DMG-Class-22-Gear Materials selection.pptx
- 1. Gear Failure The stresses that occur when the gears are in use and their surfaces in mesh. To understand gear performance as it relates to materials (properties and heat treatment), the critical failure modes must be taken into account: Bending fatigue (root fillet cracks) Macropitting (pitch line surface degradation) Subcase fatigue (sub-surface fatigue failure) Bending fatigue is caused by a load, applied along the line of action, which generates stress gradients in the root fillets of the teeth. How these stress gradients react with the inherent strength gradients in part determines the fatigue life of the tooth. The mode of failure tends to be in the form of crack propagation typically at the root fillet. Macropitting can occur on the tooth surfaces, where the combination of pressure and sliding forces is the highest. Lubrication and surface finish can either promote or prevent macropitting. Where sliding is present and the coefficient of friction is high, the applied stress reaches a maximum at the surface and can exceed the material strength. Subcase fatigue is another mechanism that can occur at the active profile face in that the applied stress level falls off gradually and can, therefore, approach or exceed the critical fatigue strength of the material. For case hardened parts, subcase fatigue usually occurs close to the case-core interface and cracking at the interface can be prevented by selecting, for a given material, the proper case depth and core hardness.
- 2. Influence of Materials Gears under load are subject to gradient stresses both on the active flank and at the root fillet. Properly selected materials and heat treatments will produce strength gradients that are adequate to withstand the stress gradients and provide an acceptable margin of safety. In all gears, the choice of material must be made only after review of the performance demanded by the application. Material choice must be a balance between overall cost and required service life. Key design considerations require an analysis of the type of applied load, whether gradual or instantaneous, and the desired mechanical properties, such as bending fatigue strength or wear resistance. Each area in the gear tooth profile sees different service demands. For example, in the root area, good surface hardness and high residual compressive stress are desired to improve bending fatigue life. On the active flank, a combination of high hardness and adequate subsurface strength are necessary for adequate resistance to macropitting and subcase fatigue.
- 3. Gear Materials and Properties
- 7. Properties of Gear Materials The material used for gears depends on the strength and service conditions. Examples of service conditions are wear and noise. Cast iron is a common gear material due to its good wearing properties, machinability, and the ease of producing complicated shapes via metal casting. Worm gears tend to use phosphor bronze because of the materials wear resistance ability. Carbon or alloy steels are commonly used due to their high strength values. These steels are heat-treated in order to achieve the appropriate combination of toughness and tooth hardness.
- 8. Properties of Gear Materials
- 9. Properties of Gear Materials Bulk metallic glass sits in the middle of metals and ceramics as where metals are best for severe wear and ceramics are best for maximum hardness. Hardness is an important factor in gear design: The higher the hardness of the material, the more that the size and the weight of the gear used can be reduced. For example, let us compare two gear sets where the first set has a Brinell Hardness Number (BHN) of 2,000 N/mm2 and the second set has a BHN of 6,000 N/mm2 . Due to higher hardness found in the second set, the gear set is smaller and has a more compact design. The weight of the second set is only 1/8 of the first set while still delivering the same power. Higher hardness also indicates surface durability or wear strength. The higher surface durability, the greater ability it has to resist tooth surface failure or pitting. Surface durability is a function of compressive strength with directionally proportional to hardness. Surface hardening is commonly achieved by case-hardening processes, producing a hard case on the gear surface, but leaving the core soft. This is done, opposed to through hardening, because increased hardness can result in more brittle material. Some examples of hardness values are for cast iron grade 35 has a BHN of 300 N/mm2 minimum. Phosphor bronze centrifugal cast has a BHN of 90 N/mm2 . AISI 9310 is a case-hardened steel alloy at 300属F used for aerospace gears that experience high loads and operate at high pitch line velocities. The BHN value for AISI 9310 is 250 to 350 N/mm2 .
- 10. Properties of Gear Materials Ceramics Ceramics excel in applications where metal alloys would traditionally faile.g., conditions that require non-magnetic parts such as vacuum or medical applications. Wear optimization is another condition in which ceramics are ideal. Metal alloys tend to break down (galling) during wear. The hardness of zirconia is 1,200 Vickers (HV) or larger. Converting this to BHN provides a hardness of 1140 N/mm2 . The bending strength of zirconia is greater than 800 N/mm2 and the heat expansion coefficient is 10 x10-6 属C-1 . Zirconia is wear and heat-resistant compared to alloy steels. In comparison, the heat expansion coefficient of steel alloys range between 11 to 13 x 10-6 属C-1 . The lower value is the less likely it is to expand. Ceramics are also non-magnetic and bio-compatible. This makes them well-suited for biotech and vacuum environments. Ceramics are well- insulated against electricity along with heat and have anti-friction properties. Unfortunately, ceramics suffer from low fracture toughness, with a typical value of less than 1 MPa*m1/2 . Their high melting points create some complications when they are being casted. The final shaping process of ceramics is also expensive. Due to the use of diamond tools, if any finishing work is required, ceramics can rise in cost. The size of the ceramic parts also factors in cost. Ceramics work best when the parts are small and in mass quantities. Large parts or a short run of parts do not offset the manufacturing costs.
- 11. Properties of Gear Materials Plastic Gears
- 12. Selection Guidelines for Power transmission In all gears the choice of material must be made only after careful consideration of the performance demanded by the end-use application and total manufactured cost, taking into consideration such issues as machining economics. Key design considerations require an analysis of the type of applied load, whether gradual or instantaneous, and the desired mechanical properties, such as bending fatigue strength or wear resistance, all of which define core strength and heat treating requirements. Consideration must be given to the forces that will act on the gear teeth, with tooth bending and contact stress, resistance to scoring and wear, and fatigue issues being paramount. For example, in the root area, good surface hardness and high residual compressive stress are desired to improve endurance, or bending fatigue life. Numerous factors influence fatigue strength, includes: Hardness distribution, as a function of case hardness, case depth, core hardness. Microstructure, as a function of retained austenite percentage, grain size, carbides (size, type, distribution), non-martensitic phases. Defect control, as a function of residual compressive stress, surface finish, geometry, intergranular toughness.
- 13. Selection of Gear Materials The choice of gear material depends on the following three factors, Mechanical property Metallurgical Manufacturing process Performance Criteria: Durability Strength Wear resistance
- 14. Selection of material for power transmission A wide variety of steels, cast irons, bronzes, and phenolic resins have been used for gears. New materials such as nylon, titanium, and sintered iron have also become important in gear work. Designers might well become hopelessly confused when faced with so many different gear materials, except that there are good and specific reasons for using each of the materials that have been adopted for gears. As their outstanding characteristics, steel gears have the greatest strength per unit volume and the lowest cost. In many fields of gear work, steel is the only material to consider.
- 15. Selection of material for power transmission The cast irons have long been popular because of their good wearing characteristics, their excellent machinability, and the ease with which complicated shapes may be produced by the casting method. The bronzes are very important in worm gear work because of their ability to withstand high sliding velocity and to wear in to fit hardened-steel worms. They are also very useful in applications in which corrosion is a problem. The ease with which bronze can be worked makes it a good choice where small gear teeth are produced by stamping or by drawing rods through dies.
- 16. Selection of material for power transmission A variety of cast irons, powder-metallurgy materials, nonferrous alloys, and plastics are used in gears. But steels, because of their high strength-to- weight ratio and relatively low cost, are the most widely used gear materials for heavy duty, power transmission applications.
- 17. Most Commonly used Gear Materials Cast iron Plain carbon steel Alloy steel Brass Phosphor bronze Al bronze Wood Nylon Bakelite