Created on:2021-10-26 08:54

Railway engineering equipment_ Failure analysis and treatment of slewing brake of hydraulic rescue crane

Railway engineering equipment_ Failure analysis and treatment of slewing brake of hydraulic rescue crane

(1) Analysis and treatment

Fault phenomenon: it is observed that when the rotary operating handle returns, it should be braked immediately, but the oil pipe connecting the brake vibrates strongly for about 10s, during which the crane continues to rotate and slide. After the oil pipe vibration stops, the rotation stops.

① Check the brake directly controlled by the combined shuttle valve for each component. First, consider that the fault may be caused by the valve. Replace the valve twice with intact components, but the phenomenon is the same. Therefore, it is judged that the combined shuttle valve has no fault. Similarly, if the swing control valve is replaced, it is judged that the component is also free of fault. Brake failure or insufficient braking force is also a possible fault. Disassemble the brake and check that all internal parts are in good condition, and the spring force can meet the braking requirements. So far, through inspection, all possible related components have no problems, but the fault phenomenon still exists.

② Braking process analysis: analyze the action process. After the rotary operation stops, A5 or B5 pressure is relieved, the two position three-way valve shall return to its position soon, and the brake shall be braked immediately after the brake oil pressure is relieved. However, the pipe vibrates for a long time, indicating that the oil circuit is blocked during pressure relief, and there is resistance in the pipeline to form throttling, so that the pressure in the pipe cannot be released in time, so it cannot be braked immediately. There is only two position three-way valve in the brake pressure oil discharge path. Remove the combined shuttle valve again for disassembly and inspection. The two position three-way valve is normal, and the spring pressure in the valve can make it return in time. The return of two position three-way valve is based on the pressure relief of A5 or B5, and the pressure relief of A5 or B5 passes through two shuttle valves and rotary control valve. When the rotation cannot be stopped immediately, the oil pipe of A5 or B5 does not vibrate, indicating that there is no throttling in the rotation control valve, so another possibility is the shuttle valve in 17-3.

The internal structure of the combined shuttle valve is shown in Figure J, which is composed of three shuttle valves and a two position three-way valve. The two outlets of shuttle valve F3 are unused control ends, which are closed with two oil plugs ml and M2.

JK.jpg

When rotating in one direction, the pressure oil enters shuttle valve F1 through A5, the steel ball in F1 closes B5 port, and the oil pressure enters F2 to push the two position three-way valve to obtain brake release oil pressure. When the swing operation stops, the swing control valve returns to the original position, and the control oil pressure returns to the oil tank through 14-2.

③ A conjecture and verification. According to the above analysis, there are great doubts about the combined shuttle valve. Based on the analysis of the dynamic process of the combined shuttle valve in braking, a possible scheme to produce braking delay is proposed.

Investigate the dynamic process of oil flow in the combined shuttle valve during braking. When the control oil pressure P enters the shuttle valve F2, the steel ball is pushed to the left because the outlet of F3 is closed. At this time, there is equal pressure P in F3 and F2. It is assumed that after the braking process begins, the oil pressure in F2 begins to drop. When it drops to P ´ (P > P ´ > 0), there is a pressure difference on both sides of the steel ball Δ p( Δ P = P-P ´ > 0), push the steel ball to move to the right. As long as the steel ball passes the midpoint of F2, the right outlet will be closed under the action of P ´ so that the oil pressure in F2 can not be discharged. When the oil pressure drops from zero to P ´, the valve core of the two position three-way valve has moved a certain distance, opening part of the oil drain channel of the brake. However, due to the small channel, throttling is formed when the pressure oil of the brake returns, strong vibration occurs in the oil pipe, and the brake begins to release pressure slowly. At the same time, due to the internal leakage of the two position three-way valve core, the oil pressure P ´ decreases gradually and the valve core returns gradually. After about 10s, the oil pressure is discharged, the oil pipe stops vibrating, the brake is fully braked, and the crane stops rotating. In short, due to the back pressure in F3, the steel ball of F2 is pushed to close the outlet, resulting in blocked oil return, unable to relieve the pressure in time, and delaying the braking time.

In order to verify this speculation, loosen an oil plug M1 so that it can not produce back pressure, and then rotate the test run for several times, which can brake quickly and reliably. It is preliminarily proved that the above speculation is correct.

After loosening the oil plug M1, it was used normally for a period of time, and the slewing brake failed again, which was exactly the same as the initial failure. It was judged immediately that another steel ball in F3 moved to the left position and blocked the original M1 port. After the oil plug M2 is released, the braking returns to normal, which further shows that the original speculation is correct.

④ Finally, the method of loosening F3 oil plug is adopted. A small amount of oil will flow out of the port during use, which is not convenient for the maintenance of the crane. Finally, take out the shuttle valve F2 steel ball to make it ineffective. When braking, the oil circuit will no longer be blocked. After the test of heavy load trial hoisting and on-site rescue, the slewing braking has completely returned to normal.

(2) Further thinking

Although the fault has been preliminarily identified and the processing results prove the correctness of the speculation, the speculation should be able to explain various existing phenomena and some other relevant questions.

① This phenomenon occurs after a period of time. The fault does not exist at the beginning. The crane works normally when leaving the factory, and this phenomenon occurs after a period of use. According to the rescue team, during the initial use period, no rotary brake failure was found, but during an accident rescue, due to special conditions, there was a large tilt and vibration during operation, and then the rotary brake failure occurred. It is possible that there is an oil flow dead angle on the left side of the steel ball. At first, due to the viscosity of the oil, the steel ball sticks to the left side of F2 and becomes loose due to large vibration.

② The randomness of the fault brake failure occurs frequently rather than every time, which has a certain randomness. Because there is static pressure in shuttle valve F3, no oil flow passes through, Δ P the thrust generated is only the elasticity of the oil stored in F3. The thrust is very limited. Sometimes the steel ball can not push the midpoint, so it will not cause braking failure. In addition, the inclination of the crane at different positions will also have an impact on the difficulty of steel ball movement.

③ There is no such phenomenon in the lifting mechanism. The working principle of the lifting mechanism is similar to that of rotation. The combined shuttle valve also controls the braking, but no braking failure is found. In addition to the hydraulic brake, the lifting mechanism is also limited by the balance valve, adding a safety link. According to the above speculation, the same phenomenon may still occur in the combined shuttle valve, so that the brake cannot brake in time, but the role of the balance valve may cover up the problem, but it is still an unsafe factor.

(3) Improvement

This fault has certain concealment and randomness, and other cranes of the same model are likely to have the same fault. Even if it does not happen for the time being, there is a hidden danger of insecurity. There are several ways to solve this problem.

① The method of loosening the two oil plugs of F3 is the simplest, but a small amount of oil will flow out of the port during use, which is inconvenient for the maintenance of the crane. If an oil return pipeline is added, the structure will be complicated.

② Taking the steel ball in F2 is also relatively simple, but it affects the interchangeability of parts. If the parts without steel balls are replaced inadvertently during maintenance and replacement, braking failure will also occur.

③ Improved design because F2 and F3 in the combined shuttle valve have no effect, the combined shuttle valve can be changed to the structure shown in Figure K, and one shuttle valve and two position three-way valve can be used to replace the combined shuttle valve. The disadvantage is that one part has become two parts. This method is also suitable for the transformation of combined shuttle valve in lifting mechanism.

For the crane in use, method ② can be considered. For newly manufactured cranes, method ③ can be adopted.

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