I’m going camping in a week equipped with a new tent. In anticipation, I’ve been designing and printing a completely unnecessary trinket to make setting up camp marginally easier. I’ve created a rope tensioner that works via a pair of geared cams. Early prototypes suggested that the gears wouldn’t be strictly necessary, but it keeps the cams in sync and makes the self-locking action more robust. The surfaces of the cams are knurled with large teeth to catch the rope. Some of my early prototypes had serrations going in just one direction or a small knurling pattern. These early teeth patterns didn’t work on the slippery surface of the paracord I was testing it on. The large teeth mesh together which forces the rope into a tortuous path when locked. This device is also designed to print in place. Once the print is complete, the cams can be gently torqued with a screwdriver to set them free.
To be completely honest, this was mostly a design exercise to see if I could create a self-locking cam mechanism. The idea turned out to be very sticky, and I couldn’t abandon it until I had a working prototype. Now I have overcome this design, I’ll probably opt to batch print a more elegant design. The simpler design employed by many rope tensioners forces the rope into tight “S” or “U” bends with a simple, monolithic design. The rope is fed through holes or around various appendages of the device. As tension is applied, the rope presses against the tensioner at the bends and friction keeps the rope from slipping. It’s ultimately the better (albeit uninteresting) design.
If I were to spend more time with this trinket, I may add a spring mechanism to bias the cams closed. Currently the cams have to close enough to catch the rope to begin the self-locking action. I thought about replacing the hinge section with a compliant spring mechanism. Imagine a central post coming from the triangular base where the hinge currently resides with spiral spokes connected it to the cam bodies. This would act as both hinge and spring, but I’m afraid that such a design would allow the cams to move apart when force is applied.
