The forward rush phenomenon of the speed regulating valve in the speed regulating circuit

The forward rush phenomenon of the speed regulating valve in the speed regulating circuit

Figure 35 shows the speed regulating circuit of the oil inlet circuit of the speed regulating valve. After the hydraulic cylinder stops moving, there is a jumping forward rush phenomenon when it is restarted.

When the hydraulic cylinder in the circuit stops moving, there is no oil passing through the speed regulating valve. When the pressure difference is zero, the valve core of the pressure reducing valve will open all the valve ports under the action of spring force. When the hydraulic cylinder is started again, the pressure drop at the valve port of the pressure reducing valve is very small, and the throttle valve receives a large instantaneous pressure difference, passing through a large instantaneous flow, showing the jumping forward rush phenomenon of the hydraulic cylinder. The hydraulic cylinder will not move at the originally set speed until the pressure reducing valve is balanced again.

After inspection, the starting process of the hydraulic cylinder in the throttle speed regulating circuit at the inlet of the speed regulating valve is shown in Figure 36 (a), in which the abscissa is time t, the ordinate is the hydraulic cylinder stroke L, and the movement speed of the hydraulic cylinder is v=l/t=tan α （ α Larger means faster).

The curve in the figure shows that before the hydraulic cylinder reaches its set speed from static, there is an instantaneous high speed (represented by a ´), which jumps by 0.7mm and reaches the set speed after 20ms, that is, the pressure reducing valve plays a regulating role.

Figure 36 (b) shows the non jump process obtained after treatment, which is obtained by adding a control oil circuit to the speed regulating valve.

Figure 37 shows that a control oil circuit a is led out from the outlet of the pump, which is connected with the spring free cavity of the pressure reducing valve at x, and the control oil circuit from the cavity to the inlet P of the speed regulating valve is disconnected at y, and made into a switchable structure associated with the reversing valve. When the reversing valve is in the middle position and the hydraulic cylinder stops moving, y is disconnected, and the pressure reducing valve closes its spool opening under the action of the pressure oil of the pump; When the reversing valve is switched to start the hydraulic cylinder, y is connected, and the pressure reducing valve fork recovers its original function (at this time, the valve core changes from the closed state to the original adjustment state, even if the hydraulic cylinder moves at the set speed). This process can be clearly seen from the curve in Figure 36 (b). It took 70ms for the valve core to change its state before the hydraulic cylinder reached its set speed without jumping. When using this control mode, care must be taken to make the pressure at joints X and Y close to equal.

As shown in Figure 38, the return oil throttling speed regulating circuit of the speed regulating valve is shown. When the hydraulic cylinder starts again after stopping, there is a jumping forward rush phenomenon, which is more serious than the oil inlet throttle speed regulation of the speed regulating valve.

The basic causes and elimination measures of the forward flushing phenomenon are exactly the same as the oil inlet speed regulation of the speed regulating valve. Here is a further analysis of the causes and elimination methods of the serious forward impact phenomenon.

After investigation, debugging and analysis, it is found that the longer the hydraulic cylinder stops moving, the worse the forward rush phenomenon when starting. The reason is that there is leakage in the oil return chamber of the hydraulic cylinder. When the hydraulic cylinder stops for a longer time, the more oil leaks in the oil return chamber, the smaller the back pressure in the oil return chamber, and the more difficult it is to adjust the pressure of the pressure reducing valve in the speed regulating valve, so the more serious the forward flushing of the hydraulic cylinder. Therefore, eliminating the oil leakage in the oil return chamber of the hydraulic cylinder will greatly reduce the forward flushing phenomenon when the hydraulic cylinder is started.

The operation of the speed regulating circuit of the speed regulating valve installed on the oil inlet and return circuits is basically the same as that of the throttle oil inlet and return throttle speed regulating circuit. However, the speed stability of the circuit is greatly improved due to the use of D-speed regulating valve instead of throttle valve in the circuit. Of course, the leakage of hydraulic valve and hydraulic cylinder, the spring force at the valve core of pressure reducing valve in speed regulating valve and the change of hydraulic pressure will actually have some impact on the speed due to the change of load, but the speed fluctuation of this speed regulating circuit will not exceed ± 4% under full load.

In the oil inlet throttle speed regulating circuit of the speed regulating valve, the speed regulating valve is mostly made into a structure with the pressure reducing valve in front and the throttle valve in the back. The advantage of this is that the working pressure P1 of the hydraulic cylinder (pressure of the rodless chamber) changes with the load, which directly acts on the pressure reducing valve, and the adjustment effect is fast. The disadvantage is that when the oil passes through the pressure reducing valve port, it heats up, and the hot oil enters the throttle valve. The oil temperature fork changes with the pressure drop of the pressure reducing valve, so the throttle coefficient C cannot be kept constant.

In the oil return throttle speed regulating circuit of the speed regulating valve, the structure with the throttle valve in front and the pressure reducing valve in the back is better. Because this form not only makes the change of pressure P2 (pressure in the rod cavity) act directly on the pressure reducing valve, which has a fast regulation effect, but also the oil temperature passing through the throttle valve is not affected by the throttling effect of the pressure reducing valve port.

The oil inlet and return throttling speed regulating circuit of the speed regulating valve is suitable for small power systems with high requirements for motion stability, such as the feed system of boring machines, lathes and modular machine tools. The oil return throttling is also suitable for the feed system of milling machines.