Snowmobile CVT, Machine Design

800-pro-rmk-155

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)

me-325-final-gearbox-closed-small-shaft

Isometric View: Final Gearbox Design, Output Shaft Side

me-325-final-gearbox-closed-assembly

Design

Design Sketch: CVT Gearbox

me-325-final-gearbox-design-sketch

Isometric Wireframe View: Final Gearbox Assembly

me-325-final-gearbox-design

Side Wireframe View: Final Gearbox Assembly

me-325-final-gearbox-wire-hidden-line-view

Top View: Final Gearbox Case Design

 

Calculations

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

gearbox-housing-enclosure-design

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

Enclosure Design: Bolt Dimensions

me3253inchbolt

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

me325feamesh

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.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s