Alternatives to Keyways in Motion Systems
- Sunday, 01 October 2006
The age-old tradition of using shaft keys in mechanical drives has served the power transmission industry well for many years. When appropriately sized, it guarantees that virtually no relative motion can take place between a shaft and its respective shaft hub in a unidirectional continuous motion application. Today’s increasing demands for speed, precision, and small size have changed the standard for shaft locking devices, and challenged motion components manufacturers to develop new methods of keyless shaft locking for dynamic loading. As motors and drives become increasingly capable of rapid acceleration and rotary positioning accuracy in smaller and smaller packages, backlash, stress distribution, and balance have all needed to be addressed in shaft locking devices, in many cases rendering the shaft key obsolete.
Keyway backlash is the area of primary concern when addressing such performance issues. Precise fitting and complicated machining can serve to reduce the clearance between the key and the shaft or hub keyway, though the backlash can rarely be fully eliminated. As an increasing number of frequent machine starts, stops, and load reversals — all at increasing acceleration and deceleration rates — emerge, keyway wear is increased in terms of both the frequency and force of the impact between the key and keyway. Backlash also will tend to increase at an accelerated pace over time. As material is compressed and removed from the keyway as the result of the impact, the keyway widens, and the velocity at which the key impacts the keyway will be higher at each load change. Under highly dynamic loading, keyways can wear to the point of problematic backlash or even failure in a very short time.
In cases where rotary positioning accuracy is critical, the problems associated with backlash are clear. A delay in angular transmission causes inaccuracy in the move, which in turn leads to an inaccurately positioned part of the machine. Small amounts of backlash can be compensated for in some servo systems; however, this is only with a compromise to the system’s speed and responsiveness to velocity and position commands. Depending on the method of position feedback employed in the motion system, backlash also can cause severe oscillations to occur in a servo motor shaft as it hunts for its true position.
Torque vs. Shaft
A further disadvantage to shaft keys in a world of increasingly compact motion components is the reduction in the torque density of the shaft and its respective locking element. The introduction of a DIN or ANSI standard keyway into a shaft normally reduces the shaft radius by 20 to 25%, and often as much as 50%. Larger diameters must be selected in order to ensure that the shaft will be able to withstand the full torque of the application. This is especially pertinent when considering that the vast majority of torsional stress is applied to the outer 40% of the shaft material. In shaft hubs, the keyway poses a similar torque density related problem. In many cases, shaft couplings, pulleys, sprockets, etc. that would otherwise be capable of transmitting all of the mechanical power required of them, must be selected in larger sizes in order to accommodate the increase in bore radius at the point of the key. This significantly can increase not only the outside diameter, but also the cost, mass, and moment of inertia of the driven element.
Yet another concern associated with shaft keys, though not quite as widely applicable, relates to balance. In high-speed motion systems, balance and the smoothness of rotation become increasingly important. A growing number of shaft hubs are manufactured from high-strength aluminum rather than steel, as this helps to reduce inertia. The use of a steel key adds an imbalance to the system, which in many cases must be compensated. The concentricity and natural balance of most keyless locking devices eliminates the need for complicated, multi-component balancing procedures.