Internal Geneva Mechanism
AIM: To create a 3D CAD model of the Internal Geneva Mechanism and to do motion analysis at different rpm.
THEORY: 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. The drive wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps. [1]
The name, Geneva drive, is derived from the device's earliest application in mechanical watches, which were popularized in Geneva, being the classical origin of the watchmaking industry. The Geneva drive is also called a "Maltese cross mechanism" due to the visual resemblance when the rotating wheel has four spokes, since they can be made small, and can withstand substantial mechanical stress. These mechanisms are frequently used in mechanical watches. In the most common arrangement of the Geneva drive, the client wheel has four slots and thus advances the drive by one step at a time (each step being 90 degrees) for each full rotation of the master wheel. If the steered wheel has n slots, it advances by 360°/n per full rotation of the propeller wheel. [1]
DESIGN: The 3D CAD models are designed in SolidWorks and motion analysis is analyzed on Motion Analysis add-on in SolidWorks. The following steps were taken to design the mechanism:-
Step-1: First, a 2D sketch of the driven wheel was designed and then extruded.
Step-2: Second, a 2D sketch of the drive wheel is designed and then extruded.
Step-3: Then both models were assembled, and constraints were provided.
MOTION ANALYSIS:
Motion analysis was conducted using the Motion Analysis add-on in SolidWorks. The motion features of the SolidWorks provide us a wide range of motion study, they can be divided into three categories:
Animation: If one simply wishes to create some nice visuals for presentation or marketing without consideration of mass and gravity effects, then animation is perfect for it.
Basic Motion: For an extra layer of complexity that takes into consideration the effects of mass, springs, gravity, and physical collision detection, then a Basic Motion study is more suited for the requirements.
Motion Analysis: This is the top tier of motion study and takes into account a wider range of physical interactions such as impact effects, damping, force, momentum, etc. [2]
Step-4: Driver part rotating at 10rpm and the rate of the frame is 60 and the following results were obtained.
- i) Contact Force
The contact force (N) generated between the driver and the driven wheel is shown as a function of time (s) without precise contact.
The contact force (N) generated between the driver and the driven wheel is shown as a function of time (s) with precise contact.
- ii) Angular displacement
The angular displacement (deg) of the driven wheel is shown as a function of time (s) without precise contact.
The angular displacement (deg) of the driven wheel is shown as a function of time (s) with precise contact.
iii) Comparison
Comparison of precise and non-precise contact in Contact force between driver and driven wheel.
No difference can be seen in the curves at 10rpm with and without precise contact.
The selection of ‘Precise Contact’ is shown in the figure below.
Step-5: New motion analysis was conducted for 20 rpm and the rate of the frame was set to 120 and the following results were obtained.
- i) Contact Force
The contact force (N) generated between the driver and the driven wheel is shown as a function of time (s) without precise contact.
The contact force (N) generated between the driver and the driven wheel is shown as a function of time (s) with precise contact.
- ii) Angular displacement
The angular displacement (deg) of the driven wheel is shown as a function of time (s) without precise contact.
The angular displacement (deg) of the driven wheel is shown as a function of time (s) with precise contact.
iii) Comparison
Comparison of precise and non-precise contact in Contact force between driver and driven wheel.
No difference can be seen in the curves at 20rpm with and without precise contact.
OBSERVATION:
In the plots of the contact forces, hikes of curves can be seen, which denotes the sudden increase in contact forces.
In the plots of the angular displacement, constant change in angular displacement can be seen.
CONCLUSION:
The magnitude of the contact force increases with increasing the rpm of the driver wheel.
Angular displacement does not affect changing of rpm of the driver wheel
REFERENCES:
[1] Wikipedia
[2] https://www.engineersrule.com/motion-studies-and-how-to-do-them