MBD simulation of Internal Geneva Mechanism in Solidworks
Aim:
To perform MBD simulation on Internal Geneva Mechanism in Solidworks.
Objective:
- To model and assemble the Internal Geneva Mechanism.
- To perform motion analysis at 10 rpm.
- To perform motion analysis at 20 rpm.
- To analyse the resulting plots.
Theory:
The Geneva drive 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. The drive wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps.
There are three basic types of Geneva motion mechanisms namely external, internal and spherical. The spherical Geneva mechanism is very rarely used. In the simplest form, the driven wheel has four slots and hence for each rotation of the drive wheel it advances by one step of 90°. If the driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel.
An Internal Geneva drive is a variant on the design. The axis of the drive wheel of the internal drive can have a bearing only on one side. 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 time the driven wheel stands still.
Setup Explanation:
1. First, the components are modelled as per the drawing.
2. Then the model is assembled using appropriate concentric and coincident mates as shown.
3. For the axial distance between driving and driven wheel, the following formula is used:
`"Axial Distance = " (crank radius)/ (sin(180//n))`
where, n = number of slots (4)
For our case it is equal to 226.274169 mm
4. With the Solidworks motion add-in enabled, click on motion study 1 and then click on motion analysis.
5. Add a motor to the axis of the driving wheel as shown. Set constant speed to 10 rpm.
6. Set contact between the two solid bodies as shown and set material to steel (dry).
7. Go to settings and set frame rate to 60 frames per second.
8. Hit calculate and wait for the analysis to complete.
9. Now duplicate this motion study and run it at same rpm but with precision contacts enabled in settings as shown.
10. Do the above processes for 20 rpm at 120 frames per second with and without precision contacts enabled. In total there shall be 4 motion studies.
11. Insert the following plots:
- The contact force between the driving pin and driven slot.
- The angular displacement of the driven wheel.
- The angular velocity of the driven wheel.
Observation:
- Case 1: 10 rpm
- The motor rpm is set to 10 and analysis is done with and without precise contacts enabled at 60 frames per second.
- Animation: https://youtu.be/LvuNv6_pwWQ
- Result: The following curves are obtained:
- Case 2: 20 rpm
- The motor rpm is changed to 20 and analysis is done with and without precise contacts enabled at 120 frames per second.
- Animation: https://youtu.be/k5Sq73roSE0
- Result: The following curves are obtained:
Conclusion:
- The angular displacement of the driven wheel increases linearly and and then becomes constant when the driver is not in contact with it. It increases faster with higher rpm as expected.
- The contact force between the driving and driven wheel starts from a maximum value and then decreases exponentially as the driving pin move further away. It again increases exponentially as the driver pin comes back. The high peaks observed are the impact forces experienced due to jerky motion. With precise contacts enabled, a smoother curve is obtained giving more accurate values. The contact forces increase with higher rpm.
- The angular velocity of the driven wheel increases exponentially as the driver pin moves further into the slot, reaches a maximum value and then decreases again. The high peaks are due to jerky motion. The angular velocity of driven wheel increases with higher rpm of driving wheel as expected.
References:
- https://en.wikipedia.org/wiki/Geneva_drive
- https://www.engineersedge.com/mechanics_machines/geneva_internal_mechanism_14920.htm
File Link: https://drive.google.com/open?id=1Otf5vJnMCRl7o60mx6QdeN1MubGYr8tK