Class #12Actuators: Linear & Rotary

• Review valve modeling• Answer reading questions• Design aspects of linear actuator• Seals: friction & leakage• Cylinder cushions• Rotary actuators

ME 8243: TOPICS IN DESIGN:ADVANCED FLUID POWER

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Review of Valve Modeling• Questions on how valves work?

– Pilot-operated relief– Pressure compensated flow control– Flow divider/combiner– Unloading– Pilot-operated check– Sequencing– Counterbalance– Shuttle

• Modeling issues:– Convergence– Hardstops - link

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Questions from Readings / Videos?

• Fitch & Hong 3.4: cylinder cushioning• Xia & Durfee paper: o-ring seal modeling• Nikas review paper: sealing technologies• Cylinder video

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What are important design aspects of a linear actuator?

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Hydraulic Cylinder Comparison244• Cylinder preloaded with tie rods

– Cylinder in compression via long tie rods

• No fatigue loading due to high pre-compression

• Tie rods provide ‘energy relief’ at high impact loads if cushion capacity exceeded

248• Cylinder is main structural member

– Heavy wall cylinder with end caps bolted on each end of cylinder

• Large cylinder cross section resists fatigue failure

• Many bolts in end cap resist failure at high impact loads if cushion capacity exceeded

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Bearing Element Comparison -Characteristics

Polymer• Thermally bonded oil lubricated nylon

bearing material– Hydrodynamic oil film supports load– Oil distribution grooves assure even

oil distribution– Oil film provides damping within the

bearing via oil squeeze film effect– Negligible wear to 6 MPa contact

pressure

Hydrostatic• Pressure balanced bearing backed by

thermally bonded oil lubricated nylon bearing material– External oil supply provides

bearing pressure– Capillary tubes provide balanced

pressure drop in bearing pads– Bearing exhibits ‘spring’ type

stiffness effect– Load supported on pressurized

film to 7 MPa

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Bearing Element Comparison – Loading

Polymer• Moderate overloading increases wear

on polymer and rod• Heavy overloading my damage

polymer surface or bond• Bearing not contaminant sensitive

unless abrasive elements are present• Oil leakage to drain up to 0.5

lpm/bearing

Hydrostatic• Moderate overloading causes rod to

contact polymer material• Heavy overloading may damage

polymer surface or bond• Bearing very contaminant sensitive

due to pressurized bearing pad oil flow effects

• Capillary tubes may become blocked due to contamination in oil supply

• Oil flow requirements up to 4 lpm/bearing

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Seal Element Package ComparisonFully sealed standard actuator configuration examples

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Seal Friction Exercise

1. Solve for friction force2. v = 0.1 m/s, F = 10,000 N. Find ηm

Piston Hub Piston Rodo-ring cross-sectional diameter 1.5 mm 1.5 mm

ID of o-ring surface 21.4 mm 12 mmOD of o-ring surface 24 mm 14.7 mmFriction coefficient 0.4Elastic modulus 3 MPa

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Straight Spear Cushioning

Cushioning Stages:

Source: Fitch and Hong, Hydraulic Component Design and Selection

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Cushioning Exercise

a. Equation: accel = f(velocity)

b. Plot

piston velocity

acce

lera

tion

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Straight Spear Cushion

Dynamic Analysis:

Source: Fitch and Hong, Hydraulic Component Design and Selection

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Rotary Actuators: Types

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Linear vs. Rotary Actuator

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Linear vs. Rotary Actuator

Why choose one over the other?

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Upcoming classes:

• Tues, 10/16: Pump architectures & pressure dynamics– 5 minute presentation on specific architecture

• Thurs, 10/18: Pump energy loss & efficiency modeling

• Tues, 10/23: Pump control & project introduction– Pump modeling report due (memo style)