Adjustable Keys (p-set 3)

If you have every tried to attach something to a shaft before, you know that in the cost-quality-performance triangle of project management, there is not much winning.

• Interference fits require careful design and high tolerance machining, plus they can fail under thermal expansion or centripetal acceleration.
• Pins can induce stress concentrations in the joint, and often require a pin and hole with high tolerances. Plus access to the pin slot can be problematic
• Keys require broaching and also create (albeit smaller) stress concentrations
• Collar clamps can be bulky and hard to integrate into the machine, plus they require compliance
• D-shafts can strip out, and require special machining
• Don’t even get me started in set screws

Each of these joinery methods have advantages too, albeit some have more than others. However, making these joints is often a big part of machine design that can be easily mistaken. So, I decided to go out and see if there are other design elements, lego bricks so-to-speak, that could be added to the list.

Because there are have been many talented engineers working on hard problems throughout history, it did not take me long to find another great way of solving the shaft joinery problem that someone else had thought of: adjusting keys. I found the concept in a book titled Textbook for Vocational Training − Machine Elements and Assemblies and Their Installation by Früngel, Geppert, and Steckling.

A simple adjusting key assembly, from Textbook for Vocational Training.

The assembly works by splitting the shaft collar into two halves, and putting an angle on the back of one of those halves. A key, with a matching angle is inserted into a groove with the angled shaft collar, and held in place with friction, a bolt, or something else depending on the design constraints. At the cost of part count (adding 2-3 components), the design is made less complicated by making the parts out of easily manufacturable geometries. The orientation of the key angle can also be reoriented to suit the application! An excellent tool to keep in the design engineer’s toolbox.