Class #4Hydraulic Fluid Properties

ME 4232:FLUID POWER CONTROLS LAB

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Notes

• Lab 2 Regrades• Regrading Policy• Undergraduate Research Position:

Compressed Air Energy Storage Testbed• Upcoming Labs:

– Lab 7: Bleed Off Circuit– Lab 8: Check Valves: Direct & Pilot Operated

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Agenda

• Feedback: Interesting / Challenging• Check Valves• Roles of Hydraulic Fluid• Fluid Properties

– Viscosity• Viscous Friction• Leakage

– Bulk Modulus– Inertia

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FeedbackInteresting• Ingenious Component

Designs• Freedom in Lab = Learning• Understandable Labs• Practical Applications• Power of Hydraulics• Variety of Configurations• Matlab for Data Analysis• In Class Activities• Analogies to Electrical

Confusing/Challenging• Learning Matlab• Circuit Diagrams• Understanding Circuit

Operation• Frequency of Labs &

Reports• Lab Handout Wording• Cavitation• TA Language Challenges• Want Lecture Schedule

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Check Valves

• Allows flow only one direction• Main uses are:

– by-pass components– cylinder locking

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Pilot Operated (open) Check Valve

• Allows reverse flow when pilot pressure is turned on

• Enables cylinder locking• Consider force balance to

calculate what pressures needed to open the check

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Cartridge Valves• Assemble Components

into Circuits in a Manifold– Manifold = metal block

with internal passages

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Cartridge Valves (screw in)

• Check Valve • Pilot Operated Check Valve

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Cartridge Valve (Screw In)

• Relief Valve • Pilot Operated Relief Valve

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Roles of Hydraulic Fluid

• Primary:– Transmit Power

• Secondary:– Lubrication– Sealing– Heat Transfer / Cooling

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Hydraulic Fluid Properties

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Viscosity Index

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Pipe Resistance

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Friction Factor – Moody Diagram

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Example: 3 MW Wind Turbine, 35 MPaHydraulic Fluid: ISO 46

– ν = 46 cSt– ρ = 870 kg/m3

Pipe: 100 m long, 20 cm diameter, perfectly smooth

Find:1. Determine the flow rate and average fluid

velocity.2. Calculate the Reynolds number. Laminar or

Turbulent?3. Find the friction factor.4. Calculate the viscous pressure drop.5. How does this compare to the pressure

difference due to gravity?

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Hydraulic Connectors

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Example

Q1: Assuming the leakage past the spool is small, find the pressure PQ2: How long does L need to be for the leakage to be < 0.01% of the

pump flow?

Fluid Properties:ρ = 870 kg/m3

ν = 46 x 10-6 m2/s

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Efficiency vs Viscosity vs Temperature

• For hydraulic components (e.g. pumps, motors, actuators) with moving parts, there is always a trade-off between – Volumetric efficiency [leakage]– Mechanical Efficiency [friction]– Total efficiencies for pumps and motors

given by:

• Optimal viscosity exists for a given component (and system)

Optimal viscosity

However, viscosity changes dramatically by temperature!

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Bulk Modulus

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Research Topic:Fluid Compressibility in Digital Hydraulics

• Major Form of Energy Loss• Widely Varying Bulk Modulus Models

Goal: Understand the Role of Fluid Compressibility on the Performance and Efficiency of Switch-Mode Circuits

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Bulk Modulus ModelsBulk Modulus: • Many Models Available in Literature

dPVdV

Basic: Akers et al.1 1 1 1 1T a a a

e T T a T a

V V V VV V V

eP

R P

oe

o

P RP

R PP P

11

1 11

11e

o o

P

P P R c P P P

Merritt

Hayward

Cho et al.0 5 10 15 20 25 30 35

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Pressure (MPa)

Bulk

Mod

ulus

(GPa

)

Bulk Modulus vs. Pressure for R = 0.020

Yu et al.Cho et al.HaywardMerritt

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ExperimentalValidation

• Vary: Entrained Air, Pressure, Vswitched

• Measure: Density, Bulk Modulus, Efficiency

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Bulk Modulus Calculation

Based on Sonic Velocity:

• Cross-correlation of Pressure Waves

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