Alternatives to Keyways in Motion Systems
- Sunday, 01 October 2006
A variety of different keyless shaft locking devices have been developed over the years in order to address the backlash, density, and balance problems associated with dynamic loading of keys while guaranteeing adequate holding torque around the shaft. Clamping hubs with a single tangential screw are the most common because of their simplicity, low cost, and relative ease of assembly. When properly manufactured, a total shaft-hub clearance between 0.0004 and 0.002" is adequate to ensure that minimal torque applied to the clamping screw will guarantee a high-force frictional clamp connection.
Industry standards for precision motion components ensure that almost any motor or gearbox will present a rough machined (not ground or polished) and properly toleranced shaft for keyless locking devices. The diameter of the shafting in nearly all cases allows for much more surface area contact to exist between the shaft and hub than the power requirements would dictate. These types of clamping hubs normally are rated with a 1.5× or greater safety factor in terms of their holding torque relative to the peak output torques associated with their respective drive elements.
Tangential clamping collars more evenly distribute the stress associated with shaft locking and torque transmission. As demonstrated in Figure 1, the entire surface area in contact between the shaft and hub is utilized, as compared to the single face of a shaft key. This represents an average of a more than 10× increase in torque-related shaft-hub engagement when the surface area of one side of an ANSI standard square key is compared to its respective shaft circumference across the same hub fit length.
Conical clamping-type hubs provide further protection from torque overload and potential shaft slipping in applications where a hard stop or aggressive load reversal takes place. These normally are used only in applications where the nominal torque will exceed 150 lbs/in or more. Conical clamping hubs typically are manufactured from steel and would cause an increase in system inertia compared to aluminum tangential clamping hubs. Their torque rating, assuming the same conditions in terms of fit tolerance and engagement length, is several times higher. Other keyless clamping devices include expansion shafts, double screw clamping hubs, as well as some others, each with their own unique purposes.
As the field of precision motion control continues to develop faster, smaller, and more accurate systems, mechanical products are required to evolve in order to keep up with demands for acceleration and control that were not possible 50 years ago. Performance issues related to backlash, size, inertia, and balance have all led mechanical component suppliers to adapt to new standards and practices in order to eliminate shaft keys. While most keyless hub designs have existed for quite some time before the surge in popularity of servo motion control, their application in precision power transmission has increased widely over the past few years.