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The use of plastics as materials for fine--pitch gears is growing. Improved materials, advances in gear mold design and molding technology, and the successful use of plastics gears in many applications have contributed to this growth. Information in this Design Manual was obtained from manufacturers of polymers, composites, stan- dard shapes and compounds, as well as of plastics gears themselves. Optimum performance of plastics gearing requires the proper design and selection of both materials and manufacturing processes. To add strength to plastic teeth, a full fillet radius is often used in the roots of the teeth.

10.5.1.1 Uses of plastics gearing

In the power transmission field most molded gears are used in the fractional horsepower area. Many cast and machined gears are used in higher horsepower applications.

10.5.1.2 Advantages of plastics gears

Some reasons for the wide use of plastics gearing are as follows:

-- Low cost;

-- Resistance to corrosion;

-- Lightweight (about 1/7 the density of steel); -- Low inertia;

-- Inherent lubricity and compatibility with com- mercial lubricants, most chemicals and common solvents;

-- Potential noise reduction;

-- Reduction of wear in mating gears;

-- Overload protection (acting as a sacrificial member);

-- Color to aid in assembly; -- Low maintenance.

10.5.1.3 Molded gearing

Molded gears can be combined with other features to create multifunctional parts, and can be produced in large quantities by the molding process.

10.5.1.4 Machined gearing

Many basic polymers are available in various shapes from which gears can be machined using essentially standard gear cutting equipment. This makes it possible to use machined plastics gears in quantities and sizes not practical for the molding process.

10.5.1.5 Limitations of plastics gearing

A major limitation of plastics gearing is reduced tooth strength in comparison with ferrous gearing. The effects of temperature and moisture on plastics materials used in gearing are not the same as for metals. It should be recognized that temperature and absorption of moisture affect the size and strength of plastics. In addition, plastics are relatively poor conductors of heat. A temperature buildup developed through the working of the gear may be difficult to dissipate. These limitations may be overcome through the proper selection of the basic polymer and the addition of reinforcements, lubricants or other additives.

NOTE: Care must be exercised when designing plas- tics gearing to account for dimensional and strength

changes due to temperature or humidity. The dimen- sional changes can either be growth or shrinkage and are generally predictable. To insure proper meshing of gears at either extreme requires adequate clearance which should be properly specified. Careful material selection can enhance the long term life of plastics gearing, and such materials are used with and without reinforcements and other additives.

10.5.1.6 Specifications for plastics gears

The proper use of plastics in gearing requires the adequate specification of the gear characteristics actually needed. It is highly recommended that all users of plastics gearing utilize a proper drawing specification format.

10.5.1.7 Available types of plastics gears

Any gear form which has been cut from metal can be cut from properly selected plastics materials. Many of the gear forms can be molded providing a suitable moldable material is selected. Certain gear forms, such as a throated wormgear which is conforming to the mating worm, are usually not considered practical as one piece moldings.

10.5.2 Plastics materials

Gears are made from various plastics, both thermo- setting and thermoplastic, with the latter by far the most widely used. These materials are available as unfilled polymers for use in molded and machined gears. Polymers can be reinforced with glass fibers, glass beads, milled glass, carbon fibers, fabrics and mineral reinforcements and fillers. Reinforcement may improve dimensional accuracy in some cases, because of the reduction in shrinkage. It may harm dimensional accuracy in other cases, where non--uniform fiber orientation or distribution results in non--uniform shrinkage. Most machined gears are not made from glass or mineral reinforced plastics since they are difficult to machine and the exposed reinforcement on the machined tooth profile may be abrasive to the mating gear as well as to the cutting tool.

Lubricants such as polytetrafluoroethylene (PTFE), silicone, molybdenum disulfide and graphite are often added to improve the inherent lubricity of plastics. Additives for stabilization (temperature and ultra--violet light) and coloring are also used. The list of candidate materials for plastics gearing continues to expand.

10.5.2.1 Thermoplastic materials

A thermoplastic (T/P) material is one that will repeatedly soften when it is heated and will harden when it is cooled. In molding, it undergoes a physical change only. The most commonly used thermoplastic materials for gearing are as follows: -- Nylon (PA) is a family of thermoplastic poly-

mers. The most widely used of any molded gear- ing material is nylon 6/6, but nylon 6 and nylon 12 are also used. Some nylons absorb moisture which may cause dimensional instability. Nylon may be compounded with various types and amounts of glass reinforcing materials, mineral fillers, and such lubricants as PTFE and MoS2

(molybdenum disulfide).

-- Acetal (POM) has a lower water absorbtion

rate than nylon and, therefore, is more stable af- ter molding or machining. Acetal polymers are used unfilled or filled, with glass and minerals, with and without lubricants, such as PTFE and MoS2, as well as one version with fibrous PTFE.

-- Polycarbonate (PC) is generally used with

the addition of glass fiber and/or PTFE lubricant and is a fine, low shrinkage material for producing consistently accurate molded gears.

-- Polyester (PBT and O\PET) are both un-

filled and with glass fiber, and are finding their way into more markets as a molded gearing material in competition with nylon and acetal. -- Polyethylene, ultra--high molecular weight (UHMW/PE) is a low moisture absorption

material that has high abrasion and wear resis- tance. Most gears of UHMW/PE are machined from extruded bar or slab stock. Some parts are made by injection and compression molding techniques.

-- Polyphenylene sulfide (PPS), when com-

pounded with 40 percent glass fiber with or without internal lubricants, has been found in certain gear applications to have much greater strength, even at elevated temperatures, than most materials previously available.

-- Polyurethane (TPU) is generally noted for

its flexibility and, therefore, has the ability to absorb shock and deaden sound.

-- Polyester elastomer (TPE) is a newcomer

to the gearing field, and has excellent sound

deadening qualities and resistance to flex fatigue, impact and creep, among other advantageous characteristics.

-- Styrene--acrylonitrile (SAN) is a stable, low

shrinkage material and is used in some lightly loaded gear applications.

Annex A of ANSI/AGMA 2004--B89, Gear Materials

and Heat Treatment Manual, has a detailed

explanation of these thermoplastics.

10.5.2.2 Thermosetting materials

A thermosetting material is one that, during mold- ing, will undergo a physical change as well as a chemical reaction called polymerization and cannot be remolded by reapplication of heat. Common thermosetting materials used in plastics gearing are phenolic (PF) and polyimide/thermoset (PI).

-- Phenolics (PF) are invariably compounded

with various fillers such as woodflour, mineral, glass, sisal, chopped cloth and such lubricants as PTFE and graphite. Phenolics are generally used in applications requiring stability, and when higher temperatures are encountered.

-- Polyimide (PI) is usually 40--65 percent

fiberglass reinforced and has good strength retention when used at high operating temperatures.

For more information, see annex A of ANSI/AGMA 2004--B89.

10.5.3 Molded plastics gearing 10.5.3.1 Molding considerations

The molding of gears requires special techniques and knowledge. Processing techniques vary with molding materials, gear geometry and size, and mold design. Gear molding is a specialized molding service.

10.5.3.2 Molding process

Molded gears are usually produced by injection or transfer molding methods in which the plastics molding material is forced under pressure through a gate(s) into the mold cavity. The teeth of a molded external gear are formed by the metal ring in the cavity in which the gear teeth have been repro- duced. The teeth of an internal gear are formed by the teeth in the metal core of the cavity set.

10.5.3.3 Mold design

Accurate molded gears require molds which are properly designed and accurately manufactured. Mold design must take into account methods of gating and other elements of the fill system, temperature control, and the ejector system. Cavity teeth must be precisely altered in form and changed in size to assure that the molded gear, after shrinkage takes place, assumes the desired config- uration. Accurate gear mold cavities can be produced by grinding, electric discharge machining (EDM) or electroforming.

10.5.3.4 Shrinkage during molding

Gear molding materials may shrink from approxi- mately 0.001 to 0.035 of an inch per inch from the cold mold dimension to the cold gear dimension, depending upon the material selected, the molding method used, and the cross--sectional mass of the gear. These same materials, when reinforced with glass fibers or other high aspect ratio fillers will shrink less in percentage but may shrink non-- uniformly because of flow orientation of the reinforcements. Therefore, a thorough understand- ing of the molding process, the selected material, and the required gear configuration is vital in order to predict whether the desired accuracy can be achieved. This will determine the size and the gear profile of the cavity required.

10.5.3.5 Accuracy of molded gears

Molded gearing accuracy depends upon selection of a suitable material ---- design of the gear, determination of the shrinkage of the plastics material, good mold design, accurate mold con- struction and use of molding techniques applicable to the molding of gears.

To obtain greater accuracy in a molded gear, the mold should be designed to have uniform cross sections, optimum gating location and adequate ejection locations. The material selected should display a highly consistent shrinkage and should be inherently stable under the environmental and use conditions.

Usually, the greater the accuracy required in a gear, the fewer the number of cavities in the mold. For example, an eight (8) cavity mold might well be used

for an AGMA quality six (Q6) gear, while a single cavity might be used for a quality ten (Q10) gear.

10.5.3.6 Mold economics

Most plastics gears are specials, inasmuch as molds for gears of all pitches and diameters and face widths are not commonly available. This means that usually a special mold must be pur- chased for each gear and, therefore, the molding process on an economic basis is usually limited to larger production quantities of gears. A molded gear from a single cavity mold will be more expensive than the same gear produced from a multicavity mold.

10.5.3.7 Use of inserts

Molded--in metallic inserts, including simple bush- ings, sleeves and shafts, are used to achieve advantages such as: more accurate bores, reduced thermal effects because of the conductivity of the metal, more closely controlled shrinkage, as in the case of a solid metal gear blank with molded plastics teeth, improved dimensional stability and usually greater load carrying capacity.

When contemplating the use of inserts, consider- ation must be given to the added cost of the insert, as well as the cost of inserting it into the mold during the molding operation, or of assembling it as a post--molding operation.

10.5.4 Machined plastics gearing 10.5.4.1 Machining techniques

These materials do not require special machines or cutting tools. Gear shapers and hobbing machines using properly selected and sharpened cutting tools at prescribed feed and speed rates will produce quality gears. Materials manufacturers generally recommend the use of coolants during machining in order to avoid stress cracking and melt outs. Climb cutting hobbing technique is also recommended for improved surface finish. It may be advisable to consult with the materials manufacturers for com- plete fabrication recommendations.

10.5.4.2 Molded blanks

Thermoset and thermoplastic gear blanks pro- duced by the molding process, and cut by conventional hobbing and shaping machines, offer alternatives to the expense of a toothed gear mold

for low quantities. Special gear requirements such as throated wormgears or double enveloping worms which may not be practical for molding, are machined from such blanks.

NOTE: Blanks should be designed and molded with special attention to preventing voids and excessive distortion after machining.

10.5.4.3 Gear and blank accuracy

End product accuracy is primarily dependent upon gear blank quality and process controls. Gear tooth finishing techniques common to metal gears, such as grinding, honing and lapping, are not usually performed on plastics gearing.

10.5.5 Other manufacturing methods

In addition to the molded and machined plastics gearing previously discussed, other manufacturing methods are also used. “As Cast” and “Forged” plastics gears will not be as accurate as machined or injection molded gears. “Stamped” and “Pressed and Sintered” plastics gears are being manufac- tured to accuracies comparable to metal gears produced by the equivalent process.

10.5.5.1 As cast

This method is used with urethanes and cast nylon materials. The mold, which is usually machined in metal, is engineered to provide adequate shrinkage allowance for the material being cast. The basic procedure closely resembles cast metal processes. Secondary operations usually include boring, key seating and machining of set screw holes.

10.5.5.2 Forgings

This method is used to produce special sprockets and gears for applications such as snowmobiles. This process is similar to metal forging. A billet of the desired material is heated below its melting point, then pressed in a metal--forming die. Secondary operations usually include boring, key seating and machining of set screw holes.

10.5.5.3 Stamping

This method is identical to that used for metal gears in instrument drives, meters, switches and timers. Stamped gears are usually finished in one operation depending on the quality of the die set and the

accuracy requirements of the part. The stamping process is generally limited to gears of narrow face widths.

10.5.5.4 Pressed and sintered

This method is similar to powder metal technology. Specially blended powders are compacted in a die, then sintered. They can be oil impregnated since the material is porous. Secondary operations will vary, depending on the complexity of the part, and the limitations of the tooling.

NOTE: The size limit of injection molded gearing is usually determined by the part wall thickness. The size limits of machined gearing is determined by the size of available extruded or cast shapes and parts. Not all plastics materials can be used for the manufacturing processes identified in this section. Discussion be- tween designer and manufacturer will usually result in considerable time savings and mutually acceptable parts.

10.5.6 Inspection

Plastics gears must be accurate to wear well, operate quietly and transmit uniform motion. In order to meet these objectives, the understanding of variations, tolerances and inspection is neces- sary. Refer to ANSI/AGMA 2000--A88, Gear

Handbook -- Gear Classification, Materials and Measuring Methods for Unassembled Gears for

additional information.

NOTE: Inspection of plastics gears must be done in a manner which accounts for size variation with tem- perature and moisture content. It is commonly ac- cepted practice to use about one--half the tight--mesh load recommended for metal gears when measuring plastics gears on a center distance measuring instru- ment. Refer to ANSI/AGMA 2000--A88. Care must be taken not to distort the gear teeth or mounting arbor of the work gear by application of excessive tight--mesh applied load.

11 Manufacturing methods

11.1 General

Many methods are used to produce the finished gear teeth including: hobbing, shaping, milling, fly--cutting, broaching, casting, powder metal proc- ess, molding, stamping, cold rolling, grinding, shaving, honing, lapping and burnishing. This clause reviews the most common methods used to

manufacture fine--pitch gear teeth but is not intended to be all inclusive.

11.2 Hobbing

The generation of a gear tooth is a continuous indexing process in which both the cutting tool and the workpiece rotate in a constant relationship while the hob is being fed into the work. As the hob is fed across the work, all the teeth in the work are completely formed. Hobbing is normally limited to external gears. Caution should be used by the designer to allow ample room on each side of the gear for entry and exit of the cutting tool.

11.3 Shaping

Gear shaping is a generation process where the workpiece and the toothed circular cutter progres- sively index in a timed relationship simultaneously with a reciprocating action of the cutter. This reciprocating action permits the cutting of gears that are closely banked against obstructions or other gears of a cluster. Moreover, the nature of shaping allows the generation of internal teeth.

11.4 Milling

Milling gear teeth is a form cutting method using a cutter having a profile matching the gear tooth space. One tooth is cut, the blank is then indexed, usually using a dividing head, and the next tooth is cut, continuing around to complete the gear.