Troubleshooting Tips

Check of the status of hydraulic accumulator

Accumulators are devices which can collect pressurized liquid and released when needed it.

parts of hydraulic accumalator

Once a system has warmed up, a quick check can be made on the status of an hydraulic accumulator with the flat of the hand. There should always be a significant temperature difference between the gas and the hydraulic oil.  The oil/gas split can be detected by the temperature change on the body of the accumulator. If the whole body is the same temperature something has gone severely wrong with the gas bladder.

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Installation of hydraulic piston motor

I will present the radial piston motor because I know this type of motor. I serviced this kind of motor on board, bench tested and assembled in the workshop. From practical side I am familiar with this type of hydraulic motor.

hydraulic piston motor

Radial piston motor is the most used hydraulic motor. A new one installed should be kept in mind that to be filled with oil, meaning that when the motor is assembled the drain connection must be located on top.

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Ship Crane slewing bearing

A sign for me if the slewing bearing of a ship crane is maintained and the maintenance chart is followed is when looking from outside the crane pedestal has spots of grease.

deck craneWhen I was troubleshooting a ship crane before climbing up into the crane house I was looking on the outside crane pedestal to see any dirt of grease. It was the first step in evaluating the maintenance condition of the crane.

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Installation of hydraulic pump of a Ship Crane

I will talk about a high pressure hydraulic pump Rexroth A4V of ship cranes. The pump is a straight-type axial piston pump with variable displacement which is controlled by a servo valve mounted on the pump housing.

hydraulic pump

According to procedure I worked when new pump is installed on the crane an adjustment has to be done.  The pumps are controlled by valves which are sealed and should be adjusted by qualified personnel.

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Entrapped air in hydraulic systems leads to the phenomenon of cavitation in pumps.

The phenomenon of cavitation is the formation of gaps or voids in a fluid flow creating high local fluid velocity which causes damage on the surface of components such as gear teeth, vanes, and pistons leading to premature pump failure. Because of entrapped air in the pump, the oil tends to get covered with foam.

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  1. Troubleshooter

    by Christopher Brunner
    (Greensboro/Winston-Salem, North Carolina Area)

    Great write up but the author left out another cavitation solution. Supercharging the pump inlet through elevation or even a pressurized reservoir.

  2. Troubleshooter

    by Al Neu

    Cavitation is caused by vaporization of the oil when the inlet pressure drops below the vapor pressure of the incoming oil. The vaporized oil causes a void in the oil flow that collapses extremely fast when the oil becomes pressurized inside the pump. This sudden collapse of the voids causes a high speed localized flow where the bubbles collapse.

    This flow causes an erosion pattern that can look like acid etching or pitting. Cavitation caused reduced pump flow, plenty of noise, and pump wear, but will not produce sponginess in the system.

    What you web page describes is aeration, a different problem altogether. Entrained air and inlet leaks will give you a spongy response in a system and foamy oil in the reservoir. Entrained air by itself will dissipate quickly in the system as the pressure builds up in the pump outlet line, but inlet leaks will cause excessive air the remains in the oil in the form of small bubbles.

  3. Troubleshooter

    Richard Throop

    Air induced into the oil stream due to a leak, or turbulence inside the reservoir, is considered aeration. Cavitation is normally defined as air bubbles being pulled out of the actual hydraulic fluid due to a low absolute pressure (vacuum), due to poor inlet conditions, then collapsing under pressure in the pumping chambers.

    Both situations have the same harmful effects on the pump as James described, the only difference between the two terms is where the air bubbles came from.

  4. Troubleshooter

    by James Oftelie
    (Des Moines, Iowa )

    Valentin, your treatise on air and cavitation is correct and useful with the exception of the actual physics of cavitation and how it occurs.

    The phenomenon happens elsewhere and is called “dieseling” in cylinders but the theory behind it is the same. Pump cavitation occurs when dissolved air comes out of solution typically at localized areas in the inlet where the absolute pressure is the lowest.

    These very tiny bubbles are then pressurized rapidly causing them to collapse so violently that, if this occurs next to a metal surface, it fractures the crystalline metallic structure, causing the “frosting” on brass and aluminum, eventually forming pits if it goes on long enough.

    An air suction leak will make the situation that much worse, but you don’t need such an air leak to have a pump cavitation problem. Solutions to the problem involve adjustment to the suction condition (reservoir location, line sizing, pump speed, etc.) or fluid anti-air additives.

    Thank you for your useful submission! Hope I haven’t confused the issue for anyone.

  5. Troubleshooter

    by Kevin Speer
    (Perth Area, Australia)

    Spot on Lawrence with regards to cavitation. Aeration is caused by air entering the system on the suction side of the pump or via worn or damaged seals on actuators.

    Aeration causes heat buildup due to compression, jerky movements of actuators and could result in a breakdown of lubrication causing catastrophic wear.

  6. Troubleshooter

    by Rob van den Brink
    (Utrecht Area, Netherlands)

    The phenomena cavitation always starts with negative pressure. So if there is unsolved air in the system but no negative pressure there will be no cavitation.

    Unsolved air might cause acceleration of the fluid in the system and this might cause negative pressure. But in hydraulic systems cavitation is generally caused by negative pressure at the suction side of a pump or motor (!).

    Watch the pictures and explanation on:

    Trapped unsolved air may cause the Diesel effect. For example, very big cylinders are vulnerable (rod side) for it. The trapped air contains oil damp and when the pressure suddenly increases the ‘gas’ will explode like in a Diesel engine. This causes severe damage, especially to seals.

    Watch the pictures of burned seals on

  7. Troubleshooter

    I’ll break the ice with a useful comment…for the proper function of the hydraulic systems the oil used has to have the correct viscosity (thickness). For example, an oil which is to tick can cause suction problems for pumps at low temperatures which leads to cavitation.
    If the oil is too thin problems can arise at high temperatures because the lubricating film between the moving parts is to thin and friction occurred between them.
    So… oil is an important factor for the proper functioning of the hydraulic installation and oil producer will specify the correct viscosity.

  8. Hi,
    I remember last time when I was on board a team of Filipino was cleaning the pedestal of deck crane. They cleaned with fresh water HP hose, and then chipped and painted. Also some rumors…joke I believe was that the same washing procedure was used inside the deck crane house -spraying with water inside the electrical and electronics panels…and when troubleshooting water was coming out from panels.

  9. About Cavitation

    I would like to return to cavitation subject, which it was on open table discussion early on. Now I want to add more about the subject. I am waiting feedback.

    A correct design of the piping system is important on the suction side of a pump. A bad design led to cavitating the pump. Cavitation is caused by too low inlet pressure, which will allow the water to ”boil” at the suction side of the impeller. Vapour takes up space so the capacity of the pump drops dramatically. In addition vibrations can cause serious damage to bearings, seals and the impeller.

    In addition, I am waiting feedback which can led to awareness and stop cavitation phenomena.

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