Ratcheting Compliant Rack & Pinion Linkage
THE PROJECT
My idea with this was to create a micro-adjusting, ratcheting clasp mechanism that could be used universally in small scale applications. To the right you can see a rendering of my first iteration of this design. After prototyping this first iteration, working through some issues, and redeveloping the linear gear to incorporate more adjustment, the mechanism has now evolved to iteration 2, which is detailed below. The goal with this is to produce with applications in belts, clothing, shoe retention, watch bands, and anywhere else where fit accuracy and increased functionality makes sense. Alongside the inner workings of the mechanism itself I'm working on a latticed version which uses the least amount of material possible while retaining all functionality.
METHODOLOGY
The design process for this was very interesting for me. I had the idea for the whole system at a coffee shop and drew out the first couple ideas while there. The next week or so I spent a good deal of time walking around and imagining variations of the mechanism in my head. A couple things had to be accomplished: smooth drive of the rack using the dial, repeatable disengagement and reengagement of the entire system from the knob, ease of assembly, feeling of mechanical quality, and staying within certain dimensions of overall thickness and width. The design I figured would work was the 5 piece system you see to the right. Of course the design has now changed to something more mechanically complex, albeit more seamless for the user, but the workings of this first iteration were vital to the project as a whole as I had successfully worked through many hurdles in functionality.
THEORY, INSPIRATION, IDEATION
Part of the inspiration for this project comes from the BOA fit system which consists of a ratcheting dial and polymer lace. My experience with this system is largely through cycling shoes which often make use of BOA over a traditional lace. Performance is the reason for this where with cycling it is ideal to have a very precise fit in the shoe. Too tight or too loose and you may injure yourself just based on there being a lot of stress on your feet when connected to the pedals. A situation where this matters for example is at the end of a race when you're preparing for the sprint. Throughout the race I'd stay at a comfortable level of retention in the shoe, whereas towards the end of the race I might reach down to tighten the BOA and make sure my connection with the pedals/cranks is more secure for the sake of efficiency when putting more power down. BOA has many applications other than just this, so the idea of a ratcheting adjustment system piqued my interest.
I was looking at bicycle hub schematics while I got the idea for this rack & pinion clasp. The ratcheting nature of a bicycle hub works similarly to BOA in the sense that there’s drive in one direction and free movement in the other. While ideating more products for my pursuits with Apreneue, I had the idea for a micro-adjusting belt that operates with a rack & pinion system—offering a new implementation of the ratcheting clasp mechanism that is more robust and ideal for certain applications.
BELOW IS AN EXPLODED VIEW OF ARTICLE 1
ASSEMBLY AND MECHANICS
There are 5 total pieces shown above: the knurled dial/knob/cap, ratcheting compliance piece, drive gear/pinion, case, and the rack.
The knurled cap/knob has directional teeth along the inside wall that the ratcheting compliance piece sits within. The compliance piece is press fit into the case and rotationally locked separate from both the dial and the pinion. The drive gear is rotationally locked with the knurled knob using a hex counterbore/insert (shown below) and vertically locked using an M2 bolt and nut. This way, the dial and the pinion spin together against the compliance piece which locks the whole rotation in place after each ratchet. The pinion sits against the inner face of the case and drives the rack which is 0.2in thick in its entirety but 0.1in thick at the rack teeth. This is for the pinion to be able to engage and disengage with the whole system. If you look at the pinion gear you can see a revolved semicircle just above the gear teeth. This portion presses into and out of the compliance piece with a separate compliant mecahnism (shown below on the underside of the ratcheting compliance piece). Pulling on the dial will pull up on the pinion as well separate from the ratcheting compliance piece. This disengages the system and lets the rack loose. Pushing down on the dial will reengage the gear and the user is free to tighten the system once again.
ARTICLE 2
This design change consists of a new system where the previously fixed linear rack is now a hinged linear gear that is incorporated into the strap (or whatever medium the clasp is applied to). The linear rail will be made up of multiple pieces that either mount flush directly to the strap or are connected to each other in a permanent degree-of-freedom-restricted way.
My drawing to the right shows the application of a belt where the linear rail portion is divided into pieces and mounted to the leather. In this case, the leather acts as a hinge for the linear gear pieces. Inserting this into the case and engaging the ratcheting internal gear will hold the whole mechanism together.
A ton of modularity comes with this design to where the band can be tightened as much as possible as long as there are enough rails to mesh with. In the case of a belt, there is now much more universality for different waist sizes, where the previous design could only account for a minor change in overall system diameter.
Additionally, with this design there is room to exepriment with end-use additive hinges that work right off of the building volume—requiring no assembly and minimal post processing.
MECHANICS
With this system the band and linear rail will tuck in behind the fixed strap on the other side of the knob. Small guide rails will be included within the case to where the linear rail fits flush behind the fixed strap on the other side, so as to create a seamlessly tighetning system.
Further, this design accounts for the slight bend of the linear rail side as it enters the case. As the backside of the strap will be in contact with the case while the strap is inserted, this leading edge on the case will be filleted/smoothed and the backside of the strap will be continous. Once the strap is insereted into the case, the linear rail is straightened out and the rail teeth are once again flush with one another to allow proper catch with the drive gear
AESTHETIC CONSIDERATIONS
LATTICE OPTIMIZATION
