Impresión 3D de polímeros y tecnología utilizada

At the start of this project a driveshaft (which was designed by Grobbelaar (2011) during his final year project) was already available. This driveshaft was previously used to connect the Yanmar engine to a hydraulic dynamometer. Due to employing the same test engine as the previous test setup, it was decided to rather adapt and reuse the existing driveshaft, as appose to designing a completely new shaft. This was done in an effort to save cost and to expedite the project. For the design of the previous driveshaft, an automobile front axle half shaft assembly was used as a departure point. An adapter flange was designed to connect the inner constant velocity joint of the half shaft, to the imperial sized output shaft of the Yanmar test engine. This was accomplished by purchasing a Fenner weld-on hub (designed to accept an imperial sized taper-lock) and welding this hub to the designed flange. Final machining of the flange was performed after the welding process to ensure that the final tolerances were met and that the flange ran true. This section of the previous driveshaft was used unaltered for the new test setup.

The other end of the driveshaft was connected to the dynamometer by implementing a completely new design. Due to the anticipated oscillation in the torque output of the engine, a Fenaflex F50 tyre coupling was selected to damp out the majority of this torsional vibrations. The flexible coupling was connected to the driveshaft through the use of a taper lock and a parallel key (an end-milled parallel keyway was machined into the driveshaft specifically for this purpose). The flexible coupling was designed by catalogue and selected based on its power rating at the rated speed of the engine. In addition, the factors of safety guarding against failure due to the maximum nominal torque output of the engine, as well as due to the peak torque output of the engine (which occurs due to spikes in the engine’s torque output profile) were calculated. The peak torque output of the engine was obtained by applying a service factor (determined based upon the system architecture and available literature) to the maximum rated torque output of the engine. Analysis showed the flexible coupling has a factor of safety of 2,4 guarding against failure given the maximum nominal torque output of the engine and a factor of safety of 1,3 guarding against short term overload of the flexible coupling during torque spikes. The detailed procedure regarding the sizing and selection of the flexible coupling is presented in Appendix C.

A second flange was designed to connect the flexible coupling to the dynamometer shaft. Here attachment was again ensured by incorporating a weld-

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on hub into the design of the flange to allow the flange to be fixed to the dynamometer shaft through the use of a taper lock. This same flange also makes provision for the mounting of a spherical bearing (SKF part no: GE 20 ES 2RS), which locates the end of the driveshaft that is extending through the centre of the flexible coupling. This spherical bearing, along with the 4 degrees of misalignment that can be tolerated by the flexible coupling, allows for relative misalignment between the output shaft of the engine and the dynamometer’s shaft. Due to the construction of the driveshaft, a small degree of misalignment is actually preferred as it prevents brinelling of the constant velocity joint that is connected to the output shaft of the test engine.

Due to the addition of a keyway to the shaft, a detailed fatigue analysis was performed on the design of the driveshaft. This was done using the distortion energy ASME elliptic failure criteria which indicated a factor of safety of 1,6 guarding against cyclic loading of the driveshaft. A check was also performed for first cycle yielding by calculating the maximum von Mises stress and comparing it to the yield strength of the driveshaft material. This produced a factor of safety of 2,65. The details of the aforementioned driveshaft analysis can be found in Appendix D. Finally, as an additional check, the structural integrity of the driveshaft design was also verified by performing a finite element analysis (FEA) on the driveshaft design. The results obtained from the linear static analysis showed good correlation with hand calculations performed for the same applied loads. The FEA results indicated a factor of safety of 1,7 guarding against yielding for a pure torsional load case. The details surrounding the finite element model used, as well as the results obtained are presented in Appendix E.

Once it was verified that the driveshaft design was satisfactory, the required modifications were made to the length of the existing driveshaft safety guard. This guard is installed to protect personnel and equipment in the event of a catastrophic failure of any of the driveshaft components. The guard encloses the entire driveshaft and will, during failure of any driveshaft component, constrain such a component, preventing it from inflicting any further damage to equipment or test cell personnel. The newly designed driveshaft and modified safety guard is shown in Figure 14 below.

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Figure 14: Driveshaft and safety guard