressure value. If it is less than 0.5MPa, the differential pressure reducing valve cannot work normally, and it cannot play the role of pressure compensation. Obviously, the pressure difference before and after the throttle valve cannot be constant, so the flow through the speed regulating valve changes with the external load, and the speed of the actuator is unstable.

To ensure that the differential pressure before and after the speed regulating valve remains within the allowable range when the external load increases, the set pressure value of the overflow valve must be increased. In addition, the speed rigidity of the actuator of this system is also affected by the leakage of the hydraulic cylinder and hydraulic valve, the spring force in the pressure reducing valve, hydraulic force and other factors. The maximum velocity fluctuation under full load can reach 4%.

In the circuit shown in Figure 40 (a), the hydraulic oil enters the rod cavity of the hydraulic cylinder through the one-way valve to jack up the weight, and the oil without the rod cavity goes directly to the oil tank. The descending stroke of the hydraulic cylinder is reduced by its own weight, and the oil in the rod cavity returns to the oil tank through the speed regulating valve, which is equivalent to the speed regulating valve oil return throttle speed regulation, so the descending speed of the hydraulic cylinder should be stable. However, the speed of the hydraulic cylinder in this circuit is unstable when it descends.

When the hydraulic cylinder descends, the hydraulic pump has been unloaded, and the pressure in the rod chamber of the hydraulic cylinder is only determined by the weight, which has nothing to do with the output pressure of the hydraulic pump. Therefore, the oil pressure in the rod chamber is determined by the load and the piston area.

The differential pressure reducing valve in the speed regulating valve must work normally, and the pressure difference before and after the speed regulating valve must reach 0.5 ~ 0.8MPa. Obviously, the reason for the unstable speed of the above circuit is the low differential pressure before and after the speed regulating valve. To increase the pressure difference between the front and rear of the speed regulating valve, the area of the hydraulic cylinder piston can be reduced, but this is often more difficult. As shown in Figure 40 (b), change the two position three-way valve to the two position four-way valve. When the hydraulic cylinder drops, the pressure oil is input into the rodless chamber. At this time, the system pressure is set by the overflow valve. Part of the pressure oil output by the hydraulic pump enters the hydraulic cylinder and part of it overflows back to the oil tank by the overflow valve. The lowering speed of the hydraulic cylinder is set by the speed regulating valve, the adjusting pressure of the overflow valve is increased, and the pressure difference before and after the speed regulating valve is also increased accordingly, which ensures the pressure difference of the normal operation of the speed regulating valve. The speed of the hydraulic cylinder conforms to the law of the speed regulating valve oil return throttling speed regulation, and will not change with the load change, and the hydraulic cylinder can drop steadily.