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1994 by Alexander H. Slocum

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Aesops1 Surface Self Compensated Profile RailLinear Hydrostatic Bearing System: TheHydroRail

1 Patents pending. Aesop, Inc. PO Box 2126, Concord, NH 03302-2126, Fax 603-224-5369

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The carriage contains all the hydrostatic compensation andpockets on its unique surface shape2

2 This patented design was the subject of Ntahn Kanes Ph.D. thesis with Prof. Slocum at MIT.

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The small protrusion on the rail center acts as the surfaceself compensation system:

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9 Defined as max. deviation of carriage relative to rail reference edges10 Quantity shown is typical for rolling element guides, according to S. Futami, A. Furutani, and S.Yoshida, Nanometer Positioning and its Microdynamics, Nanotechnology, Vol. 1, No. 1, 1990, pp.31-37

Performance of a size 45 HydroRailTM bearingverses a high end rolling element linear guide.

The HydroRailTM

Vs. a high Size 45, @1190 psi Size 45

end rolling element bearing HydroRailTM Bearing Rolling Element Bearing

Nominal gap = 0.001 in. Ultra Precision, 0.03 Preload

Compress. Tension Lateral Compress. Tension Lateral

Static Load which causes permanent damage lb 250,0001

75,000 110,000 29,7505,7

29,750 29,750

Dynamic Load Capacity at given Life lb 2,5002,3

2,500 6,500 3,0006,7

3,000 3,000

Life km no limit 1,000

Static Stiffness lb/in 3.03

2.7 4.6 5.17

4.7 7.1

Dynamic Stiffness at Resonance lb/in 2.03,4

1.8 3.0 0.88

0.8 1.1

Q (Damping Factor) - 1.54

6.28

Maximum Speed m/s no limit 27

Maximum Acceleration m/s2

no limit 507

Static Friction lb none 3.27

Motion Resolution in infinite 4010

Friction @ 0.5 m/s lb 4.23

9.47

Friction Power @ 0.5 m/s W 9.43

21.07

Straightness (over 15.5 inches travel)9

in 204

458

Straightness Repeatability in 84

178

Hydraulic Oil Flow (m=76 cSt @ 90 F) gpm 0.23

Pumping Power required W 1003

Will cause permanentcarriage deformationaccording to FEAanalysis2Load which causes 50%

closure of bearing gap3From fluid circuitmathematical model4Based on data from pastnon-modular hydrostaticbearings designed atMITAll analysis done byNathan R. Kane at MIT

Will cause permanentdeformation of rollersand races6Equal to theroeticaldynamic load divided

by load factor of 3 toaccount for the mostdemanding applications7Based on productcatalogue data8Based on experimentaldata obtained byNathan R. Kane at MIT

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Technical advantages and tradeoffs associated withthe HydroRailTM

HydroRailTM Feature Benefits for Various Applications

Very high "crash" resistance Greatly reduces the chance that a crash will result in expensive downtime forbearing repair.

Potentially infinite bearing life If fluid filter is replaced regularly, cost of rebuilding linear ways can be eliminated.

High damping Reduces problematic chatter, increases surface finish.

Zero static friction Reduces servo reversal error; increases motion resolution, allowing 20 in infeedsor smaller.

Low dynamic friction Reduces heat generation and hence reduces thermal errors at high speeds.

High Straightness Results in straighter surfaces, more accurate parts.

Swarf particles can pass freelythrough bearing gap

Virtually immune to the extremely destructive effects of ceramic swarf whengrinding ceramic materials. Allows bearing to remain accurate over several yearsof operation without a rebuild.

Estimated Carriage andRail Costs

Fluid System Cost

Bearing Size 3 5 4 5 5 5 6 5 To Power 4 Carriages of anysize

$3,800

Per Carriage $570 $600 $650 $700 To Power 8 Carriages of anysize

$4,800

Per meter ofrail

$770 $800 $850 $900 To Power 12 Carriages of anysize

$5,600

1 Estimates are based on an ultra precision rolling element linear guide system with similar manufacturing complexity2 Includes DC permanent magnet motor (1 hp per 4 carriages), power supply, gear pump, 3m high pressure filter, properly sized

oil chiller with a 10 gallon tank, pressure relief value, a pressure measurement and control system, 10 foot hydraulic hose, and 15gallons of hydraulic oil. Does not include fittings and hoses going to individual trucks, fluid gutters and drains, or a way protectionsystem such as bellows.

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New Possibilities for Machine Tool Manufacturers

Offer Check the Box Hydrostatics to Customers Buying a N e wMachine

Linear Bearing Option: Standard Linear Guides HydroRailTM

Hydrostatic Linear Guides

Upgrade an Entire Product Line Quickly and InexpensivelyWith minimal additions (such as adding drainage gutters for hydraulic oil), an oldproduct line that uses standard linear guides can be rejuvenated by equipping themachines with HydroRailTM bearings.

Introduce A New Ultra Performance Machine Tool Which is Very CostCompetitiveDesign a new ultra performance high speed machining center which takes fulladvantage of the performance capabilities of the HydroRailTM bearing. Promote theunique crash resistant, accuracy enhancing, and long bearing life features, and alsothe high value added to the machine tool by the HydroRailTM technology, whichtranslates directly into a great value for the customer.

Replace Costly Hand Scraped Way SystemsIn an effort to reduce assembly time and costs, countless machine tool manufacturershave spent thousands of R&D dollars trying to make rolling element guides work forthem, only to give up and go back to using their old labor intensive, costly custom waysystems (such as sliding and hydrodynamic systems which require hand scraping).For many of these companies, the HydroRailTM can offer the ultra high performancethey require while also providing them with much of the savings that they are lookingfor.

New Possibilities for Machine Tool Users

Retrofit a Machine to Bring it to A New Level of AccuracyDuring a rebuild, a machine that uses standard linear guides can be economicallyupgraded to HydroRailTM bearings. For a modest investment, the machine can then canperform at a new level of accuracy.

Retrofit a Machine to Greatly Increase Bearing Life in AbrasiveEnvironmentsWhen grinding ceramics or other hard materials, the abrasive swarf created always getspast even the best bearing seals, and then quickly wears the surfaces of rolling elementsand races. Retrofitting such a machine with HydroRailTM bearings can eliminate thisproblem, since the bearing gap is large enough to allow swarf particles to pass throughfreely.

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Porous GraphiteAerostatic bearings

Aerostatic bearings utilize a thin film of high pressure air(typically 690 kPa) to support a load:

Po Pa

Low clearance side

centered

High clearance side

Po

Pa

Axialpressureprofile

Since air has a very low viscosity, bearings gaps are small,on the order of 1-10 microns.

Aerostatic bearings bearings can be configured in virtuallyall the ways hydrostatic bearings can.

Because air bearings have essentially zero friction and arevery clean running:

They have extensive use in metrology equipment and in

machines used in clean rooms.

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Modular fluidstatic bearing systems can be easily made fromoff-the-shelf components:3

Porous graphite bearing pads (e.g., from Devitt

Machinery in Aston PA).

Ceramic Hydroguide pads from Wilbanks International.

Hollow alumina beams (e.g., from Wilbanks Internationalin Hillsboro OR).

Hollow cast iron square tube (e.g., from Smith Tool Co.in Manasquan, NJ)

Epoxy potting compound (with metal fillers to increase

modulus) (e.g., DWH epoxy from Devitt Machinery).

Outer structurePotting epoxySteel backingPorous graphiteMoving member

3 Patent by Drew Devitt, Devitt Machinery, Aston, PA, "Method for ManufacturingExternally Pressurized Bearing Assemblies", #5,488,771, Feb. 6, 1996.

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Assembly procedure:

The outside of the cast iron tube is finish ground.

The modular porous graphite air bearing pads are held tothe precision alumina beam by a vacuum.

The beam is positioned with jigs with respect to theoutside of the cast iron tube.

The potting epoxy is injected between the pads and thecast iron.

After curing, when the air is off, the alumina beam is lockedin place (self-braking).

When air is applied, the cast iron tube expands a few m andthe alumina beam floats.

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Advanced Hydrostatic Bearings for High SpeedSpindles4

Since 1986, Prof. Slocum has been working on self

compensated hydrostatic bearings.

Dr. Kevin Wasson earned his Ph.D. working with Prof.Slocum on high speed hydrostatic bearings.

Dr. Wasson has developed advanced designs andmathematical models, that allow him to deterministicallydesign high speed surface self comepnsated hydrostaticbearings.

These bearings can run on oil, water, or cryogenic

fluids.

These bearings have widespread application, frommachine tool spindles, to rocket motors!

4 This section was prepared by Dr. Kevin Wasson, Principal Engineer, Aesop, Inc. PO Box 2126, Concord,NH 03302.

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Conventional Hydrostatic Bearings

CompensationResistance

Some type of external compensation resistances arerequired:

Orifices.

Capillaries.

Servo Valves.

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Advantages/Disadvantages of HydrostaticSpindles

Compared to rolling element spindles, hydrostatic spindleshave:

Much lower static and dynamic run-out. Usually equal or better static stiffness at the tool-point. Much higher damping, higher dynamic stiffness. Higher tolerance to impact loads.

No wear, potentially infinite life. Enables machining forces to be measured by measuringpocket pressures.

Much higher power consumption, heat generation,temperature rise at high speeds (if oil is used).

Typically higher cost, mostly due to the requiredsupport equipment.

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Water Hydrostatic Machine Tool Spindles

Compared to oil, using water (or coolant) as the bearinglubricant results in:

Much lower bearing power consumption (drag). Much lower temperature rise, thermal growth. Better ergonomics. No problems with cross-talk between oil and coolant

systems.

Requires tighter clearances to keep flow rates low.

Can create clogging problems if issue is not addressed.

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Self-Compensated Hydrostatic Bearings5

P

PExternalCompensation

Resistance

CL

CL

Conventional Self-Compensation

Eliminates clogging.

Enables higher bearing stiffness. Eliminates the need to hand-tune inlet restrictors. Bearing load capacity is independent of the clearance. Enables the bearings to be located on the surface of the

shaft; much more easy to manufacture.

5 U.S. Patents 5,281,032; 5,466,071; 5,533,814; other patents pending.

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Prototype Test Spindle

Pulley-DriveShaft

Air Seal

Pulley-DriveSupport Bearing

Thrust

Rear BearingCompensator

Rear Bearing

Through-the-Tool

Coolant SupplyChannel

Front BearingFront BearingCompensator

Air Seal

56 mm

Flange

Unique features:

Self-compensated hydrostatic bearings. All bearing detail CNC-milled into the outer surface of

the shaft with a ball end mill.

Damping areas left in the centers of the bearing pockets. Unique thrust bearing provides preferential load capacity

in the pushing direction.

Air seals on front and rear of spindle prevent coolantleakage.

Unique method to supply through-the-tool coolant.

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Modeling Bearing Stiffness

R1

L2

L1La

La

Ro

Ri

hpad

La

La

R

R

RR

R

R3

R3

2

3

13

2

Supply

Pump

R1a

R1b

R2a

R2b

R3a

R3b

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Modeling Static Spindle Stiffness

Finite Element Representation:

Fs

f

f

a

bfc

Complications:

Often many changes in cross-section. Shear deformations are not negligible for short spindles. Bearing characteristics are a strong function of the shaft

bending.

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Modeling the Effect of Pad Tilt

R1

L2

L1La

La

Ro

Ri

h pad

La

La

R

R

RR

R

R4

R5

3

5

14

2

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Predicted and Measured Spindle Stiffness

Fs

f

f

a

b fc

Front Bearing Rear Bearing

Pulley Bearing

Predicted Stiffness : 131,700 lbf/in

Measured Stiffness : 130,500 lbf/in

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Measured Dynamic Spindle Stiffness

Pump NoiseResonance

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Measured Spindle Radial Error Motion

Speed(rpm)

TotalRunout

(in)1,000 29.02,000 31.2

5,000 36.27,000 76.510,000 67.0

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Modeling Viscous Friction

hp

La Lp

ha

Shaft Rotationy

Flow on the lands typically remains laminar up to

speeds of 2 million DN.

Flow in the pockets typically becomes turbulent at

speeds greater than 0.1 million DN.

2.06 mm1.04 mm0.53 mm

Experimental Data

Laminar AnalyticalSolution

Turbulent AnalyticalSolution

Re

T

V D2

for Pocket Depth of:

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New Method to Reduce Bearing Friction andIncrease Bearing Damping

bm

bm

bm

Bearing

Pocket Damping-

Enhancingand

Friction-

Reducing

Land Area

Grooves

Machinedwith a

Ball-Nose

End Mill

Theoretical Manufactured

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Relative Pocket and Land Friction

0.0008 Nm/s

0.0013 Nm/s

0.005 Nm/s

Calculated for a clearance of 20 m, pocket depth of 2mm, bearing diameter of 90 mm.

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Power Consumption of Prototype Spindle

Temperature Rise

topic 18b

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