Hydraulic system fault of ladle lifting device
Hydraulic system fault of ladle lifting device
The hydraulic system of ladle lifting device in the process of installation and commissioning of a steel-making plant's imitation Demark type Ultra-low Head multi-point straightening slab caster exposed the problems of poor lifting and lowering of the ladle turret as a whole and rhythmic vibration and noise at the same time.
(1) Hydraulic system principle of ladle lifting device
Figure m is the schematic diagram of hydraulic system of ladle lifting device. The overall lifting of ladle turret is driven by one hydraulic cylinder, which avoids the asynchronous phenomenon of multi cylinder driving. When 1dt is powered off and 2DT is powered on, the pressure oil enters into the rod less chamber (lower chamber) of the hydraulic cylinder to make the piston move upward to realize the lifting arm rising. The oil in the rod chamber returns through the double one-way throttle valve channel to control the rising speed. When 1dt is powered on and 2DT is powered off, the pressure oil enters the rod chamber, and the lifting arm descends. At this time, 3DT is powered on, and the control oil circuit of the hydraulic control check valve is connected, so that the oil in the rod free chamber of the hydraulic cylinder returns through the hydraulic control check valve in the reverse direction, and the descending speed is controlled through the throttling channel of the one-way throttle valve.
(2) Failure analysis
After checking, all the components are normal, so the system fault is not caused by the quality of the components themselves. Later, through the structural analysis of the hydraulic control check valve, it was confirmed that the cause of the failure was the improper selection of components. The original structure diagram is shown in Figure n. When the liquid flow is reversed, the force balance expression of the valve core is as follows:
Where PK -- control oil pressure;
PA -- oil pressure in reverse outlet chamber;
Pb -- oil pressure in reverse oil inlet chamber;
FM -- total friction resistance of poppet valve;
FKM -- control piston friction resistance;
FS -- spring force;
W -- valve core gravity;
AK -- control piston area;
A -- valve seat area.
Therefore, the control oil pressure is as follows:
pK=[pBA+ (AK-A)pA+FKM+FS+FM+W]/AK (4-2)
This value is the control oil pressure to ensure reverse flow. When the valve is closed and the reverse flow of oil stops, PA = 0,
This value is the minimum control pressure for opening the hydraulic control check valve, which remains unchanged after being set by 3 (Fig. m).
Because the one-way throttle valve is used in the system to regulate the descending speed of the hydraulic cylinder, when the oil flows in the opposite direction, PA > 0. The control oil pressure is still the setting value of the relief valve, so the force balance of the valve core is destroyed. The valve core moves to the left to close the valve port, and the oil flows in the reverse direction. In the reverse oil outlet chamber, the back pressure is generated, and the hydraulic control one-way valve is closed. Once it is closed and opened repeatedly, the piston drops and stops intermittently, which produces vibration and noise.
① Scheme I increases the control oil pressure and the set pressure of relief valve 3 to compensate the throttling pressure loss, so that the hydraulic control check valve can always be opened, but the system pressure loss is too large.
② In scheme II, hydraulic control check valve with external oil drain port is selected, and its structural diagram is shown in Fig. o.
It can be seen from the figure that the control oil pressure is:
Where, A1 is the area of chamber a pressure acting on the control piston rod.
Generally, A1 < A, so (a1-a) Pa < 0. Therefore, as long as PK = [PBA + FKM + FS + FM + w] / AK, even if PA > 0, the hydraulic control check valve can always be opened without intermittent opening and closing.
③ In scheme III, the leakage type hydraulic control check valve with pilot valve is selected (see Fig. P).
As shown in Figure P, due to the high oil inlet pressure at port B, the pilot valve can be used for pre relief. A smaller taper spool B (some are steel balls) is installed in the taper spool of the check valve, which is called the pilot spool (or pressure relief spool). Because the pressure bearing area of the valve core is small, it can be pushed open first without much pressure. Cavity a and B communicate with each other through the small notch C on the round rod of the pilot valve core, so that cavity B gradually relieves pressure until the control piston pushes the main valve core away from the valve seat, so as to open the reverse channel of the one-way valve. In this way, the control oil pressure PK can be further reduced.
(4) Transformation effect
In case of spare parts, scheme II is adopted. After the transformation, the phenomenon of intermittent opening and closing of the check valve no longer appears, and the vibration and noise are eliminated.
In the system design, when selecting the relevant components, we should not only understand its function, but also know its structure type, and select the appropriate hydraulic components. In this example, in addition to understanding the function of the hydraulic control check valve as a hydraulic lock, we also need to understand its structure. When the system needs back pressure, we should choose the hydraulic control check valve with an outlet.