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MIT Self-Compensating Hydrostatic Bearings

26 February 2010Professor Alex SlocumGerald RothenhoferMatt Angle

A Quick Primer on Hydrostatic Bearings

• Are Used to Support the Crankshaft in Automotive Engines

• Pump Forces Fluid Into a Channel and then Between a Shaft and Journal

• Shaft Rides on Thin Film of Fluid• Metal-to-Metal Contact and

Friction are Reduced

QinF/2

m

Rout=const/h3

h

F/2

Qout

Aeff

p

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Functional Principle of a Hydrostatic Bearing II

constant flow supply: When the load increases the bearing gap decreases. Since the flow is kept

constant the recess pressure increases. =>note: spring like behavior but no spring/capacitance

constant pressure supply: The lubricant is pumped through a restrictor into the recess. The supply

pressure, upstream the restrictor is kept constant. With increasing load the gap decreases. Thus the flow decreases and therefore also the pressure drop at the restrictor. Consequently the recess pressure in the bearing increases. =>note: spring like behavior but no spring/capacitance

Issues with Non-Self Compensating Hydrostatic Bearings

• Small Orifices– Used to Create Constant-Pressure Fluid Supply– Are Easily Clogged by Contaminants in Fluid

(Especially Sea Water)• Control Loops

– ??• Water Hammer

– High Flow Rates Cause Turbulence in Fluid Exit– Shaft Can Oscillate in Unsteady Flow

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Hydrostatics Thrust Bearing Test Setup

7000N magnetic and gravitational bearing preload

Theoretical open loop stiffness at 20μm bearing gap: ≈1000N/μm

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Hydrostatic Bearing: Stiffness

Theory assumes constant flow In reality the flow is pressure dependent. Linearization of Bearing gap:

00 0

0 0

1 1

3 3

h Qh h W h

W Q

Pump performance is essential to bearing stiffnessAt a 25μm bearing gap a 1.5% to 2% flow change due

to 70N load change would lower the stiffness to ≈ 200N/μm

Why ours is cool

• Supply Grooves– Oriented on Inside of

Bearing Assembly (Made of Two of These Pieces)

– Extend ~180° to Pocket on Opposite Side of Bearing

– Function as Orifices in Non-Compensating Bearings

• Are Self-Cleaning and Difficult to Clog

• Pockets– Bear Load of Shaft– Fed by Supply Channels

• How it Works– When Shaft is Displaced,

Supply Grooves are Blocked, Pressure Reduced in Pockets on Opposite Side of Shaft

– Produces Restoring force