Milestone 12: Driveshaft and Loops

Driveshaft Design

The Wildcat Pulling Team has used a 1-inch diameter solid rotary shaft from McMaster-Carr for over 10 years. This shaft is manufactured from AISI 1566 medium-carbon steel, which has high torsional strength. This shaft transmits torque from the clutch assembly to the dropbox and, therefore, is subjected primarily to torsional loading. Secondary loading should also be considered due to any misalignment. This shaft is located before any gear reductions; the torque on the shaft is directly correlated to the engine torque.

Figure 1: SolidWorks FEA simulation results for von Mises stress on the driveshaft

Figure 2: SolidWorks FEA simulation results for factor of safety

Finite Element Analysis

To verify the use of the 1-inch driveshaft, we performed a static structural analysis in SolidWorks Simulation to evaluate the stress distribution and factor of safety. The shaft was meshed with second-order solid elements with refinement applied in regions of stress concentration, specifically in the keyways. As seen in Figure 1, the maximum von Mises stress on the shaft was 20,000 pounds per square inch (psi) near the end of the shaft that was fixed for simulation. AISI 1566 carbon steel has a yield strength of 90,000 psi, which contributes to a minimum factor of safety (FOS) of 4.6. While a decision could be made to downsize the driveshaft, a larger driveshaft has greater angular momentum, thus assisting in pulling performance.

Additional Components

Figure 3: SolidWorks FEA simulation results for the previous 11-gauge clutch support bracket used on the Wildcat 3525 (previous year's tractor)

Figure 4: SolidWorks FEA simulation results for the proposed 14-gauge clutch for the Wildcat 3526 (this year's tractor)

Figure 5: 16-gauge driveshaft loop, used to divide the driveshaft into equal lengths in accordance with ASABE IQS Rules Committee.

Figure 6: 2-piece shaft coupler from McMaster-Carr proposed to use on the drivetrain of the Wildcat 3526

Clutch Support Bracket

A critical component to secure the drivetrain to the tractor is the clutch supporting bracket. The bracket bolts to the clutch housing design in Milestone 11 and is bolted to the frame rails of the tractor. Last year, the team designed and manufactured this piece out of 11-gauge sheet steel. This year, we decided to do an FEA study on this piece to see if there was any weight we could remove from the piece. As seen in Figure 3, the 11-gauge model was estimated to have an FOS of 9.4. While a high FOS is good, this indicates there is some room for change to reduce weight if necessary. Therefore, as seen in Figure 4, we redesigned the piece in 14-gauge sheet steel and performed the same FEA study, resulting in an FOS of 4.6, while removing over 1-pound of weight. The rules committee of the competition does not set a minimum FOS for components, so it is up to a team's discretion to decide what is acceptable. For the Wildcat Pulling Team, a FOS of 1.5 to 2.5 is acceptable.

Driveshaft Loop

As required by competition rules, the driveshaft includes at least one drive shaft loop around the driveshaft regardless of length. Additionally, loops must be placed no more than 18 inches apart and must equally divide the length of the driveshaft with a tolerance of 2 inches. There are no restrictions on the thickness of this piece, so the Wildcat Pulling Team has utilized 16-gauge sheet steel for loops for many years. Our team has conferred with the members of the Wildcat Pulling Team and has determined that only one loop is necessary.

Shaft Couplers

Additionally, required according to competition rules, all drivetrains must incorporate a disconnect located anywhere between the engine and all driven wheels. The powertrain must also be able to be disconnected in 4 minutes and reconnected in 4 minutes by two people. This time also includes the removal and installation of any shield guards, ballast, or body components. Before switching to the Kawasaki engine, the Wildcat Pulling Team used a Lovejoy coupler for a disconnect. However, the Kawasaki engine uses a vertical output shaft compared to the horizontal output shaft of the previous engine. This creates issues related to the location of the disconnect. Last year, the Wildcat Pulling Team utilized a single-piece straight coupler on the end of the driveshaft connecting to the output of the clutch and then a Lovejoy connector on the other end to connect to the dropbox. This setup was quick for disassembly, but slow for reassembly. The straight coupler could slide back after loosening two set screws, allowing for the shaft to be easily disconnected. However, to reconnect, the Lovejoy connector had to be disassembled to put the shaft back into place. To resolve this issue, our team is proposing to use two, 2-piece shaft couplers (Figure 6). With the 2-piece coupler, half the coupler can be removed, while the other half is connected to the second shaft. This should speed up the reassembly time as the shaft can easily be placed back together. Disconnecting the driveshaft should be similar in timing to last year, as only 4 bolts need to be removed to disconnect the drivetrain.

Final Assembly

Figure 7: Final manufacturing picture of the clutch support bracket and driveshaft loop

Pictured on the right is the manufactured clutch support bracket and the driveshaft loop. Not pictured due to purchasing delays are the driveshaft and shaft couplers

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