RC Car
This project was part of a class at UT - ME 338 "Machine Elements". This group project consisted of designing and manufacturing the hardware for an RC Car, given electronic components. We were tasked with engineering the chassis, steering, and drive train given a limited budget. We also conducted analyses for these components and delivered an engineering report on our final assembly. In this 7 person group, I focused heavily on the design, and assisted with manufacturing.
Materials and Budgeting
We decided to keep the overall design of the car simple due to the limited budget resources we had. Most of the materials and machines we used on this project were from Texas Inventionworks. Due to the timeframe and budget, we utilized a lot of rapid prototyping manufacturing techniques, such as laser cutting and 3D printing.
In addition to the electronic parts we received, we spent our budget on critical pieces of our car that needed to be machined, or bought. This included bearings, fasteners, gears, and RC car wheels. These parts were also picked out prior to our design so that we can effectively design around these components.
In terms of resources, we also did research into the materials we have at our disposal, and the methods to manufacture those materials. For our design review, we constructed a table to show the pros and cons
Steering System
After some research, I decided to implement a steering system similar to the Ackerman steering system. The system was designed based on an Ackermann steering system, however we chose to have both our front wheels turn at the same angle to reduce complexity. The steering system is essentially a 4 bar linkage driven by a servo motor. It consists of two linkages laser cut out of acrylic and three 3D printed linkages. One of the acrylic linkages is fixed in the back to the chassis and does not move while the other, in the front is not fixed. Both of these pieces or connected to a pieces that mounts to the tire.
The entire mechanism is driven by a PLA “Step Link”. This link incorporates 5 holes that mount to the servo motor, and rotate with the servo. The servo mount was designed so that the servo rests flat so that the main rotating motor faces upward. When the servo rotates, it causes the step link to rotate as well. This motion is then transmitted to the parallel linkage in the front, but not to the linkage in the back. This causes the system to move similar to a parallelogram, turning the 3D-printed wheel mounts and causing the wheels to turn.
The linkages were connected with bolts as pins, and the nuts on the linkages were not tightened all the way to allow the motion to be smoothly transmitted. The wheel mounts were the most complex as they needed to be flush with the chassis and included extrusions to press fit the bearing. These pieces were printed to ensure the press fit cut had higher tolerances. Since rotary bearings were placed in the wheel mounts, it easily allows the two shafts used to rotate with the wheel.
Chassis
The initial phase of the design process for the car was the chassis. The chassis as a subassembly includes the frame of the car as well as the holding mechanisms for the electronics parts, which includes the battery, ESC, and receiver.
The chassis was designed with a few requirements in mind. We needed to fit all the electronics and components for the steering and drive train assemblies, and we also needed some protection from collisions. We decided to laser cut the chassis because it is one of our largest parts and laser cutting decreases the speed of manufacturing significantly. With this in mind, we chose plywood as it was not as brittle as acrylic and not as dense, despite the lower strength.
In order to effectively mount the electronics mentioned above to the chassis, I decided to 3D print holders. These holders were designed to hold the components with tight tolerances. These mounts were also designed with channels for any wiring. The holder were to be mounted onto the plywood chassis with fasteners, and they had through holes for m3 bolts. As shown in the image, the chassis itself had m3 sized slots for these holders. This decreased the chance for error in mating and assembly and worked well with these pieces as they did not need to be in an exact position or orientation.
Drive Train Assembly
The last critical subassembly for the dynamic components of the car is the drive train. The major design part of this system is the motor mount. The mount we designed once again has through holes that interfaced with slots on the chassis as the location was not too critical. The mount also had extrusions to hold the standard DC motor, with through holes to mount to the face. Lastly, I needed extrusions that could house bearings on either side for support of the back axle. This piece took some trial and error to get perfect as the press fit extrusion for the bearings were too weak, and the distances between the shaft and the pinion also was not accurate initially. The drive train revolves around the gear ratio we chose for our desired max velocity. We purchased metal gears and used a 17:64 ratio to achieve this. These were all assembled on the back axle.
Lastly, the wheels of our car were initially going to be off the shelf RC car wheels, however, we soon realized the interfacing of the wheels to the 5mm shaft was not simple. To solve this problem, I stripped the rubber cover off the wheels and designed a 3D wheel hub to fit inside. This hub was designed to press fit into a 5mm axle, but also had an extrusion that I inserted a heat set insert to. This allowed me to screw in an m4 bolt that acted like a set screw for clamping onto the axle.
Manufacturing and Integration
Manufacturing proved to be one of the more tedious parts of the process. Initially we found some of our designed components did not fit well and needed to be reevaluated, such as the step link for the servo being too short, or the holing pattern on the drive train mount not being oriented correctly.
But a big focus of the manufacturing came in the assembly, and ensuring we put everything together correctly. All the holders and mounts needed ot be screwed tight as after some driving and collisions, they loosened very quick. The press fit bearings needed to be malleted and hammered into place. And, axle needed to be cut down to size. I spent some time reviewing the design thoroughly, this helped to make sure the manufacturing process was smoother.
Issues and Ideas
After the first build, I saw a major issue with the car. The large gear we purchased that interfaced with the pinion and press fit into the 5mm shaft did not effectively engage with the shaft, and the tolerance was very loose. Unfortunately, this was not something we accounted for or could redesign so we had to think outside of the box. We worked to designed a piece that could interface with both the shaft and the gear to remedy this. I worked on a gear hub that utilized heat set inserts and bolts to clamp onto the gear, and a heat set insert onto the shaft. Although this worked when practicing, under high torque and longer drive times, the set screw stripped and the heat insert sheared out of the PLA.
We also further implemented the clamping and location of the pieces on the front and back axles with shaft collars and o-rings that keep the parts from translating along the shaft.
System Performance
Overall, our project was a success as we were able to drive and turn the car effectively. We did not win our race, but competed in multiple heats and did win a couple. We held up against a rough terrain, and ultimately failed due to a crash that sheared our 3D printed tire linkage.
There are some major limitations and improvements we could work on. The press fit for some of the bearings was a little loose, and although these pieces did not majorly affect our performance, it did come undone, especially with fatigue. A fix for this would be to incorporate a set screw, or other clamping mechanism to hold the bearing, or design the piece out of a less malleable material. Another issue we had was the set screw stripping and the heat insert shearing out of the PLA for the gear hub. To improve upon this we can design this piece to have a larger and longer heat sent insert for more engagement, or design this piece with a material stronger than PLA. Lastly, the bumper for the front broke easily due to it being small, this was simply an oversight and this could be increased to help with crashes and absorbing some impact.
