Pneumatic and hydraulic maintenance questions.

MODULE TITLE : APPLICATIONS OF PNEUMATICS AND

HYDRAULICS

TOPIC TITLE : SPECIFICATION, SELECTION AND

MAINTENANCE OF EQUIPMENT

LESSON 6 : MAINTENANCE OF HYDRAULIC EQUIPMENT

APH - 3 - 6

© Teesside University 2011

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School of Science & Engineering

Teesside University

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INTRODUCTION ________________________________________________________________________________________

In this lesson we deal with the functions of a hydraulic fluid, and consider the

importance to the operation of the system of keeping the fluid in a satisfactory

condition. The second part of the lesson is concerned with the general routine

maintenance requirements of a hydraulic system.

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YOUR AIMS ________________________________________________________________________________________

On completion of this lesson you should be able to:

• state the functions of a hydraulic fluid

• explain the fluid properties required to fulfil these functions

• list the commonly used fire resistant fluids

• describe the precautions to be taken for correct storage of hydraulic

fluids

• design a preventative maintenance programme for a simple hydraulic

system.

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PROPERTIES OF HYDRAULIC FLUIDS ________________________________________________________________________________________

The prime function of the fluid in a hydraulic system is to transmit power to

perform useful work.

The hydraulic fluid must transmit an applied force from one part of the system

to another, and reproduce immediately any change in direction or magnitude of

the force transmitted.

In order to perform its prime function, as efficiently as possible, the fluid must

be virtually incompressible and exhibit very low resistance to flow.

The system fluid must also perform a number of secondary functions of equal

importance.

These secondary functions will include:

• the provision of adequate lubrication for all system components

• the fluid must have sufficient film strength to seal clearances between

moving parts and minimise the rate of internal leakage in the system

• the fluid must cool and clean the system by transporting generated

system heat and contamination back to the filter and the reservoir

• the fluid must protect the component parts of the system from

corrosion and not react chemically with any of the materials that it

comes into contact with.

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For a modern day hydraulic fluid to be able to fulfil both its primary and

secondary functions it must possess certain properties. The more important of

these properties are as follows.

Viscosity

Hydraulic fluid must have a suitable viscosity. Viscosity is the internal

resistance of the fluid to flow. A fluid with a low viscosity is said to be thin

and flows quite easily whereas a fluid of high viscosity is thick and will not

flow so readily.

In a hydraulic system viscosity is always a compromise: a fluid of low

viscosity is best for transmitting power but a fluid of a higher viscosity will be

better for reducing internal leakage and lubricating components.

Viscosity may be either dynamic or kinematic; however, for hydraulic fluids it

is usual to express viscosity as kinematic and measure it in mm2/sec or

centistokes (cSt.) at 40 °C.

What do you think the consequences will be of

(a) using a fluid of too low a viscosity

(b) using a fluid of too high a viscosity?

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Answer

If a fluid of too low a viscosity is used this will result in increased leakage,

delayed response, increased wear and a reduction in pump and actuator

efficiency.

If a fluid of too high a viscosity is used this will result in cavitation, sluggish

operation, increased pressure drop and excessive heat generation.

Viscosity Index

A hydraulic fluid's viscosity should remain relatively stable at varying

temperatures and most fluids used in hydraulic systems have a high Viscosity

Index (V.I.).

This is an arbitrary indication of a given fluid's rate of change of viscosity with

a certain change in temperature. A low viscosity index signifies a relatively

large change in viscosity with temperature, while a high V.I. shows a relatively

small change in viscosity with temperature.

Initially, the V.I. scale ranged from zero to 100 representing the poorest to the

best V.I. characteristics known. However, over recent years chemical additives

and improved refining techniques have increased the Viscosity Index of some

oils to well beyond the 100 mark.

Most hydraulic fluids will have a viscosity index in excess of 80.

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Oxidation Resistance and Chemical Stability

A hydraulic fluid should be chemically stable and resist chemical change due

to the adverse effects of the following:

(a) high operating temperature

(b) contact with air

(c) the catalytic effects of metals and contaminants such as dirt and water.

Oxidation is the chemical union of the fluid with oxygen and results in the

fluid becoming increasingly acidic. It also promotes the formation of

insoluable gums and varnishes that plug orifices, increase wear and cause

valves to stick.

One of the main factors that governs the rate at which oxidation will take place

is operating temperature. The normal operating temperature for most

hydraulic systems, using mineral oil, is between 40 and 55 degrees centigrade;

this should be strictly adhered to as operating the system at a higher

temperature will dramatically increase the rate at which oxidation will occur

and reduce the working life of the oil.

Premium grades of hydraulic oil contain inhibitors that tend to decrease the

factors causing oxidation. However, the best defenses against high oxidation

are low operating temperature, periodic draining of water from the bottom of

the reservoir and frequent fluid changes.

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Lubricity

Hydraulic fluids must possess good lubricating properties to prevent undue

friction being created between the moving parts of hydraulic components.

Increasing operating pressures and speeds together with the reduction of

component clearances make component lubrication increasingly more difficult.

Good quality hydraulic fluids contain anti-wear additives which reduce wear

and provide adequate lubrication even under extreme operating conditions.

Demulsibility

Demulsibility is the ability of a hydraulic fluid to separate out water. Small

quantities of water can be tolerated in most systems but too much water will

result in emulsions being formed with the fluid. These can cause sticking

valves, accelerated wear and corrosion of system components. However, if the

oil has a good demulsibility characteristic, then entrained water will separate

out easily and fall to the bottom of the reservoir where it can be drained off.

Where do you think that this water contamination originates?

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Answer

Water will be ingressed into a hydraulic system through inefficient quality

seals on actuators and reservoirs; it will also be ingressed as humidity through

the tank breather and condense out on the tank inner walls when the system is

not in use.

Foaming and Aeration Resistance

Foaming and aeration of the fluid will cause erratic operation of system

actuators, cause pump damage, and lead to accelerated oxidation of the fluid.

Some of the more common causes of foaming are too low an oil level,

excessive turbulence of return oil and air leakage into the pump suction line.

Good quality hydraulic fluids will contain additives that control foaming and

encourage separation or air bubbles from the fluid.

All of the properties mentioned are essential for the efficient operation of a

modern hydraulic system. Most brands of fluid will have these properties to a

greater or lesser degree depending upon quality of fluid. It is very important

that you use the correct type of fluid of the correct quality and maintain the

fluid correctly if you wish to avoid serious hydraulic failures.

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HYDRAULIC FLUIDS ________________________________________________________________________________________

Mineral Oils

Mineral oils are probably the most widely used hydraulic fluid because they

are relatively inexpensive, readily available and can be supplied in various

viscosity grades.

Mineral oil has good lubricating properties, is not corrosive and is compatible

with most seal materials. Premium grades of hydraulic oil contain additives

which will increase the oil's resistance to wear, oxidation and foaming. The

additive package will also improve viscosity index and lubricity.

The principle disadvantage of mineral oils is that they will burn and this

precludes their use in applications where fire could be a hazard.

FIRE RESISTANT HYDRAULIC FLUIDS

In situations where fire is a hazard then a fire resistant hydraulic fluid must be

used.

There are four types of fire resistant fluid.

1. Water In Oil Emulsions are mixtures which include 30 – 40% water, oil

and an emulsifying additive package. Each drop of water is encased in a

skin of oil that breaks at high temperature to release steam that will

combat any fire outbreak.

2. Oil In Water Emulsions are mixtures of 5% oil and 95% water with an

emulsifying additive package. Because of the high water content of the

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fluid it is extremely fire resistant: its characteristics are more like water

than oil. It is highly incompressible with good cooling properties. The

main disadvantages are poor lubricity and low viscosity resulting in high

internal leakage losses.

3. Water Glycol Fluids consist of three main components and an additive

package. They are 40% water, glycol and a water soluble poly glycol.

The additive package is required to impart corrosion resistance, metal

passivation, anti-wear, and lubricity to the fluid. This type of fluid is

probably the most effective of all the fire resistant hydraulic fluids

showing the least tendency to burn. However, water glycol fluid will

attack certain metals so that components containing zinc, magnesium

cadmium and aluminium cannot be used in the system. Water glycol will

also attack most paints.

Fire resisting fluids containing water should only be used for medium temperature

applications; why do you think this is so?

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Answer

At higher temperatures the water content tends to evaporate resulting in a

decrease in the fluid's fire resistant properties. For optimum life, operating

temperatures of hydraulic fluids containing water should not exceed 20 °C.

4. Phosphate Esters are synthetic hydraulic fluids which also exhibit

excellent fire resistant properties but because they do not contain water

they can operate at high temperatures. They also have good lubricity and

for these reasons are used in plastic moulding and die-casting machines

where the risk of fire is particularly high. Unfortunately phosphate ester

fluids will attack certain seal materials including the commonly used

Nitrile (Buna) and Neoprene seals. They are also by far the most

expensive of all fire resistant hydraulic fluids, being approximately 8

times the cost of straight mineral oils.

Note that particular care should be taken when handling synthetic fluids

as many are toxic or give off toxic fumes.

CHANGING A SYSTEM FLUID

If it becomes necessary to change the fluid in a system from a straight mineral

oil to a fire resistant fluid then certain precautions must be taken.

(a) All materials of construction must be checked for compatibility with the

chosen fluid.

(b) It is usual to de-rate the system by running the pumps at slower speeds

and lower pressures.

(c) The system must be thoroughly flushed to remove all traces of the

original fluid.

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Why do you think it is necessary to de-rate equipment when it is being used on a

system using a fire resistant fluid?

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Answer

Most hydraulic equipment is designed to be used with mineral oil as the

working fluid. Because fire resistant fluids have reduced lubricity, and

increased specific gravity, and because there is a tendency for water based

fluids to separate, the working life of most hydraulic equipment will be

reduced if it is not de-rated.

A list indicating the compatibility of common system materials with fire

resistant hydraulic fluids is given in FIGURE 1.

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FIG. 1

The fluid in a hydraulic system is to be changed from a water glycol mix to a

Phosphate Ester fluid. What changes with regard to system materials of construction

need to be made?

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Phos. Ester & Oil syn. blends

NC

C

C

C

C

C

C

C

C

NC

NC

C

C

C

Paints

Metals

Seals

Key

Common Industrial

Epoxy & Phenolic

Ferrous

Brass Copper

Zinc

Aluminium Unanodised

Aluminium Anodised

PTFE

Viton

Neoprene

Buna N

Butyl Rubber

EP Rubber

Silicone Rubber

Water-Glycol

NC

C

C

C

NC

NC

C

C

C

C

C

C

C

C

W/O Emulsion

NC

C

C

C

C

C

C

C

C

C

C

NC

NC

C

C = Compatible NC = Non-Compatible

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Answer

From the chart shown in FIGURE 1, to change from a water glycol to a

phosphate ester requires that a check should be made to ensure that the seals in

use in each component are not made from Neoprene or Buna N.

Hydraulic Fluid Maintenance

The hydraulic fluid is the life blood of the hydraulic system. Eighty percent of

all hydraulic faults occur as a result of fluid being contaminated, or as a result

of fluid properties being depleted due to overuse. It is therefore of extreme

importance that the fluid is maintained in a satisfactory condition and

adherence to some basic rules can save downtime and prevent system damage.

(i) Store hydraulic fluid drums on their side, keep them dry and cool and

preferably under cover.

(ii) Ensure the utmost cleanliness when topping up or renewing system fluid.

(iii) When filling or topping up the system, pump the fluid through a pre-fill

filter.

(iv) Sample and test the fluid to establish fluid change intervals so that

oxidation and fluid breakdown do not occur.

(v) Prevent fluid contamination at all costs and use proper fluid and air

filtration.

(vi) Repair all leaks immediately.

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Hydraulic System Maintenance Requirements

Prior to initiating a programme of planned preventative maintenance for any

hydraulically powered or hydraulically controlled equipment it is essential

that:

(a) the operation of the machine is fully understood by the maintenance team

(b) personnel involved have received adequate training in the maintenace and

operation of hydraulic equipment to allow them to efficiently and safely

provide the level of maintenance required

(c) all manufacturer's information, including circuit diagrams and data sheets

of all system components, should be made readily available to all

personnel involved with the maintenance of the machine.

From this information it should be possible to produce a data card for each

system that will briefly describe the hydraulic operations of the machine. The

information given on the data card would normally include:

(i) optimum operating pressure

(ii) the set pressure of relief valves or pump compensator controls

(iii) pump flow rates and running speeds

(iv) type of fluid used

(v) normal operating temperature.

The function of this data card is to provide maintenance personnel with all

relevant information regarding the hydraulic operation of the machine as

quickly and easily as possible. Attached to the data card should be a copy of

the circuit diagram.

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This data sheet now becomes the system benchmark and is used as a

comparator when any functional checks or inspections are made.

First line preventative maintenance on hydraulic equipment is normally the

responsibility of the machine operator. On a daily basis the operator will be

required to carry out routine checks of a simple nature. Typical routine checks

carried out by the operator are shown below.

A Daily Inspection of

(a) fluid level in the reservoir

(b) fluid temperature

(c) system operating pressure

(d) inspection of filter tell-tales

(e) general inspection of equipment for signs of leakage and malfunction.

This inspection would normally take place at the beginning of each day or shift

and take less than 15 minutes.

All data collected, system pressures, temperatures and so on, should be logged

and compared with the norm indicated on the data card. Fluid level should be

topped up as required and the amount of fluid added to the system should also

be recorded.

The data card will indicate the type of fluid to be used. Tell-tales should be

inspected for indication of filter blockage which should be reported

immediately to maintenance for changing. Dates of filter changes should be

logged.

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On a Weekly Basis the above inspections should be carried out by a member

of the maintenance team.

He/she will carry out a much more detailed general inspection of the system

for signs of component faults, loose and damaged pipe work, leaks, and so on.

He/she should also total up the oil additions to the system for the week, log all

results and countersign the machine hydraulic log.

At Six Monthly Intervals, in addition to carrying out the normal weekly

inspections, maintenance personnel will also do the following.

(a) Measure the gas pre-charge pressure of any system accumulators and

adjust as necessary.

(b) Sample the hydraulic fluid in the system and compare visually with a

sample of unused fluid. The use of a Patch Test Kit will make this

comparison much easier and much more meaningful.

Patching a hydraulic fluid involves removing a representative sample of

fluid from the system and drawing it through a fine filter membrane (1 – 5

micron). Particulate contaminant including sludges and varnishes will be

deposited on the filter membrane which can be inspected under a simple

microscope. Comparison with a 'patch' of unused fluid will give an

indication of the degree of contamination.

If a much more precise measure of oil degradation is required, the sample

can be sent to the fluid manufacturer who usually provides a fluid

analysis service for customers.

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(c) Examine piston rods for signs of scratching and scoring. Log any

remedial action taken.

(d) Total the oil consumption of the machine for the six month periods and

multiply by current cost per litre to give an estimate of the cost of oil

leakage for this period.

What, useful purpose do you think is being served by estimating the actual cost of oil

leakage?

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Answer

The purpose of this exercise is to make operators and maintenance personnel

aware of the high cost of fluid leakage and to increase their diligence in

preventing unnecessary fluid loss.

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At Twelve Monthly Intervals a member of the maintenance team should:

(a) remove and clean the pump suction filter/strainer

(b) carry out a pump performance test to ascertain the actual pump output at

normal working pressure

(c) disconnect the pump coupling and measure 'lift and float' on the pump

and drive motor shafts to determine shaft bearing condition, record and

compare data with manufacturer's data.

What do you understand by the term 'lift and float' of the pump or motor shaft?

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Answer

By 'lifting' the pump or motor shaft the amount of radial bearing clearance will

be sensed; it can be measured with a dial test indicator; 'floating' the shaft

involves moving the shaft along its axis to check for end-float on the shaft

bearings.

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This continual monitoring of system operation will establish very quickly a

machine history which will allow undesirable trends to be spotted and

corrected before they have had time to develop into full blown problems. Any

reading that varies by more than 10% from the norm should be investigated

immediately. System monitoring of this type will greatly reduce machine

downtime due to unplanned failure of hydraulic equipment .

However, because it is so successful at preventing failure then it is possible to

become complacent and start to reduce the level of system care provided. This

should be resisted at all costs as a reduction in attention will bring about an

increase in system failure and machine downtime.

The information given will form the basis for a preventative maintenance plan

that is general to all hydraulic systems. When designing a programme for a

specific machine it will be necessary to consider certain other factors to ensure

that the level of maintenance provided is sufficient to keep the system at the

operational level required.

These factors will include:

(i) the importance of the h ydr aulic system to the pr ocess and the

consequences of system failure

(ii) the working environment with regard to temperature, dirt or moisture

(iii) the proportion of operating to idle time for the machine

(iv) the degree of hydraulic expertise and equipment available within your

maintenance organisation.

When hydraulic system faults do occur, maintenance personnel must be

encouraged to think and adopt a logical approach to locating and rectifying the

fault.

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The fault must be correctly identified and all possible causes of that fault be

listed before any attempt is made at a repair.

Each possible cause must be investigated, the most probable cause being dealt

with first.

Use should be made of test equipment, pressure gauges, flow meters and so on,

to identify the failed component, and components should not be changed at

random to try and identify the fault as is often the case.

This completes this lesson on hydraulic fluids and hydraulic system

maintenance requirements. Now attempt the Self-Assessment Questions.

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SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. List four functions of a hydraulic fluid.

2. Define 'viscosity' and state the S.I. unit by which it is measured.

3. What effect does temperature have on viscosity?

4. What is demulsibility?

5. How does moisture get into a hydraulic system?

6. List the features of a hydraulic system that should be monitored every day

during a routine maintenance inspection.

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ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Functions of a hydraulic fluid will include:

(a) the transmission of power

(b) the lubrication of system components

(c) sealing internal clearances and reducing leakage

(d) the transmission of heat and contamination back to the filters and

system reservoir

(e) the protection of system components from corrosion.

2. Viscosity is a measure of a fluid's resistance to flow, and in hydraulic

systems we refer to its kinematic viscosity which is expressed in mm2/sec,

or centistokes, usually at 40 °C.

3. Viscosity is affected by temperature; increase in temperaure will cause a

reduction in viscosity, with the oil becoming thinner, and a decrease in

temperature will increase the viscosity of the fluid. The rate of change of

viscosity due to temperature change is controlled by the fluid's viscosity

index and most hydraulic fluids will have a viscosity index in the region

of 100.

4. Demulsibility is a fluid's ability to separate out water.

5. Moisture will be ingressed into a hydraulic system through inefficient

seals on cylinder rods, for example, and will also be ingressed into the

reservoir in the form of humidity.

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6. Features of all hydraulic systems that require checking every day are:

(a) the level of fluid in the reservoir

(b) the condition of filter tell-tales

(c) the system operating temperature

(d) the system operating pressure

(e) any evidence of external leakage.

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SUMMARY ________________________________________________________________________________________

The prime function of a hydraulic fluid is to transmit power through the

hydraulic system. It does, however, fulfil several secondary functions which

include cooling and sealing of the system, lubricating and protecting

components against corrosion.

The most commonly used fluid is straight mineral oil but this is unsatisfactory

for systems where fire is a major hazard and for this reason a range of fire

resistant hydraulic fluids has been developed. Common fire resistant fluids are

oil/water emulsions, water glycol mixes and synthetic fluids which are mainly

phosphate esters.

To get the best out of the system fluid it should be stored correctly prior to

being used and once in the system should be maintained in a satisfactory

condition by good filtration to remove contamination, by operating the system

at appropriate operating temperatures and by regular inspection of fluid

condition.

For trouble-free operation of hydraulic equipment it is usual to initiate some

programme of maintenance inspections and checks in an attempt to detect

deterioration of the system which could lead to major faults. First line

maintenance checks would normally be carried out by the machine operator

checking such things as fluid level and temperature. However, he/she should

then be supported by maintenance personnel carrying out a much more

detailed programme of inspections and tests at less frequent intervals.

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