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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)