Quality gears, other components are the product of many different factors. But it all begins with the materials chosen, especially when it comes to inspection.

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Probably the most critical aspect of inspection is making sure the part has been produced as specified and is flawless. The detail of this inspection is dependent upon the documents and test certificates received from the supplier. The following certificates may be required: chemical composition, physical properties, hardness, ultrasonic or equivalent test results, furnace times and temperature records, details on each process and inspection, and a written record of the dimensions and identifying details.

ASME has expanded the scope of its standard for the dimensioning and tolerancing of casting and forging technologies to include molded parts, as the same principles apply. The standard Y14.8 -2009 has been renamed “Castings, Forgings, and Molded Parts.” Recommendations for the uniform description of features that are to be included in drawings and documents are included. New symbols and CAD requirements are also part of this standard. Tolerances are included for those unavoidable variations created during the manufacturing process. These features include die closure, mismatch, draft, parting lines, flash, and ejector marks. Parting lines are very important, and a symbol to indicate their location then the other features can be toleranced. The latest version of Y14.5 -2009 Dimensioning and Tolerancing includes a movable datum symbol to improve the clarification of the drawing. Other helpful information includes how to relate machined datum features to cast, forged, or molded datum features (here I have quoted liberally from an article by D.E. Day, chair of the Y14.8 committee).

The quality of a forging is especially vulnerable to the material quality. To guarantee the level of hardenability the metal’s “H-Band” should be specified. Bar material are specified with a “D.I.” value that provides the ideal critical diameter that can be quenched and still provide a proper core microstructure. Hardenability is not to be confused with hardness. The former indicates the depth below the surface to which the material can be hardened. It is the property that determines the distribution and depth after quenching. Hardness, on the other hand, is the quality of a surface to resisting indentation and is directly proportional to strength and wear resistance. Material strength is the ability to resist an applied stress without fracture (ultimate tensile stress) or permanent deformation (yield strength). Ductility is the material’s ability to deform without fracturing and is inversely related to its strength. Toughness implies impact toughness, or in other words the material will not fracture from an impact based on the results obtained from a Charpy test. The basis of testing a material is to verify a specific quality based on simulating the anticipated service conditions. The basic testing requirements for composition and properties are provided by ASTM specifications.

Testing may not be required if the material has been previously known and approved. Testing is always required for those situations considered critical. Mechanical tests reveal the response of material to the forces applied. There are two basic categories: destructive, and non-destructive. The former includes tension—tensile and traverse rupture—compressive, fatigue, notched bar impact, and bend, creep, stress-rupture, hardness, and residual stress. Compressive testing provides the compressive yield strength, yield point, modulus of elasticity, and compressive strength. Non-ductile materials fail by shattering fracture. The ductile material assumes a barrel shape, and an arbitrary compressive strength is calculated by the unit stress corresponding to the maximum load or the load corresponding to a specified prior compressive deformation. Non-destructive testing may include radiography, magnetic particle crack detection, dye penetration, ultrasonic flaw detection, eddy current, and electromagnetic testing.

The condition of castings is frequently determined by testing samples at the commencement and completion of each cast. Test procedures are instituted for sand, and pre-tap analyses with a spectograph. Final testing of coupons determines the chemistry, microstructure, and mechanical properties.

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is former director of the National Conference on Power Transmission, as well as former chairman of the AGMA's Marketing Council and Enclosed Drive Committee. He was resident engineer-North America for Thyssen Gear Works, and later at Flender Graffenstaden. He is author of the book Design and Application of the Worm Gear.