Project Proposal Change
The initial project proposal called for the execution of a single print rubber band launching mechanism. After evaluating the goals of the ME514 course project, it was determined a different assembly could be 3D printed that may present more interesting challenges when attempted to be made in one print. The project proposal was changed to be the creation of a 3D printed differential gear box.
Differential Gear Box
The differential gear box is used to transport rotational motion from the driveshaft to the wheels of an automobile. It is geared to allow different shafts on opposite sides to rotate in the same direction. A picture of a simple differential gear box is shown below in Figure 1. The input rotation comes from the shaft labeled 3 in the figure, which rotates the large ring gear labeled 1. This ring gear has mounts that attach to the two spider gears – labeled 2 – that are faced toward one another. The spider gears are intermeshed with the side gears attached to their respective axles – causing them to spin when the spider gears rotate [1].
Figure 1 – Differential gear diagram. The above diagram is an early, simple, but effective design of a differential that could be found under the chassis of a car.
The differential gear box has a large number of intermeshed and rotating parts that make 3D printing difficult, but a gearbox made with additive manufacturing capabilities would open up a large number of possibilities for smaller mechanical applications with low torque requirements.
First Gearbox Design
The first design and print was used to test the feasibility of this project. Some conclusions the first print targeted included the ability for the gears to be printed, interlock, and rotate. The design was based off the center of the differential, the two spider gears and two side gears. They were put in a container that allowed for easier printing and spaced to interlock with each other which is shown in Figure 2. The cubic structure the gear structure was built around was specified to be 3.5” in each direction. The spacing between the gear faces and the side wall was kept at 1/16”. The diameter of the gear shafts were 1/16” smaller than the wall holes they were placed in to allow for rotation. The gears used were ISO standard bevel gears, allowing for a 90 degree interface that translated rotation across to the other gears.
Figure 2 – Initial gearbox print design. The design was used as a proof of concept to help determine the spacing needed between the gear shafts and the wall.
First 3D Print Slicing
Initially the SLA FormLabs Form 2 printer was going to be used to print the gearbox. As shown in Figure 3, the SLA printer required support structure in the gearbox itself, which would be difficult to remove and may have prevented the gears from moving due the added polymer in construction.
Figure 3 – PreForm sliced gearbox. The red colored structure in the interior of the walls of the gearbox indicated a need for support structure for the print.
The printer used instead was the Ultimaker S5 FDM printer. The gearbox was made with PLA filament for a main material and PVA filament for a support structure material. PVA is water soluble, allowing for a support structure that could be removed in difficult to reach locations a cutting tool would not be able to. Since two materials were to be used, the print time was longer than expected. The print was supposed to take around 23 hours when using the original dimensions of the gearbox along with a slow print speed (layer thickness ~ 0.1mm). The model was shrunk to half of the original size and the print speed was doubled to decrease the total print time. The final settings led to a print that took 6 hours and 36 minutes. The Cura sliced gearbox is shown in Figure 4. Support structure for the gearbox is shown in blue.
Figure 4 – Cura sliced gearbox. The support structure added around a 30% increase in print time. Although it was necessary, support will be revisited for the next print.
First 3D Print
After the print was finished, the PVA was removed using a warm water bath overnight shown in Figure 5. After the print was processed, the part was inspected. As seen in Figure 6 there appeared to be some warpage on the bottom of the frame in the corners of the box. The gears would not turn – possibly due to the reduction in dimensions when scaling down using the slicing software – the gaps that were 1/16” became 1/32”.
Figure 5 – Finished first printed gearbox (left) and part post processing (right). The gearbox was placed in a warm water bath to remove PVA.
Figure 6 – Finished part (left) and deformities (right). The first print’s gear were unable to be turned.
First 3D Print Conclusion
Although the first print was physically correct with few deformities, the mechanical functionality of the assembly did not suffice. The first trial was not a success. Scaling down the part using the slicing software to reduce print time caused the spacing to change between part interfaces, leading to an immobile gear box. For the next design, less part volume will be used on aesthetic parts such as the longer shafts and large gear stoppers that kept gears in place on the box. More attention must be placed on designing structures that require less support structure. If print time can be reduced, more trials will be able to be conducted to further the success of this project. The gears must be placed closer together to ensure movement translation while maintaining enough distance to prevent gears from printing together. The addition of a drive shaft and ring gear would also allow for a more complex geometry that closely mimics a differential gear system. The FDM techniques is suitable for the current geometry, and the PVA allows for added flexibility that will be crucial for when the geometry becomes more complex.
References
[1] “Differential (mechanical device),” Wikipedia, 04-Apr-2019. [Online]. Available: https://simple.wikipedia.org/wiki/Differential_(mechanical_device). [Accessed: 05-Apr-2019].
[1] P. S. Foresman, Line art diagram of a differential gear.