Introduction
Compared with boom-type cargo winches, hydraulic cranes have the advantages of strong lifting capacity, easy operation, stable performance, and high cargo handling efficiency.
For shipowners, hydraulic cranes can increase market competitiveness and economic benefits, and are therefore widely used on ships.
As the service life increases, marine cranes will gradually develop complex faults such as wear, aging, failure, and malfunction.
Based on years of onboard work experience and combined with typical fault cases of LIEBHERR cranes, the author analyzes how to find the cause of faults and solve problems occurring during crane operation.
I. Analysis of Typical Faults of LIEBHERR Cranes
A certain vessel’s LIEBHERR crane has been in operation for over 20 years, with numerous and complex faults.
In the first few years after a new ship is launched, this type of crane generally only experiences simple and repairable faults such as hose bursts and joint oil leaks.
As the wear of moving parts intensifies year by year, it can lead to faults such as wear of the hydraulic variable pump and hydraulic motor, spring aging, seal failure, and control part malfunction.
Apart from frequent faults in the electrical control parts, mechanical wear and seal failure are the main causes of control part faults.
Mechanical wear refers to the failure of the sealing surfaces and spring steel plates of the hydraulic variable pump and hydraulic motor due to metal fatigue caused by long-term use, resulting in insufficient oil discharge or no oil discharge.
Generally speaking, if a crane is found to be slow during operation, it is mainly due to insufficient oil discharge from the hydraulic variable pump or hydraulic motor.
The diagnosis and repair of this type of hydraulic fault can also be extended to other hydraulic automatic control equipment (same principle).
1. Troubleshooting of the Servo Variable Mechanism of the Hydraulic Pump
(1) Fault Phenomenon of Hydraulic Control Failure
While the vessel was discharging cargo in Thailand (high ambient temperature, a season with frequent crane faults), the operator found that when the control handle was returned to the neutral position, the boom or hook drum continued to rotate slowly, causing the boom or hook to descend accordingly. At the same time, significant resistance was felt, the mechanical brake had already engaged, and the oil pump emitted a sound of oil being trapped.
(2) Fault Analysis and Solution for Hydraulic Control Failure
Inspection confirmed no faults in the electrical control part, no jamming of the variable displacement pump’s swashplate, correct rotation angle, correct brake control timing, etc. Therefore, the most likely cause was that the differential piston (see Figure 1) of the servo variable mechanism controlling the swashplate action could not promptly return to the neutral position from the operating state along with the hook, preventing the hydraulic variable pump from stopping oil discharge in time, causing the return-to-neutral time to lag behind the brake solenoid valve action time.
▲Source: Internet, Infringement must be deleted
This fault occurs when the seal ring wear of the differential piston (generally with pressure equalizing grooves, this vessel has seal rings) is not severe.
When the differential piston seal ring is severely worn, during the lifting and lowering operation, after the control handle is returned to the neutral position, the boom or hook drum will continue moving at the original speed for a distance before stopping. A sudden increase in load is clearly felt, and in severe cases, the safety valve opens, accompanied by the sound of oil being trapped from the hydraulic variable pump, boom, or hook drum.
This fault phenomenon indicates complete failure of the differential piston seal ring seal, causing the pump’s return to neutral to lag significantly behind the brake solenoid valve action time.
Therefore, the primary cause of this fault is the failure of the differential piston seal ring seal, preventing correct displacement to change the swashplate angle of the hydraulic variable pump (i.e., changing the oil discharge volume of the hydraulic variable pump).
Aging and wear of the seal ring (severe wear of the pressure equalizing groove can also cause this fault) will affect the seal, leading to leakage, causing high-pressure oil to leak into the low-pressure side, preventing the pump from resetting in time.
When the control handle is placed in the neutral position, the hydraulic variable pump is still discharging oil.
At this time, the brake solenoid valve has already acted and applied the brake, causing a sharp increase in load, even severe overload.
At this point, the servo variable mechanism should be disassembled and inspected.
By removing the end cover of the variable displacement pump, the hydraulic servo variable mechanism assembly can be taken out;
Remove the four screws on the oscillating cylinder end cover and extract the differential piston (see Figure 2), check the aging, deformation, and wear of the seal ring.
Figure 2 Physical object of the servo variable mechanism ▲Source: Internet, Infringement must be deleted
If the outer plastic seal ring is not significantly worn, replace the inner rubber ring, then install the plastic seal ring. Afterwards, heat the outside of the plastic seal ring to a maximum of 80°C. After cooling, the plastic seal ring will automatically shrink into the ring groove;
The inspection standard is that the plastic seal ring protrudes 0.2~0.3mm above the ring groove to ensure sealing.
Special care should be taken during repair: when removing the plastic seal ring, be careful not to break it. If no spare parts are available, the plastic seal ring can be cut shorter and placed at the bottom of the ring groove, with a thick rubber ring placed over it. This can also restore the seal to normal.
This fault first occurred on the No. 4 crane of this vessel.
After checking that other systems were normal, the servo variable mechanism was disassembled and inspected, revealing that the plastic seal ring had worn down below the ring groove.
With no spare plastic seal ring available, the plastic seal ring was cut shorter and placed at the bottom of the groove, with a thick rubber ring added on top, restoring the crane to normal operation.
Other cranes subsequently experienced this fault, and after repair using the same method, they all returned to normal operation, completely eliminating this potential fault.
2. Troubleshooting of the Brake Mechanism
(1) Fault Phenomenon of Hook Brake
While loading cargo in Russia in January of a certain year (ambient temperature reaching -30°C, a season with frequent crane faults), it was found that the hook drum experienced significant resistance and difficulty rotating during operation. The hydraulic pump emitted a sound of oil being trapped, and in severe cases, there were intermittent pauses.
(2) Fault Analysis and Solution for Hook Brake
After confirming that the brake solenoid valve and brake oil circuit were normal, the drum brake clutch was disassembled and inspected. It was found that the clutch friction plates were severely deformed, reducing or eliminating the free clearance, causing the clutch friction plates to be unable to fully disengage after the brake solenoid valve acted, meaning the brake remained engaged.
Whether the crane was operating under no-load or full-load conditions, it was working under overload.
Upon discovering this sign, the crane operation must be stopped immediately. Disassemble the hydraulic motor, open the end cover of the brake clutch, inspect the clutch friction plates (241, 242 in Figure 3), and replace the deformed friction plates with new ones.
If no new spare parts are available, 1~2 friction plates can be appropriately removed to increase the free clearance.
The free clearance should be maintained at 5~6mm. Too large or too small a clearance will seriously affect the clutch braking effect:
Too large a clearance will cause the brake to fail to hold the load, causing the cargo to slip. This situation is absolutely unacceptable;
Too small a clearance means that even slight deformation of the clutch friction plates will prevent them from fully disengaging.
To ensure both reliable braking and complete disengagement of the clutch friction plates during operation, the number of springs (251, 252 in Figure 3) can be appropriately increased to ensure reliable braking.
▲Source: Internet, Infringement must be deleted
When disassembling and inspecting the clutch, pay attention to checking the sealing condition of the clutch hydraulic brake cylinder (3 in Figure 3).
If its seal ring (12 in Figure 3) shows any sign of deformation or aging, it should be replaced with a new one to keep the brake clutch in normal working condition and ensure safety.
This is one of the common faults of the brake clutch.
If oil seepage is found at the flange connection of the hydraulic motor for the hook and boom drum during operation, the most likely cause is leakage of the hydraulic motor oil seal.
In such cases, the hydraulic motor should be disassembled and the oil seal replaced as soon as possible. Otherwise, hydraulic oil can easily penetrate the clutch friction plates, contaminating them (especially in low-temperature conditions like in Russia, which may cause the friction plates to stick together), leading to poor braking or brake failure, resulting in a major accident.
3. Analysis of Electrical Control Faults of LIEBHERR Hydraulic Cranes
Check the tightness of wiring inside the crane control box, the condition of electromagnetic contactor and relay contacts, the status and function of each switch, and whether the /stop interlock functions of each relay are normal;
If carbon brushes and slip ring surfaces are found to produce sparks and burn during contact operation, remove the carbon brushes and grind them smooth or replace them. Grind the burned parts of the slip ring surface smooth, or build them up by welding, then turn and polish them smooth, ensuring the carbon brushes and slip ring surfaces always maintain good contact;
Check the operation of the anti-condensation heaters inside the control box and motor; measure the output voltage of the automatic control system, ensuring it remains within the normal working voltage range specified in the manual;
Check the function and condition of the solenoid valves, temperature switches, limit switches, level switches, and pressure switches used for protection and control.
II. Discussion on Methods for Finding Crane Faults
In crane management, the most fundamental method is to perform good routine maintenance.
Proper maintenance can eliminate faults in their infancy, thereby reducing the fault incidence rate.
However, for old cranes, faults are still inevitable.
Based on work experience and practical operation, the author has compiled a fault finding table to facilitate finding solutions based on fault phenomena and fault cause analysis.
