Created on:2021-09-18 11:27

Construction machinery_ Hydraulic cylinder expansion failure of excavator Outrigger

Construction machinery_ Hydraulic cylinder expansion failure of excavator Outrigger

Cylinder expansion is a frequent fault phenomenon in hydraulic machinery. It will lead to abnormal operation of the machine and must be repaired in time. Because the middle of the expanded hydraulic cylinder is large and the two ends are small, the repair is very difficult and the repair cost is high, so it is more harmful.

(1) Cause analysis of cylinder expansion

When a wyl60 wheeled excavator found that the outrigger could not support up for the first time, it worked normally after replacing the piston oil seal. After three months of normal use of the machine, the outrigger could not be supported or retracted, but the operator did not stop immediately, but used the auxiliary device of the excavator to support it for maintenance after one day. During disassembly and inspection, it was found that the piston oil seal was damaged. Replace the piston oil seal again, and the outrigger still could not work. Disassemble the hydraulic cylinder, measure the piston and piston oil seal, and the dimensions are normal. When measuring the cylinder barrel of the hydraulic cylinder with the cylinder gauge, it is found that the dimensions at both ends of the cylinder barrel are normal, while the dimension in the middle of the cylinder barrel has increased from 125mm to 128mm, resulting in cylinder expansion.

The hydraulic system diagram of outrigger operation is shown in Figure D.

D.jpg

In order to prevent the phenomenon of soft legs during operation, hydraulic locks are set at the oil ports of the large and small cavities of the outrigger hydraulic cylinder (see Part 4 in Figure d). When the machine is normal, if the outrigger is not retracted, the hydraulic oil in the oil chamber will be closed no matter how much external load it bears. The stress of hydraulic cylinder during operation is shown in Figure E.

If piston diameter is d= φ 125mm, piston rod diameter D= φ 80mm, as shown in Figure e (a), the force balance formula when the piston oil seal is intact is:

G1=A1p1 =πD2p1/4=1252πp1/4 (7-2)

The force balance formula shown in Figure e (b) (after the piston oil seal is damaged) is:

G2=A2p2=πD2p2/4=802πp2/4 (7-3)

During normal operation, the external load shall be the same, i.e. G1 = G1

Therefore, a1p1 = A2P2, that is, P2 = a1p1 / A2 = 2.44p1

It can be seen that after the piston oil seal of the outrigger hydraulic cylinder is damaged, the internal pressure of the outrigger hydraulic cylinder rises sharply during operation, which is 2.44 times higher than that before the piston oil seal is damaged. Coupled with the additional impact load during operation, the pressure of the hydraulic cylinder is even higher. With such a high working pressure of the hydraulic cylinder, cylinder expansion is inevitable.

(2) Countermeasures to avoid expansion of outrigger hydraulic cylinder

The structure can be improved as follows: remove the valve B of the hydraulic lock 4 with rod cavity of the outrigger hydraulic cylinder. In this way, if the piston oil seal is damaged, the excessive working pressure will open the safety valve 7 of the leg hydraulic cylinder branch and return the oil, so that the hydraulic cylinder barrel can be protected from damage. The service condition is normal after improvement.

When using machines with such structures, in case of soft legs, stop the machine immediately for inspection and eliminate all faults of the outrigger system before continuing to use. Do not work with diseases, let alone forcibly prop up the outrigger by other auxiliary methods.

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