Snowmobile CVT, Machine Design


For my machine design class, my group and I was assigned to design a gearbox for a power transmission application. We decided to design a transmission that would transmit power from a snowmobile engine, to the tracks of the snowmobile. This design consisted of two stages of power transmission. The first stage was a CVT which would vary the gear ratio for the range of engine RPM’s (our group analyzed the CVT across a high and low gear ratio). The second stage was a gearbox which was connected between the CVT and the track of the snowmobile.

Even though it was not required for this design project, I ended up designing the gearbox assembly of this project. As you can see below, the gearbox assembly contained one input shaft (smaller on left) and one output shaft (larger on right). The CVT was transmitting power to this gearbox through the small input shaft, and the gearbox outputed the power to the track of the snowmobile through the large output shaft.

Isometric View: Final Gearbox Design, Input Shaft Side (CVT Input Shaft)


Isometric View: Final Gearbox Design, Output Shaft Side



Design Sketch: CVT Gearbox


Isometric Wireframe View: Final Gearbox Assembly


Side Wireframe View: Final Gearbox Assembly


Top View: Final Gearbox Case Design



To determine the design parameters of our gear set, a group member and I wrote a Matlab code. This code was designed to work both with spur and helical gear sets.

Important Matlab Code Deliverables:

  • Minimum number of teeth for pinion
  • Minimum number of teeth for gear
  • If contact ratio was met
  • Maximum theoretical velocity
  • The force exerted on gear tooth
  • Factor of safety against bending
  • Factor of safety against pitting

–>Gear Design

Final Gear Design Choice: 75 tooth gear on left, 40 tooth gear on the right


Bolt torque specifications were also determined. Below is an excel spreadsheet that I created to calculate the required torque and the factor of safety of a bolt under static loading conditions (highlighted in orange below). Calculations were based off of a cast iron enclosure material and an SAE Grade 8 fine threaded bolt. It was also assumed that there was absence of oil which would affect the coefficient of friction between the bolt and the enclosure.

Enclosure Design: Bolt Torque Specifications


The final choice of fastener was four SAE grade 8 fine threaded bolts. These bolts were sourced from a parts bin. Below are the dimensions of the bolt provided by McMaster-Carr.

Enclosure Design: Bolt Dimensions


The final component to study was the input shaft of the gearbox. Below is a fine mesh that I generated for performing the FEA analysis. Split lines were used to simulate locations for the roller supports. Torsional and normal forces were then used to generate the forces caused by the CVT pulley and transmission gear.

Shaft Analysis: CVT Input Shaft Mesh


The FEA analysis was performed and Von Mises stresses were determined. The area of highest stress occurred at the stress concentration of the shaft. It is also important to note that the yield strength of our keyway material was below that of the yield strength of the shaft. This ensured that the keyway material would fail before the shaft in the event of overloading.

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