Robotics Education & Competition Foundation
Inspiring students, one robot at a time.

Linear Ratchet


Entry ID #: 8914
Created: Tue, Dec 8, 2020 2:48 AM

VEX U Team QUEEN - Queen's University, Kingston, Ontario, Canada Introduction Linear motion is one of the many things that we feel has been somewhat left behind in the world of VEX Robotics. After our submission of the Low-Friction Slider Truck last year, we felt that we should create another part to add to our linear motion toolbox. An issue with the current linear motion systems is that there is no clear-cut way to ensure one-directional motion, meaning that a mechanism can move in one direction, but not the other. Additionally, many existing solutions only provided a way to completely prohibit a mechanisms movement in both directions once actuated. Previous solutions to this problem included, among other things, pneumatics that pushed a pin into a hole on the sliding mechanism, however, this was very inconsistent as it required a very high accuracy to make sure the pin aligned properly. To solve this problem, we created a custom, 3D-printable Linear Ratchet designed to work seamlessly with our existing Low-Friction Slider Truck. The slide-ratchet allows a mechanism to move and become locked in place once a threshold has been reached. One of the many applications that this part provides is allowing mechanisms to become fixed after a robot has expanded in the initial stages of a match. The main benefit of the ratchet over other solutions is the high reliability it provides due to the multiple teeth. This means even if the mechanism fails to deploy completely, as long as it is within approximately +/- 1” of its desired location, the mechanism will be restricted from backwards movement. Part Explanation The Linear Ratchet integrates seamlessly with the Low-Friction Slider Truck, winner of the 2020 Make It Real CAD Challenge as well as a Pawl from the VEX Winch and Pulley kit. The part can be mounted with between 2-4 screws to any flat surface with 1/2” spaced holes, in our assembly we used a VEX C-Channel. Note that if the ratchet is mounted on the same C-Channel as the Slider Truck, it should be mounted ahead of the slider truck (with respect to the desired direction of motion), otherwise the pawl will interfere with the movement. The Linear Ratchet relies on smooth linear motion to function correctly, so we designed the part specifically to work with our Low-Friction Slider Truck. The teeth of the ratchet clear the edge of the C-Channel that truck slides in, allowing for a no modification to be made to our existing parts. Once the assembly is created, the pawl can be mounted within the same C-Channel used to contain the slider truck, this greatly increases the ease of manufacturing as no other custom parts are required. Finally, an elastic band is required to apply a force onto the pawl such that it is pushed into the teeth of the ratchet. The mechanism mounted on the slider-ratchet assembly requires a linear force to be applied on it to engage the ratchet, one example of how this can be done is through the use of tensioned elastic bands that pull the mechanism in the direction of motion. The same outcome can be achieved through pneumatics, although the motion is limited to the maximum stroke of the pistons used. Since the Slider Truck requires two opposing C-Channel to constrain the mechanism, a ratchet should be mounted adjacent to both sets of sliders to ensure that the mechanism is securely locked in place once stopped. Additionally, the part can be lengthened and shortened to match any application. Use of Autodesk Inventor Software The part and all assemblies were designed using Autodesk Inventor Professional 2021. The part was constructed by creating a 1x2” sketch and extruding it out 1/8” to create the base of the part. Next, a sketch of one tooth profile was created on the side of the part and was extruded to half the width of the entire part. The tooth was then patterned down the edge of the part to create a total of 8 teeth. Finally, 4 holes were created on a 1/2” spacing to provide easy mounting of the part with existing VEX parts. Chamfers of 1/16” and 1/32” were added to all edges for ease of 3D-Printing and to give the part a professional look. We also rely heavily on a version control system know as GrabCAD Workbench. Similar to Git, Workbench allows many members to work seamlessly on the same CAD projects allowing for rapid creation and prototyping of CAD models and assemblies. Conclusion This is one of many parts and mechanisms which our team has designed using Autodesk Inventor this season, which we plan to continue to use it for future projects. We design all mechanisms in CAD software before building them to ensure tolerances are reasonable and to enable the creation of custom parts, such as this one, to be printed. This allows for more advanced designs, as well as makes our robots more professional. Everyone who has taken on 3D design projects expects the skills which they learned as a result to be useful in the future, both in industry and in future personal projects.  


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