MBD Simulation - Internal Geneva Mechanism

Aim

1. Create 3D models for driver and driven wheels.
2. Perform motion analysis by rotating the driver wheel at 10rpm.
3. Obtain the following plots
   • Contact force (between driving and the driven wheel) as a function of time.
   • Angular Displacement of the driven wheel.
   • Compare the contact forces with and without "Precise contact"
4. Make a plot of the driver wheel's angular velocity and discuss the results

Introduction

Geneva mechanism is the mechanical mechanism that converts continuous rotation motion into intermittent rotary motion (alternate periods of motion and rest). The dwell period for driven part is based on the design of the mechanism.

The driver carries a pin/roller that fits in the radial slots in the driven wheel. The pin will enter one of the slots and it will move into the slot and rotate the driven on further rotation of the driver. After the pin leaves the slot, the driven wheel dwells. The lowest practical number of slots in a geneva mechanism is 3, more than 18 are seldom used.

Types of Geneva mechanism

1. External Version
2. Internal Version
3. Spherical Version

Applications

• Turrets of CNC,Lathes.
• Mechanical watch.
• Old film cameras, projectors.

Internal Geneva Mechanism

Unlike external type, internal version slots has opening at inner side of the driven whee. Also driver wheel will don’t have any convex shapes as that driver of external wheel. The axis of the driver wheel have a bearing.

The angle by which the drive wheel has to rotate to effect one step rotation of the driven wheel is always smaller than 180° in an external Geneva drive and always greater than 180° in an internal one, where the switch time is therefore greater than the driven wheel dwell period.

3D Models

Drive link that contains

1. Part files
2. Assembly file
3. Zip file of above to avoide part missing error
4. Videos (at 10 rpm, 60fps and 20 rpm, 120fps)

Link:https://drive.google.com/drive/folders/1Dgd70KaaCtPIvvLyL9LLYGJNieKGMLGp?usp=sharing

Assembly

Driver

Driven

Motion analysis

1. 4 cases has to be studied for contact force, Angular displacement of driven wheel.
   • 10 RPM without precision
   • 10 RPM with precision
   • 20 RPM without precision
   • 20 RPM with precision
2. Angular velocity of driver

Plots

For 10RPM and 20 RPM 60 frames and 120 frames selected respectively in settings.

During precision selection

1. contact force at 10 RPM without precision

2. Contact force at 10 RPM with precision

3. Compare the contact forces with and without "Precise contact"

4. Angular displacement at 10 RPM without precision

5. Angular displacement at 10 RPM with precision

6. Angular velocity at 10 RPM with precision

(For selecting driver alone, it would produce straight horizontal line.)

7. Contact force at 20 RPM without precision

8. Contact force at 20 RPM with precision

9. Compare the contact forces with and without "Precise contact" at 20RPM

10. Angular displacement at 20 RPM without precision

11. Angular displacement at 20 RPM with precision

12. Angular velocity of driver at 20 RPM with precision

(For selecting driver alone, it would produce straight horizontal line.)

Conclusion

1. Using precise contact option, we can receive smooth and accurate plot over without precision. Moreover it will be almost true to reality. But execution time increased considerably (i3 processor with inbuilt intel graphics).
2. The increase in contact force at start and decease and then increase is due to the motion of pin that suddenly hits the driver during it enter.
3. Also the force increase as the RPM increases.
4. The angular displacement plot shows that, it is periodic and the within -180 to 180 degree it turns.
5. (The angular velocity of driver alone will be straight line as the speed is constant eiher at 10 or 20 RPM.)
6. The angular velocity of driver wrt to driven have changes, i.e, the peak occurs at its enter. As the acceleration occurs again the angular velocity increases and on further rotation the angular velocity decrease.. Also the angular velocity increases when RPM increased os the aceeleration.