By Joyce Laird, Contributing Editor
Andy Lechner, Product Manager for R+W America is an expert in this field. He explains that for specifying keyways, it must be noted that there are very well established standards for dimensions, which can be found in Machinery’s Handbook, or ANSI B17.1 for imperial dimensions and DIN 6885 for metric dimensions. Every design engineer should have access to this information.
“Manufacturers of keyless clamping devices generally have holding torque ratings, which are contingent upon proper shaft/bore fit tolerances, and the assembly screws having the appropriate level of torque applied to them,” he adds.
A Wide Variety to Choose from
Lechner goes on to say that shaft keys and keyways are one of the oldest forms of power transmission connection.
“Generally speaking, with a square key situated between notches in the hub and the shaft, they form a positive lock between the driving and driven components. Over the course of the twentieth century a large amount of development took place with respect to the design and implementation of keyless shaft clamping systems. There is a wide variety available, including clamping collars with a single tangential clamping screw, fully split clamping collars, conical clamping hubs, which wedge a conical bushing between the shaft and the hub, shrink discs, which fit around the outside of the hub, sandwiching it over the shaft, and hydraulic locking hubs, to name a few. These devices are engineered to clamp between the shaft and the hub instead of driving on a shaft key.”
Keyed vs. Keyless Connections
Keyed connections tend to be the default selection for many designers, having been the incumbent technology for decades, and since many drive components come with a keyed shaft as a standard configuration. Lechner notes that depending on the sizing they can be considered a fail-safe solution.
“However, in applications where there will be load changes, reversals, or indexing, it is possible to wear out the keyway connection, causing increasing amounts of free play or “backlash” between the shaft and the hub, until failure results,” he says. “With keyless clamping hubs, there is no relative movement between the shaft and the hub, and stresses are distributed more evenly around the shaft. The result is a connection which is considered to more accurate and reliable for many power transmission applications.”
“For light duty continuous motion applications, keyed connections can often be the least expensive, since they are so common,” Lechner says. “For applications with heavy vibration, dynamic motion, or precise motion requirements, keyless is definitely the way to go.”
“Some popular keyed shaft applications would be conveyors or light duty pumps, since they are generally required to run in one direction without many stops or reversals. Servo driven machines typically require keyless connections, since the performance of the motion system depends on backlash free operation.”
Lechner concludes that the final decision remains with the design engineer and should be based on the exact requirements of each application.