Logical Troubleshooting in Hydraulic Systems

��

LogicalTroubleshooting

in HydraulicSystems

Vickers®

General Product Support

2

Index

Introduction 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Shutting Down Machines 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Line Service 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Checking Faults 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Instrumentation Principles and Measuring Instruments 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Measuring instruments 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pressure Gauges 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pressure Gauge Installation 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Flow Meters 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Flow Meter Installation 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Machine malfunction procedure 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Machine faults 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System faults 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System malfunction 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System faults 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Unit Fault Preliminary Check 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Excessive Temperature 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Excessive Noise 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Excessive Vibration 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Excessive Leakage 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for gear and vane pumps 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for piston pumps 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for pressure relief valves 20. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for sequence valves 21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for pressure reducing valves 22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for flow control valves 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for directional control valves 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for pilot operated check valves 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for cylinders 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for hydraulic motors 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for accumulators 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for coolers 29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for air leaks 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System test for fluid contamination 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pump cavitation 32. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Aeration of fluid 33. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Re-start procedure 34. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Bar trimming machine – Troubleshooting exercise 35. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Bar trimming machine layout 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Circuit diagram for bar trimming machine 37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

“Logical Troubleshooting in Hydraulic Systems” is intended as a guide to systematic faultfindings in a hydraulic system. Vickers systems (to the extent permitted by law) accept noliability for loss or damage suffered as a result of the use of this guide. If in doubt, alwaysrefer to the original equipment manufacturer. Refer at all times to the “safety procedure” onpages 3 and 34.

Warning

3

IntroductionIt would be a virtually impossible task totry to document the cause and remedyof every possible fault that could occuron even the simplest hydraulic system.For this reason it is necessary to adopta logical approach to troubleshooting, inorder to locate a fault as quickly andaccurately as possible. Down time onmodern production machinery is veryexpensive, so an hour saved in locatinga problem may make hundreds, orsometimes thousands, of pounds worthof saving in lost production.

Inevitably, hydraulic systems arebecoming more and more complex asmethods of controlling machines becomeincreasingly sophisticated. The last tenyears has seen rapid technologicaladvances in the components used inmany hydraulic systems, and it is vital thatequipment, or machine manufacturer’sservice information or software’ keepspace with the actual hardware beingused.

It is probably true to say that there is still ageneral lack of understanding of hydraulicsin some areas of industry, and in reality,the job of a hydraulic maintenance engi-neer is now a specialized occupation withmany similarities to that of an instrument orelectrical engineer.

The object of this book is to provide pro-cedure for a logical approach to trouble-shooting, which can be extended whennecessary to cover specific machines inall areas of industry. The fundamentalsaround which this procedure is devel-oped, that is, the control of flow,pressure and direction of flow, appliesequally as well to a rolling mill in a steel-works or a winch drive on a trawler.

The Hit and Miss ApproachThe only alternative to a logical trouble-shooting method is the ‘hit and miss’approach, where units are changed atrandom until the failed component islocated. Eventually the problem may befound, but on all but the simplest ofsystems this method proves to beexpensive in terms of time and money. Itis usually the case that a large numberof perfectly servicable units are changedbefore the right one is found.

As with all troubleshooting techniques,knowledge of components and theirfunction in a system is vitally important.It is probably fair to say that when all thecomponents of a hydraulic system havebeen identified, their function deter-mined and the operation of the systemas a whole understood, the trouble-shooter has gone 51% of the waytowards finding the problem. It is impor-tant therefore, that to make use of thisbook effectively, a good understandingof the basic principles of hydraulicstogether with a knowledge of the opera-tion and application of hydraulic compo-nents should first be obtained.

Shutting Down MachinesWhenever servicing work is carried outon a hydraulic system, the overridingconsideration should be one of safety; tothe maintenance engineer himself, hiscolleagues and the machine operators.Although safe working practices relylargely on common sense, it is veryeasy to overlook a potential hazard inthe stress of a breakdown situation.Maintenance personnel should thereforediscipline themselves to go through aset procedure before commencing anywork on a hydraulic system. Becausehydraulic fluid is only slightlycompressible when compared with gas,only a relatively small amount ofexpansion has to take place to releasethe static pressure. However, wherecompressed gas can be present in ahydraulic system, either throughineffective bleeding, or where an

accumulator is fitted, extra care must betaken to release the pressure gradually.

Safety procedure for shuttingdown machines

Lower ormechanically secureall suspended loads

�Exhaust any

pressure locked inthe system

�

Drain down allaccumulators

�Discharge

both ends ofintensifier

�Isolate the

electrical controlsystem�

Isolate theelectrical power

supply

Line ServiceWhen adopting service or trouble-shooting procedures, it is useful todefine three distinct areas similar tothose used in the armed forces, that is,first, second and third line service.

First LineService or troubleshooting carried outon the machine itself and resulting in thefailed component being identified andrepaired whilst still in situs, or replaced.

Second LineInvestigation and repair of a failed orsuspect component away from themachine, possibly in a user’s ownworkshop

Third LineInvestigation, overhaul and retest of acomponent carried out at the maker’sfactory or service depot.

It should be the responsibility of themaintenance manager to decide wherethe dividing line is drawn between eacharea for his particular equipment.

4

First Line ServiceRepairon site.

Second Line ServiceRepair inworkshop.

Third Line ServiceManufacturers repair service

For example, if a pump fails on a systemit may be possible to repair the unitwhilst still on the machine i.e. first lineservice. In another case it may benecessary to replace the unit with a newone sending the failed pump to a work-shop or second line area where adecision can be made either to repair,return to manufacturer (third line) or ifthe unit has reached the end of its use-ful life, to scrap it. Obviously severalfactors will affect this decision, such asspares availability, time factors, etc.Wherever the line is drawn, the proce-dure should be clearly defined for thebenefit of maintenance personnel.

This book is confined to the first linearea i.e. working from a fault to thefailed component. Second line serviceinformation can be found in otherVickers publications together withdetails of specialist test equipment.

Checking FaultsThe troubleshooting procedure in thisbook will endeavor to answer the follow-ing questions:

What do I check?

Where do I check?

What do Icheck with?

What do I expectto read?

What do I check?Which things can be measured in ahydraulic system that will indicate wherethe problem lies? A doctor will very oftencheck a patient’s heartbeat andtemperature when making a diagnosis,to what do these correspond in ahydraulic system?

What do I check with?Knowing what to check, it is thennecessary to determine any specialinstruments or equipment that will berequired (corresponding to the doctor’sstethoscope and thermometer).

Where do I check?Whereabouts in a hydraulic system is itnecessary to carry out the checks andwhich should be done first? Asmentioned, a doctor will very oftencheck a patient’s heartbeat i.e. thehuman pump; should the hydraulicpump be check first?

What do I expect to read?Having taken a measurement at acertain point in a system, it is obviouslynecessary to know what the correctreading should be in order to drawconclusions if the reading is anydifferent from normal. Again, a doctorknows that the body temperature shouldbe 37�C so if there is any variation, adiagnosis can be made.

What do I check?A hydraulic system is a means oftransmitting and controlling power.Mechanical power is a function of forcemultiplied by distance moved persecond or force � velocity. If a hydraulicactuator is considered as a device toconvert hydraulic power to mechanicalpower, then the force (or torque) exertedby the actuator is governed by theapplied pressure and the velocity (orangular velocity) is governed by the flowrate. It follows, therefore, that flow andpressure are two basic elements of ahydraulic system that control the poweroutput. In engineering terms, velocityusually implies both speed anddirection, speed, as discussed beingcontrolled by flow rate and the directionof the actuator movement beingcontrolled by the direction of flow.

5

The three factors therefore that transmitand control power in a hydraulic systemare:

FlowPressure

and Direction of flowand it follows that in order to assess theperformance of a hydraulic system oneor more of these factors will have to bechecked. In order to decide which, it isnecessary to obtain the full facts of theproblem.

Very often when a problem is reportedon a machine, it is described in vagueterms such as “lack of power”. Aspreviously mentioned, power is afunction of both force and velocity and itis necessary to define the problem interms of one or the other. In practice,relevant questions must be asked inorder to determine exactly what theproblem is i.e. when lack of power isreported does it mean that the actuatoris moving too slowly, or is it not givingthe required force or torque?

Having defined the problem as one ofSpeed, Force (Torque) or Direction it isnow possible to define the hydraulicproblem as one of Flow, Pressure orDirection.

Although the troubleshooting procedureis based upon checking flow, pressureand direction, there are other aspects ofa system which can be measured bothas an aid to locating a failed componentand also to determine the reasons for acomponent failure. such properties are:

– Negative pressure (vacuum),especially in the area of the pumpinlet to check for problems in thesuction line.

– Temperature,generally when one component orpart of a system is hotter than therest, it is a good indication that flowis taking place.

– Noise,when checked on a regular orroutine basis is a good indicator ofpump condition.

– Contamination level,when repeated problems occur thecleanliness of the fluid shouldalways be checked to determinethe cause of the failures.

Instrumentation Principles andMeasuring InstrumentsWhat do I check with?When an electrician checks an electricalcircuit, he usually has available a meterthat will measure electric current andvoltage. In a hydraulic system thevoltage corresponds to the pressure,and is usually measured with a pressuregauge, the current corresponds to theflow and is usually measured with a flowmeter. Although the electricians meterwill measure positive or negativevoltage, if the hydraulic engineer wishesto measure a negative pressure i.e.vacuum, then a separate instrument isrequired, namely a vacuum gauge.

Apart from the basic requirements of apressure gauge, vacuum gauge andflow meter, there are several otherinstruments that can prove useful to thehydraulic engineer i.e.:

– Pressure Transducer and Recorder.If the pressure in a system needs tobe measured to an accuracygreater than that which can beobtained with a pressure gauge, orif transient pressure peaks orshocks need to be measured, thena pressure transducer can be usedwhich produces a varying voltageaccording to the pressure applied.

– Measuring Jar and Stopwatch.For measuring very small flowssuch as leakage, a graduated jarand watch may be used. This canvery often give a more accuratereading than a flow meter workingat the bottom of its range.

– Temperature Gauge orThermometer.To measure the general systemtemperature a temperature gaugecan be immersed in the fluidreservoir (sometimes incorporatedwith the level gauge). Very often thetemperature gauge incorporates aswitch to give a warning if the fluidtemperature is too low or too high.

– Thermocouple.Temperature can be measuredlocally in a system by means of athermocouple. If one part of asystem is very much hotter than therest, it is a good indication thatpower is being wasted (such as aleakage point).

– Noise Meter.Excessive noise is again a goodindication of a fault in a systemespecially the pump. In the middle ofa noisy factory it may be difficult tojudge whether a pump is more noisythan usual, so a noise meter enablesa comparison to be made between asuspect pump and a new pump.

– Particle Counter.The condition of the system fluidfrom a contamination point of viewis obviously a major factor in the lifeand performance of a system. Intrying to determine the reasons fora component failure it may benecessary therefore to measure thecleanliness of the fluid.

Although equipment may not beavailable on site to check the fluid,such a service is offered by most ofthe major fluid suppliers and filtermanufacturers.

In an electrical circuit thecurrent and voltage acrossany component can be easilychecked when fault finding.

In a hydraulic system:

The ‘voltage’ correspondsto the pressure

and is usually measured bya pressure gauge.

The ‘current’corresponds tothe flow

and is usuallymeasured by aflow meter.

Instrumentation Principles

6

System pressure isusually measured witha pressure gauge.

For very accurate measurement ofpressure or for measuring transientpressure shocks, a pressuretransducer and recordermay be used.

Flow is usually measured by a flowmeter which can be of severaldifferent types.

For very smallflows (e.g.leakage) ameasuring jarand stopwatchmay be used.

Temperature is usuallymeasured with athermometer ortemperature gauge.

For certain applications athermocouple and electricalmeter is used tomeasuretemperature.

Noise can bemeasured with anoise meter.

Fluid contaminationcan be measured

by a particlecounter.

Measuring Instruments

Considering the two basic requirementsof pressure/vacuum gauges and flowmeters, thought should now be given onhow they are to be connected into thesystem, bearing in mind the type ofinstrument concerned.

Pressure Gauges

Pressure gauges are usually of theBourdon Tube type consisting of acurved tube attached to a pointer. Whenpressure is applied to the curved tube, ittends to straighten out, exactly as agarden hose does when the tap is turnedon. As the tube straightens, the pointer ismoved around the dial, indicating theapplied pressure. Being a delicateinstrument, it is necessary to protect thegauge as much as possible frompressure shocks in the system. Usuallysome form of snubber arrangement isfitted to the stem of the gauge, and thecomplete case may be filled withglycerine to damp down vibrations.

Pressure gauges are available in severaldifferent ranges and obviously a gaugemust be chosen to suit the expectedpressure reading (if in doubt as to whatthe pressure is likely to be, start with ahigh pressure gauge first). Most pressuregauges however tend to be moreaccurate around half scale deflection i.e.a 0 – 100 bar gauge would be mostaccurate around pressures of 50 bar.

Pressure Gauge InstallationThere are several ways of connecting apressure gauge into a system as follows:

60

Case maybe filled withglycerine toaid damping.

Bourdontube

Pressureinlet

Tube tends tostraighten

under pressurecausing pointer

to rotate.

0

10

20

30

4050

70

80

90

100

Threadedplug

Porous elementrestricts flow

Metering pin

Long, narrow,spiral passagefor flow

Restricted opening

Snubber Arrangements

ÂÂÂÂ

1. The gauge can be directly connectedinto the pipework by means of a ‘tee’piece. Obviously, the gauge will besubject to all the pressure shocks in the

system so over a period of time theaccuracy will inevitably drop off.

2. The gauge can be installed with anisolating valve so that the valve is onlyopened when a pressure reading needsto be taken and the gauge is normallyisolated from shocks in the system.

3. A venting isolation valve may also beused normally of the ‘push-to-read’ or‘twist-to-read’ type which both isolates thegauge from the system and also vents thegauge to tank when the button is released.

4. A multi-station isolating valve allowsthe pressure to be read at six differentpoints in the system using only onepressure gauge. The valve is alsousually of the push-to-read type, ventingthe gauge when the button is released.

5. Most hydraulic units are providedwith gauge points in inlet and outletports usually a screwed plug. If thesystem designer has not allowed for agauge to be permanently installed in apart of the system, it is usually possibleto connect one in without having todisturb pipework, etc., provided thegauge points can be identified.

6. Quick release, self sealing test pointscan be provided around the system (oreven connected into unit gauge points)allowing the maintenance engineer tocheck pressure in the system with aportable gauge kept in his toolbox, fittedwith the appropriate male probe. (Byconnecting the male probe to the test pointwithout a pressure gauge, they can alsobe used for bleeding air from the system.)

7

1. Pressure gaugepermanentlyinstalled in pipe-work.

2. Pressuregauge installedwith shut-offvalve.

3. Pressure gaugeinstalled withventing isolationvalve.

4. Pressuregauge installedwith multi-pointselector valve.

Pressure Gauge Installation

1

2

3 4

5

6

5. Pressure gaugeplugged into unitgauge point.

6. Pressure gaugeplugged intosystem testpoint.

Flow Meters

Flow meters are available of severaldifferent types such as the float type orturbine type. In addition, test units areavailable which combine flow meter,pressure gauge and temperature gaugein one portable unit. In practice, flowmeters are rarely connected into asystem permanently since flow setting ina system is usually accomplished bymeasuring the speed of an actuator.When it is necessary to check flow in asystem, however, careful considerationshould be given to the positioning of theflow meter in the system.

3. An electrical device will beconnected to the sensor toconvert the pulses to flow rateinformation

Flowthroughtubecausesindicatorto rise intube

Flow rateis readdirectly onscale ofindicator

Float Type

2. Sensing devicedevelops anelectrical signalevery time aturbine bladepasses

1. Flow causes turbine tospin at rate determined bythe rate of flow

Hydraulic Test UnitA hydraulic test unit combines...

Pressure gaugeTemperature gauge

Flow meter

...in one unit

BAT-TER

YCHECK

BAT-TER

YAD-JUS

T

C PM

Flow Meter InstallationFlow meters may be installed in ahydraulic system to check while themachine is operating normally (on-line)or while the machine is shut down formaintenance purposes (off-line).

On-lineFigure (A) illustrates a flow meter installedin the main flow line from the pump. Byincorporating two 3-way valves in the line,a flow meter can be connected to thesystem and the 3-way valves selected todivert flow through the meter. The meterused must obviously be capable ofwithstanding the full system pressure andflow. The meter reading will indicate theflow available to the system, but if thereading is less than specification it is notimmediately apparent whether the pumpis giving less flow than required or therelief valve is leaking a proportion of thepump flow to tank. If a problem is foundhowever, the list of possible causes hasbeen narrowed down to two units, and acheck on the tank return line from therelief valve should then confirm which ofthe two units is at fault.

Measures pumpoutput atnormal systempressureDoes not allow forrelief valveleakage

LP

HP

Monitors pumpperformanceby measuringcase leakage

(B)

(A)

8

In the case of a variable pump, the pumpoutput flow will only be that actuallyrequired by the system at any time. Agood indication of pump performancehowever, can be obtained by measuringthe internal leakage in the pump, i.e.measuring the case drain flow asindicated in Figure (B). A certain amountof case leakage is inherent on brand newpumps (caused by design clearances,lubrication drilling etc.) so it will benecessary to compare the leakageactually measured with that for a pumpwithin full specification. When measuringcase leakage it is important that it isdone under steady state conditions, ie.with the pump delivering a constantvolume. The flow meter required will onlyhave to withstand pump case pressure(normally around 0.3 bar) and very lowflows so in fact a measuring jar andstopwatch may be used. It is importanthowever that the drain line is NEVERallowed to be blocked off.

Figure (C) illustrates a typical off-linearrangement. The addition of two shutoffvalves in the system allows the systemitself to be isolated and the pump flow

Off-line

Measures pumpoutput at lowpressure

Does not allow forrelief valveleakage

Measuresflow availableat actuator

HP

LP

(C)

(D)

diverted through the flow meter. Again alow pressure meter can be used since thepump flow is measured at low pressurealthough this may not give a true indicationof the pump outlet at normal workingpressure. As mentioned previously, a flowdeficiency could be caused by low pumpoutput or relief valve leakage so a furthercheck will be required if this is the case. Itis possible to incorporate a restrictor in theflow meter line in order to developpressure, in which case of course, a highpressure meter will be required.

If the output from the pump unit provesto be satisfactory, it may be necessaryto connect flow meter into other areas ofthe system. Again the addition ofisolating valves as illustrated in Figure(D) considerably simplifies the operation.

CAUTIONWhenever a flow meter isconnected into a system, it is

vital to ensure that the pump always hasdirect access to the relief valve, and therelief valve tank line is never allowed tobe blocked off or unduly restricted.

Where do I check and what do Iexpect to read?When considering failures in a hydraulicsystem there can be two alternativestarting points namely:

� Machine Malfunction –where a fault occurs in a hydraulicsystem causing a malfunction on themachine itself ie. an actuator fails tooperate correctly.

� System Malfunction –where a fault occurs in the hydraulicsystem without necessarily affectingthe machine performance in the shortterm, eg. excessive leakage, tempera-ture etc.

The two can of course occur together.For example, a pump failure wouldresult in the machine failing to operatecorrectly and would most likely beaccompanied by an increase in noiselevel. Experience has shown that it isusually better to start at the fundamentalproblem and work through the checkingprocedure, using symptoms such asheat, noise, leakage etc, as clues.

Again, common sense must prevailwhen working through this procedure,as some symptoms may point straight tothe problem area. A fountain of oilgushing from a valve points immediatelyto the problem area, but somesymptoms may not be so obvious.When a unit leaks flow from high to low,pressure heat is usually generatedlocally in that part of the system whichmay not be immediately obvious.

Erraticmovement

Incorrectthrust

Incorrectspeed

Nomovement

Movementwhere thereshouldn’t be

Wrong direction

Wrong sequence

Step 1An actuator can fail to operatecorrectly in the following ways:

Incorrect SpeedIncorrect ThrustNo MovementMovement in the Wrong DirectionErratic MovementIncorrect SequenceCreep

Whichever fault or faults haveoccurred, the fundamental problemarea should be defined wherepossible as one of FLOW,PRESSURE or DIRECTION.

Step 2

From the circuit diagram eachcomponent in the system can beidentified and its function in thesystem determined.

Step 3

A list of units that can possibly affectthe problem area can now be drawnup. e.g. A slow actuator speed can bedefined as a problem of FLOW (eventhough this may in turn be due to lackof PRESSURE, the fundamental prob-lem area is FLOW). A list of units thatcan possibly affect flow to the actuator(including the actuator itself) is drawnup bearing in mind that, for example aleaking or wrongly adjusted relief valve(i.e. a PRESSURE control valve)could affect flow to the actuator.

Step 4

The list of units can be arranged inrough order of priority based on pastexperience and also ease of checking.

Step 6

If the preliminary check does not revealthe unit at fault, a more exhaustive teston each unit can be carried out usingadditional instrumentation but withoutany unit from the system.

Step 5

A preliminary check can now be carriedout on each unit on the list in turn tocheck such things as installation,adjustment, signals etc. and also todetermine if any unit exhibits abnormalsymptoms such as excessivetemperature, noise, vibration etc.

MACHINE MALFUNCTION

Step 8

Before restarting the machine, thoughtshould be given to both the causeand the consequence of the failure.If the failure was caused bycontaminated or over-heated fluid,then further failures can be expectedand remedial action should be taken.If a pump has broken up on a system,there is a possibility of pump debrishaving entered the system whichshould be thoroughly cleaned outbefore a new pump is fitted.

THINK ABOUT WHAT CAUSEDTHE FAILURE AND ANYCONSEQUENCES OF IT.

Step 7

The instrument checks should nowreveal the failed unit and a decisioncan be made whether to repair orreplace the unit at fault.

9

Whichever the starting point, certainquestions should be answered beforeproceeding. When a problem is reported, itis important to gather as many factstogether as possible. It could be that thesame problem occurred six months agoand is recorded somewhere on a log sheetor record card in which case a good dealof time can be saved. It should beascertained whether any recentmaintenance work or adjustments havebeen made to the system. The exactnature of the failure should be determined,was it a sudden or gradual breakdown,which parts of the machine have beenaffected and which have not? It may bedifficult always to get the complete story,but an effort should be made to gather asmuch information as possible.

The philosophy of the trouble shootingprocedure is to start at the fundamentalproblem and determine which aspects ofthe hydraulic system is at fault; flow,pressure or direction. By consulting thecircuit diagram, a list of possible causescan be drawn up. The next stage is tothen look for the obvious. It is perhapshuman nature when faced with achallenging problem to search too deeplytoo quickly, in the process overlookingwhat in hindsight appears a very obvioussolution. There are certain checks that canbe carried out on a hydraulic system usingthe human senses of sight, touch andhearing and which can be carried out veryquickly. If a rigid procedure is adoptedeach time it will ensure that no obvious orapparently trivial problem is overlooked. Inpractice very many problems will be solvedat this stage without having to resort toadditional instrumentation.

Only if this stage fails to reveal theproblem is it necessary to resort to extrapressure gauges, flow meters etc. andagain a logical approach should beadopted using the algorithm charts in thisbook.

Machine malfunction procedure

10

Machine faults

StepMachine

fault

Flow Press. Direction1

Circuitdiagram

List ofunits to bechecked

Order ofchecking

Prelim.check

ALGOtest**

Failedunit**

Think!**

2

3

4

5

6

7

8

Noise&Heat

Vibration&Leaks *

* Algo’s 0.1 to 0.5** Algo’s A to L

System malfunction

System faults

A hydraulic system may exhibitsymptoms of excessive heat, noise, vibration orleakage without necessarily affecting the machineperformance in the short term.

Whichever the problem, a preliminary check can becarried out to try and identify the problem area byusing the human senses of sight, touch and hearing.

A unit may exhibit abnormal symptoms due to aproblem elsewhere in the system eg. cavitation oraeration on a pump inlet line. In this case theFAULT, CAUSE, REMEDY (F.C.R.) charts can beused to locate the problem area.

If the problem is not identified by the preliminarycheck, then a more exhaustive check must becarried out on suspect units and a similar procedureto the ‘Machine Fault’ exercise can be carried out.

SYSTEM MALFUNCTION

VibrationNoise

Heat

Leakage

11

System faults

SystemFault

HEAT NOISE VIBRATION LEAKS

Prelim.CheckAlgo 0.2


F.C.R.Prelim.CheckAlgo 0.4


SystemInspection

Continue on ’Machine Fault’diagram step 3



ProblemArea

12

Unit Fault Preliminary Check

Is the unitmodel

numbercorrect?

Check orchange

unit

Is the unitinstalled

correctly?

Remedyinstallation

Is the unitadjustable?

Re-adjustunit

Is the unitadjusted

correctly?

Does theunit have

an externalsignal?

Check thesignalsource

Is the signalcorrect?

Check theunit

Does the unitrespond tothe signal?

Check theunit

Does the unitexhibit abnor-

mal symptoms?(noise, heat)

Continueon nextchart

NO

YES

YES

YES

YES

YES

YES

YES

NO

NO

NO

NO

NO

NO

NO

YES

Algo 0.1

13

Excessive Temperature

Is a coolerfitted to the

system?

Check thecooler (see

chart Algo J.2)

Is thereservoir fluid

level low?

Top up thereservoir

Is the fluidcorrect (seechart Algo

L2)?

Drain andrefill with

correct fluid

Is the heatlocal to one

unit?

Continueon chartAlgo 0.1

List all unitswith tank ordrain returns

YES

NO

NO

NO

YES

YES

YES

Algo 0.2

Excessivetemperature

NO

Continueon chartAlgo 0.1

14

Excessive Noise

Is the noiseassociated with

the pump?

Top upreservoir

Is thenoise a high

pitchedscreaming

noise?

ConsultFCR 2

Can thenoise betraced toone unit

Insert vacuumgauge in

pump inlet

Is the readingmore than 5 inHg (-0.21 bar)

Check settingsand adjustments

of all valves

YES

NO

NO

NO

YES

Algo 0.3

Excessivenoise

Is the reservoirfluid level too

low?

YES

Is there air inthe system?

YES

NO

NO

Check systemrelief valvesand repair or

replace

YES

Checkthe unit

YES

Pump iscavitatingconsultFCR 1

NO

Is thealignmentcorrect?

NO Re-alignpump/motor

YES

Removepump andexamine

15

Excessive Vibration

Is the vibrationassociated with

the pump?

Replaceor repaircoupling

Is the pipeworkadequatelysupported?

Realigndriveshafts

Is thevibration

associatedwith one unit?

Is the pumpmountingcorrect?

Is there air inthe system?

Check settingsand adjustments

of all valves

YES

NO

YES

NO

YES

Algo 0.4

Excessivevibration

Is the flexiblecoupling

unbalanced?

YES

Is the align-ment correct?

NO

NO

YES

Tighten orinstall clamps

NO

Check settingand adjustment

of unit

YES

NO

Removepump andexamine

Tighten orstrengthen

pump mounts

NO

ConsultFCR2

YES

16

Excessive Leakage

Clean downthe system

Can theleakage be

traced to oneunit?

Repair orreplace

unit

Can the leak-age be tracedto one area of

pipework?

Isolate each partof the system inturn and check

for leaks

NO

NO

Algo 0.5

Repair orreplace

pipework

Check fordamaged

cooler

YES

YES

Is a watercooler fitted tothe system?

YES

NO

Excessivevibration

17

System test for gear and vane pumps

NO

Algo A.1

Correctrotation

Is the driverotationcorrect?

Check forleakage in

system

Is there aflow fromany valvereturn line

Is theflow fromthe pumpcorrect

Check nextunit on list

Reassemblepump

correctly

Is the pumpassembledcorrectly for

rotation

Check thevacuum at the

pump inlet

Is there flowpassing downthe relief valve

tank line

Check therelief valve

Is theremotion taking

place

Check pressurewhen motionhas stopped


place

Insert flowmeter in

pump outlet

Loadsystem

Is the systemoff-loaded

Re-adjustrelief valve

Is the reliefvalve adjusted

correctly

Is thesystem problem

lack of flowCheck nextunit on list

Check driveto pump

Is the pumpshaft rotating

Is pressurereading normal

systempressure

Is there apressure read-

ing at pumpoutlet

START

NO YES

Primepump

Is the pumpprimed

Pump iscavitating

consult FCR1

Is thevacuum more

than 5” Hg.(-0.21 bar)of mercury

Check thepump


Is their flowpassing downthe relief valve

tank line

Check thepump

NO

YES

YES

NO

YES NO

NO YES YES

YES

NO NO

NO YES YES

YES NO NO

NO YES

YES NO

NO YES

YES

YES NO NO

OUTLET

INLET

OUTLET INLET INLET OUTLET

Vane Pump Intra-Vane PumpGear Pump

INLET & OUTLET

18

System test for piston pumps

Algo A.2

Correctrotation

Is the driverotationcorrect?

Is theflow fromthe pumpcorrect


Adjustcompensator

setting

Is the pumpcompensator

adjustedcorrectly

Is there flowfrom any valve

return line


tank line



place


setting


correctly

Insert flowmeter in

pump outlet

Loadsystem

Is the systemoff-loaded

Re-adjustcompensator

setting

Is the pumpcompensator

adjustedcorrectly

Is thesystem problem

lack of flow


Check driveto pump

Is the pumpshaft rotating

Is pressurereading normal

systempressure

Is there apressure read-

ing at pumpoutlet

NO YES

Primepump

Is the pumpprimed

Check thecase drain

flow

Is the pumpon maximum

delivery



tank line

Check thepump

NO

YES

YES

NO

YES NO NO

NO YES YES

YES

NO YES

NO YES YES

YES NO NO

NO YES

YES NO

NO

YES

YES NO

OUTLET

INLET

INLET &OUTLET

PVB 5PFB 5

PVB 10 & 15PFB 10 & 15 PVB 20 & 29

INLET &OUTLET

INLET &OUTLET

START


YES


place


YESCheck forleakage inthe system

NO

YES NORe-adjust max.

stop and/orcompensator

setting

Is thedrain flowexcessive

YES

NO

Continued on next page

19

System test for piston pumps (cont’d)

Algo A.2

Check thevacuum at the

pump inlet

OUTLET

INLET

INLET &OUTLET

PVB 5PFB 5

PVB 10 & 15PFB 10 & 15 PVB 20 & 29

INLET &OUTLET

INLET &OUTLET

Pump iscavitating

consult FCR1

YES

NO

Continued fromprevious page

Is thevacuum more

than 5” Hg.(-0.21 bar)of mercury

Checkthe

pump

20

System test for pressure relief valves

Algo B.1

Insert pressuregauge in TP1

Continue onnext chart

YES

Is the pres-sure maxi-

mum systempressure

Checkthe relief

valve

CS-06CS-10

CG-06CG-10

TP1

TP1 TP1

Is the systemproblem lack

of flow

YES

Is there flowpassing down

relief valvetank line

YES

NO

YES

Is thesystem on

load

Is theremotion

taking place

NO


correctly

YES

Is there flowfrom any

valve tankreturn line

YES

Checkthe relief

valve

Load thesystem



Check forleakage inthe system

NO NO

YES

NO

NO

21

System test for sequence valves

Algo B.2

Check thepressures atTP1 and TP2

YES

Are thepressures

correct

Checknext unit

on list

RG-06 & 10

TP1

TP2

Is thesystemproblem

lack of flow

YES

Is there ex-cessive flowdown valvedrain line

YES

Is thevalve

adjustedcorrectly

Is the pilotsignal correct

YES

Is the drainline blocked

NO

Is thespool free

Shut the systemdown in a safe

condition & removevalve end caps

Re-adjustthe valve

Check thepilot signal

Check thedrain line

NO NO

NO

YES

RT-06 & 10 RCT-06 & 10 RCG-06 & 10

TP2

TP1

TP2

TP1

TP2

TP1

TP2

TP1

NO

Checkthe valve

NO

YES

Checkthe valve

NO

Checknext unit

on list

YES

22

System test for pressure reducing valves

Algo B.3

Insert pressuregauges in TP1

& TP2

YES

Is there apressurereading at

TP1

XG-06 & 10

TP1

TP2

YES

Is thepressurereadingcorrect

Is thesystem on

load


place

YES

Is the valveadjustedcorrectly

YES

Load thesystempumps

Check forleakage inthe system

Re-adjustvalve

NO NO

NO

NO

XT-06 & 10 XCT-06 & 10 XCG-06 & 10

TP2

TP1

TP2

TP1

TP2

TP1

TP2

TP1

XGL–03

TP1

TP2

Check thegauge in

TP2

Check thepressure when

motion hasstopped

YES

Is thesystem

problem lackof flow

YES


Is thedrain linerestricted

Check thedrain line

Is thedrain flowexcessive

YES


Check thevalve

Check thevalve

NO

NO

NO

YESNO

NO

Is thereading equalto full system

pressure

23

System test for flow control valves

Algo C.1

Correctinstallation

YES

Is thevalve

installedcorrectly

TP1

TP2

Is the valveadjustedcorrectly

YES

Is the read-ing normal

systempressure

Is reading normalsystem

pressure

YES

Is readingnormalworkingpressure

Check forloss insystem

NO

NO

FCG-02-1000

TP2

TP1

Insertgauge at

TP1

YES

Check thevalve

Are thereadings at

TP1 and TP2the same

Insert gaugeat TP3

Valve is notcompensating.Check valve

NO

NO

A

TP3TP3

Is thesystemproblem

flow

Re-adjustvalve

Check forloss insystem

Insertgauge at

TP2

System isstalled. Checkwhen actuator

is moving

Insertgauge at

TP1

Insertgauge at

TP2

Checknext unit

on list

Does gaugeread thesame as

TP1

Checknext unit

on list

NO

YES

NO

NO YES

NO

YES

YES NO

24

System test for directional control valves

Algo D.1

Insertgauges

TP1 & TP2

Is thesystem on

load

YES


place

Is readingfull systempressure

Loadthe

system

NO

Check pilotsupply


Is there areadingon TP1

Operate valvemanually

P to A

Check thevalve

Wait untilmotion has

finished

Checkvalve

NO

NO

DG5S4–102

A B

P T

TP1 TP2

DG3S4

DG17S4–06TP1 TP2

TP1 TP2TP1 TP2

Is readingfull systempressure

YES

NO

Operate valvemanually

P to B

Is there areadingon TP2

Is thesystem on

load

YES


place

Check pilotsupply

Wait untilmotion has

finished

YES

NO

NO

YES

Load thesystem

NO

YES

NO

YES

YES

25

System test for pilot operated check valves

Algo E.1

Insertgauges

TP1 & TP2

Checkthe

valve

YES

Is the systemproblemcreep

Insert gauge inpilot pressure

line

Is thepressuredifferenceexcessive

Check pres-sures with flow

passing thruthe valve


Checkpilot

supply

TP1

TP2

TP1

TP2

Is pilot pres-sure sufficientto open valve

NO

NO

YES

4CG-06 & 10 4CT-06 & 10

TP1

TP2

Does pilot pres-sure decay whenvalve is required

to close

YES

NONO

YES Checkthe

valve

Check forrestrictionin pilot line

26

System test for cylinders

Algo G.1

YES

Fully extendcylinder to end

of stroke

Is thesystem

problem erraticmotion

Bleed airfrom cylinderbleed points

NO

TP

Double acting

TP TP

TP

Off load orrelease system

pressure at cylinder ports

Disconnectannulus

connection ofcylinder

Apply pressureto full bore end

Is oil leakingfrom annulus

conn. oncylinder


Check thecylinder

Double acting double rod

27

System test for hydraulic motors

Algo G.2

Is pressurenormalworkingpressure

Check pressureat motorinlet port

M2-200 25M, 35M45M, 50M

MFB-**

TP

TP

TP TP TP

Check pressureat motor

outlet port

Is backpressureexcessive

Check motordrain flow

Is drain flowexcessive

Disconnectmotor couplingand rotate by

hand

Check for lossin the system

Check forrestriction inreturn line

Checkthe motor

NO YES

YES

NO

Is shaftfree

Checkthe motor

YES

Checknext unit

on list

NO

NO

YES

28

System test for accumulators

Algo J.1

Is the pre-charge

pressurecorrect

Insert test gauge ingas side of accumu-lator as manufactur-

ers instructions

TP

Stop pumpsand dischargeaccumulator

Is the pres-sure switch

settingcorrect

Charge withnitrogen to

correctpressure

Check the accu-mulator accordingto manufacturers

instructions

NO YES

NO

Is shaftfree

Re-adjustpressureswitch

YES

TP

Discharge fluid fromaccumulator and

check gas pressure

Insert pressuregauge in fluid

line

Charge accumu-lator to fluid sys-

tem pressure

Is thepressurereadingcorrect

Is the reliefvalve of com-pensator ad-

justed correctly

YES

YES

Checknext unit

on list

NO

Re-adjustrelief valve orcompensator

NO

29

System test for coolers

Algo J.2

Is the temptoo high

Check the oil tempat the outlet port

of the cooler

Is the watersupply

switched on

YES

NO

Is shaftfree

YES

Is thethermostat

adjusted

Is the water out-let temp higherthan water inlet

temp

Is the waterinlet temptoo high

NO

Is there a differ-ence between oilinlet and outlet

temps

Cooler is working.Check next unit

on list

YES

Check when oilhas reachedworking temp

Open watervalve

NO

NO

YES

Re-adjustthermostat

NO

YES

Check thecooler

NO

Check watersupply

NOIs the oilflow thru

the coolercorrect

Check forlosses in the

system

YES

YES

Check thecooler

30

System test for air leaks

Algo L.1

Check pipefittings

Continuealong the

joints

Smear eachjoint with grease

one at a time

Are fittingstight

Has thepump noise

changed

Check thepump shaft

seal

Smear withgrease

YES

NO Tighten orreplace worn

fittings

NO Has thepump noise

changed

NO

YES

The last jointwas causing

aeration

YES

Has thepump noise

changed

Check thatpump and

strainers arebelow fluid

level

NO

Remove pumpand replace

shaft and seal ifneeded

Fill tank tocorrect level

Are theybelow fluid

level

Check forporous hoses

and pipefractures

YES

YES

NO

31

System test for fluid contamination

Algo L.2

Draw off asample and

compare withfresh fluid

This oil hasoxidized

Does thesystem sam-ple appear

to be darker

Can a pop-ping soundbe heard atthe test tube

YES

NO Does thefluid appearmilky white

Allow fluidto stand

YES

The fluid iswater laden

Replace

NO Allow fluidsamples to

stand for onehour together

Set up aninclined planewith a shallow

angle

Pour both sam-ples on to theincline at thesame time

Pour a quantityinto a test tube

and heat

YES

NO

Do airbubbles

appear at thesurface

YES

The fluid isbadly

aerated consultFCR 2

This fluid ispollutedReplace

NO

Did theyboth flow atthe same

speed

The fluid isof the wrong

viscosity

NO

Fluid viscosityis correct

YES

32

Pump cavitation

FCR 1

PumpCavitation

FAULT CAUSE REMEDY

Suction strainer cloggedor too small Clean or renew

Bore of suction linetoo small Fit larger bore pipes

Too many bends insuction line

Modify pipe layout

Suction line too longReduce length or fit

larger bore pipes

Fluid too cold Heat fluid torecommended temperature

Unsuitable fluid Replace with correct fluid

Air breather blockedor too small

Clean or replace element

Local restriction in suction line e.g.partly closed valve or hose collapse

Open or modify valves,renew houses, etc.

Failure of boost pumpRepair or replace boost

pump

Pump running too fast Reduce torecommended speed

Pump mounted too highabove oil level

Modify pump installation

33

Aeration of fluid

FCR 2

Aeration offluid

FAULT CAUSE REMEDY

Reservoir fluid level low Fill to correct level

Poor reservoir design Modify design

Return line in reservoirabove fluid level

Extend return pipebelow fluid level

Unsuitable fluid Replace with correct fluid

Pump shaft seal wornor damaged Renew seals

Suction line jointsallowing entry of air Renew or tighten joints

Porous suction hose Renew hose

Improper bleeding Re-bleed system

34

Re-start procedureAlgo 0.6

Has thecause of thefailure beenremedied

Can dirt haveentered the system

by the unitfailure or during

replacement

NO

YES

Is theunit the

correct one

YES

Clean or flushthe system as

required

NO

YES

Modify orreplace withcorrect unit

Remedyinstallation

Is the unitadjustable

YES

Are all hydraulic

connectionscorrect

Have thecases of pumps

& motorsbeen filled

Have all safetyinterlocks

been removed

YES

NO

Start thesystem

YES

YES

Bleed orprime as

necessary

Removesafety

interlocks

Are all personnelaware themachine isre-starting

Sound alarmor inform allpersonnel

NO

Will the systemrequire bleeding

or priming

Carry outfurther

investigation

Adjust toa safe

condition

Fill up case

Are all electrical

connectionscorrect

Remedyinstallation

Is the unitadjusted to a safe

condition forstart-up

YES

NO

NO

NO

YES NO

YES

NO

YES

NO

NO

35

Bar trimming machine – Troubleshooting exerciseIntroductionTo illustrate the procedure described inthis book, an example will be workedthrough in detail for a typical machine asshown on page 36.

The Bar Trimming Machine consists of acarriage which is driven backwards andforwards by a hydraulic motor. Mounted onthe carriage is a traverse cylinder whichmoves a cutting torch across a metal bar.Two clamp cylinders hold the bar in placeand a semi-rotary actuator pushes the baronto a roller table at the end of the cycle.

Assume that the operator has reportedthree problems which occurredsimultaneously and prior to which themachine was operating satisfactorily.

Symptoms� The carriage drive motor is slow in

both directions.

� The traverse cylinder is slow whenextending.

� The system is running hotter than usual.

Starting PointStart at the fundamental problem, i.e. theslow movement of the carriage motor andtraverse cylinder, using the heat problemas a clue. Since two machine faults haveoccurred it is not impossible that two faultshave occurred in the hydraulic systemsimultaneously. It is more likely however,that one fault in the hydraulic system iscausing both machine faults. The logicalprocedure therefore is to look at eachmachine fault in turn and arrive at a list ofunits which may be responsible for thatparticular symptom then look for unitscommon to both lists, i.e. units which couldcause both symptoms.

Machine Fault 1.Slow speed of carriage drive motor.

Step 1. The symptom is one of speed,therefore, the problem is Flow.

Step 2. Consult the circuit diagram andidentify the units and their function.

Step 3. List the units that could affectthe flow to the carriage drive motor.

(The object of this step is to narrowdown the number of units to be checkedas much as possible. It is importanthowever not to apply too muchjudgement at this stage, since it is betterto have a long list rather than a shortone which misses out the vital unit.

It is very easy to overlook a componentwhen drawing up the list, so for mostsystems it is safest to go through theparts list numerically and consider eachunit in turn.)

Referring to the circuit diagram on page37 the following units may cause theproblem:

Unit # Comments9 A partially closed inlet valve

could starve the pump andhence the system of fluid (Un-der these circumstances thepump would probably be cavi-tating and hence noisy but asmentioned above. Do not applytoo much judgement at thisstage.)

10 A blocked inlet filter couldcause the same effect

11 Low output from the pumpwould affect the carriage drivemotor since the flow to themotor is not throttled by anyflow control valves.

13 A partially closed shut-offvalve may have the sameeffect as a low pump output.

16 A partially closed check valvewill also have the same effect.

17 A leaking relief valve againreduces the effective pumpoutput.

18 A blocked return line filter mayrestrict the exhaust flow fromthe motor. (Although a by-passis fitted, the filter may havebeen installed the wrong wayaround.)

20 It can be assumed that thepurpose of the accumulator inthis system is to supplementthe pump flow, therefore anincorrect pre-charge pressuremay affect the flow rate fromthe accumulator.

21 A partially closed isolatingvalve would tend to restrictthe flow from the accumulatorand hence to the motor.

22 A blow-down valve left openwould reduce the effectiveflow from both the accumula-tor and the pump.

30 A directional valve leaking Pto T could reduce the flow tothe carriage drive motor.

31 As above.34 An excessive leakage down

the drain line of the pressurereducing valve could againreduce the flow to the carriagedrive motor.

35 A restriction in the directionalvalve controlling the carriagedrive motor (caused by a largepiece of swarf, incomplete spoolmovement or wrongly adjustedstroke adjusters for example)could reduce the flow to themotor.

Unit # Comments36 &37

Although a leaking relief valvemay affect the flow to themotor, two faults would haveto occur to affect the speed inboth directions ie. either 36 &37 both leaking or 36 & 38leaking or 37 & 39 leaking.While this is not impossible, itis probably very much lesslikely.

40 As for 30 and 31.42 As above.48 A worn or damaged motor

having excessive drainleakage would reduce theeffective flow to the motor.

49 &50

From the solenoid energiza-tion chart it can be seen thatneither the traverse cylindernor the eject rotary actuatorare operating at the sametime as the carriage drivemotor. Therefore, a leakage ineither of these two actuatorswould not affect flow to themotor.

51 Because the clamp cylindersare held down under pressurewhen the carriage drive motoris operating (refer to solenoidenergization chart) a leakageacross the piston seals couldaffect the flow to the carriagedrive motor.

52 As above.

Machine Fault 2.Slow speed of traverse cylinder whenextending.

Step 1. Again the symptom is one ofspeed, therefore the problem is Flow.

Step 2. Consult the circuit diagram andidentify the units and their functions.

Step 3. List the units that could affectthe flow to the traverse cylinder whenextending.

Unit # Comments

9 See Machine Fault 1.









36

Unit # Comments Cont’d








44 Failure of the by-pass checkvalve, i.e. jammed closed, inthe flow control valve wouldrestrict the flow to the traversecylinder when extending.

49 A leakage across the pistonseals of the traverse cylinderwould reduce the effectiveflow rate to the cylinder.



The units common to both lists ie.units which on their own could causeboth machine faults are as follows:

9, 10, 11, 13, 16, 17, 18, 20, 21, 22,30, 31, 34, 35, 40, 42, 51, 52

Therefore from a total of 52 units on thesystem the search has been narroweddown to 18 by doing no more than lookat the circuit diagram, highlighting theimportance of being able to identify andunderstand circuit diagrams.

Step 4. Arrange the list in order ofchecking

The order in which units are checked ispurely arbitrary and may be influencedby such things as past experience,layout of components, position ofgauges etc., however, some units willinevitably be easier to check thanothers.

� Shut-off valves can be checked veryeasily to ensure they are in the correctposition, so these may come first onthe list – 9, 13, 21, 22.

� Assuming indicators are fitted to thefilters, these can also be checkedeasily – 10, 18.

� A pressure gauge is fitted to the pumpoutlet port so the setting of the pumpcompensator and relief valve can bechecked quite readily – 11, 17.

� A pressure gauge is fitted to reducingvalve 34 so the setting of this valvecan be checked – 34.

� The clamp cylinders can be quicklychecked for abnormal symptoms – 51,52.

� The directional valves can now bechecked for any abnormal symptoms –30, 31, 35, 40, 42.

� Finally check valve 16 can be checkedfor abnormal symptoms giving thecomplete list in order as –9, 13, 21,22, 10, 18, 11, 17, 34, 51, 52, 30, 31,35, 40, 42, 16.

Step 5. Preliminary check

Before going to the trouble of fittingadditional pressure gauges, flow meters,etc., or of removing pipework, there arecertain things that can be checked withthe instrumentation already installed, orwith the human senses of sight, touch,and hearing. Unless this step is carriedout, it is very easy to overlook what inhindsight appears a very obviousproblem, i.e. look for the obvious first).

For each of the units on the list thefollowing questions should be answeredwhere applicable.Is the Unit correct? (model number)Is the unit installed correctly?Is the unit adjusted correctly?Is the external signal correct?Does the unit respond to the signal?Are there any abnormal symptoms(heat, noise, etc)?Assume that the preliminary check iscarried out for each unit up to the clampcylinder 52, which is unusually hot.

Step 6. Algotest.

Having discovered that one unit exhibitsabnormal symptoms, i.e. heat, the unitcan now be checked in more detail byreferring to the system test sheet forcylinders ie. Algo G.1. Assume thathaving removed the connection from therod end of the cylinder and pressurizedthe full bore side fluid is found to beleaking from the rod end connection,i.e. the piston seals have failed.

Bar trimming machine layout

Step 7. The failed unit has now beenlocated and a decision can be madewhether to strip the unit in situ and makea repair or replace the complete unit,removing the failed unit to the secondline service area for further examination.

Step 8. Think!

Having made the repair or replaced theunit, thought should be given to both thecause and the consequence of thefailure. Assuming the cylinder pistonseals were damaged, the followingquestions should be answered beforethe machine is restarted:What caused the seals to becomedamaged?– contamination?– heat?– wrong seals?– wrong installation? etc.

Will the failure have had any effect onthe rest of the system?– If the seal has broken up, have

particles of rubber entered the system?– Has the oil become overheated and

oxidized?– Have any valves been adjusted to

compensate for the leakage and willnow require re-adjusting, etc.

By thinking about the cause and theconsequence of the failure, the sameproblem (or a consequential one) maybe prevented at a later date.

Carriage traverse cylinder

Clamp cylinders

Carriagedrivemotor

Ejectorrotaryactuator

37

Circuit diagram for bar trimming machine

A B

P T

A B

P TP T

A B

P T

A B

P T

A B

P T

A B

1

2

3

4

5

67

8

9

10

11

12

13

14

15 1617

18

19

20

21

22

23

24

25

26

2730

31

32 33

34

35

36

37

38

3940

41

42

43

44

45

46 47

48 49

50

51 52

M

a

80 bar

Pumpset

Reservoirassembly

Manifoldblock

Accumulatorstand

Pre-charge50 bar

Manifold block

35 barPressure

Tank

51 bar

DRa DRb

DR T

b a

a b

b ab ab a

A B C D E F

G H I J

EjectorCarriage traverse

DRc

Carriage driveClampClamp

70 bar

24bar

P

a b a b a b a b a b a0 0 0 0 0 0 0 0 0 0 10 1 0 1 0 0 0 0 0 0 10 1 0 1 0 1 0 0 0 0 10 1 0 1 0 0 1 0 0 0 10 1 0 1 0 0 0 1 0 0 10 1 0 1 1 0 0 0 0 0 11 0 1 0 0 0 0 0 0 0 10 0 0 0 0 0 0 0 1 0 10 0 0 0 0 0 0 0 0 1 1

30 31 35 40 42 17ValveSol

Solenoid Chart

Logical Troubleshooting in Hydraulic Systems

Documents

carriage drive motor

operate valve manually

valve adjusted correctly

venting isolation valve

motion taking place

pump shaft rotating

water inlet temp

cavitating consult fcr1