Fault Analysis of Free Hook Drop of 200t Hydraulic Crawler Crane

In 1984, Shandong Electric Power Construction Second Engineering Company purchased two 200-ton hydraulic crawler cranes from abroad. They have been in use for over a decade without any malfunctions. In November 1996, during the construction of a certain power plant, one of the machines suddenly fell off the hook freely while carrying a load. Through timely analysis and handling, both personal and mechanical accidents were avoided.

The working principle of the lifting system

The drive system of this 200t crawler crane is a fully hydraulic open working oil circuit. The hydraulic pilot control operating system is equipped with four hoists, namely the main and auxiliary hoisting hoists and the main and auxiliary luffing hoists. The hoist is driven by a hydraulic motor with an integrated planetary gear reduction transmission, and the brake is a spring brake with hydraulic release multi-disc brake. The lifting mechanism mainly relies on a swashplate axial piston variable pump for oil supply, with a maximum working pressure of 32 mpa, and the working pressure of the pilot control system is 3MPa.

Main hook lifting action: Through operation, the operating system supplies operating pressure oil to the C3a and B1-II oil circuits. Then, both the main and auxiliary hook switching valves and the main and auxiliary hook hoist braking switching valves are in the straight-through position, and the working oil circuit reversing valve is connected through the cross passage, allowing the working oil to enter the H side of the main lifting motor. When the oil pressure in the B1-II oil circuit reaches 0.7MPa, the pressure contact switch connected to this oil circuit operates, causing an electromagnetic valve to supply operating pressure oil to the C2 oil circuit. When the brake of the main winch is released, the main winch immediately starts to lift. The maximum working pressure during lifting is set at 32MPa.

Main hook descent action: Through operation, the operating system supplies operating pressure oil to the C3a and A1-II oil circuits. The working oil enters the S side of the main lifting motor. Meanwhile, the other path of the working oil passes through the shuttle valve to release the brake of the main winch, enabling the main winch to achieve the descent action. The maximum working pressure during descent is set at 7MPa. The lifting and lowering action of the auxiliary hook: It supplies oil to the C3 oil circuit, connects the main and auxiliary hook switching valves and the main and auxiliary hoist braking switching valves through the cross passage, and the other principles are the same as those of the main hook. When the luffing mechanism and the traveling mechanism are not in operation, another main working oil pump (not shown in the schematic diagram) can simultaneously supply oil to the hoisting mechanism to achieve rapid action of the hoisting mechanism.

2 Fault Phenomena

The malfunction occurred during the landing operation. When the load needed to be stopped after landing, during the process of returning the control handle to its original position, just as it was about to return to the neutral position, the load suddenly dropped rapidly in free fall. The descent time was very short, and the load did not stay on the ground all the way. When it reached a sufficient height above the ground, it would always come to a stop in the air again. During this period, there was no loud braking sound. This kind of malfunction occurs in the colder winter mornings, especially during the first few operations at the beginning of work. However, after working for a period of time, this phenomenon does not occur again.

3. Causes of the fault and handling methods

After analysis, it is initially determined that there may be the following two causes for this malfunction:

(1) The pressure oil pumped out contains air bubbles.

After observing the state of the hydraulic oil in the oil tank during the operation of the crane and testing the oil quality, it was found that the state of the hydraulic oil was no different from that before the failure. There were no problems with the components in the oil tank. However, the failure still occurred after replacing the new high-quality hydraulic oil. Therefore, it can be determined that the problem does not lie in the hydraulic oil.

(2) At the end of the landing operation, the valve on the motor return oil circuit failed to close in time.

By analyzing the hydraulic schematic diagram, it can be known that when the descent action is completed and the operating handle has returned to the neutral position, if the return oil circuit of the motor is not closed in time, the inlet and return oil circuits of the motor are connected and both are connected to the oil tank, that is, the motor is in a floating state, and thus the heavy object will descend by itself. Why does the phenomenon of heavy objects falling freely occur when the motor displacement is fixed and the oil return line oil pipe is also filled with hydraulic oil? The key issue lies in the oil inlet side of the motor at this time. If the motor sucks in sufficient oil at this time, the heavy object will descend at the normal maximum working speed. In fact, at this time, the motor sucks oil from the oil tank and the return oil line. Due to the motor being far from the oil tank and the inner diameter of the oil pipe, the resistance caused by the filter in the oil tank to the oil suction, etc., simply cannot meet the flow rate required for the normal operation of the motor. As a result, a negative pressure is formed on the oil intake side, creating a vacuum. The motor then stalls, causing the heavy object to fall freely (in a normal falling action, due to the effect of the balance valve, the oil intake pressure of the motor is always maintained at 7MPa).

Now let's analyze why the return oil circuit cannot be closed in time. The valve core of the three-position six-way directional control valve on the working oil circuit is an integral rigid structure. There will be no possibility of asynchronous opening and closing at the inlet and return oil ports. Therefore, the problem can only lie in the two-position two-way directional control throttle valve and the check valve block. Since the free descent of the load does not always stop at the ground but can come to a stop in the air after a momentary descent without any braking sound, it can be determined that the check valve and throttle valve cores are not stuck. Otherwise, the load would keep falling to the ground or a loud braking sound could be heard.

Therefore, the problem lies in the two-position two-way throttle directional control valve and the two-way damping valve block. The following is an analysis of the reasons for the malfunction caused by the function of this valve group.

When performing the landing action, one of the pressure oil enters the oil intake side of the motor. Another path is connected to the bidirectional damping valve. Through the one-way valve and the damping valve Z, pressure is applied to the valve core, gradually opening the throttle port, which connects the return oil circuit of the motor. Due to the effect of the damping valve Z, the load begins to drop steadily. During the descent process, when the intake oil pressure is less than 7MPa, the small orifice will be closed under the action of the spring to ensure that the intake oil pressure returns to 7MPa. When the descent action stops, the intake oil pressure disappears. Under the action of the spring, the throttle port is closed. Due to the effect of the damping valve R, the load stops descending smoothly. From this, it can be known that the key to the problem lies in the damping valve R. When the load stops falling, the pressure oil of the compression spring is depressurized through a check valve and the damping valve R. The opening of R is too large to provide damping, and a sudden stop of the load will cause an impact on the crane. If the opening of R is too small and the pressure relief time is too long, that is, the return oil circuit closes slowly, the free hook drop phenomenon discussed in this article will occur. If the viscosity of the hydraulic oil is too high, the damping effect of R will be greater, and the possibility of this phenomenon occurring will be higher. This is why the probability of failure is higher in the cold winter morning.

The opening of R has been set at the factory and cannot be adjusted at will during use. However, due to over a decade of use or some accidental factor, the opening of R may have become smaller, which led to the occurrence of the fault. After increasing the opening of R, the fault disappeared. But when the opening of R is too large, it will cause an impact when the hook drops. The adjustment data of this damping valve is not mentioned in the vehicle's user manual and maintenance manual either. Therefore, Adjustments can only be made by conducting tests with an empty hook based on the accumulated experience from past use, or by inviting experts from the manufacturer to make the adjustments.