It would be a virtually impossible task to try to document the cause and remedy of every possible fault that could occur on even the simplest hydraulic system. For this reason it is necessary to adopt a logical approach to troubleshooting, in order to locate a fault as quickly and accurately as possible. Down time on modern production machinery is very expensive, so an hour saved in locating a problem may make hundreds, or sometimes thousands, of pounds worth of saving in lost production. Inevitably, hydraulic systems are becoming more and more complex as methods of controlling machines become increasingly sophisticated. The last ten years has seen rapid technological advances in the components used in many hydraulic systems, and it is vital that equipment, or machine manufacturer’s service information or software’ keeps pace with the actual hardware being used. It is probably true to say that there is still a general lack of understanding of hydraulics in some areas of industry, and in reality, the job of a hydraulic maintenance engineer is now a specialized occupation with many similarities to that of an instrument or electrical engineer. The object of this book is to provide procedure for a logical approach to troubleshooting, which can be extended when necessary to cover specific machines in all areas of industry. The fundamentals around which this procedure is developed, that is, the control of flow, pressure and direction of flow, applies equally as well to a rolling mill in a steelworks or a winch drive on a trawler.
The Hit and Miss Approach
The only alternative to a logical troubleshooting method is the ‘hit and miss’ approach, where units are changed at random until the failed component is located. Eventually the problem may be found, but on all but the simplest of systems this method proves to be expensive in terms of time and money. It is usually the case that a large number of perfectly servicable units are changed before the right one is found.
As with all troubleshooting techniques, knowledge of components and their function in a system is vitally important. It is probably fair to say that when all the components of a hydraulic system have been identified, their function determined and the operation of the system as a whole understood, the troubleshooter has gone 51% of the way towards finding the problem. It is important therefore, that to make use of this book effectively, a good understanding of the basic principles of hydraulics together with a knowledge of the operation and application of hydraulic components should first be obtained.
Shutting Down Machines
Whenever servicing work is carried out on a hydraulic system, the overriding consideration should be one of safety; to the maintenance engineer himself, his colleagues and the machine operators. Although safe working practices rely largely on common sense, it is very easy to overlook a potential hazard in the stress of a breakdown situation. Maintenance personnel should therefore discipline themselves to go through a set procedure before commencing any work on a hydraulic system. Because hydraulic fluid is only slightly compressible when compared with gas, only a relatively small amount of expansion has to take place to release the static pressure. However, where compressed gas can be present in a hydraulic system, either through
ineffective bleeding, or where an accumulator is fitted, extra care must be taken to release the pressure gradually.
Safety procedure for shutting down machines
- Lower or mechanically secure all suspended loads
- Exhaust any pressure locked in the system
- Drain down all accumulators
- Discharge both ends of intensifier
- Isolate the electrical control system
- Isolate the electrical power supply
When adopting service or troubleshooting procedures, it is useful to define three distinct areas similar to those used in the armed forces, that is, first, second and third line service.
Service or troubleshooting carried out on the machine itself and resulting in the failed component being identified and repaired whilst still in situs, or replaced. Second Line
Investigation and repair of a failed or suspect component away from the machine, possibly in a user’s own workshop
Investigation, overhaul and retest of a component carried out at the maker’s factory or service depot. It should be the responsibility of the maintenance manager to decide where the dividing line is drawn between each area for his particular equipment.
The troubleshooting procedure in this book will endeavor to answer the following questions:
What do I check?
Which things can be measured in a hydraulic system that will indicate where the problem lies? A doctor will very often check a patient’s heartbeat and temperature when making a diagnosis, to what do these correspond in a hydraulic system?
What do I check with?
Knowing what to check, it is then necessary to determine any special instruments or equipment that will be required (corresponding to the doctor’s stethoscope and thermometer).
Where do I check?
Whereabouts in a hydraulic system is it necessary to carry out the checks and which should be done first? As mentioned, a doctor will very often check a patient’s heartbeat i.e. the human pump; should the hydraulic pump be check first?
What do I expect to read?
Having taken a measurement at a certain point in a system, it is obviously necessary to know what the correct reading should be in order to draw conclusions if the reading is any different from normal. Again, a doctor knows that the body temperature should be 37_C so if there is any variation, a diagnosis can be made.
What do I check?
A hydraulic system is a means of transmitting and controlling power. Mechanical power is a function of force multiplied by distance moved per second or force _ velocity. If a hydraulic actuator is considered as a device to convert hydraulic power to mechanical power, then the force (or torque) exerted by the actuator is governed by the applied pressure and the velocity (or angular velocity) is governed by the flow rate. It follows, therefore, that flow and pressure are two basic elements of a hydraulic system that control the power output. In engineering terms, velocity usually implies both speed and direction, speed, as discussed being controlled by flow rate and the direction of the actuator movement being controlled by the direction of flow. The three factors therefore that transmit and control power in a hydraulic system are:
and Direction of flow
and it follows that in order to assess the performance of a hydraulic system one or more of these factors will have to be checked. In order to decide which, it is necessary to obtain the full facts of the problem. Very often when a problem is reported on a machine, it is described in vague terms such as “lack of power”. As previously mentioned, power is a function of both force and velocity and it is necessary to define the problem in terms of one or the other. In practice, relevant questions must be asked in order to determine exactly what the problem is i.e. when lack of power is reported does it mean that the actuator is moving too slowly, or is it not giving the required force or torque?
Having defined the problem as one of Speed, Force (Torque) or Direction it is now possible to define the hydraulic problem as one of Flow, Pressure or Direction. Although the troubleshooting procedure is based upon checking flow, pressure and direction, there are other aspects of a system which can be measured both as an aid to locating a failed component and also to determine the reasons for a component failure. Such properties are:
- Negative pressure (vacuum), especially in the area of the pump inlet to check for problems in the suction line.
- Temperature, generally when one component or part of a system is hotter than the rest, it is a good indication that flow is taking place.
- Noise, when checked on a regular or routine basis is a good indicator of pump condition.
- Contamination level, when repeated problems occur the cleanliness of the fluid should always be checked to determine the cause of the failures.