Plastic Gear Materials

Plastic Gear Materials

Tolerances.
Under certain operating conditions, the tolerances for plastic gears may be less critical than for metal gears for smooth and quiet performance. Ordinarily, however, the same care in manufacturing, testing, measuring, and quality level specifications should be utilized in plastic gearing as in metal gearing. The inherent resiliency of some of the plastic used may result in better conjugate action. The resiliency of many plastic gears gives them the ability to better dampen moderate shock or impact type loads within the capabilities of the particular plastics materials.
Operating Characteristics.
Generally, plastic gearing materials are noted for low coefficient of friction, high efficiency performance, and quiet operation.
Many plastic gearing materials have inherent lubricity so that gears require little or no external lubrication.
They can perform satisfactorily when exposed to many chemicals which have a corrosive effect on metal gears.
Plastic gearing, when operating at low stress levels in certain environments, have been known to outwear
equivalent metal gears.
Load Carrying Capacity.
The maximum load carrying capacity of most plastic gears decreases as the temperature increases more than with metal gears. The upper temperature limit of most thermoplastic gears is 250_F(121_C) at which point they lose approximately 50 percent of their rated strength. The upper operating temperature limit of thermosetting gears now exceeds 400_F(250_C). Very little degradation of mechanical properties in certain thermosetting materials occurs at temperatures up to 450_F(232_C).
Plastic Materials.
Many different plastics are now used for gearing. Both thermosetting and thermoplastic material are used, with the latter being by far the most prevalent.
1. Phenolic(T/S - indicates thermosetting).
  Phenolics are invariably compounded with various fillers such as woodflour, mineral, glass, sisal, chopped cloth, and such lubricants as PTFE (polytetrafluorethylene) and graphite. Phenolics are generally used in applications requiring stability, and when higher temperatures are encountered.
2. Polyimide (T/S).
  Polyimide is usually 40-65 percent fiber glass reinforced and has good strength retention when used at high operating temperatures.
3. Nylon(T/P - indicates thermoplastic).
  Nylon is a family of thermoplastic polymers. The most widely used of any molded gearing material is nylonbr> 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).
4. Acetal (T/P).
  Acetal has a lower water absorption rate than nylon and, therefore, is more stable after 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.
5. Polycarbonate (T/P).
  Polycarbonate 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.
6. Polyester (T/P).
  Polyesters are both unfilled and with glass fiber, and are finding their way into more markets as a molded gearing material in competition with nylon and acetal.
7. Polyurethane (T/P).
  Polyurethane is generally noted for its flexibility and, therefore, has the ability to absorb shock and deaden sound.
8. SAN(Styreneacrylonitrile) (T/P).
  SAN is a stable, low shrinkage material and is used in some lightly loaded gear applications.
9. Polyphenylene Sulfid (T/P).
  When compounded with 40 percent glass fiber with or without internal lubricants, it has been found in certain gear applications to have much greater strength, even at elevated temperatures, than most materials previously available.
10. Polymer Elastomer (T/P).
  Polymer elastomer 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.
Part Combinations.
Several plastic gears can be molded together as a gear cluster. Combinations of gears, pulleys, sprockets, and cams can also be produced as a single part.
Gear Blanks.
Many of these plastic materials, notably unfilled nylon and acetal, are available in standard extruded shapes, such as rounds, squares, and rectangles of various sizes from which gears can be machined. Gears can be molded at less cost if large quantity warrants the cost of the mold.
Machined Plastics Gears.
The quality of machined gears may be generally better than their molded counterparts, but the molded tooth surface is superior to the machined surface in smoothness and toughness. Final tooth strength is generally better inr> a molded gear, than an equivalent machined gear, because of the flow of the material into the tooth cavity of the mold. Gear cutting is done on standard machines and with standard tools. The following considerations will assist in obtaining higher quality machined parts.
1. Inspection.
  The modulus of elasticity is so low in plastics that errors in measurements are very difficult to control. The use of controlled load checking equipment is almost mandatory to avoid errors in measurements.
2. Tools.
  Sharp cutting tools are necessary to avoid tooth profile and size variation due to deflection.
3. Burrs.
  Feather edge burrs, if not eliminated by back up discs or subsequent removal by other means, will impair inspection of gearing and possibly contribute to noise during operation.
Laminated Phenolics Plastics.
1. Industrial Laminated Thermosetting Products.
  These products, whether in sheet or rod form, contain laminations or plies of fibrous sheet materials such as cellulose, paper, asbestos, cotton fabric, glass fabric, or mat. These materials are impregnated or coated with a enolic resin and consolidated under high pressures and temperatures into various grades which have properties useful for gearing.
  Fabric base grades are chosen to withstand severe shock loads and repeated bending stresses, and to resist wear. Fabric base grades are tougher and less brittle than paper base grades. The linen grades made with finer textured lightweight fabrics will machine with less trouble. Gears of linen base phenolic are abrasive, and thus may require a hardened steel mate and adequate lubrication.
  Asbestos-phenolic grades have excellent thermal and dimensional stability.
  The glass fabric base grades have good heat resistance and very high tensile and impact strength.
2. Performance Characteristics.
  Phenolics are used for fine pitch gears due to economy, high resiliency, and high wear resistance. Lower
  density than metals often provides higher strength to weight ratios. It should be noted that all grades have
  some dimensional change due to humidity.
3. Chopped Fabric Molding Compound.
  Chopped fabric impregnated with phenolic resin is capable of being molded as a gear but may require
  finish machining to meet most commercial quality requirements.