580

INTRODUCTION

581

The side mounted hydraulicreservoircommonlyreferredtoas a saddle mounted tank has been utilized on the Yaletractor shovels for a number of years. The hydraulic re-servoir is mounted on the rear frame near the operatorcompartment; and the exterior surfaces are utilized toprovide apcess ladders, operator platform, handraillJIountings,and accessory items, such as the steeringframe lock and brackets. The fuel tank mounted on theopposite side is similar in design, thereby lJIaintainingthe overall styling of the maphine. It also allowsoperator access to the machine from either side. Thesloped bottom of the tank provides the vehicle groundclearance when working on side slopes. Additionally,the sloped bottom affords easier cleaning and drainingof the hydraulic reservoir. The drawing on Page 3illustrates the design concept of the Yale hydraulicreservoir.

Performance requirements of the hydraulic system

OPERATORPLATFORM

VACUUM BREAKERVALVE

The Yale tractor shovel uses a closed and pressurizedsystem. This design approach prevents external contam-ination from entering the hydraulic system, and improvesthe inlet conditions imposed on the hydraulic pumps.Entrained air in the hydraulic fluid is also minimized.The air volume maintained within the tank provides thecushioning effect during surge flow conditions.

SYSTEM

~ RETURN

HYDRAULIC RESERVOIR DESIGN CONSIDERATION

There are four major topics to be considered in the de-sign criteria of a hydraulic reservoir.

1)

2)

Overall machine styling

3)

4)

Serviceability

Cost consideration of the design

Overall Machine Styling'- The Vehicle EngineeringGrouphas the responsibility for the overall vehicle styling,and the Systems Engineering Group work in close coordi-nation in the design and placement of components on themachine. Vehicle appearance is considered a valuablemarketing asset for the sale of the construction equip-ment machines today; and it is for this reason, compro-mises must be made when designing the hydraulic reservoir.

ACCESSSTEPS

EMERGENCYSTEERING PUMPINLET

CLEAN-OUTPORT

TYPICAL HYDRAULIC TANK ASSEMBLY,

582583

load analysis must be performed. There are two operatingparameters that are taken into consideration for the heatload analysis.

The design variables of: ground clearance, platformsizing, location of tanks are obtained from the vehiclegroup to be used as general guidelines for the initialenvelope sizing of the reservoir. The Hydraulics En-gineer must now predict the system demands and designparameters which will ultimately establish the designfor the hydraulic reservoir.

First, the heat load is calculated on the basis of thesystem valving open center pressure losses, with themachine being roaded at maximum speed. It is assumedthat under these conditions, no useful work is being donewith either the main or steer hydraulic systems~ andtherefore,a maximum heat input occurs. .

PerformanceRequirements- The actual hydraulic reservoirsizing must be based on the circuit requirements and themachine operational design parameters. In this endeavor,the design is aimed towards adequate air and oil capacityto achieve the desired oil circulation for maximum heattransfer, and at the same time to minimize variation ofthe oil level within the reservoir. Before precedingwith the sizing of the reservoir, an oil volume require-ment must be established. This is a ratio of the totalpump flow to the tank oil volume of approximately 3 to 1.This ratio will be modified based on other circuit con-siderations. If the reservoir oil volume selected islow, and the return flow velocities are of large magni-tude, turbulence in the tank will result. This willcause surface agitation of the oil, allowing entrainmentof air in the oil. Operating the hydraulic pumps withheavily entrained air, oil mixtures will cause pumpdamage and greatly decrease the life of the hydraulicpump.

Differential pressure losses in the discharge circuit ofthe hydraulic pump are calculated including the lossesacross open center valving in both the main and steerhydraulic systems. Once the differential pressure hasbeen established for the discharge system, from the mainhydraulic and steer hydraulic pumps, the pump overall in-efficiency is established and then summated to obtain thetotal heat load in the system. 3If the heat generationlevel is 5000 BTU's per hour or lower, it is probablethat the system will dissipate the heat by itself. Ifthe heat generation level exceeds 10,000 BTU's per hour,it will be necessary to perform the heat balance charac-teristics of the system to establish whether a hydrauliccooler will be required for the system operation. Theheat balance equation for steady state condition is asfollows:

Air entrained oil passing through the hydraulic systemwill be subjected to a compression and expansion cycle.The energy absorbed in the air bubble during this con-dition is given up to the fluid in the form of heat, thusraising the overall operating temperature of the system.The added heat in the hydraulic fluid will also causedeterioration of the fluid additives and the formationof coagualted pentane. These semi-solids will causeblockage of the filters in the system.

Another factor that effects the oil volume requirementsare the grade and slope on which the machine is designedto operate. A minimum oil level must be maintained abovethe pump suction tube to prevent vortexing of the hydrau-lic fluid entering the suction tube. An oil level offour inches is recommended. Also the addition of a per-forated sleeve on the suction tube redistributes the en-trance velocities and aids in preventing the formationof the vortex. The perforated tube also acts as astrainer to prevent large particle contaminants fromentering the hydraulic system.

qe = qa + qd

Secondly, the heat load is analyzed dynamically based ona maximum machine work cycle and now includes the workcircuit portion of the main and steer hydraulic systems.In order to fully evaluate the energy loss during theworking functions of the machine, it is necessary to es-tablish both the flow and pressure profiles versus timefor the work circuit of both systems.

Before establishing the final oil volume requirements tomeet the system operating parameters, a preliminary heat

Where q = Heat loss generated (1)e internally in the

hydraulic system(BTU/hr.)

q = Heat absorbed by oil,a tank, and hydraulic

system components(BTU/hr.)

qd = Heat dissipated ortransferred to atmos-phere (BTU/hr.)

584 585

qe = KlQt.P

ep

(2)

With the heat balance calculations completed, the finaloil volume will be determined. The next step is to es-tablish the air volume requirements within the tank. Theair chamber in the hydraulic reservoir provides a cushion-ing effect for the thermal expansion of the oil, for theaccitional oil returning to the reservoir when the cylin-ders are retracted, and the surge flow caused by over-running loads. The air pressure developed in the reser-voir sets the working stress level requirements for thetank structure. In some cases, it is necessary to adda pressure relief valve to the tank. This is generallytrue for the high flow rates encountered in the largermachines. The pressure relief valve is set at 15 PSIG,and the air volume is.calculated so that it does not ex-ceed this setting under extreme conditions. This mini-mizes the replenishment of air required during non-opera-tion of the machine, and reduces the intake of moistureinto the fluid.

From this data, an average rate of work (dG) can be cal-culated using the equation: dT --

qeA

= 0.22Ii 0.25

(T - Ts)1.25 + 0.171 F [(0.01;m)4s4

- (0.01 Ts) ]

-K dGsaT

(3)

The air pressure developed in the reservoir is alsoutilized to prevent cavitation at the pumps and in thework circuit. The main control valve contains anti--cavitation valves in the cylinder work ports. Thesecheck valves allow oil to flow from the reservoir to theunpressurized side of the cylinder during over-runningload conditions.

D Average diameter of tubing,reservoir, and system components

D 4 x System VolumeArea

(4)

This is an important factor inliness of the hydraulic fluid.develop on the rod side of thecontaminates and moisture willthe life of the component.

maintaining the clean-If voids are allowed to

work cylinder, externalenter the system reducing

T Average Maximum System Temperature of

Ambient Temperature of

To determine the peak reservoir air pressure and resul-tant required air volume, the following operating condi-tions are considered:

Ts =

FS

E

Gray body shape factor ~ E = 0.5 a) The oil volume increases due to thermalexpansion of the oil. (Limited to 100°F ~T)Emission Factor

b) The increase in oil volume when the hoistcylinders are lowered with the engine stoppedIf it is found necessary to supply an oil cooler to meet

the stabilization temperatures of the hydraulic systemwithin the design parameter, the heat load equations mustbe modified to include pressure losses in the coolingcircuit. Since the heat load analysis is based on es-timated duty cycles and also include empirical data, themachine hydraulic heat load is always evaluated duringthe performance testing of the prototype machine to in-sure hydraulic heat stabilization does not exceed thedesign objective (100°F over ambient).

c) The increase in air pressure resulting fromthe differential temperature of 100°F

Where: Kl= 1.481 BTU/hr.-PSI-Ga1.

-3K2 = 1.285 x 10 BTU/ft. lb.

t.P = Differential Pressure PSI

ep= Pump Overall efficiency

586587

Using the gas laws:

Initial conditions: PlVlTl

The pump suction tube must be sized to meet the require-ments of the pump manufacturer. It is advisable to posi-tion the pump suction tube at the neutral axis of oillevel in the reservoir, so that machine attitudes willhave minimal affect on oil level. To reduce the entrancevelocity affect at the throat of the tube, the tube maybe scarf cut. This will vary the velocity at the en-trance of the tube. A strainer or perforated tube sleevecan also be used to vary the entrance velocity. In thecase of the hydraulic reservoir design used on Yaleloaders, the perforated tube sleeve is a removable memberto allow for cleaning and servicing.

V3 = Vz - Vc (5)

The pump suction tube within the reservoir and the hy-draulic lines to the pump inlet port must now be evalu-ated for the size and number of bends to insure that anyentrained air in the oil will not come out of solutionand cause damage to the pump.

If multiple pumps are to be used in the system, the suc-tion tube must be sized for a flow velocity of four (4)to five (5) feet per second based on the total systempump flow.

To further evaluate the inlet conditions imposed on thehydraulic pumps, the pressure drop calculations are per-formed under cold start up conditions to insure that themaximum pump inlet vacuum does not exceed five (5) in.hg.

P + [P +'P] - 14 5max 3 4 .

P < PSIG (Design)m -

Where: Vs

VT

VI

Vz

= Total System Oil Volume The position of the return line manifold filter box inthe hydraulic tank may now be establsihed. The filterbox is positioned in a manner that allows the filter re-tainer plates to protrude above the static oil level.In some cases, the hydraulic filters are located abovethe static oil level of the tank. Generally, this occurswhen filter elements are stacked to meet the flow require-ment of the machine. The filter element protruding abovethe oil level does not cause air entrainment due to thecascading affect of the returning oil above the surfacelevel. Additionally, the filter element with its inside/out flow capability on the return line provides an excel-lent diffuser to disperse the return velocity in the hy-draulic tank. The filter element is a specially designedcomponent for the Yale front end loader. Considerabledevelopment work by both the supplier and Eaton Corpora-tion were required to achieve the design performance andreliability of the element.

= Total Reservoir Volume

= Air Volume Initial

Vc

= Air Volume due to thermal

expansion of oil

= Oil volume increase from hoist

cylinders (head end) with controlvalve in power down and bucketcylinders in roll back positionand the engine off.

P4 = Increase in air pressure due to6T of 100°F = 3.Z PSIG

C = Coefficient of cubica14expansionvOl/vol/oF = 4.5 x 10- Since the element has an inside/out flow requirement, the

pleat design of the media, and the supporting members fo~the media had to be modified over conventional filter

Vz = VT - [VI + C Vs] (6)

Pz = PlVl(7)

VZ

P3 = PZVZ(8)

-v;-

588 589designs to withstand the high return line surge flow,under certain conditions. Early in the development phaseof the filter, it was observed that the column strengthin the pleat was insufficient to withstand the forces in-duced by the high surge flow.

The graph, Figure 1, describes the empirical frictioncoefficient of the filter media. With this design tool,it was then possible to predict the ambient temperaturecondition at which the by-pass valve would open, and whatthe flow ratio between the by-pass valve and the filterelement is under varying speed conditions.When this condition existed, the pleats would bunch toge-

ther, blocking the flow through the element and increas-ing the pressure drop across the element. Also the con-ventional 'V' shaped pleat design elements fatigued atthe tip of the pleats causing the media to open, resul-ting in loss of filtration. The filter element designwas flow tested utilizing the standard Itest procedureprocedure developed by Oklahoma State University andfurther field tested in actual operation to insure thedesign parameters had been achieved.

~

w

6P._- _ELEIIEIITIPSID>Q.FLOWACIIQSSEtEIIEIfT (GPM).p. SPECIFIC ...vrrv OF FLUID.V' VISCOSITYOF 'WID ICENTlS1OKESI.f .FRICTION coo..

.01

.008

,.MPQ'

A single filter element has a flow rating of 70 GPM anda 8-10 2 ratio of 6. The micron rating is 25 ~ absolute.This is compatible with the contamination level require-ments of the hydraulic pumps to achieve the required de-sign life. Additional performance characteristics ofthe hydraulic filter, such as the loaded burst pressureof the filter media and the pressure drop characteris-tics versus oil visocity had to be established to pro-vide the design parameters for the filter by-pass valve.

To confirm the filter burst pressure data supplied by thevendor, a prototype element was placed on test using a600 weight base stock gear oil, and the flow increasedacross the element until media failure was observed. Theburst data obtained from this test was used to establishthe maKimum over-ride pressure requirements of the by--pass valve.

i

15004(j

itw8 .002Z0>=2II:IL .001

.1 .2 .4

FLOW/VISCOSITY RATIO

.8 1.0Qv

2D 4D

Figure 1 - Flo~ Factor for HydraulicFilter Element

To define the performance of the filter/by-pass valvecombination, including variations of oil viscosity dueto ambient temperature conditions, it was necessary todevelop an empirical flow factor for the filter media.

Using the flow factor equation, P = fPQ2, it is possibleto predict the differential pressure across the filterelement with varying conditions of flow and oil tempera-tures. Graph Figure 2 illustrates a family of flowcurves and the resultant P versus oil temperature char-acteristics of each.

1 OSU-F-5 - Method for Verifying the Flow FatigueCharacteristics of a Filter Element, Recommended Pro-cedures for Evaluating Fluid Power Components and Systems,FPRC No. 72-1, March 1972

2 OSU-F-2 - 8-10 ratio definition, Multi-pass Method forevaluating the Filtration Performance of a Fine HydraulicFluid Power Filter, Recommended Procedures for EvaluatingFluid Power Components and Systems, FPRC No. 72-1,March 1972

This data may also be plotted to describe the flow re-quirement in the system at different operating oil tem-peratures to cause opening of the by-pass valve. GraphFigure 3 describes the performance of the filter/by-passvalve combination.

590

Ll.PC.fpQ2

p- SPECIFIC GRAVITY.a'RATE OF FLOW (GPM).LI.Pc =CAL. PRESSURE DROP.f=FRICTION COEF. BASED

ON RATIO .9..11"

v= KINEMATIC VISCOSITY

(CENTISTOKES).

U)U)00 25a:-uU)<1:0.

wen 20.a: I-:::::IZU)wU):E 15Wwa:...Io.w

ci! ~ 10-I-!z--

WU) 5a: a:WwH:!:iaU:

DESIGN CRACK PRESSURE --- - - - - -- - OF SYSTEM BY.PASS VALVE.

0 20 40 60 80 100 120

SYSTEM OPERATINGTEMPERATURE (FO)

140 160 180

Figu~e 2 - Diffe~ential P~e88u~e Aa~OBB Filte~Element VB. System Ope~ating OilTempe~ature at Varying Flow Rates

591

30IIII

-,- ~

40"-

~60" I

ENGINE HIGHIDLE SPEED

IIIIIII

.JII

BY.PASS

~---T---'--------III

ENGINE IDLESPEED

IIIII

1BO"t

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

TOTAL RETURN OIL FLOW (GPM)

.BY.PASS VALVECRACK PRESSURE

Figu~e 3 - Performanae Cha~aateri8tias of Filte~/By-PassValve System vs Operating Tempe~ature &Engine Speed

This design analysis also allows the Hydraulic Engineerto evaluate ~he number of filter elements required fora particular system to achieve the desired filtration.It may be used to evaluate the performance of thesystem filter/by-pass combination on other types ofhydraulic fluids.

To insure the proper air volume requirements are main-tained with the hydraulic reservoir, a fill tube is pro-vided. This limits the amount of oil that can be putinto the hydraulic reservoir. The filler cap is designedwith a bleed port to allow decompression of the hydraulic'tank before removal of the cap. The tank mounting

35 T 60 gpm-

50 gpma: 30w!:ii:i:

25! 40 gpmU)U)

20I 30 gpmW

a::::::I

15U)U)wa:.

20 gpm0.0 I...I en 10<1:0.

§;:ffi 5

w:Eu..w!:!::...Iow

592 593

bracke~s are welded to the reservoir utilizing a threebolt mounting to prevent induced torsional strain in thehydraulic reservoir structure.

full flow return filtering system is designed to meetthis criteria. It also reduces the possibility of con-taminants entering the system during service maintenanceperiods, thereby increasing life of the hydraulic com-ponents within the system.The hydraulic reservoir supplies a dual function, when

it becomes the reservoir for the emergency steer systemaccessory package. The emergency steer system pump suc-tion is located near the bottom of the tank and well be-low the main pump suction tube, thus providing a reserveoil supply in the event of a main system hose failure.The design criteria is to provide one minute of machinesteering time on the emergency system before depletionof the oil in the tank. The emergency steer motor/pumpunit is normally mounted high in the machine, thusforcing the pump to operate on a negative inlet condi-tion. It is for this primary reason, the pump suctioninlet port does not use a screen or perforated tubearrangement.

The oil level sight gauge is positionedthe tank in step area to allow operatorhydraulic oil level before mounting thework period. In this location, the oilclass is also out of the splash area ofits protected from damage.

on the face ofobservation ofmachine for thelevel sightthe tires and

Construction Detail--Cost Reduction - The constructiondetails of the tank design were evaluated to achieve theoptimum method of fabrication with cost being ~ majorconsideration. The basic construction of the tankutilizes a 900 bent sheet, which forms the rear wall andone side wall of the tank. The front wall in the steparea of the tank is formed to also provide the bottomsurface of the tank. A single flat sheet forms the re-maining side wall. Positioning problems during weldingof the tank are virtually eliminated by the use of thisdesign. Filter box and pump suction tube can be fabri-cated to the rear wall of the tank prior to main assemblyof the tank structure. The fuel tank on the oppositeside of the machine, with its common styling, allows thesame formed members to be used for its construction.

REFERENCES

1. Oklahoma State University Recommended Proceduresfor Evaluating Fluid Power Components and Sys-tems, March, 1972, FPRC No. 72-1, OSU-F-5 -Method for Verifying the Flow fatigue Character-istics of a Filter Element

The vacuum breakers and relief valve components aremounted on a 'T' fitting on the back side of the hydrau-lic reservoir. The vacuum breaker valve contains abreather to prevent contaminants from entering the hy-draulic reservoir when the vacuum breaker valve is opera-tional. This valving group is located on the inside ofthe rear frame to reduce its exposure to the dusty en-vironment during machine operation.

2. Oklahoma State University Recommended Proceduresfor Evaluating Fluid Power Components and Sys-tems, March, 1972 FPRC No. 72-1, OSU-F-2 -Multi-pass Method for Evaluating the FiltrationPerformance of a Fine Hydraulic Fluid PowerFilter Element

Serviceability and Maintenance Considerations - The Hy-draulic Engineer must always address himself to the ser-viceability and maintenance requirements of the hydrauliccomponents used in the systems he designs. The locationof the upper access cover on the side wass of the hydrau-lic tank is positioned above the oil level to allow itsremoval without the need to first drain the oil. Thefull f10w return filters are also positioned slightlyabove the oil level to allow accessibility during filterchange-out. The lower access cover is positioned nearthe bottom of the tank to allow thorough cleaning and in-spection of the tank prior to filling. The by-pass boxcan be removed, and its components easily serviced.

3. Henke, Russell W., Introduction to Fluid PowerCircuits and Systems

The intent of the Hydraulic System Design Engineer of to-day is to increase the interval between service and main-tenance of the components. As described earlier, the

594

30

595

OPERATORPLATFORM C/)

C/)

00 25a:-oC/)«D.

wen 20'a:~:JZCl)WCI):E15Wwa:...ID.w

...10 10«:!=

~t

~f2 5Wwtt!:ioU::

II

BY.PASS :~~,~

SPEED

~'IDLE

/.600,71

. ENGINEIDLE SPEED

I

. I

IIII

180~ ~II

VACUUM BREAKERVALVE

c-.---

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

TOTAL RETURN OIL FLOW (GPM)

'BY.PASS VALVE CRACK PRESSURE

SYSTEM

~ RETURN

PERFORMANCECHARACTERISTICSOF FILTER/BY.PASS VALVESYSTEMvs OPERATINGTEMPERATURE& ENGINE SPEED.

iCLEAN-OUTPORT !Z.004

w!;i......w8 002

~~II:... 001

1

TYPICAL HYDRAULIC TANK ASSEMBLY,TRACTOR SHOVEL.

.04

ACIOOSS E1.EIiEHT"""Q. now ACIOOSS mOO!... -,.f' ...e.", "'" w I'U'"V' , , OF"... fe,...........,.,."""", COEI'.

02

01

008

,.~

4

FLOW FACTOR FOR HYDRAULIC ALTER ELEMENT.

FLOW/VISCOSITY RATIO

8 10

Qv

20 40

596

ffi 30!:Ji:i:

en 25en00:~ 20w0::;)en 15enw0:.Q..-0..J- 10cren-Q.....-Z...wz 5o:ww:eLLW!:!::..JOw

35 60 gpm- b.pc'f,oa2p.SPECIFIC GRAVITY.a'RATE OF FLOW (GPM).b. Pc . CAL. PRESSURE DROP.f = FRICTION COEF. BASED

ON RATIO Q.V

11=KINEMATIC VISCOSITY(CENTISTOKESI.

50 gpm

40 gpm

30 gpm

DESIGN CRACK PRESSURE- - - - - - - - OF SYSTEM BY.PASSVALVE.- -

20 gpmVENDOR PERFORMANCERATING.

0 20 40 60 80 100 120

SYSTEM OPERATING TEMPERATURE (Fe)

140 160 180

fig. 2

DIFFERENTIAL PRESSUREACROSS FILTER ELEMENT vs SYSTEMOPERATING OIL TEMPERATUREAT VARYING FLOW RATES.