Design & Development of Precision Plastic Gear Transmissions · 1 © 2012 Ticona Gears Webinar Gear_DesignPPT_AM_0212_016.pdf Design & Development of Precision Plastic Gear Transmissions

1 © 2012 Ticona Gears Webinar Gear_DesignPPT_AM_0212_016.pdf

Design & Developmentof PrecisionPlastic Gear

Transmissions

David SheridanSenior Design EngineerTICONA


Overview

Methodical and rational procedure for designing and developing high-precision injection-molded plastic gear transmissions that function satisfactorily across the entire range of manufacturing tolerances and operating conditions.

Calculations to examine the effects of tolerances and environmental influences on gear geometry, operating center distance, and gear performance.


Outline

1. Why plastics for gears2. Gear types and arrangements for plastics3. Design and engineering4. Specifications5. Prototype parts and mold development6. Measurement and inspection7. Testing and validation8. Production molding


Plastic vs. Metal Gears

Lower cost– Injection molding vs. machining

• Especially for large quantities– As-molded, no finishing

Greater design flexibility– Parts consolidation– Molded-in features– Allow other gear geometries

• Easy to mold, difficult to machine, e.g., internal and cluster gears

Less noise–Lower modulus

• Do not transmit sound• Greater tooth deflection

increases load sharing and reduces transmission error effects

–Light weight, low inertia • Reduce dynamic loading

and noise

Plastic Advantages


Plastic vs. Metal Gears

Inherent lubricity– Do not need lubrication in

many low-load applications– Internal lubricants

• For applications that cannot use external lubricants

– Computer printers– Motorized toys

Chemical and corrosion resistance– External lubricants

• Grease• Oil

– Water• Lawn sprinklers• Water meters• Shower heads

Plastic Advantages


Outline



Base Circle

Involute Curve

Taut String

Involute Gearing


Involute Gearing

Advantages of Involute Gear Teeth Provide constant angular velocity (or ratio)

between two gears– “conjugate action”

Conjugate action is independent of changes in center distance (CD)– design flexibility– insensitive to manufacturing tolerances,

material expansion and contraction Manufacturing ease and accuracy with basic rack

Conjugate Action Independent of CD


Gear Types and Arrangements

Parallel Axis– Spur– Helical– External or Internal

Non-Parallel– Intersecting Axis

• Bevel, On-Center• Face, On-Center

– Non-Intersecting Axis• Worms• Bevel, Off-Center• Face, Off-Center


Straight Bevel Gears

Dudley

Critical on Mounting and Alignment


Dudley

Not critical on center distance

Not critical on axial position

Recommended for Plastics

Face Gears


Worm Gearing

Characteristics High ratio Low part count Low cost Low noise Low capacity

Types of Worm Gears Involute worm (crossed-helical) Worm thread profile straight in

axial plane Worm thread profile straight in

normal plane Worm produced by conical mill or

grinding wheel with straight sides

These are NOT interchangeable!


Worm Gearing

Dudley

Critical on Mounting and Alignment

Semi-enveloping

Single-enveloping

Double-enveloping Cylindrical


Crossed-Helical Involute Gears

Not critical on center distance Not critical on axial position Theoretical POINT contact Useful for

– low power– low cost– low noise

Recommended for Plastic Worm Gearing


Gear Types and Arrangements

Parallel Axis– Spur– Helical– External

or Internal

Non-Parallel– Intersecting Axis

• Bevel, On-Center• Face, On-Center

– Non-Intersecting Axis• Worms• Bevel, Off-Center• Face, Off-Center

Epicyclics


Input Output

Epicyclic Drives

Epicyclic Arrangements Simple epicyclics Multi-stage epicyclics Compound epicyclics Coupled epicyclics Fixed differentials

Simple Epicyclics Planetary Star Sun

Characteristics Large reductions High power density Small space Split power path

Simple Planetary

Dudley


Single Reduction100%(∑ f·d2)

Double ReductionSingle Branch40%

Double ReductionDouble Branch25%

2-StagePlanetary9.5%

Relative Gear Train Size15:1 Reduction


Outline



Low Noise GearsLow Noise Gears Large Noise Gears

58.3dB

56.8dB61.1dB

68.5dB

Gear NoiseInfluence of Gear Quality

Low Noise Gears High Noise Gears


The Plastic Gear Development Team

Molder

MaterialSupplier

Plastics Engineer

Quality ControlEngineer

ManufacturingEngineer

Gear Engineer

Project Engineer

Purchasing

ToolBuilder


Plastic Gear Development

Prime mover – Torque and speed– Inertia, natural freq.

Load(s)– Torque and speed– Special conditions– Inertia, natural freq.

Duty cycle Life Physical limits

Ratio Precision Efficiency Lubrication Environment

– Temperature– Chemical exposure– Moisture exposure

Test requirements Other

Identify Application – Voice of the Customer (VOC) Define Operating Requirements

Anticipate Future Applications

© 2011 Ticona Gears Webinar Gears-007r1 EN 12/11


Select Overall Transmission Geometry From requirements

– Minimum weight?– Minimum size?– Good plastic designs may use more

gears with split power path Carefully consider added features

– Runout– Distortion

Shafting and bearings– Precision– Efficiency

Housing considerations– Stiffness– Tolerances


Preliminary Gear Sizing

Select materials

Select preliminary gear geometry– Number of teeth– Size (pitch or module)– Profile (tooth proportions)

Nominal ambient conditions

Simple load analysis– K-factor– Unit load

For more information see ANSI/AGMA 1106-A97, Tooth Proportions for Plastic Gears


Select Materials

Suit operating environment– Temperature range

• Dimensional behavior• Property behavior

– Chemical environment• Dimensional behavior• Property behavior

Appropriate property mix– Fatigue– Stiffness– Impact– Creep

For more information see AGMA 920-A01, Materials for Plastic Gears

Interaction with other components– Friction– Wear


Determine Production Tolerances

Gears– Diameters and tolerances– Tooth thickness and tolerance– Tip radius and tolerance– Accuracy grade

Housing– Center distance and tolerance

Shafts, bearings, and bushings– Diameters and tolerances– Runout

26 © 2012 Ticona Gears Webinar Gear_DesignPPT_AM_0212_016.pdf© 2011 Ticona Gears Webinar Gears-007r1 EN 12/11

Precision Engineering Components

Dimensional Requirements (i.e., Tolerances)

MUST Equal Manufacturing Capabilities


Preliminary Cost Estimate

Overall geometry Components sized Materials selected Tolerances

Cost Estimate

Alternative concepts


Decision Point

Evaluate changes Refine cost estimate Commit tooling

Gear Design Begins Engineering data for materials Analyze “theoretical” gear tooth geometry

– Extreme geometry conditions– Extreme load conditions

Iterate


Plastic Properties & Dimensions

Mechanical properties vary– Temperature– Moisture

Obtain strength & modulus data for load analysis– At operating conditions

Dimensions vary– Temperature

• Thermal expansion > metals (x10)

– Moisture

Obtain material data for dimensional stability considerations– Thermal expansion– Moisture expansion


About Gear Tooth Geometry and Assembly

Always perfect in analytical models Always perfect in CAD models Always imperfect in production Operating environment alters geometry


Gear Engineer’s Job

To develop gear tooth geometry and assembly specifications that will produce gears that function satisfactorily under all operating conditions and across the entire range of manufacturing tolerances and environmental influences on dimensions.

© 2011 Ticona Gears Webinar Gears-007r1 EN 12/11


One Approach

Use analytical models for gear tooth geometry and load analysis

Include all possible tolerances and environmental influences on dimensions in “effective” operating center distance

Design “Perfect” gear geometries– Develop gear geometry at tight

mesh– Re-analyze at open mesh– Analyze worst load condition


Develop at Tight Mesh Condition

Maximum Material Condition

Maximum tooth thickness Minimum tip radius External gear

– Maximum outer diameter– Maximum root diameter

Internal gear– Minimum inner diameter– Minimum root diameter

Select Tight Center Distance

External gear set– Minimum effective

operating center distance

Internal gear set– Maximum effective

operating center distance


Develop at Tight Mesh Condition

Optimize geometry– Maximize contact ratio– Minimize root clearance

• Tip interference?

– Minimize backlash– Minimize specific sliding

Load analysis at temperature– Minimize or balance stresses– Excessive tooth deflection?– Tip relief?


Determine Effective OperatingCenter Distance RangeAssembled center distance range Mounting center distance and

tolerance Bushings, bearings, and shafts

– Maximum and minimum radial play– Runout

Gears– Total composite tolerances (Accuracy

grades)

Environmental effects Environmental conditions

– Temperature range– Moisture exposure

Dimensional response between housing and gears

– Thermal response (CLTE)– Moisture response

Examine when– Cold-Dry– Cold-Wet– Hot-Dry– Hot-Wet

Determine extreme CD range and conditions


Assembled Center Distance RangeMounting Center Distance

CM

CMmin

CMmax

Housing– Center distance and tolerance


Shafts, bushings, bearings– Diameters and tolerance

• Maximum and minimum radial play

Maximum radial play Minimum radial play

Maximum bushing diameter Minimum shaft diameter

Minimum bushing diameter Maximum shaft diameter



Shafts, bushings, bearings– Concentricity or runout

Nominal Radius

Runout



Gears– Accuracy grade

• Total composite tolerance (TCT)

Total Composite Error, TCE



)/2BRP(BRP)/2BRO(BROCC

GminPmin

GPMminAmin

GPGmaxPmax

GPMmaxAmax

TCTTCT)/2BRP(BRP)/2BRO(BROCC

Assembled Center Distance RangeExternal Gear Set

tolerance composite Totalplay radial Bearing

runout Bearingdistance center Mounting

distance center Assembled

TCTBRPBRO

CC

M

ASubscripts:

A - assembledM - mountingP - pinionG - gear


)TCT(TCT)/2BRP(BRP)/2BRO(BROCC

GPGmaxPmax

GPMminAmin

)/2BRP(BRP)/2BRO(BROCC

GminPmin

GPMmaxAmax

Assembled Center Distance RangeInternal Gear Set

tolerance composite Totalplay radial Bearing

runout Bearingdistance center Mounting

distance center Assembled

TCTBRPBRO

CC

M

ASubscripts:

A - assembledM - mountingP - pinionG - gear


Add change in center distance due to temperature and moisture effects to assembled center distance range

Examine at temperature and moisture extremes– Cold-dry– Cold-wet– Hot-dry– Hot-wet

Find overall maximum and minimum operating center distance

CCC

CCC

AO

AO

maxmax

minmin

Operating Center Distance RangeEnvironmental Effects


RHTTd

RHTTdC

PMPPMMP

GMGGMMG

)(2

)(2

Operating Center Distance RangeEnvironmental Effects- External gear set

expansion moisture of tCoefficien expansion thermal of tCoefficien

diameter pitch Operatinghumidity relative in Change

etemperatur in ChangeTdistance center in Change C

d

RH

A

AGG

APP

AMM

RHRHRHTTTTTTTTT


Operating Center Distance RangeEnvironmental Effects- Internal gear set

expansion moisture of tCoefficien expansion thermal of tCoefficien

diameter pitch Operatinghumidity relative in Change

etemperatur in ChangeTdistance center in Change C

d

RH

A

AGG

APP

AMM

RHRHRHTTTTTTTTT

RHTTd

RHTTdC

PMPPMMP

GMGGMMG

)(2

)(2


Analyze at Open Mesh Condition

Minimum Material Condition

Minimum tooth thickness Maximum tip radius External gear

– Minimum outer diameter– Minimum root diameter

Internal gear– Maximum inner diameter– Maximum root diameter

Open Center Distance

External gear set Maximum effective

operating center distance Internal gear set

– Minimum effective operating center distance


Check geometry– Contact ratio > 1?– If not, go back to beginning,

• Select new diametral pitchor module

• Change tooth proportions• Renegotiate tolerances

Load analysis at temperature– Load capacity– Excessive tooth deflection?– Tip relief?

Analyze at Open Mesh Condition


Analyze Other Load Conditions

Tight mesh condition is often hot and moist

Open mesh condition is often cold and dry

But worst load condition– Open mesh - minimum load

sharing– Hot and moist - minimum

material properties Transient conditions

– Cold housing and hot gears– Hot housing and cold gears


Finally

Iterate until all of the above works– Design time cheap– Changes during/after development costly ($ and )

Computer programs are necessary– Analytical programs preferred– Graphical programs often

cause problems

Write specifications


Outline



Specifications

Plastic gear transmissions require significant engineering effort. Components

– Gears– Housings– Shafts– Bearings

Variations– Manufacturing tolerances– Operating conditions

(i.e., temperature, moisture)• Dimensions• Material properties

Making certain the resulting design intent is specified clearly, accurately, and precisely to the gear manufacturer is essential to ensuring performance, cost, and delivery requirements are met.


Specifications

For more information see AGMA 909-A06, Specifications for Molded Plastic Gears


Specifications

For more information see AGMA 909-A06, Specifications for Molded Plastic Gears


Outline



High-Precision Gear Molding

Accurately predicting and consistently controlling (precision) shrinkage.


The Controlling Principle

Shrinkage is only affected by material’s:– Orientation (polymer and reinforcement)– Temperature– Pressure

Almost everything can have an effect on at least one of these three things and will effect shrinkage.


Accuracy vs. PrecisionThe Target Analogy

High Accuracy Low Precision

High Precision Low Accuracy


Precision

Material– Crystalline resins vs.

amorphous resins– Shrinkage data– Fiber reinforcement– Viscosity

Part Geometry– Wall thicknesses– Features/ribs/holes/cams/etc.– Fillets inside corners– Gate(s)

Mold– Tolerances– Cavitation– Cooling

Process– Temperatures– Pressures– Cycle time


m = 1.0 mm; z = 28; = 20 °; b = 15 mm; Hostaform C 27021

Web CenteredWeb Off-Center

PrecisionGeometry


m = 1.0 mm; z = 28; = 20 °; b = 15 mm; Hostaform C 27021

6 Ribs 12 Ribs No Ribs

PrecisionRibs


PrecisionGates


Filling Roundness

PrecisionGates


FillingRoundness

PrecisionGates


PrecisionMold Cooling

Bad Better


Best

PrecisionMold Cooling


Prototyping Verification of part and material performance Verification of manufacturing capabilities with dimensional

control Represent production as mush as possible

– Mold• Mold material• # cavities, runners, gates, etc.• Cooling channels

– Molder– Molding machine

• Barrel size– residence time

• Injection rate• Clamp tonnage

– Molding conditions• Temperatures

– Mold– Melt

• Cycle profile– Injection speed– Hold time & pressure– Cooling time

• Screw RPM & back pressure

Identical to Production


Prototype Mold DevelopmentPrecision

Follow material supplier’s molding recommendations Establish appropriate processing window

– Maximize material properties• Resist “correcting” dimensions with extreme

processing conditions– Wide, stable processing window

• Minimal variational effects on properties and dimensions– Maximize dimensional stability

• Consistent as-molded dimensions– Precision vs. cycle time

• Minimize post-molding shrinkage– Mold temperature must exceed operating temperature

• Pay now, or pay later! Design of experiments (DOE)

Stability Equals Precision


Prototype Mold DevelopmentAccuracy

Then correct tooling for shrinkage– Cut molds “steel safe”

• Undersized cavities• Oversized cores

– Use inserts Iterate Measure thoroughly

– Make what you designed

Then Correct for Accuracy


Outline



Plastic Gear Development Cycle

Design & Engineering

Prints & Specifications

Prototype Tool & Parts

Measurement & Inspection

Testing

Production


Inspection and Geometry Verification

During development Elemental inspection (CNC)

– Profile error (involute error)– Lead error (helix angle error)– Pitch error (spacing error)– Runout (radial position error)

General inspection– Outside radius– Root radius– Tooth thickness

• Measurement over pins or balls

Elemental Inspection for Development


Inspection and Geometry Verification

During production Composite inspection (Double-flank roll checker)

– Total composite error (TCE)– Tooth-to-tooth error (TTE)– Runout

Composite Inspection for QC


Outline



Testing and Validation

Geometry verification! Realistic

– Properly represents end-use conditions– Continuous testing when end-use is

intermittent• Overheating• No thermal or dimensional recovery

time• Temperature control

Effective– Static loads

• Creep and creep rupture– Impact loads

• Motor stall load• Motor rotor inertia load

Appropriate– Test procedures often developed for

metal gears

Keep It Real


Plastic Gear Development Cycle

Design & Engineering

Prints & Specifications

Prototype Tool & Parts

Measurement & Inspection

Testing

Production


Outline



Production

Utilize prototype knowledge– Minimize deviations

• Mold• Mold machine• Molding conditions

– Wide, stable process• Maximum material properties• Consistent dimensions

– Correct tooling for accuracyMeasure thoroughly

– Make what you designed and verified Run capability study Establish production QC methodology Produce!

77 © 2012 Ticona Gears Webinar Gear_DesignPPT_AM_0212_016.pdf© 2012 Ticona Gears Webinar Gear_DesignPPT_AM_0212_016.pdf

Thank You

Questions?


David SheridanSr. Design Engineer

[email protected]

Product Information800-833-4882

[email protected]

www.Ticona.com

© 2012 Ticona Gears Webinar Gear_DesignPPT_AM_0212_016.pdf


Information is current as of February 2012 and is subject to change without notice.

The information contained in this publication should not be construed as a promise or guarantee of specific properties of our products.

Any determination of the suitability of a particular material and part design for any use contemplated by the user is the sole responsibility of the user. We strongly recommend that users seek and adhere to the manufacturer’s current instructions for handling each material they use.

Any existing intellectual property rights must be observed.

© 2012 Ticona. Except as otherwise noted, trademarks are owned by Ticona or its affiliates.

NOTICE TO USERS:

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Design & Development of Precision Plastic Gear Transmissions · 1 © 2012 Ticona Gears Webinar Gear_DesignPPT_AM_0212_016.pdf Design & Development of Precision Plastic Gear Transmissions