Lubrication Fundamentals 2011 Part 5 pps

Rotary Fluid Motors Rotary fluid motors are used instead of hydraulic cylinders to convert fluid energy tomechanical motion, especially when rotary motion is required or continuous or lo

Figure 7.18 Two-pump circuit with unloading valve.

C Unloading Valves

In some circuits, two pumps may be used to meet substantially variable flow requirements

In the case, shown in Figure 7.18, flow from the larger volume pump is used to ensurethe rapid advance of the machine tool to a certain point but then, as the work activity isperformed, the volume of the flow decreases markedly During the rapid advance cycle,the discharge volume of both pumps is required but while actual work is being performed,only the small pump volume is required, causing a rise in system pressure As the pilotpressure rises (pilot pressure set below relief valve pressure setting), the unloading valveopens, allowing the flow volume from the larger pump to be discharged to the reservoir

at low pressure If one constant volume pump were used, most of the oil pumped duringthe work cycle would be discharged at full system pressure through the relief valve Itsenergy would be wasted and excessive heating could occur Use of a variable volumepump could be an alternative to the method featuring two pumps and an unloading valve

D Sequence Control Valve

In some machines, two or more movements may need to be hydraulically operated insequence When one movement must not begin until another has ended, a pressure-operated

sequence valve may be used Referring to the circuit shown inFigure 7.19,when oil flowstops at the end of the clamping-cylinder stroke, pressure rises sufficiently to open valve

A Full line pressure is then available to activate the feed cylinder Sequence valves can

also be activated by pressure-sensing pilot valves or electronically, by position or otherpressure-sensing devices

Figure 7.21 Flow control valve: the orifice shown in the meter-in circuit inFigure 7.20usuallywill not maintain constant speed throughout the work stroke The workload will usually vary, causingthe pressure drop across the orifice and flow to vary To maintain constant pressure drop and flow,

a pressure compensator such as shown may be used.

the right Since the two pressures act on equal areas, the pressure at A will be greater than the pressure at B by a constant amount determined by the spring tension Any imbalance will cause the spool to move in a way that opens or throttles the inlet, at C, and restores

the balance With constant pressure drop across the orifice, the flow remains constant

F Accumulators

Although not control valves, accumulators can act as flow control mechanisms As we

briefly discussed in connection withFigure 7.3,accumulators can be used to store energy

of an incompressible fluid Some presses and other machines require large volumes of oilunder pressure for short duration cycles with relatively long periods of time betweencycles A pump and motor large enough to generate the necessary flow and pressure forsuch an application could be very costly Instead, accumulators (Figure 7.22),in conjunc-tion with small pumps and motors, can often be used as shown inFigure 7.23.Energy isstored in the accumulator by the pump during the long periods between high energyrequirements This is done by pumping hydraulic fluid into the accumulator and raising

a weight, compressing a spring, or compressing a gas charge The energy is returned tothe system when required

In addition to energy storage capacity, accumulators serve other functions Manyhydraulic systems are subject to rapid or sudden flow changes where the dynamics of

fluid (incompressible) in motion can create high levels of system shock In these situations,

accumulators act as shock absorbers They are able to reduce the severity of the systemshock by allowing the instantaneous pressure rises to be taken up by the compressiblemechanisms within the accumulators This helps reduce the potential for line breakageand component failures in those high flow, high pressure systems that are subject to abruptchanges in flow They can also be used to smooth out flow by absorbing pump pulsations,maintain constant pressures for long periods of time, such as in clamping operations, andmake up for system internal leakage Most hydraulic applications use gas-pressurizedaccumulators

IV ACTUATORS

A Hydraulic Cylinders

At the point of use, hydraulic energy must be converted to mechanical energy and motion

The device most frequently used is the hydraulic cylinder (Figure 7.24), sometimes called

a reciprocating motor Hydraulic cylinders may be single-acting (pressure applied in onedirection) or double-acting They are made in sizes ranging from less than an inch toseveral feet in diameter Working strokes of up to several feet and pressure ratings of up

to several thousand psi are available Single-rod-end cylinders, as shown in Figure 7.24,are most common, but double-rod-end cylinders (rod extends through both heads, as shownearlier:Figure 7.9)can be obtained By means of flanges, extended tie rods, and adapters,cylinders can be mounted in many positions Cylinders are usually made of steel tubing,bored and honed Rods are often hard chrome-plated steel, ground and polished All sur-faces in contact with seals and gaskets are finely finished Typical double-acting cylindersare illustrated The left-hand view of Figure 7.24 features a cartridge-type rod gland thatcan be removed without dismantling the cylinder Accessibility of the rod gland is impor-tant from the maintenance point of view The piston is sealed by means of cast iron pistonrings, which are very satisfactory where a small amount of leakage can be permitted O-ring static seals, which are very popular for hydraulic service, are used The right-handview of Figure 7.24 is cushioned at both ends As the piston nears the end of its stroke,the cushion plunger enters the head counterbore, shutting off oil flow The stroke can then

continue only as fast as oil can flow through passage A and past the cushioned valve At

the beginning of the next stroke, oil flows past the spring-loaded ball-type cushion checkvalve and builds up pressure against the full face of the piston Unless other means areprovided to decelerate the piston, cushioning becomes more and more necessary at higherpiston speeds The piston in this case is sealed by means of cup-type seals, which permitlittle or no leakage V-ring packings are similar in this respect

Figure 7.24 Hydraulic cylinders

In addition to single and double-acting cylinders described earlier, and the ram anddouble-rod cylinders also discussed earlier in this chapter, several other types are com-

monly used in industry These are spring return, telescoping, and tandem types Spring

return cylinders are components of the single-rod type with pressure to activate the workstroke, and the spring in the nonpressurized end returns the piston to its original positionwhen pressure is released Telescoping cylinders are used where long strokes are requiredbut space limits the length of the cylinder These applications also have lower load require-ments as the cylinder rod extends These are made of multitubular rod segments fit intoeach other to provide the telescoping action Tandem cylinders are used when high force

is required and length is not limited but larger diameter cylinders cannot fit in the availablespace These are mounted in-line with a common piston rod

B Rotary Fluid Motors

Rotary fluid motors are used instead of hydraulic cylinders to convert fluid energy tomechanical motion, especially when rotary motion is required or continuous or long move-ments in one direction are involved They compete with electric motors under the followingconditions: (1) when a variable speed transmission having a wide range of closely con-trolled speeds and torques is required, (2) when space limitations demand a very compactpower source, or (3) when torques or loading might occasionally be severe enough tooverload an electric motor Rotary fluid motors can be of gear, vane, or piston (radial oraxial) design They are similar to their counterparts in pumps but differ in certain detailsthat affect efficiency In fact, some radial and axial piston pumps are designed to act asboth a pump and a motor, as described in connection withFigure 7.11,but in these designs,they do lose some efficiencies Rotary fluid motors are supplied with oil under pressureand rotate at a speed and torque dependent on the available volume of oil that flowsthrough the motor

Radial and axial piston motors are usually of the constant displacement design butare available in variable displacement forms In a constant displacement axial piston motor

(Figure 7.25), the motor shaft is supported by ball bearings A and B The cylinder barrel

Figure 7.25 Axial piston fluid motor

is supported by the motor shaft and is keyed to by means of a splined joint The drive

plate is supported at an angle by ball bearing C Ports are arranged so that oil under

pressure starts to enter a cylinder when its piston is at the end of its inward stroke Thepressure causes the piston to push against the drive plate Part of this force is perpendicular

to the drive plate and part is tangential The tangential force causes the drive plate, cylinderbarrel, pistons, and motor shaft to rotate The torque developed depends on oil pressureand the motor dimensions The speed depends on the rate of oil flow

Axial piston motors exhibit high volumetric efficiencies and excellent operationover a wide range of speeds They can be used where torque requirements are up to morethan 17,000 ft-lb or speeds up to 4000 rpm Radial piston motors can attain several hundredthousand foot-pounds of torque or speeds up to 2000 rpm Gear-type motors will provide

up to 6000 ft-lb of torque or up to 3000 rpm It is important to recognize that since thetorque of hydraulic motors is inversely proportional to rotational speed, their highesttorques will occur at low speeds

V HYDRAULIC DRIVES

Hydraulic drives are classified as hydrostatic or hydrodynamic These can be designed to

produce power in three ways: variable power and torque, constant power and variabletorque, and variable power and constant torque Hydrostatic drives use oil under pressure

to transmit force while hydrodynamic drives use the effects of high velocity fluid totransmit force Engine-driven transmissions widely used in main and auxiliary drives ofmobile construction and farming equipment is an example of hydrostatic drives Torque

converters (sometimes referred to as hydrokinetic drives) are commonly found in

automo-tive applications but are finding increased use in industrial applications Another form of

a hydrokinetic drive is the hydroviscous drive This form uses the viscosity characteristics

of the fluid rather than the energy from fluid in motion to develop the drive torque.Hydrodynamic drives include hydrokinetic and hydroviscous drives

A Hydrostatic Drives

In a hydrostatic system, power from an electric motor, internal combustion engine, orother form of prime mover is converted into static fluid pressure by the hydraulic pump.This static pressure acts on the hydraulic motor to produce mechanical power output.While the fluid actually moves through a closed-loop circuit between the pump and motor,energy is transferred primarily by the static pressure rather than the kinetic energy of themoving fluid

The hydraulic pump in a hydrostatic system is of the positive displacement type.Either fixed or variable displacement is acceptable, but the majority of systems use variabledisplacement Axial piston pumps are the most commonly used, although radial pistonpumps are used in some applications The motor in a hydrostatic system can be any positivedisplacement hydraulic motor Axial piston motors are usually used for most drives, butboth gear motors and radial piston motors are used for specific designs The motor isusually of fixed displacement type, but variable displacement is acceptable The motor isreversible, with the direction of rotation dependent on the direction of flow in the closed-loop circuit from the pump.Figure 7.26shows a diagram of a typical hydrostatic drive

VI OIL RESERVOIRS

The oil reservoir is also a very important component of the hydraulic system It containsthe oil supply, provides radiant and convection cooling, allows solid contamination andwater to drop out, and helps reduce entrained air from circulating to the critical controlcomponents In addition, in relation to oil levels and pump location, the oil reservoirfacilitates easy flow of the oil to the pump suction, reducing the potential for cavitation orstarvation conditions Proper design and care of oil reservoirs will help assure satisfactorycomponent service life and long oil life

A Reservoir Design

When possible, the oil capacity of a hydraulic reservoir should be at least 2.5 times therate of oil circulation (pump capacity) at full operating conditions This rule does notapply to closed-loop systems, such as the hydrostatic drives, in which the oil flows backand forth between the pump and motor without returning to the reservoir In these systems,the reservoir is used only for oil makeup to the system necessitated by internal leakage(or other system leakage) Figure 7.27 shows a typical reservoir configuration for non-closed-loop systems The proportions shown are suitable If the reservoir is too shallow,there may not be enough sidewall area in contact with the returning oil for effective coolingand conditions that allow air to enter the pump suction (vortexing) may be promoted Onthe other hand, if the reservoir is too deep and narrow, there may not be enough surfacearea for separation of the air in the oil The baffle aids cooling and contamination separation

by promoting flow along the sidewalls The baffle also helps to prevent short-circuiting

of hot oil from the return on one side to the pump suction line on the other The bottom

of the reservoir is dished, and the drain is located at the lowest point This design notonly permits complete drainage at oil change time, but allows occasional removal of waterand other heavier-than-oil contaminants that separate in the reservoir Space below the

Figure 7.27 Oil reservoir design

reservoir permits the use of hoses, pans, and so on, to permit oil to be drained withoutspillage The reservoir cover is welded on in this example, but bolted and gasketed coversare often used Large clean-out doors are provided at one or both ends Other designfeatures include a filler hole with cap and screen, a level gage, a breather with a filter,and gasketed seals for clearance holes where pipes pass through the cover Return linesshould be extended well below the oil level to reduce misting and aeration of the oil.The reservoir design discussed is a separate component of most typical hydraulicsystems and is exposed to atmospheric pressure In a pressurized reservoir, sometimesused to positively charge the pump suction, the level of pressure must be considered

in the design The pressurized reservoir reduces the potential for atmospheric tion such as moist air or other airborne contamination to enter the reservoir In addition

contamina-to these designs, a space-saving integral design can be used In these, the reservoir

is built into the machine as, for example, a fluid-tight base or hollow member of thesupport structure that can hold oil without requiring additional space for a separateoil reservoir The same rules and precautions apply to these alternate designs Theneeds remain for the oil to be cooled, contamination removed, and adequate pumpsuction supply provided

VII OIL QUALITIES REQUIRED BY HYDRAULIC SYSTEMS

As the discussion of hydraulic system components has indicated, the hydraulic fluid is avery important component that is often casually considered Most often, satisfying therequirements of only the pump seems to be the primary consideration for fluid selection.Although the costs of hydraulic pump failures are generally one of the more costly occur-rences within hydraulic systems, erratic operation of valves and actuators due to inadequateoil performance characteristics such as oil degradation (oxidation) that causes deposits toform in critical clearance areas, often leads to costly production losses With the closeclearances, different metallurgies, various elastomers, and high pressures and temperatures,service life and performance of all the system components depend on proper selectionand maintenance of the hydraulic fluids

Hydraulic fluids perform many functions in addition to transmitting pressure andenergy These include minimizing friction and wear, sealing close-clearance parts fromleakage, removing heat, minimizing system deposits, flushing away wear particles andcontamination, and protecting surfaces from rust and corrosion The important characteris-tics of a hydraulic fluid vary by the components used and the severity of service.Chapter

3dealt with product characteristics and testing in some detail A number of the physicalcharacteristics and performance qualities of hydraulic fluids commonly required by mosthydraulic systems are listed as follows:

ViscosityViscosity index (VI)Wear protection capabilityOxidation stabilityAntifoam and air separation characteristicsDemulsibility (water-separating characteristics)Rust protection

Compatibility

Some specific applications may require the following:

Soluble oilsHigh water content fluidsFire-resistant fluidsEnvironmental performance

A Viscosity

The single most important physical characteristic of a hydraulic fluid is its viscosity.Viscosity is a measure of the oil’s resistance to flow, so in hydraulic systems that aredependent on flow, viscosity is important with respect to both lubrication and energytransmission Although viscosity requirements are to some extent dictated by the compo-nents (pumps, valves, motors, etc.) and by system manufacturers, certain effects of im-proper viscosity selection need to be recognized Too low a viscosity can lead to excessivemetal-to-metal contact of moving parts, as well as to wear and leakage Too high a viscositycan result in excessive heating, sluggish operation (particularly at start-up), higher energyconsumption, lower mechanical efficiencies, and increased pressure drops in transmissionlines and across filters Since viscosity decreases as temperature increase, viscosity require-

ments are generally specified at operating temperature If temperatures are higher than those specified for normal operation, a higher viscosity oil may be required to provide

long service life of the components If start-up and operating temperatures are lower thanthose specified, then a lower viscosity oil may prove to be better for overall systemperformance Systems operating over a wide range of temperatures may require oil thatexhibit high VIs Hydraulic systems normally use oil with a viscosity range of 32–68 cSt

at 40⬚C (150–315 SUS at 100⬚F) To ensure flow to the pump, most hydraulic equipmentbuilders require that the viscosity at start-up temperatures not exceed 1515 cSt (7000SUS) Some builders, however, limit the start-up viscosity to 866 cSt (4000 SUS)

B Viscosity Index (VI)

The viscosity of all oils varies substantially with changes in temperature In some hydraulicsystems, subjected to wide variations in start-up and operating temperatures, it is desirable

to use an oil that changes relatively little in viscosity for a given temperature range An

oil that does this is said to have a high viscosity index High viscosity indexes can be

achieved by using mineral oil base stocks that have been refined through a severe cessing technique(Chapter 2)in conjunction with the use of long chain polymers called

hydropro-viscosity index improvers (VIIs) Mineral oil base stocks refined through conventional

methods can also achieve wide temperature range performance by the addition of viscosityindex improvers Synthetic hydraulic fluids with naturally high VIs are an alternative forsevere application temperatures—both high and low

Hydraulic fluids are subjected to high shear rate conditions, particularly as pressuresand speeds rise Viscosity index improvers are generally long chain polymers and, depend-ing on the type of polymer used, are subject to shearing over time This shear can result

in loss of viscosity and, if severe enough, in poorer hydraulic system performance owing

to increased potential for metal-to-metal contact, increased leakage, and loss of some ofthe friction-reducing qualities In the selection of a mineral oil based hydraulic fluid with

a high viscosity index obtained by the use of viscosity index improvers, good shear stabilityperformance should be required

C Antiwear (Wear Protection)

To assure satisfactory hydraulic component life, the hydraulic fluid must minimize wear.Wear results in loss of mechanical efficiency as well as higher costs due to shorter compo-nent life In some hydraulic systems, such as low pressure, low temperature systems withgear pumps, antiwear additives are not necessary Other high pressure, high temperaturesystems using vane pumps do require antiwear additives in the hydraulic fluid Pistonpumps may or may not require antiwear additives depending on the metallurgy and design

A properly refined petroleum oil has naturally good wear protection (without the use ofantiwear additives) This quality, sometimes referred to as lubricity or film strength, ispresent to a greater degree in oils of higher viscosity than those of lower viscosity How-ever, certain additive materials are used to improve antiwear performance in hydraulicoils These additives work by chemically reacting with the metal surfaces forming a strongfilm preventing metal-to-metal contact under boundary lubrication conditions The use ofeffective antiwear additives may allow the use of lower viscosity fluids without sacrificingpotential wear

Antiwear fluids are generally required in gear and vane pumps operating at pressuresabove 1000 psi and over 1200 rpm Piston pumps may or may not require antiwear additivesdepending on the specific manufacturer and the metallurgy used For example, DenisonHydraulics Inc prefers R & O oils for their piston pumps, whereas antiwear additives arerequired for Vickers piston pumps Denison Hydraulics typically uses bronze piston shoesagainst a steel swashplate, while Vickers may use steel on steel Steel against steel at highpressure will always require antiwear formulations Premium quality antiwear hydraulicfluids are formulated to provide performance in all pumps and hydraulic systems Because

a given industrial plant will use many different pumps and other component, it may beadvisable to consolidate the number of hydraulic fluids by using antiwear formulationsthat meet all the requirements

The antiwear performance of hydraulic fluids is evaluated in several standard try-recognized tests The major ones include the ASTM D 2882 (Vickers V-104C Pump),the Vickers M-2952-S (Vickers 35VQ25 Pump), and the Denison Hydraulics HF-0 (combi-nation of the P-46 piston pump test and the TP-30283A Vane Pump Test) These testswere discussed in some detail inChapter 6in connection with environmental hydraulicfluids

indus-D Oxidation Stability

Oil is circulated over and over during long periods in hydraulic systems It is heated bythe churning and shearing action in pumps, valves, tubing, and actuators Also, the energyreleased as the oil goes from high pressure to low pressure in a relief valve is converted toheat, which raises its temperature Oil can be further increased by convection or conductionheating while performing its work in applications such as the hot molds in plastic injectionmolding operations and continuous caster hydraulics in steel mills

The oil is in contact with warm air in the reservoir Air is also dissolved or entrained

in the oil Because of this contact with air, oxygen is intimately mixed with the oil Underthese conditions (exposure to temperature and oxygen), the oil tends to chemically combinewith the oxygen, creating oxidation products The tendency to oxidize is greatly increased

as temperatures increase, as agitation or splashing becomes excessive, and by exposure

to certain materials that catalyze the oxidation reactions Catalysts such as iron, copper,rust, and other metallic materials are commonly present in hydraulic systems

Slight oxidation is not harmful, but if the oil has poor resistance to this chemicalchange, oxidation may become excessive If this occurs, substantial amounts of both solu-ble and insoluble oxidation products are formed, and the oil gradually increases in viscosity.Some variation of viscosity within a range that has proved satisfactory in service is notnecessarily harmful However, viscosity higher than necessary is accompanied by higherfluid friction and more heating With many of today’s critical systems using electrohy-draulic servovalves with extremely close clearances, slight oxidation could result in theformation of deposits on the servovalve spools, restricting their movement and resulting

in production problems Low quality oils have poor resistance to oxidation under severeconditions With such oils, troubles of the kind just described often occur In high qualityhydraulic oils, the natural ability of well-refined, carefully selected base oils to resistoxidation is greatly improved by the use of additives that retard the oxidation process

E Antifoam/Air Separation Characteristics

The positive and accurate motion of actuators within a hydraulic system is dependent onthe virtual incompressibility of the hydraulic fluid Under high pressure, mineral oils cansee a very slight reduction in volume (4.0% at 10,000 psi and 140⬚F) and a correspondingincrease in density For purposes of the vast majority of hydraulic systems, this is consid-ered to be insignificant Introduction of air into the fluid can substantially change the

compressibility Air causes spongy or erratic motion, which will result in poor system

performance, particularly during the production of close-tolerance parts Antifoaming andair separation characteristics are two different concepts, although somewhat connected

‘‘Air separation’’ means that the entrained air is released from the oil, while ‘‘antifoam’’means that the air bubbles getting to the surface of the oil are readily dissipated Bothaspects are important to the performance of a hydraulic oil Contamination can alter boththese characteristics so it is not only important to select an oil that will provide goodantifoam and air separation performance but it is necessary to minimize contamination inorder to maintain this good performance

F Demulsibility (Water-Separating Ability)

Water contamination is sometimes a problem in hydraulic systems It may be present as

a result of water leaks in heat exchangers or washdown procedures, but more commonly

it accumulates because of condensation of atmospheric moisture Most condensation occursabove the oil level in reservoirs as machines cool during idle or shutdown periods Aclean hydraulic oil of suitable type will have little tendency to mix with water; and in astill reservoir, the water will tend to settle at a low point During operation, the water may

be picked up by oil circulation, broken up into droplets, and mixed with the oil, forming

an emulsion The water and oil in such an emulsion should separate quickly in the reservoir,but when solid contaminants or oil oxidation products are present, emulsions tend to persistand to join with other deposit-forming materials present to form sludge The emulsionmay be drawn into the pump and made more permanent by the churning action of thepump and the mixing effect of flow at high velocities through control devices

To prevent such contamination from occurring, it is essential that a hydraulic oilhave the ability to separate quickly and completely from water Properly refined oils havethis ability when new, but only oils having exceptionally high oxidation stability are able

to retain good water-separating ability over long service periods In addition to using

such an oil, every effort should be made to keep systems free of water, dirt, and othercontaminants.

G Rust Protection

Water and oxygen can cause rusting of ferrous surfaces in hydraulic systems In as much

as air is always present (except in specialized nitrogen blanket systems), oxygen is able, and some water is often present The possibility of rusting is greatest during shutdown,when surfaces that are normally covered with oil may be unprotected and subject togathering condensation as they cool This is particularly important for operations thatexperience high humidity conditions and temperature changes within the reservoirs.Rusting results in surface destruction, and rust particles may be carried into thesystem where they will contribute to wear and the formation of sludge like deposits certain

avail-to interfere with the operation of pumps, actuaavail-tors, and control mechanisms Rusting ofpiston rods or rams causes rapid seal or packing wear, resulting in increased leakageand system contamination; moreover, external contaminants can enter the hydraulic fluidthrough worn seals or packing

High quality hydraulic fluids contain an additive material, called a rust inhibitor,which has an affinity for metal surfaces It plates out on the surfaces, forming a barrierfilm that resists displacement by water and, therefore, protects the surfaces from contactwith water The rust inhibitor must be carefully selected to provide adequate protectionwithout reducing other desirable properties, especially water-separating ability

VIII SPECIAL CHARACTERISTICS IN HYDRAULIC FLUIDS

A Soluble Oils

Water serves as a hydraulic fluid in some very large noncritical systems Large forgingand extrusion presses with vertical in-line pumps or special axial piston pumps, for exam-ple, operate at 2000–3000 psi (136–204 bar) Because water has little ability to lubricate,seal, or prevent rust, it can be mixed with 2–5% soluble oil to form an oil-in-water emulsionfor these systems Operating temperatures are limited to 60⬚C (140⬚F) to prevent excessevaporation

B High Water Content Fluids (HWCF)

High water content fluids, sometimes referred to as 95/5 fluids, contain 2–5% soluble chemicals that impart some lubricity, rust protection, and wear protection of spe-

water-cially designed pumps, valves, packing glands, and cylinders They work well at pressures

as high as 10,000 psi (680 bar) in reciprocating plunger pumps, or in axial piston pumps

at pressures in the range of 1000 psi (68 bar) Because their antiwear protection is limited,they are used in vane pumps up to 1000 psi (68 bar) and are not recommended for systemswith gear pumps Again, operating temperatures must be limited to avoid excessive loss

of water by evaporation

C Fire-Resistant Fluids

When the possibility exists that the hydraulic fluid may come in contact with a source ofignition, fire-resistant fluids may be used The use of any fire-resistant fluid will meanadditional costs, but these may be offset by safety factors as well as high costs of equipment

in the event of fires The potential for fires exists in applications such as die-castingoperations, and continuous casting hydraulics in steel mills or presses that are operatednear furnaces or ovens In these and many other types of service, hydraulic line breakagecould result in serious fires

Three approaches to this problem follow One or a combination of them may beadopted, depending on cost for any particular application: (1) redesign or relocate machines

to remove or isolate sources of ignition; (2) install fire control measures such as firesuppression devices to assure the safety of the personnel and prevent or localize property

damage; and (3) use a fire-resistant hydraulic fluid.

In addition to the use of soluble oils and HWCFs just discussed, there are threebasic types of general-use fire-resistant fluid: synthetic, emulsion, and water glycol Theterm ‘‘fire-resistant’’ does not mean that the fluid will not burn For example, emulsionsand water glycol fluids depend on the water content for their fire-resistant characteristics.Once the water has evaporated, these materials will burn when subjected to a source ofignition Even the synthetics will burn when subjected to high enough temperatures

It should be noted that pumps and other system components must be specially signed or operating conditions derated for use of certain fire-resistant fluids In most cases,pumps are designed with special materials to provide satisfactory service life in equipmentusing fluids with high water content In addition, the pumps have specified maximumoperating parameters placing limits, for example, on pressures, temperatures, and speed.Component as well as system manufacturers should be consulted before designing systems

de-or using fire-resistant fluids

1 Synthetics

Synthetic fire-resistant fluids (typically phosphate ester materials) are the most expensivechoice They provide good overall performance compared to the other fluids, and lowermaintenance Their densities are high (heavier than water) and the VIs are low They havepotential problems with compatibility with paints, elastomers (seals and hoses), and othersystem materials They are not compatible with emulsions or water glycol fluids, andspecial precautions are necessary when converting to these from one of the other fire-resistant fluids A synthetic fluid may be partially or fully compatible with conventionalmineral oils Because the densities of such fluids are relatively high, additional pumpingrequirements may be necessary Polyol esters are also used as fire-resistant fluids andprovide better compatibility but may not provide the same level of fire resistance

oil as the continuous phase Conventional emulsions contain about 95% water, and theinverts contain between 40 and 45% water Fire resistance characteristics are provided bythe water, and maintenance of the correct water content is necessary to assure fire resis-tance Invert emulsions provide good lubrication characteristics, and various productsrange in viscosity from 65 to 129 cSt at 40⬚C (300–650 SUS at 100⬚F) The viscosity ofboth invert and conventional emulsions is affected by water content Invert emulsions willincrease in viscosity as the water content is increased, while conventional emulsions willdecrease in viscosity as water content increases To maintain proper viscosity and fireresistance, operating temperatures should be limited to 60⬚C (140⬚F) and preferably shouldremain below 49⬚C (120⬚F) to prevent evaporation of the water In properly designed andmaintained systems, invert emulsions will provide a reasonable cost alternative where fire-resistant fluids are used.

3 Water Glycols

Water glycols are true solutions that contain about 40–50% water and glycol (typicallydiethylene glycol), and additives to impart specific performance levels Because of thewater content, they contain liquid and vapor phase rust inhibitors Since glycols do notprovide good natural antiwear protection without the use of antiwear additives, these areused in most formulations for hydraulic applications Water content controls the viscosity

of the fluid as well as the fire-resistant characteristics Maximum operating temperaturesshould be limited to 60⬚C (140⬚F)

D Environmental Hydraulic Fluids

Chapter 6provided a detailed discussion of environmental hydraulic fluids The mance characteristics of fluids of these types are the same as those of conventional hy-draulic fluids, but, in addition, high levels of biodegradability and low levels of toxicityare required

perfor-E Changing or Converting to Fire-Resistant Fluids

Changing to a fire-resistant fluid generally requires draining of existing fluid and thoroughflushing and cleaning of the system to assure minimal contamination If the system containspainted surfaces, compatibility of the fluid with the paint should be verified, as well ascompatibility with seals, hoses, and other system components In some instances, it may

be necessary to change suction pipe size (to allow adequate flow of fire-resistant fluid topump suction) and filters to accommodate the fluid being used Specific manufacturers

of both the fluid and the system components should be consulted for any additional tions

precau-IX HYDRAULIC SYSTEM MAINTENANCE

The degree of system maintenance is based on specific performance expectations, the fluidused, and the system operating parameters The various hydraulic fluids, ranging frommineral oil based to synthetic to water-containing fire-resistant fluids, demand variouslevels of maintenance to assure performance Water-containing fluids require higher levels

of maintenance to assure not only that the fire protection properties are retained but thatthe fluid will provide proper lubrication characteristics while in service This topic wasdiscussed earlier in this chapter, but selecting the proper, fluid matched to system needs

and understanding the limitations of that fluid comprise the basic starting point for aneffective maintenance program.

Once the proper fluid has been selected, the equipment and operating conditionswill dictate the degree of maintenance required to keep that fluid in service for longperiods of time while retaining its lubrication characteristics These maintenance procedureobjectives should include the following:

Keeping fluid clean/controlling contaminationMaintaining proper temperatures

Maintaining proper oil levelsPeriodic oil analysis

Routine inspectionsNoise levelsVibrationPressuresShock loadsLeakageFluid odor and colorFiltration

TemperaturesFoaming

A Keeping Fluid Clean/Controlling Contamination

The first maintenance objective starts at the time the oil is received and stored and continuesthrough the period of time it is in service in the system Contamination, such as moisture,can enter ‘‘sealed’’ containers while in storage, through normal expansion and contraction

of the fluid due to temperature changes Moisture thus allowed to enter then condensesinside the containers Contamination can also result while the oil is being transferred fromstorage (or containers) to the system Dirty transfer containers might be at fault, or equip-ment that has been sitting in the open or has been used for other materials such as gearoils, engine oils, and coolants Dirty reservoirs and debris around the fill location are alsosources of contamination during filling or adding makeup oil to the system In criticalsystems, sometimes quick-disconnect fittings are installed on reservoirs, or portable filtercarts are used to facilitate adding oil to clean oil to these systems Such measures minimizethe potential for contamination

The fluid in service must be clean, and the level of cleanliness depends on thesystem Numerically controlled (NC) machine tools, for example, require high levels ofcleanliness to accommodate the close-tolerance servovalves, whereas the hydraulics used

to operate hydraulic lifts in automotive repair shops can run satisfactorily with minimalfiltration It should be noted that conventional filters will not remove water- or oil-solublecontaminants Special coalescing-type filters are available for the removal of limitedamounts of water

B Filtration

Full flow filtration is the most common type used on hydraulic systems to control the

levels of solid contaminants These filters are generally installed in the supply (pressure)line but can also be installed in return (low pressure) lines to the reservoir Full flow filter