02 - cortez - cortez.pdf · alcohols, aldeídos, esters, ethers, glycols, gasoline, minerals oils....

GEARS & TRANSMISSIONS Workshop paper II [29]

Faculdade de Engenharia da Universidade do Porto, Portugal, 5th June 2003

05 - 06 - 2003 FEUP- Gears 2003 1

Thermoplastic Gears1) Gears Introduction and Terminology

2) Properties of Materials used.

3) Manufacture process.

4) Kind of Tools.

5) Competitive Advantages and Disadvantages

6) Design Notes

7) Appliances and some examples

05 - 06 - 2003 FEUP- Gears 2003 2

Thermoplastic Gears1. Gears Introduction and Terminology

Introduction and Requirements for Spur and Helical Gears

Example of Feasibility Checking with Nomograf

[30] paper II GEARS 2003


05 - 06 - 2003 FEUP- Gears 2003 3

Gears Introduction and Terminology

Introduction – Spur Gears:

Thermoplastics gears have found wide application,particularly in precision drive system for watches, time switches, water and electricity meters, etc. which are produced in large quantity. In such applications, the low production costs made possible by injection moulding is a decisive factor in the choice of plastics.

A more demanding application with regard to the drive power being transmitted is gearing for small electrical appliances such as a food equipment, mixers, ironing press, floor polishers etc. In addition to low cost production, a compelling argument for the use of engineering thermoplastics is their good sliding properties which provide silent running and maintenance free operation.For special applications in which high chemical resistance, e.g. against acids, is required, machined gear wheels are made from the ultrahigh molecular weight Polyethylene.

05 - 06 - 2003 FEUP- Gears 2003 4


Introduction – Worm and Helical Gears:

This type of gears are characterized by silent running, high transmission ratio in one step and a relatively compact, light weighted design. This has made them the preferred type of drive for small electrical appliances such as food slicers, flour mills, electrical knives , carpet and upholstery cleaning equipment, etc. This gears can be manufactured with a low cost from thermoplastics - especially Acetal resins.Worm gears are frequently used in automotive engineering as drives for Windshield Wipers, window cranks, seat and mirror adjusting mechanisms, etc.A characteristic feature of worm Gear is high sliding speed between the flanks of the Worm and Helical gear.The heat generated places a limit on the transmissible output of worm gears.



05 - 06 - 2003 FEUP- Gears 2003 5

Gears Introduction and TerminologySpur Gears - Requirements:

The requirements for Spur Gears vary greatly according to the type of gear operating conditions. In Watch movements, constant torque transmission with low friction loss is necessary. In drives for Meters and recording instruments, it is important to avoid jamming as a result of dirt contamination. Power gears including drives for food process, Windscreen Wipers, vehicle seat adjustment etc. must be capable of operating over a wide temperature range and guaranteeing adequateservice life with low tooth flank wear. Another important requirement which has become more relevant in recent years and has actually favoured the use of plastics is the demand for gears to operate as silently as possible.

These various requirements are met by different tooth designs and gear quality classes.

In power gears, normally we only use gears with involut theet.

05 - 06 - 2003 FEUP- Gears 2003 6

Gears Introduction and TerminologyWorm and Helical Gears - Requirements:

This type of gears may be called on to meet widely varying requirements with regard to speed ratio, transmission output, service temperatures and service life. While almost negligible transmission outputs are required for electricity meters, speedometers and time switches, outputs of up to about 400 watts may be needed for the stirrer arms of kitchen mixers or for mincers. Drives for food slicers are normally operated at room temperature, while wind shield wiper drives in motor vehicles have to function at service temperature ranging from about – 40ºC to + 80 ºC. Some worm gears operate for short periods at time only while others must be designed for continuous operation. The service life required in each particular case may be anything from a few to several hundred operating hours.In many cases, shock loads have to absorbed such as when the mechanism runs up against a stop or when the motor is blocked byoutside effects.



05 - 06 - 2003 FEUP- Gears 2003 7

We can also check the feasibility with the following Nomograph

For example:What is the required face width, f, (mm) for a gear which will have a diametral pitch,

Pd = 18, with Module M=1.4 mm, a tooth load, F=9.1 kg and operate at speed of 100 rpm, with 1Mio cycles and initial lubrication?.

Gears Introduction and TerminologyExample of Feasibility Checking with Nomograf

The estimated design stress, S (kg/cm2) is 210 kg/cm2.

05 - 06 - 2003 FEUP- Gears 2003 8

Gears Introduction and TerminologyExample of Feasibility Checking with Nomograf

A straight line is drawn on the nomograph between the stress, S=210 kg/cm2 and the tooth load, F= 9.1 kg until the reference line is intersected. A line is drawn from the Module M= 1.4 mm through the intersect on the reference line to the tooth face width line, f. Thus the suggested width of tooth of 5 mm is determined.



05 - 06 - 2003 FEUP- Gears 2003 9


52,10.61.4

9.1S0.6M

Ff ≈∗∗

=∗∗

=

Example of Feasibility Checking with Nomograf

We can also check the feasibility with the following equation, finding the same value ofNomographFor example:What is the required face width, f, (mm) for a gear which will have a diametral pitch, Pd = 18 mm, with Module M=1.4 mm, a tooth load, F=9.1 kg and operate at speed of 100 rpm?

The design stress, S (kg/cm2) estimated is 210 kg/cm2= 2,1 Kg/mm2

05 - 06 - 2003 FEUP- Gears 2003 10

Thermoplastic Gears

2. Properties of Materials used:

POM (Polyoxymethylene)= Acetal Resin

PA (Polyamide) = Nylon Resin



05 - 06 - 2003 FEUP- Gears 2003 11

POM- Polyoxymethylene- Acetal ResinThe acetal resins are considered as technical polymers (homopolymer and Copolymer) and they have the following combinations of properties:

Good dimensional stabilityHigh tensile strength, shear strengthExcellent resilience,abrasion and vibration resistance.Excellent temperature resistance(-40 at 110ºc).Good electrical insulating characteristics.Low friction.Excellent resistance to moisture, gasoline, lubricants, solventsand others neutral chemicals.Easy processing and low costs.Enhancement of material properties by use of additives.

05 - 06 - 2003 FEUP- Gears 2003 12

Density: between 1.41 and 1.43 g/cm3Structure: they are thermoplastics with high crystalline. They chains are linear without ramifications, such those crystalline could reach 75%. Practically they don’t absorb water.Materials of reinforce and charges: we can use glass fibers, mineral, PTFE (Polytetrafluoroethylene) MoS2 and carbon black.Color: Due its high crystallinity, it only exist in white (opaque), however we could have it in any opaque color. It have a good superficialbrightness.Mechanical properties: It have good resistance and high rigidity accompanied with good tenacity, even at low temperatures. The homopolymers, due its higher crystallinity have bigger densities, hardness and flexural modulus (E), as well as a better resistance to abrasion in comparison to copolymers. However they have minor capacity of deformation and for that minor resistance to impact. The resistance to wear and low coefficient of friction (µ) could be improved through the addition of MoS2 and PTFE. The addition of glass fibers, allows the increase of its mechanical resistance, flexural modulus and resistance to deformation by heat.

POM- Polyoxymethylene- Acetal Resin



05 - 06 - 2003 FEUP- Gears 2003 13

Electric properties: They have good electric insulation as well as high dielectric rigidity, basically independent of air humidity.

Thermal properties: Its use temperature could vary between -40ºC and 110ºC. It burns with blue flame.

Chemical resistance: chemically they resist to many organics composts, for example:

alcohols, aldeídos, esters, ethers, glycols, gasoline, minerals oils. They have good resistance to hydrolyze. However they don’t resist to:Oxides composts and strong acids (pH<4)The homopolymers aren’t capable to be under water for a long time if her is at temperatures biggest than 65ºC.They deteriorate be UV radiation. To avoid this it is advisable to use an stabilizer (carbon black)


05 - 06 - 2003 FEUP- Gears 2003 14

Type of Applications:

The POM is very used on small parts with high precision and thin walls (until 0.15 mm)

GearsCounting mechanism CamsSliding elementsValves bodiesMotorcycles oil pumpsHelices, etc.




05 - 06 - 2003 FEUP- Gears 2003 15

PA – Polyamide

The polyamide are considered engineering plastics for many technical applications, by the reason of having:

Good mechanical resistanceHigh tenacityHigh impact resistanceGood behavior to slidingGood wear resistance

They have a good fluidity which allow the moldability of very complex parts. However, the polyamide absorbs water (as well the inverse), which imply the change of there properties.

From the polyamides, we can distinguish the followings types:Poyiamida 6 PA6Poyiamide 46 PA46Polyamide 66 PA66Polyamide 610 PA610Polyamide 612 PA612Polyamide 11 PA11Polyamide 12 PA12CopolyamidesPolyamide 6-3-T amorphous, transparent

05 - 06 - 2003 FEUP- Gears 2003 16

Density: between 1.01 a 1.6 g/cm3 depending if they have charges and kind of charges

Structure: Most of them are thermoplastics semi crystalline with a crystallinitynot higher than 60%. In some kinds of PA, the water absorption could be intense.

Materials with reinforcement and charges: we can use glass fibers, carbon, glass spheres, minerals charges,MoS2 and graphite

Color: its natural color is opaque white, although we can get any opaque color using pigments. The amorphous polyamides are almost transparent.

Mechanical properties: They have a big fatigue and impact resistance. They have good abrasion resistance and good friction properties, which can be improved by using MoS2, PTFE and graphite. The use of glass and carbon fibers allows the improvement of mechanical resistance and flexural modulus, which at the same time decrease the shrinkage indices and improves the resistance to deformation by heat.

PA – Polyamide



05 - 06 - 2003 FEUP- Gears 2003 17

Electrical properties: they depend of the content of water. The good superficial resistance prevent most of electrostatics charges. They are not appropriated to insulation on areas of high frequency due its dielectrics losses caused by its polarity.

Thermal properties: Its use temperature could vary between -40ºC and 120ºC, depending on the type of PA. Most of them resist to boiling water. It burns with blue flame and yellow aureole. Its possible to increase the heat resistance adding stabilizers.

Chemical resistance: Chemically they resist to:

Aromatics hydrocarbons, oils, gasoline, some alcohols, esters, acetones, ethers. Its chemical resistance increase with it level of crystallinity.

However they don’t resist to:Inorganic acids, solution of oxidants composts, formic acid, phenols, chloroform. The amorphous PA don’t resist to ethylic alcohol, acetone and dicloromethane.

PA – Polyamide

05 - 06 - 2003 FEUP- Gears 2003 18

Types of Applications:The PA is very used on small parts where high precision is required:

GearsConnectors for automotive industry BobbinsFilters of oilHelices for boatsEngine coversDomestic appliances

PA – Polyamide



05 - 06 - 2003 FEUP- Gears 2003 19

Thermoplastic Gears

3. Manufacture process:

Machined by special Milling machines

Moulding by injection

Details of process -

05 - 06 - 2003 FEUP- Gears 2003 20

Manufacture process:Effects of mold design variables on part shrinkage



05 - 06 - 2003 FEUP- Gears 2003 21


05 - 06 - 2003 FEUP- Gears 2003 22

Manufacture process:

Welding Lines / N.º and position of gating points

Effects of mold design variables on part shrinkage



05 - 06 - 2003 FEUP- Gears 2003 23

Manufacture process:

Welding Lines / N.º and position of gating points

Effects of mold design variables on part shrinkage

05 - 06 - 2003 FEUP- Gears 2003 24


1. Part Thickness - ↑ Wall Thickness ⇒ ↑ Mould Shrinkage2. Gate Size - ↑ Gate Size ⇒ ↓ Mould Shrinkage3. Gate location - dependent of part geometry for its effects (gate

must be in the larger thickness)

Effect of wall thicknessand gate size on theshrinkage of a POM part.



05 - 06 - 2003 FEUP- Gears 2003 25

Manufacture process:Effects of Injection Parameters on the quality of the part

(e.g. shrinkage)

1. Pressure - ↑ Pressure ⇒ ↓ Part Shrinkage

2. Mould Temperature ↓ Mould Temperature ⇒ ↓ Part ShrinkageA hot mould out-weights the benefits of a Could Mould:

Makes the cavity easier to fill (lower level of internal stress)Improves the surface appearance (more gloss)Is essential for long term dimensional stability of part

Hot mould ⇒ high cycle time

3. Material (Cylinder) Temperature (dependent of wall thickness)↑ Cylinder Temperature (thin parts) ⇒ ↓ Part Shrinkage

05 - 06 - 2003 FEUP- Gears 2003 26

Thermoplastic Gears

4. Kind of Tools:

3 plates (different n.º of gates)

Hot runner (different n.º of gates)



05 - 06 - 2003 FEUP- Gears 2003 27

Kind of tools

05 - 06 - 2003 FEUP- Gears 2003 28

Kind of tools



05 - 06 - 2003 FEUP- Gears 2003 29

Kind of tools

05 - 06 - 2003 FEUP- Gears 2003 30

Kind of tools



05 - 06 - 2003 FEUP- Gears 2003 31

Kind of tools

05 - 06 - 2003 FEUP- Gears 2003 32

Thermoplastic Gears

5.Competitive Advantages and Disadvantages

There are a high number of advantages in the use of plastic gears that cannot be reached with metal gears:

No subsequent operations of mechanization or flash elimination are necessary;Possibility to combine gears with other elements, such as: springs, roller-bearings, racks and other gears;Less weight;Lower costs (for high series);Corrosion resistance (within some limits);



05 - 06 - 2003 FEUP- Gears 2003 33

Thermoplastic Gears

5.Competitive Advantages and Disadvantages

Electrical Insulation;In several cases, better working in dry than metals;Better sound level;It allows the combination of an unlimited number of elements and functions in a sole piece, reducing the number of components;Possibility to produce the same gear with different materials (Polymer base + different additives) and, consequently, the same gear with differentperformances

05 - 06 - 2003 FEUP- Gears 2003 34

Thermoplastic Gears5.Competitive Advantages and Disadvantages

If in a determined application that requires high physical properties, which plastic hardly endures, in that case, there is no alternative to metal;

Temperature limitations (140°C max.)Temperature generated in the teeth’s surface by friction;Flexion’s tension on the tooth’s root;Fatigue or impact resistance;Wearing;Chemical resistance.



05 - 06 - 2003 FEUP- Gears 2003 35

Thermoplastic GearsDesign Notes:

Combinating a metal gear wheelwith a plastic gear wheel, thetransmissible tangential force Ftcan be increased by the use ofcomplementary profiles (accordingto picture);

In this case, the tooth thicknessof the plastic gear wheel, can beincreased at the expense of tooththickness of the metal wheel. Theratio of the tooth thickness S to tooth space e, can be up to 2:1.

Complementary Profile

05 - 06 - 2003 FEUP- Gears 2003 36


By thin, long teeth.

By radial slits in the carrier;

By flexible connection (spring elements) of the gear wheel

Reduction of operation noise



05 - 06 - 2003 FEUP- Gears 2003 37


For any given material, the allowable stress is dependent on a number of factors:

Total lifetime cycle

Intermittent / continuous duty

Environment (temperature, humidity, chemicals, etc.)

Change in diameter and centre to centre distance with temperature and humidity

Pitch line velocity

Diametral Pitch (size of teeth) and tooth form

Accuracy of tooth form, helix angle, pitch diameter, etc.

Mating gear material including surface finish and hardness

Type of lubrication (frictional heat)

Factors affecting Gear Design

05 - 06 - 2003 FEUP- Gears 2003 38


Factors affecting Gear DesignSelection of proper stress level can best be made based on experience with successful gear applications of similar nature.

The picture on the right plots a number of successful gear applications in terms of:

peripheral velocity

Tooth bending stress

Where suitable experience is not available, the allowable tooth must be based on careful consideration of:

all the factors previously outlined (previous slide)

Available test data on the gear material of choice (SEE EXAMPLE NEXT SLIDE)



05 - 06 - 2003 FEUP- Gears 2003 39


Factors affecting Gear Design: EXAMPLE

05 - 06 - 2003 FEUP- Gears 2003 40

Thermoplastic GearsAppliances and some examples



05 - 06 - 2003 FEUP- Gears 2003 41


05 - 06 - 2003 FEUP- Gears 2003 42




05 - 06 - 2003 FEUP- Gears 2003 43


02 - cortez - cortez.pdf · alcohols, aldeídos, esters, ethers, glycols, gasoline, minerals oils....

Documents