Low-Friction Slider Truck
Entry ID #: 7428
Created: Sat, Jan 11, 2020 5:09 PM
VEX U Team QUEEN - Queen's University, Kingston, Ontario, Canada
There are several existing methods for creating linear motion structures on VEX Robots. These include both VEX's linear slides, and existing designs for custom systems, using C-Channels, spacers, and other standard VEX parts. However, these all have issues that prevent them from being used in demanding applications. The VEX slides are notoriously poor at handling any amount of torque about an axis normal to the slide surface (which is a key component of our application), in addition to requiring the use of unnecessarily heavy steel parts. Custom slider mechanisms that use VEX EDR tend to be large and bulky (a common pattern is to have rollers wrap around the outside of a C-channel), and can have limited strength due to the compliance of the parts used to hold the rollers. To solve this problem, we created a custom, 3D-printable truck which is designed to slide on the inside of a 2-hole-wide C-channel, which can be used to assemble a slider mechanism with a small profile and low friction, even with high torque loads.
The part requires four 0.375" outer diameter nylon spacers (0.250" length) as rollers, which are attached on screw joints, allowing the spacers to only barely stick out from the side of the part. These roll along the inside walls of the C-channel, while the bottom of the part slides along the bottom inside surface of the C-channel. This means that the kinetic friction is limited to the nylon-on-steel friction between the spacer and its screw and the PLA-on-aluminum friction between the bottom surfaces. As nylon on steel has a very low coefficient of friction, the part is able to handle high normal forces against the walls (caused by the aforementioned torques) without significant losses.
The same screws that the roller spacers sit on attach the truck to the structure that should be sliding. Using four screws for this purpose should be more than enough structural support for any VEX mechanism. On the other side, the enclosing C-channel can be attached only by the center holes; there is a groove in the bottom of the part specifically to allow screwheads to fit on these holes. No other holes in this C-channel can be used, as such use would interfere with the truck's motion.
It should be noted that, unlike the VEX linear slides, our trucks are not intrinsically restricted from moving away from the C-channel. To avoid this, two opposing C-channels should be used for any application of the mechanism. Additionally, the truck length can be changed as desired for different applications.
Use of Autodesk Inventor Software
The part and all assemblies were designed using Autodesk Inventor 2020. The part was constructed by creating a sketch detailing the side profile of the part and then extruding it to form the base solid. Next, a sketch was created on the bottom of the part to cut out the four screw holes. Then a plane was created to place the profile of the spacer hole which was then cut. The spacer hole was then mirrored twice to place it in all four corners. The center was then cut and all the edges were chamfered either 1/16” or 1/32” dependant on clearance requirements.
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.