Rotary shafts are elongated, rod-shaped devices that rotate about a longitudinal axis and transmit torque. They are similar in shape to linear shafts, but are designed to withstand torsional forces. Some rotary shafts have tapped or untapped axial holes for mounting to support structures. Others are grooved for the placement of snap rings or channeled for keyways. Solid or hollow rotary shafts with male or female threads and stepped or chamfered ends are also available. Rotary shafts with break edges are deburred, but not chamfered. Precision ground products are also available. Most rotary shafts are made of aluminum, alloy steel, carbon steel, stainless steel, composite materials, or plastics. Alloy steel is harder than carbon steel and provides superior durability. Stainless steel is well-suited for applications in which corrosion resistance is an important consideration. Aluminum rotary shafts provide good electrical and thermal conductivity, high reflectivity, and resistance to oxidation. Composite materials are often made of carbon fibers bonded together by resins. They are not as strong as metal shafts, but are lightweight and help reduce energy requirements.
Rotary shafts are usually coated or hardened to improve durability. Anodizing is a protective surface coating process used mainly with aluminum products. Black oxide coatings are applied to steel or stainless steel shafts to prevent ion corrosion. Ceramic coatings provide a wear-resistant finish while chromium coatings improve corrosion resistance and reduce friction. Rotary shafts with nickel or nitride coatings are also available. Teflon®, a registered trademark of DuPont Dow Elastomers, is a class of fluoropolymer resins that is resistant to high temperatures, chemical reactions, corrosion, and stress cracking. Rotary shafts that are coated with polytetrafluoroethylene (PTFE) are used in a variety of applications. Some steel shafts are case-hardened with carbon or nitrogen. Others are through-hardened to ensure that the entire shaft has the same hardness.
Measuring the Hardness of Rotary Shafts
There are several ways to measure the hardness of rotary shafts. The Rockwell hardness test presses a steel or diamond cone against a test sample and measures the depth of the resulting indentation. Higher measurements indicate harder materials. For rotary shafts, common Rockwell hardness ranges are 50 to 59, 60 to 69, and 70 to 79. The Brinell hardness test subjects a test material to a load of 3000 kg with a hardened steel or carbide ball that is 10 mm in diameter. The Knoop hardness test also measures a material’s hardness through its resistance to indentation. The Vickers hardness test indents a test material with a diamond indenter that is shaped into a right pyramid with a square base and an angle of 136° between opposite faces.
Important specifications for rotary shafts include shaft diameter or width, maximum length, weight, height, and tolerance. Most rotary shafts have circular or square cross sections and are produced in standard lengths that can be cut to size for specific applications. Weight is measured in per unit distance, typically pounds per feet. Height is the distance from the guide or rail base to the center of the guide or rail. Ultra precision shafts have a very tight tolerance. Standard grade and precision grade shafts are also available.
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Linear shafts are elongated, rod-shaped devices that provide linear or rotary motion for power transmission applications.