Robotics Education & Competition Foundation
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Vex Pulley Block


Entry ID #: 8900
Created: Tue, Dec 8, 2020 1:42 AM

Team 8059Z's "Make It Real" Online Challenge Submission: Vex Pulley Block   Introduction The Pulley Block consists of a bracket that holds the pulley wheels and three mounting holes, of which any two can be screwed into a Vex C Channel. Pulleys increase the pulling force of a winch at the cost of increasing the length of the string connected to the winch. Pulleys have many applications in other machines and we feel as if the Pulley Block can help us bring such applications into Vex. Compared to gears and sprockets, pulleys are simpler and lighter, are generally more compact, and can be built over any distance. Unlike gearboxes, where the diameter of the gear determines the size of the mechanism, or chain systems, where the chain’s tension must be tuned to be kept consistent, pulley systems can be built to differing sizes simply by changing the length of the string. Pulleys are typically used in conjunction with winches, however, pulley blocks can provide an easier and lighter alternative for producing mechanical advantage compared to gearboxes or sprocket systems. We have attempted to build pulley blocks with normal VEX parts, but the products turned out to be too bulky, or not structurally sound, negating a large advantage of pulley blocks. As such, we have designed a 3d-printed pulley block in Fusion 360 to fill this need.    The Process The pulley block ideally is only slightly larger than the width of two pulley blocks and has mounting holes to be easily attached to other VEX parts such as C channels or Angles. It would also need to be sufficiently sturdy to withstand the high stress it may have to deal with. We would use it in conjunction with winches to achieve great pulling force on another part of the robot from an arbitrary distance, for example pulling part of a lift or even a catapult arm downwards, or in the case of tetherbots and wallbots, reeling part of the robot back in. We used Autodesk Fusion 360 version 2.0.9313, Windows 64-bit to CAD the pulley block component by first sketching the shape of the pulley block on the X and Y planes, resulting in a 2D sketch that would be used when creating the 3D body. The Sketch consisted of 4 different components, 2 center diameter circles -  one at the top of the pulley block and one at the bottom that served as the holes the axles would go through. The body was shaped like a rounded teardrop split into 2 segments using a linear line perpendicular to the vertical height of the body 2.04cm from the upper tip. With one segment that would serve as the connector and another that served as the walls for the pulley wheels. A rounded teardrop was sketched by creating two center diameter circles and making them tangent to two lines at the sides that converged at the top of the teardrop. After the sketch of the object on a 2D plane was completed, the next step in the design process was to transfer it from a 2D plane to a 3D plane, making it a true body. To achieve this, extrude and split body tools were used. The two segments were first extruded to a height of 2.6cm resulting in a solid body with one hole of 0.4cm diameter in the center of the lower segment and a similar hole with a center 0.65cm perpendicular to the top of the upper segment.  Now to create the space which would house the pulley wheels, we had to hollow out the middle of the lower segment, leaving 0.5cm solid walls on either side. We used the split body function to remove a 1.6cm thick segment in between the lower segment, resulting in a hollow area that could now be used to house the pulley wheels  After the pulley body’s completion, the axles were created in the same fashion - making a 2D sketch and extruding it into a 3d body. The axles consisted of 2 components, a larger axle of 2.7 cm length and a smaller axle of 0.7 cm length. The sketch of the larger axle consisted of 3 circles on the same plane, all of them with the same center and of the diameters 0.2cm, 0.4cm, and 0.6cm respectively. The smaller axle consisted of 0.2cm and 0.6cm circles on the same plane with their centers in the same position. The largest circle on each axle was extruded to a height of 0.1cm, the 0.4cm circle extruded to a height of 2.7cm on the larger axle, and the 0.2cm circle extruded to a height of 2.1cm on the larger axle. The 0.2cm circle was extruded to a height of 0.7cm on the smaller axle, resulting in it being able to fit perfectly into the hole created on the larger axle by the differing extrusion heights. This results in an axle that is secured in the hole.   Conclusion We learned quite a bit from this project. We learned to appreciate the difficulty and nuances of designing even a simple part or component, and also learned more about predicting the use cases of the component while designing it to make sure it is useful and properly applicable to the problem at hand. We have used Fusion and Inventor many times in the past for designing 3d-printed components and designing our robots and will continue to do so in the future. CAD softwares are invaluable to communicating, simulating, and refining ideas in the prototype stage to easily and quickly iterate through robot designs, therefore saving precious manpower, time and resources. As aspiring engineers, we will definitely continue to use CAD software in our future careers to prototype and design components and products to contribute to solving the world’s problems.    


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