17XT Drivetrain
The 17XT drivetrain was my second iteration of a 4WD system and I had lots of changes to make based on what I learned while building and testing the 16XT drivetrain. While the 16XT used purchased front and rear differentials, I decided to switch back to the traditional rear spool (both rear wheels being powered by the same shaft) and made the team’s first custom front differential. I was responsible for designing both of those components and as the drivetrain lead I organized and
coordinated the design and manufacturing of the rest of the drivetrain. The 17XT was the team’s first 4WD vehicle to go to competition, where it successfully passed the technical inspection and 4WD test.
Transfer Case
Overview
The 17XT transfer case was significantly different than the 16XT’s, switching from silent chain to gears and, because it was rotated 90 degrees from the orientation of the 16XT’s, the 17XT uses bevel gears to transfer power to the center driveshaft and front differential. I decided to switch from a rear differential to a rear spool because it could save significant weight and make a tighter rear package. The rear spool will make the vehicle handle differently than the rear differential through turns, so instead of the differential allowing the outer wheel to rotate faster through the turn, the inside wheel will pick up and the vehicle will tricycle.
Gear Design and Analysis
The overall gear ratio was selected using a MATLAB script to calculate our top speed based on the available power coming from the engine. I used a weighted decision matrix to decide between using silent chain like the 16XT transfer case and using gears, ultimately landing on gears because the reduction of the center-to-center distances of the shafts made gears the lighter option. I used AGMA’s spur and bevel gear standards and equations to design the gears and calculate the bending and root stresses in the teeth. I used KISSsoft to validate these calculations as well as calculate their fatigue lifetimes.
Case Design and Analysis
The case itself was analyzed with a SolidWorks FEA, where I applied the reaction loads from the shafts on the bearing surfaces and adjusted the case wall thicknesses until the stress was below my allowable and it had an acceptable deflection to maintain gear locations. The case was split into four parts to make it easier to machine and to reduce the size of stock that it would be cut from.
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Shifter Design
I decided to use a dog tooth clutch to engage/disengage the output to the center driveshaft because of its simplicity to both design and manufacture.
One side of the clutch is cut from the hub of a bevel gear which is riding on a bearing on a shaft and the other side of the clutch is splined onto the center output shaft. The clutch is spring-loaded to be disengaged and the driver uses a shifter handle to pull a cable and engage the shifter.
17XT Transfer Case Shifting Functionality
Manufacturing + Results
The transfer case manufacturing process involved a variety of different tools - I turned the gear blanks and shafts on a manual lathe, EDM’d all of the splines, and cut some of the keyways/gear webbing on a manual mill. After final assembly, including o-ring installation and filling with oil, the transfer case was tested on the 17XT. It was able to shift from 2WD to 4WD and was about 20 lbs lighter than the 16XT’s transfer case and rear differential combination.
Front Differential
Overview
Making a custom front differential would also allow for significant weight savings (-10 lbs) because the stock differential used on the 16XT was designed for a Polaris vehicle that saw higher loads than a Baja car and required a longer lifetime. I used a weighted decision matrix to decide on what type of front differential, with tractive and cornering capabilities, weight, and output shaft height being the most important factors. The reference differential was the purchased 16XT Hilliard and was compared to a mechanically
locking diff, limited slip diff (LSD), and an open differential. Based on the results of the decision matrix, I decided to make a mechanically locking differential.
Output Shaft Height
The height of the front diff output shaft is an important factor in the design because it dictates the max steering angle we can achieve. As the output shaft is moved lower, more of the allowable misalignment in the output CV joints can go towards a sharper steering angle. In order to lower the height, I reversed the direction that the ring gear mounted to the differential cage. Traditionally, the cage would have to fit inside of the front face of the ring gear,
requiring the outer diameter of the bevel gear to be larger than if the cage was mounted to the back face of the ring gear. Ultimately, I was able to lower the output shaft height by 1”, which, along with other factors in the suspension points, was able to increase the steering angle by 3 degrees.
Gear Selection
The front diff used six bevel gears: the pinion, ring gear, and four smaller gears inside of the cage. I decided to use purchased bevel gears that we would post machine as this was faster and cheaper than making custom gears and would give us the most time for testing before competition. Like the transfer case gears, the preliminary gear analysis was done using AGMA standards and a spreadsheet, with the final selection validated in KISSsoft.
Shifter
The front diff shifter was another dog tooth clutch, resulting in three possible drive configurations: 2WD (TC and FD shifters disengaged), 4WD open (TC shifter engaged, FD shifter disengaged), and 4WD locked (TC and FD shifters engaged). Each shifter was controlled with a sheathed cable which was actuated by the driver using a lever. The height of the dog teeth were varied so that as the cables are pulled, the dog teeth of the transfer case are engaged before the front diff.
Manufacturing + Results
Other than the front diff case itself, the majority of the manufacturing was done in-house. I turned the purchased bevel gears to add the bearing bores, ring clip grooves, and bearing surfaces. I EDM’d the diff cage to ensure the bores would be precisely located and the gears would properly mesh. The car successfully passed the 4WD inspection at competition, resulting in an additional 80 bonus points (+30% to overall score).
