Internal Geneva Mechanism

AIM: To perform the MBD analysis of an internal geneva mechanism by

• Createing 3D models for driver and driven wheels.
• Perform motion analysis by rotating the driver wheel at 10rpm.
• Obtain the following plots
  • Contact force (between driving and the driven wheel) as a function of time- with and without precise contact.
  • Angular Displacement of the driven wheel.
  • Use 60 Frames per second for 10 rpm and then 120 Frames per second for 20 Rpm
  • Make a plot of the driver wheel's angular velocity and discuss the results

GENEVA MECHANISM:

The Geneva drive or Maltese cross is a gear mechanism that translates a continuous rotation movement into intermittent rotary motion. The rotating drive wheel is usually equipped with a pin that reaches into a slot located in the other wheel (driven wheel) that advances it by one step at a time.

Types of geneva mechanism:

• External
• Internal
• Spherical

Applications:

This mechanism can be used wherever there is a requirement of intermittent motion.

• Mechanical watches
• CNC machines
• Lathe turrets
• Film camera projectors (old)

2D DRAWINGS:

Steps to generate the 3D model:

• Driver wheel
  • Open a new part
  • Begin sketching on the front plane
  • Create a circle with centre at origin and dimension it using smart dimension (1120 mm)
  • Create two circles of diameter 240 mm and 1040 mm, with centre located at the origin
  • Draw a line connecting the origin and the circle of diameter 1040 mm.
  • Offset this line to either sides by 20 mm.
  • Trim the unnecessary part within the smaller circle
  • Pattern the feature such that 4 instances are created in 360 deg.
  • Fillet the edge near the smaller circle with a radius of 50 mm
  • Exit sketch
  • Extrude the sketch using midplane option and 40 mm length
  • Save the part
    
• Driven wheel
  • Open new part
  • Draw a circle of diameter 840 mm with centre located at origin
  • Extrude the sketch by 20 mm
  • Exit sketch
  • Click on the top face of the cylinder and click sketch
  • Draw a circle of diameter 40 mm
  • Using smart dimensions, align the circle to the desired position
  • Extrude this face by 40 mm and exit sketch
  • Save the part
    
• Assembly
  • Import both the parts
  • Provide the mates as follows
    
  • The driven wheel is positioned 160 mm from the right and top planes of the assembly. This is because the distance of stub from the centre of driving wheel is 160 mm.
  • Save the file
    
• Enable motion study
  • Provide solid body contacts and assign materials
  • Insert a motor at the centre of driving wheel axis
  • Setup all plots required
  • Perform the simulation

RESULTS AND DISCUSSION:

The analysis has been performed and the results are as follows:

1. Contact force vs time at 10 rpm
   

We see that the contact reaction force abruptly increases at intermittent time steps. This is peak is noted at the time when the driver wheel stub comes into contact with the driving wheel slot.

2. Angular displacement of the driven wheel vs time- 10 rpm
   

We see that the angular displacement of the driven wheel is continuous and fluctuates between ± 180°.

3. Angular velocity of the driver vs time- 10 rpm
   

We notice an angular constant velocity for the driving wheel.

Enabling the precise contact, in motion study properties tab.

4. Precise contact enabled for Contact force vs time at 10 rpm
   

On enabling the precision contact option, the peaks have similar height unlike the previous case.

5. Contact force vs time at 20 rpm
   
6. Angular displacement of driven wheel vs time at 20 rpm
   
7. Angular velocity of the driving wheel vs time at 20 rpm
   
8. Precise contact enabled for Contact force vs time at 20 rpm
   

All plots for 20 rpm display the same properties as that of 10 rpm.

Outcome: The MBD analysis of the internal geneva mechanism has been performed and the various results have been plotted.

Please find the model at: https://drive.google.com/drive/folders/1IbpzAI9XhzTSurHX8Abxl5fMfaBZFI-0?usp=sharing