In the process of spiral welded pipe, the next process is to cut the end of hot-rolled coil after uncoiler and straightener.
Because the head and tail of steel plate are usually irregular shapes after hot rolling, as shown in Figure 1.
It is necessary to align and tailor weld the plate head and the plate tail after they are cut together.
The practice has proved that under the same plate width, the cutting speed of plasma cutting is slower than that of plate shears, so most of the welded pipe units adopt plate shears for cutting.
The plate shears are divided into swing beam shear and guillotine shear.
The guillotine shears have many advantages, such as high cutting precision, high cutting force, high cutting speed, etc.
So, they are widely used in spiral welded pipe sets with large diameter and large wall thickness.
Fig. 1 Schematic diagram of the head
1. Schematic diagram of braking-type plate shear system
As shown in Figure 3, it is the hydraulic schematic diagram of a guillotine shear.
When the shearer is not working, the pump is idling and the electromagnet is not charged.
When the shearer needs to adjust the shearing angle, it is controlled by the reversing valve 5.
Figure 2 shows the shearing process.
Fig. 2 Schematic diagram of presser foot cutting
The electromagnet Y4 of the directional valve 7 is powered on to control the presser foot hydraulic cylinders 13 and 14 to compress the plate head or plate tail.
The directional valve 4 controls the hydraulic cylinders 11 and 12 connected in series to complete the shearing action, and the directional valve 9 is opened for oil return.
The reversing valve 4 can control the lifting of the cutting edge at the same time.
The specific electromagnet power sequence is shown in Table 1.
Table 1 The power on sequence of electromagnet
|Shearing||Y1||Y2||Y4||Y7 is powered on after 1 s delay|
- 1 – Internal gear pump;
- 2 – Motor;
- 3 – Electromagnetic relief valve;
- 4 – Electromagnetic directional valve;
- 5 – Electromagnetic directional valve;
- 6 – Hydraulic control check valve;
- 7 – Electromagnetic directional valve;
- 8 – Relief valve;
- 9 – Electromagnetic ball valve;
- 10 – Hydraulic control check valve;
- 11 – Auxiliary shear cylinder;
- 12 -Main shear cylinder;
- 13 – Presser foot cylinder;
- 14 – Presser foot cylinder
Fig. 3 Hydraulic schematic diagram of guillotine shear
At present, the shear force estimation of plate shears usually uses the Nosari formula:
In the formula:
- f – shear force;
- σb – material strength limit;
- ξx – elongation of the cut sheet;
- h – sheet thickness;
- α – blade angle;
- z – bending force coefficient of the sheared part;
- y – relative value of the lateral clearance of the front edge;
- x – influence coefficient of the press.
According to the data, ξx is 0.25, z is 0.95, y is 0.083, x is 7.7.
According to the quantitative analysis of σb, h and α, as shown in Fig. 4, the strength limit and thickness of the plate are directly proportional to the shear force F, while the inclination angle of the blade is inversely proportional to the shear force.
Based on this conclusion, the main hydraulic system faults of this kind of guillotine-type plate shear are analyzed and summarized.
2. Problem analysis
2.1 No pressure in the system
It should first determine whether the motor is reversed, and then check whether the coupling between the motor and the pump is damaged.
After the above two points are determined, if there is still no pressure, the fault of relief valve 3 can be determined.
It may be that the damping hole of relief valve is blocked, or the directional valve of relief valve is stuck or there is serious leakage.
2.2 System pressure does not go up
Most of the faults are related to the valve.
Serious internal leakage of the valve and stuck valve core may make the system pressure unable to go up, which can be eliminated one by one by controlling the corresponding solenoid valve.
But before troubleshooting the valve, it should check the system tank first.
If there are a large number of bubbles in the oil tank, it means that the pump is vacuuming.
At this time, check the oil level of the oil tank first.
If the hydraulic oil is sufficient, make sure whether the plum blossom pad or nylon pin of the coupling is damaged individually.
After removing the above two faults, it can be determined that the pump has been damaged.
If there are iron and copper chips in the oil, it can be judged that the pump and valve are seriously worn, resulting in insufficient pressure.
There is no cooling system in this kind of shears.
When the operator does not pay attention to the fact that Y1 and Y3 electromagnets are not powered off when the work is completed.
If the motor is not turned off all the time, a large amount of heat will be generated in a short time, which is easy to make the oil temperature rise and deteriorate.
After troubleshooting the pump and valve, the sealing problem of the hydraulic cylinder can be directly located, resulting in the failure of the system pressure.
Fig. 4 The relationship between the parameter and F
2.3 Automatic falling of presser foot and scissors
Figure 5 shows the presser foot structure of a plate shearer.
Due to its own weight, the presser foot hydraulic cylinder has a downward trend.
Now the reason why the presser foot hydraulic cylinder falls is found from the schematic diagram.
As shown in the schematic diagram, the rod cavities of the hydraulic cylinders 13 and 14 are connected with the rod cavities of the hydraulic cylinder 12 and the solenoid valve 9.
Assuming that the hydraulic cylinder 12 has no fault, so it should first consider the way of the solenoid valve 9.
If the solenoid valve 9 is removed, the oil in the rod cavities of the hydraulic cylinders 13 and 14 will be connected with the port B of the solenoid directional valve 4 and the control oil port of the hydraulic control check valve 10.
Therefore, it is inevitable that there will be leakage after a long time of oil supply, so the reversing valve 9 uses a seat valve structure, which can effectively prevent leakage.
Once the electromagnet Y7 is powered on in disorder or the sealing effect of the seat valve sealing surface becomes worse, the presser foot will automatically fall again.
If the hydraulic cylinder 11 and 12 have seal damage, it is also a common reason for the presser foot to fall automatically.
Fig. 5 Presser foot mechanism of shearing machine
As for the automatic falling of the cutting edge, as shown in Figure 3, the scissors are controlled by two hydraulic cylinders in series.
The electromagnetic directional valve 4 and the electromagnetic directional valve 5 control the different actions of the scissors through different actions.
The rod diameter, cylinder diameter and stroke of hydraulic cylinder 11 are 212 mm, 320 mm and 185 mm.
The rod diameter, cylinder diameter and stroke of hydraulic cylinder 12 are 212 mm, 240 mm and 185 mm.
When the seal and joint of two hydraulic cylinder leak, the shear blade will fall down automatically.
Just like the presser foot, the electromagnetic ball valve 9 will also fall down automatically.
Another possible reason is the solenoid valve 5 and the hydraulic lock 6.
When the O-ring on the hydraulic lock 6 is not installed properly or the oil temperature is too high, the O-ring gets into the hydraulic lock and gets stuck in the oil circuit, which makes the hydraulic lock unable to close normally.
The oil in the two hydraulic cylinders returns to the oil tank through the electromagnetic directional valve 5 (“J-type function”) and causes the cutter to fall.
The “O” ring can be replaced to solve the problem.
2.4 The cutting edge can not move the plate
It can be seen from the previous conclusion that the shear is related to the strength limit of steel plate, the thickness of steel plate and the shear angle.
For example, X70 steel plate is made of 15.9 thick steel plate, and the required pressure is about 12.5 MPa.
However, in actual production, it often occurs that the steel plate cannot be cut when the pressure is adjusted to 15 MPa or even 20 MPa, and the equipment has no oil leakage fault.
At this time, it is necessary to find the problem by combining with the equipment structure.
Figure 4 shows that the shear force difference between 2.5 ° and 10 ° blade angle is close to 5 times, so the failure of the shear plate basically depends on the blade angle.
During the operation of the equipment, improper setting of the limit of the shear hydraulic cylinder often leads to the failure of the shear angle adjustment to reach the required angle, which can be solved by moving the limit of the shear hydraulic cylinder.
As shown in Figure 6, when cutting, the cutting edge often cuts down first, but the presser foot cylinder does not press down, resulting in the steel plate curling and unable to cut.
According to the power on sequence of the electromagnet, the electromagnet Y2 controlling the shearing and the electromagnet Y4 controlling the presser foot are powered on at the same time, so the fault is not in the power on sequence.
The speed of the shearing hydraulic cylinder 11 is v1=q/s11, and the speed of the presser foot hydraulic cylinder is v2=q/2/s13.
Among them, S11 is the piston area of hydraulic cylinder 11, with 0.08 m2.
S13 is the piston area of hydraulic cylinder 13, with 0.0095 m2, so v2 ≈ 4v1.
Therefore, in this system, the synchronization of shearing and presser foot can be adjusted by adjusting the direct acting relief valve 8.
Fig. 6 Schematic diagram of shear failure
The main function of relief valve 8 is to increase the oil return back pressure and prevent the shear cylinder from crawling.
Another function of this valve is to adjust the speed of shear and presser foot.
The flow characteristic equation is known as:
It can be determined that the flow g through relief valve 8 is proportional to the pressure difference △p between P and T.
When shearing, the pressure p12 at the joint of rod cavity of hydraulic cylinder 12 is greater than the sum of the pressure p1314 of rod cavity of two presser foot hydraulic cylinders 13 and 14.
Therefore, when the return oil flow g cannot be greater than or equal to p12 ＋ p1314, p12 will exert a reaction force on the hydraulic cylinders 13 and 14 to slow down the pressing speed of the hydraulic cylinders 13 and 14, resulting in the failure shown in Figure 6.
At this time, by adjusting the set pressure of relief valve 8 to change the return oil flow qT, the presser foot effect shown in Figure 2 can be achieved.
As shown in Figure 7, the shear hydraulic cylinder is fixed to the rack through the steps on the hydraulic cylinder.
When the step at point A is worn like point B, the steel plate gives an upward force to the cutting edge to make the hydraulic cylinder move upward through the reaction of force.
In an instant, the inclination angle of the blade becomes larger and the shear force is reduced, which is also an important reason for the failure of shearing the steel plate.
This article analyzes some faults of the hydraulic system of a guillotine shear.
According to the working experience in recent years, the faults of the equipment are often comprehensive.
Mechanical faults are often accompanied by hydraulic faults, and hydraulic faults are accompanied by electrical faults.
However, as long as the reference drawings, combined with on-site analysis, the establishment of an equipment failure library, it can quickly determine the equipment failure to ensure the normal operation of the equipment.
Fig. 7 Schematic diagram of shear hydraulic cylinder