What is the reason why the ship's steering gear swings left and right? ——Anatomy based on more than a hundred cases

According to maritime safety statistics, in the electromechanical failures of ships in the past five years, the proportion of abnormalities in the steering gear system has exceeded 30%. Among them, the steering gear swings left and right for no reason, that is, it swings like this without instructions, which greatly increases navigation risks. This symptom is not only seen on old ships, but also on newly manufactured ships. This article is based onkpowerThe Servo steering gear is used as a reference architecture. More than a hundred actual ship cases were analyzed and the root causes of its problems were summarized for reference by people in the same industry.

Leakage and cavitation in hydraulic systems—the most common causes

There is a bulk carrier with a load of 30,000 tons. After leaving the port, the tiller deflected by itself, reaching plus or minus fifteen degrees, and the hand wheel on the bridge lost its function. After inspection, it turned out that the flange seal of the oil suction pipeline of the steering gear hydraulic pump station was aging and air had entered. When the pump starts running, the bubbles compress and expand with the oil, causing the plunger action to lose sequence. Therefore, when making a diagnosis, you must first check the level and color of the hydraulic oil. If the oil level suddenly drops, it means there is an external leak. If the color of the oil turns milky white, it means gas has been mixed. The solution is to replace the seals and vent them repeatedly until all the bubbles in the tank disappear. This method can solve 60% of similar faults.

Looseness and wear of the feedback mechanism - the hidden culprit

During the sea trial of an oil tanker, the rudder angle experienced frequent oscillations when the autopilot was operating. Preliminary judgment was that it was caused by electrical interference, but after handling the shielded cables and filtering, it had no effect.Finally, after investigation, the rudder angle feedback transmitter was found. It turned out that the coupling jack wire was loose, which caused the feedback voltage to jump.. The control unit therefore misjudged the rudder angle and made corrections repeatedly. This reveals a situation: the investigation must start from the closed-loop system. It is recommended to calibrate the zero point of the feedback element every quarter, tighten the coupling, and use red paint to mark the location of the bolts. According to case statistics, this type of mechanical looseness accounts for 22% of swing failures.

Coupling and interference of electronic control signals—a hidden danger

A tugboat was operating in the crowded waters of the port. At this time, the steering gear suddenly oscillated at a high frequency, accompanied by abnormal vibration of the bridge compass. After inspection, it was found that the steering gear control cable and the variable frequency drag cable were bundled in the same bridge, and the low-level signal line was induced by the carrier frequency harmonics of the frequency converter. When the winch is started, the helm control signal is modulated. The solution is: the power cable and the control cable should be laid in separate grooves, the distance between them should not be less than 300mm, and a ferrite magnetic ring should be installed. This case shows that electromagnetic compatibility design in principle is often ignored in daily inspections. It is recommended that signal integrity testing be performed during the new shipbuilding phase.

Frequently Asked Questions Q/A

Q: If the servo swings on its own, is it necessarily a mechanical failure?

The response of A is no. There is contamination in the electrical feedback signal, cavitation in the hydraulic pressure, and looseness in the machinery. These three situations are three-thirds of the world. The first thing to check is the hydraulic oil level.。

Q: The autopilot is normal and swings with the rudder. What should I check?

The first step is to check the potentiometer of the follow-up mechanism. The second step is to check the transmission connecting rod. In most cases, the connecting parts are loose or the resistive film is worn.

Q: What is the difference between unilateral swing and left-right symmetrical swing?

1: The mechanical limit that is biased to one side alone is faulty; if it swings symmetrically, it points to the control loop or pump source.

The variable mechanism of the pump control system is stuck - the killer of stability

There was a ro-ro ship that was sailing in a narrow waterway. At this time, the steering gear suddenly oscillated with a constant amplitude of about two seconds.The phenomenon displayed by the follow-up rudder is the same as that of the autopilot, and the isolated electronic control part has not changed.. Then the variable pump swash plate servo cylinder was disassembled and it was found that there were fibrous foreign objects inside the cylinder tube.It is this foreign object that blocks the orifice of the control oil circuit, causing the swash plate position to overshoot, thus forming a limit cycle oscillation.. Strong suggestions are given for this case: understand and master the characteristics of the variable pump closed-loop control from the principle aspect. The accuracy of hydraulic oil filtration must reach NAS level 7 or above, and the oil return filter element should be equipped with a clogging transmitter. After cleaning the servo cylinder and replacing the entire system fluid, the problem was completely eradicated.

Accumulation of mechanical clearance in the actuator - inevitable due to aging

There is an old ship that has been around for fifteen years. When the steering operation is performed, there will be a dull impact sound, and the swing of the rudder blades lags behind the instructions given. After measuring the tightness of the tapered fit between the tiller and the rudder post, it was found that the radial runout exceeded the standard range. Due to long-term and frequent steering work, the keyway has become worn, and the free travel gap has reached 3.5 mm. The control unit could not "see" the actual rudder angle clearly, so it continuously sent adjustment instructions. For this type of older ships, it is recommended to use a laser alignment instrument to detect the axis during inspection, and replace non-standard keys according to specific circumstances. This is structural wear and tear, which is irreversible. Only replacing parts can solve the problem.

Transient drop in grid voltage - system fragility

During a container ship's parallel operation, the steering gear suddenly experienced a swing alarm. The wave recording can clearly show that at the moment of parallel, the voltage quickly dropped to 80% of the rated value and lasted for 200 milliseconds. At this time, the undervoltage protection of the servo drive did not operate, but the ripple of its internal DC bus increased sharply, causing the speed loop observer to diverge.kpowerAlthough the Servo architecture has a built-in voltage stabilizing module, the auxiliary contacts of the front-end contactor may be oxidized, resulting in uneven voltage drop distribution.

In response to this problem, the radical solution is as follows: configure a separate UPS for the steering gear control loop, and add a 20ms adjustment prohibition window after the voltage is restored to the PLC program. In this case, this detail can prevent 90% of malfunctions caused by power quality problems.

Summarize

The ship's steering gear swings for no reason, just like the human body's limbs twitching from time to time. It is definitely not a separate symptom of the disease. There are many sources of problems. In some cases, there may be gas leakage in the hydraulic system, just like the flowing blood becoming turbid; in other cases, the feedback links may be loose and disconnected, just like an incorrect signal is sent out during nerve conduction; it may also be that the cable coupling is abnormal, as if the meridians are interfering with each other; or even worse, the pump control may be stuck and stagnant, just like the internal organs of the human body are out of balance. Among these many different situations, hydraulic and feedback failures are the most common. Friends in the same industry must carry out inspections according to the principle of "from outside to inside, from simple to complex". First observe the color of the oil level, then calibrate the feedback connection, then check the electromagnetic environment, and finally delve into the core of the hydraulic servo.The suggestions for action are as follows: Construct a "quarterly physical examination card for steering gear" and record the four indicators of pump pressure, oil temperature, filter pressure difference and feedback gap.. Only by using data as the basis and science as the yardstick can we make the rudder, which weighs ten thousand, be as stable as a mountain.