LINEAR ACTUATOR BASICS ELECTROMATE Toll Free Phone (877) SERVO98 Toll Free Fax (877) SERV099 www.electromate.com [email protected]
LINEAR ACTUATOR BASICS
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LINEAR ACTUATORS• Convert electric motor rotations into linear motion• Two primary methods for rotary motors:
– Screw-drive– Belt-drive
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WHAT IS A SCREW-DRIVE?• A mechanism producing linear motion by rotating either
a screw or nut in an assembly– Screw: a cylindrical element
upon which are formed threads– Nut: a mating component to the
screw threads– Each component is capable of
rotating independently– By restraining one element,
linear motion occursELECTROMATE
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SCREW TERMINOLOGY• Turns
– Number of revolutions per inch of travel (turns per inch)
• Lead– The distance the nut advances
with each revolution of the screw (inches per rev)
• Pitch– The axial distance between threads
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SCREW TERMINOLOGY• Backlash
– The distance between screw and nut engagement when reversing direction or force
• Lead Accuracy– Possible variation in travel over a standard length of screw (inch/foot)
• Load Rating– Static – maximum load that may be applied before damage occurs– Dynamic – a value used in the life calculation of a system
• Backdriving– The ability of an external force or load to cause rotation of the screw
through the nut
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SCREW TYPES• Three types of screw assemblies
Acme Screw
Ball Screw
Roller ScrewELECTROMATE
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WHAT IS AN ACME SCREW?• Resin (plastic) or Bronze nut • Sliding contact surface to lead
screw surface• Also referred to a Trapezoidal
Screw for Metric designs
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ACME SCREW ADVANTAGES• Cost effective solution• Quiet operation (Acme)• May reduce back driving in vertical applications • Ideal for applications with
– Slower speeds– Low duty cycle or intermittent motion– Low positioning requirements– Low accuracy requirements
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ACME SCREW LIMITATIONS• Solid nut design will wear• Lower efficiency ratings• Unpredictable service life• External factors can affect the life
– Environment – Lubrication– Load– Duty cycle
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WHAT IS A BALL SCREW?• Recirculating ball bearing nut
assembly rolls along the ball screw threads
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BALL SCREW ADVANTAGES• Higher thrust capabilities compared to solid nut • Longer and predictable service life• Increased efficiency• Low Backlash • Ideal for applications that require
– High duty cycles– High thrust– Higher speeds
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BALL SCREW LIMITATIONS
• In vertical applications the ball nut can back drive or free fall with loss of motor torque
• Higher initial cost compared to Solid Nut• Generates more noise
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WHAT IS A ROLLER SCREW?• Multiple threaded rollers are
assembled in a planetary arrangement around a threaded shaft
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ROLLER SCREW ADVANTAGES
• Very high thrust capabilities• Extremely long life in continuous duty applications• Higher speed and acceleration rates possible• Low Maintenance• High Efficiency
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ROLLER SCREW LIMITATIONS
• Highest cost of three screw types• In vertical applications the screw can be back driven or free
fall with loss of motor torque• Nut assembly has a larger outside diameter• Longer lead times (8-12 weeks)
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WHAT IS A BELT-DRIVE?• A mechanism to produce linear motion by the rotation of
pulleys and a belt in an assembly• By rotating one of the pulleys, linear motion is achieved
with the use of a carrier attached to the belt
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BELT TERMINOLOGYTENSILE CORD CHARACTERISTICS
Material Tensile modulus Bending flexibility Cost
Fiberglass Low Highest Low
Aramid (Kevlar) Medium Medium High
Steel Highest Low Highest
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SCREW OR BELT?• The application requirements will dictate one
technology over the other – Stroke length– Max. speed– Max. thrust– Accuracy– Repeatability– Resolution
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THRUST• Screw
– Thrust will vary by:• Diameter of screw• Turns per inch
– Ball screw 140-4500 lbs– Acme screw 70-500 lbs– Bronze screw 70-3500 lbs
• Belt– Max thrust is dependent
upon belt width, tooth design and construction materials
– 150-325 lbs
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ACCURACY• Ball Screw
– Industry standard classifications exist
• Class 10 - .008 in/ft• Class 7 - .002 in/ft• Class 5 - .001 in/ft• Class 3 - .0005 in/ft• Class 1 - .0002 in/ft
• Belt– Typically within 0.006 in/ft
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REPEATABILITY• Screw
– Generally very repeatable• Ball or Roller = +/- 0.001• Solid or Bronze = +/- 0.0035
– Solid nuts will wear over the course of life and increase in backlash
• Belt– Generally repeatable
• +/- 0.002 in– Belt stretch
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RESOLUTION• Screw
– A finer lead screw will provide a system with higher resolution
• 2 TPI Screw– 0.0014 in / deg
• 5 TPI Screw– 0.0005 in / deg
• Belt– Belt systems will have
considerably lower resolution• 1.003 PCD = 3.151 in travel
– 0.0088 in / deg
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THE ADVANTAGES:ACTUATOR CHARACTERISTICS
ACME Screw
Ball Screw Roller Screw
HTD Belt
Stroke medium medium short - medium longSpeed slow medium medium - high highThrust low – medium medium - high high lowAccuracy medium medium - high high low - mediumBacklash medium - high low - medium low mediumRepeatability low - medium medium - high high mediumResolution medium - high medium - high medium - high low - mediumBackdriven difficult moderate moderate easy ELECTROMATE
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QUESTIONS??
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