Tolomatic presentation belt-vs-screw stages

LINEAR ACTUATOR BASICS

ELECTROMATEToll Free Phone (877) SERVO98

Toll Free Fax (877) SERV099www.electromate.com

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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

Toll Free Phone (877) SERVO98Toll Free Fax (877) SERV099

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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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http://nookindustries.com/ball/BallHome.cfm

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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Technology

Tolomatic presentation belt-vs-screw stages