DESIGN AND MOTION ANALYSIS OF INTERNAL GENEVA MECHANISM

INTRODUCTION: 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.

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.

OBJECTIVE:

1. Creating 3D models for driver and driven wheels (SOLIDWORKS).
2. Performing motion analysis at different RPMs.
3. Obtaining plots:
   • Contact Force (b/w driving & driven wheel) vs Time
   • Angular Displacement (driven wheel) vs Time
   • Angular Velocity (driver wheel) vs Time

PROCEDURE:

• Initially, a 2D drawing is generated with defined dimensions from the reference provided. The reference has been provided below.

• To create a 3D model, extrude the sketches made in the previous step using the ‘Extruded Boss’ command in SolidWorks. The dimetric view of the driven and driver wheel is shown below.

• Now, both the parts are assembled in the assembly file. While inserting the first component, caution has to be taken to float it in order to have free movement or the one desired by the designer. The position of the driver wheel has to be judged accordingly in order to have hinder free movement. The snap of the assembly is shown below.

• The Multi-body Dynamics are calculated using ‘Motion Study’ in SolidWorks. The motor with the desired RPM is assigned to the Driver Wheel. The contacts are also specified to have appropriate force transmission.

ANIMATION: The link for the animation is given below –

https://youtu.be/CETVyMDyVA0

PLOTS:

• ANGULAR VELOCITY = 10 RPM

• Contact Force vs Time (No Precise Contact)

The above plot signifies that the contact force varies significantly at points when the Driver Wheel makes contact with the Driven Wheel. It is due to the sudden jerk produced while making contact. also, there is a bump visible at time points when the Driver Wheel loses contact with the Driven Wheel.

• Angular Displacement vs Time (No Precise Contact)

The above plot intends to project the gradual decrement in the displacement after a sudden increment at 2.5s.

• Angular Velocity vs Time (No Precise Contact)

From the above plot, we can conclude that the angular velocity remains somewhat constant throughout the motion with the exception of some time steps.

• Contact Force vs Time (Precise Contact)

In the above plot, the use of Precise Contact is clearly visible due to sharp and accurate results obtained.

• Angular Displacement vs Time (Precise Contact)

The above plot shows a smoother curve as a result of Precision Contact between the Driver Wheel and the Driven Wheel.

• Angular Velocity vs Time (Precise Contact)

The plot obtained has greater refinement, thus gets better to interpret at certain time intervals.

• ANGULAR VELOCITY = 20 RPM

• Contact Force vs Time (No Precise Contact)

The consistency in the plot is visible after changing the RPM of the Driver Wheel. Also, the jerk felt during contact is more proportionate due to higher RPM.

• Angular Displacement vs Time (No Precise Contact)

The trend observed in the plot is followed throughout with consistency.

• Angular Velocity vs Time (No Precise Contact)

The angular velocity remains constant int he entire cycle of movement. The variations observed at higher RPM are somewhat minimal.

• Contact Force vs Time (Precise Contact)

Sharper plot obtained due to precise contact between the Driver Whell and the Driven Wheel.

• Angular Displacement vs Time (Precise Contact)

The above plot depicts smoothness and accuracy when compared to its unprecised contact twin.

• Angular Velocity vs Time (Precise Contact)

The transition is smoother, although some variation exist. The higher RPM makes the mechanical motion of the mechanism much more reliable.

LINK TO SOLIDWORKS PART & ASSEMBLY FILES:

"https://drive.google.com/drive/folders/1eEzCZImrAv9Pt-2xeThbbo_e8emwqA4b?usp=sharing"

CONCLUSION: The Motion Study helps us to simulate the real time performance of the Internal Geneva Mechanism and thereby give an outline to develop our model for better output.