Created on:2021-07-09 10:04

Fault diagnosis and maintenance of CNC hydraulic plate bending machine (2)

Fault diagnosis and maintenance of CNC hydraulic plate bending machine (2)

The causes of vibration in hydraulic system are complex, but the harm is very serious, such as the vibration caused by pressure or flow pulsation, which may make the system unable to work normally and reduce the service life of components. In this paper, the abnormal phenomena of noise and vibration of two CNC bending machines (63T and 100t) in debugging are compared and diagnosed.

63T and 100t CNC bending machines have hydraulic noise and machine vibration when they work. Because the oil tank and hydraulic components are combined on the upper part of the machine bed, it is difficult to distinguish the vibration from the hydraulic noise. During the preliminary inspection of these two equipments, the main problems of 100t are as follows:

① The cover plate of oil tank is not sealed normally, and the oil has been emulsified and deteriorated; ② The liquid level of the pump suction filter and the oil tank is basically the same. There is oil leakage at the oil suction port of 63T pump and the oil supplement valve block at the upper end of hydraulic cylinder. As a result, the 100t oil tank and the filter element of the oil absorption filter were cleaned, the hydraulic oil was replaced, and the liquid level after adding new oil was above the oil absorption filter. At the same time, the 63T oil leakage was solved.

It is found that the noise and vibration are light in low-pressure and pressure maintaining operation, but serious in high-pressure and pressure maintaining operation. It is concluded that the abnormality is related to the pressure. After shielding the electromagnetic proportional relief valve, the safety valve is used as the system pressure regulating valve, and the system works normally from low pressure to high pressure. According to this conclusion, the electromagnetic proportional relief valve is faulty. Check that the electrical control is normal. After replacing several new valve cores and valve seats, the problem still exists. The valve core and valve seat of the proportional valve in question can be replaced on other equipment which can work normally. It is found that the problem is only accompanied by the machine tool, and has nothing to do with the proportional valve migration. Therefore, it can be inferred that the proportional valve and the machine tool may not match well, and the two throttling and damping holes in the proportional valve seat may not match. Check and adjust the two throttling and damping holes (0.5-1.0 mm) on the proportional valve seat again, the effect is not obvious, but it is still judged that this is the key to the problem.

In conclusion, it is speculated that resonance may occur when the proportional valve works.

According to the principle of mechanical control system, the mathematical model function of resonance should be a typical oscillation link, and its transfer function is as follows:

G(S)=V(S)/q(S)=K/[(1/ ω 2n)S2+(2 ξ/ω n)S+1]

Where ω N -- undamped natural frequency;

ξ—— Damping ratio.

According to the transfer function, ξ The smaller it is, the easier it will oscillate. ξ< 1 is under damped oscillation, which is characterized by fast response, but there is oscillation before reaching a stable value, that is, there is overshoot; commonly ξ= 7, which can provide the fastest response without overshoot; ξ< 7, the overshoot is large; When ξ= At this time, there is no oscillation, that is, there is no overshoot; ξ> The response is slow and the lag is large.

Undamped natural frequency ω N and damping ratio ξ Is the main parameter of the oscillation link, and its transient response depends on these two parameters. Finally, it was decided to change the damping ratio. As shown in Figure o, a cylindrical pin was added into the process hole between Part 14 and the proportional valve to change the volume of the cavity and the cross-sectional area of the oil flow, so the vibration and noise were significantly reduced. After carefully matching the clearance between the cylindrical pin and the damping hole, the vibration and noise were completely eliminated.

The electromagnetic proportional relief valve uses input signal to adjust and control the hydraulic system, which can achieve fast, stable and accurate control.

In this case, it belongs to the open-loop control system (see Figure P). Due to the power amplifier, slight disturbance of hydraulic source will cause great fluctuation of output pressure or flow.

P.jpg

This case shows that the influence of oscillation link in hydraulic system on proportional control is an important link that can not be ignored in modern hydraulic control.

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