Internal Geneva Mechanism

Aim:

• Create 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.
• Angular Displacement of the driven wheel.
• Compare the contact forces with and without "Precise contact"
• Show image for precise contact selection
• Export the results that you get to Excel and make the plots over there

Theory:

The Geneva drive is also commonly called a Maltese cross mechanism. The Geneva mechanism translates a continuous rotation into an intermittent rotary motion. The rotating drive wheel has a pin that reaches into a slot of the driven wheel. The drive wheel also has a raised circular blocking disc that locks the driven wheel in position.

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.

In an internal Geneva drive, the axis of the drive wheel of the internal drive is supported on only 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 is always greater than 180° in an internal one. The external form is more common, as it can be built smaller and can withstand higher mechanical stresses. Both the Geneva mechanisms can be used for light and heavy-duty applications. Generally, they are used in assembly machines. This type of movement is basically required in packaging, assembly operations, stamping, embossing operations in manufacturing automation.

CAD Model:

This is the 3D view of the driven wheel.

This is the 3D view of the driver wheel.

2D diagram with the dimensions of the driven wheel.

2D diagram with the dimensions of the driver wheel.

Result:

Case 1: Speed of 10 RPM and 60 FPS

• Contact Force vs Time without Precise Contact (10 RPM )

This plot describes the contact force at each point when the driver wheel continues to rotate over a period of time. The major spikes in the graphs are generated when the driver is in complete contact with slots provided in the driven wheel and it tries to transfer the rotational energy to the driven wheel eventually making it rotate.

• Contact Force vs Time with Precise Contact (10 RPM )

When you select Use Precise Contact, the computed contact is analytically correct, but the computation can take longer than an approximate solution. The results here are more accurate than in the case where there's no precise contact. The user is provided with features where the frame rate and accuracy are varied to get better results. In this case frame rate of 60 fps is selected along with the accuracy mentioned in the below figure and other selections.

• Angular Displacement without Precise Contact (10 RPM)

This plot describes the angular displacement with respect to the time of the driven wheel. As the graph shows the displacement starts from a higher value and it continues to decrement as the time goes on, it completes a full revolution and the value becomes 0.

• Angular Displacement with Precise Contact (10 RPM)

This plot describes the angular displacement with respect to the time of the driven wheel. As the graph shows the displacement starts from a higher value and it continues to decrement as the time goes on, it completes a full revolution and the value becomes 0. The results here are more accurate than in the case where there's no precise contact. The user is provided with features where the frame rate and accuracy are varied to get better results. In this case frame rate of 60 fps is selected along with the accuracy mentioned in the below figure and other selections. The change might not be evident or noticeable here but it has an impact on the final result.

• Angular Velocity (10 RPM)

The Angular Velocity of the driver tends to remain the same throughout the simulation with very negligible divergence from the original value. It is because the rotor is provided with a constant 10 RPM throughout.

• Precise Contact Selection:

These are the precise contact settings used while performing the simulation.

Case 2: Speed of 20 RPM and 120 FPS

• Contact Force vs Time (20 RPM ) without Precise Contact

This plot describes the contact force at each point when the driver wheel continues to rotate over a period of time. The spikes in the graphs are generated when the driver is in complete contact with slots provided in the driven wheel and it tries to transfer the rotational energy to the driven wheel eventually making it rotate.

• Contact Force vs Time (20 RPM ) Precise Contact

When you select Use Precise Contact, the computed contact is analytically correct, but the computation can take longer than an approximate solution. The results here are more accurate than in the case where there's no precise contact. The user is provided with features where the frame rate and accuracy are varied to get better results. In this case frame rate of 120 fps is selected along with the accuracy mentioned in the below figure and other selections. The variation can be observed from the previous plot and the one where the simulation was performed at a lower rpm.

• Angular Displacement without Precise Contact (20 RPM)

This plot describes the angular displacement with respect to the time of the driven wheel. As the graph shows the displacement starts from a higher value and it continues to decrement as the time goes on, it completes a full revolution and the value becomes 0. Then it completes another revolution which is evident from the graph.

• Angular Displacement Precise Contact (20 RPM)

This plot describes the angular displacement with respect to the time of the driven wheel. As the graph shows the displacement starts from a higher value and it continues to decrement as the time goes on, it completes a full revolution and the value becomes 0. The results here are more accurate than in the case where there's no precise contact. The user is provided with features where the frame rate and accuracy are varied to get better results. In this case frame rate of 120 fps is selected along with the accuracy mentioned in the below figure and other selections. The change here is quite noticeable from the earlier plot where there was no precise contact.

• Angular Velocity (20 RPM)

The Angular Velocity of the driver tends to remain the same throughout the simulation with very negligible divergence from the original value. It is because the rotor is provided with a constant 20 RPM throughout.

• Precise Contact Selection:

These are the precise contact settings used while performing the simulation.

References:

https://nptel.ac.in/content/storage2/courses/112103174/pdf/mod4.pdf

Animation Link:

https://www.youtube.com/watch?v=l8-7-ztyJ8A&feature=youtu.be