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Gears, pulleys, couplings, shaft extenders, shaft reducers, flexible shafts and shaft accessories are critical rotating and power transmission components. They are found in assemblies for applications as diverse as robotics, 3D printers, machine tools, automobiles, military and defense equipment and aircraft controls.

A key aspect of the proper design and operation of these assemblies is how components are fastened to the rotating shaft in order to maintain position and alignment. To optimize manufacturing and operating functionality, many criteria should be considered during the design phase, including some that are frequently overlooked.

Holding force: Understanding the required holding force is critical to ensure that the component will not slip or move during operation. Designers should determine the operating torque and holding force required to prevent slippage.

Alignment and phase adjustment: Assemblies that require greater control and accuracy in axial positioning and angular alignment (phase adjustment) may dictate the type of hub fastener used. If phase adjustment is necessary, methods such as machine keys that fix the orientation of the component on the shaft should be avoided.

Costs: Cost is always a concern. Design engineers should consider manufacturing, purchasing, assembly and maintenance costs when specifying the best hub fastener for an assembly. While the initial cost may seem small, it can add up over the life of the product and for high-volume production.

Machining variables that contribute to higher costs are shaft tolerances and features, such as keyways, drilled or tapped holes, flats and shoulders on shafts. Purchasing additional components such as clamps, keys and pins add to the overall cost of the design.

Assembly and maintenance costs should be considered as well. Ease of maintenance is sometimes overlooked but may be important, especially on assemblies that require routine or frequent maintenance. Hub locking methods such as set screws may mar the shaft, requiring repair upon removal or replacement. Pinning a part to a shaft can result in having to replace multiple components during removal.

Standard vs. custom product: Many designers think they are always better off selecting a standard product over a custom design. While this may be true for one-off or very low production designs, it is not the case in all situations. When all of the design variables have been taken into consideration, a custom solution may be the optimal choice.

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