Internal Geneva Mechanism

Aim:

1. To create driver and driven model of Internal Geneva Mechanism and perform Multi-Body Dynamics on it.
2. To plot the following graphs and compare them at both given cases,i.e;10 Rpm at 60 fps & 20 Rpm at 120 fps :-
   i. Contact force (between driving and the driven wheel) as a function of time.
   ii. Angular Displacement of the driven wheel.
   iii. Angular Velocity of driver wheel.
3. Also,compare plots of Contact forces with and without "Precise contact".

Introduction:

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 Drive.These types are listed below :-

1. External Geneva Drive
2. Internal Geneva Drive
3. Spherical Geneva Drive
   
   

1. External Geneva Drive:
In Internal Geneva Drive, the driven wheel is connected with the rotating drive wheel externally.Given below is an animation of External Geneva Drive for better understanding.

2. Internal Geneva Drive:
In Internal Geneva Drive, the driven wheel is connected with the rotating drive wheel internally.Given below is an animation of Internal Geneva Drive for better understanding.

3. Spherical Geneva Drive:
In Internal Geneva Drive, the driven wheel is connected with the rotating drive wheel externally but in a spherical motion.Given below is an animation of Spherical Geneva Drive for better understanding.

There are some concepts to be known for this project which are listed below:
1. Multi-Body Dynamics(MBD):-
A multibody dynamic (MBD) system is one that consists of solid bodies, or links, that are connected to each other by joints that restrict their relative motion. The study of MBD is the analysis of how mechanism systems move under the influence of forces, also known as forward dynamics.

2.Mechanism:-
An assembly of moving parts performing a complete functional motion, often being part of a large machine; linkage.

Procedure:

1. Model Design:
Includes making the cad model of the system that is to be analyzed.

2. Assembly:
With the help of mates parts are combined to form an Assembly.

3. Applying Set-Up:
Includes setting up the motor, contacts and material.

4. Motion Analysis:
The solver is run giving out the results for plots including forces,velocities,accelerations and identifying the minimum and maximum values of those to determine the critical regions.

Case Study:

1. Model Design:

i. 2D Model:
Given below are the 2D sketches of driver and driven part of Internal Geneva Mechanism.

Fig 1: 2D Sketch of Driving part of Internal Geneva Mechanism

Fig 2: 2D Sketch of Driven part of Internal Geneva Mechanism

ii. 3D Model:
Now the 3D model for the driving and driven part is done by using the commands such as extrude,trim,etc. are used.

Firstly we will see the modelling of driving part:

• Disc of diameter 420 mm is extruded by 20 mm.
• Pin of diameter 40 mm is extruded by 40 mm on the other side.

Given below is the 3D model of driving part.

Fig 3: 2D Model of Driving part of Internal Geneva Mechanism

Now,we will see the modelling of the driven part:

• The driven disc is extruded by 40 mm.

Given below is the 3D model of driving part.

Fig 4: 2D Model of Driven part of Internal Geneva Mechanism

2. Assembly:
Now that both the required parts are created we need to combine them with the help of mates.The mates used are listed below:

Starting with imported the driving part and turn it into floating.

• Coincident Front Plane of Assembly with Front Plane of Driving part.
• Coincident Top Plane of Assembly with Axis of Disc(select the temporary axis to be visible option).
• Coincident Right Plane of Assembly with Axis of Disc.

Now import the driven part.

• Distance mate of 20 mm from Front Plane of Assembly to Front Plane of Driven part.
• Coincident Top Plane of Assembly with Axis of Disc.
• Finally apply distance mate between axis of driving and driven part(Distance found from model).

Given below is the Assembly of Internal Geneva Mechanism.

Fig 5: Assembly of Internal Geneva Mechanism

3. Applying Set-Up:

Study 1: Motion Analysis with 60 frames per second and motor at 10 rpm constant speed :-

• Setting up time frame for our motion study to 10 seconds.
  
• In motor,select axis of driver with 10 RPM in clockwise direction.
• In contact,select driver wheel and driven wheel.
• Select Steel (Dry) material for both driver and driven wheels.
• Select Friction.
• In Motion Study Properties, enter 60 fps without the use of precise contact.

Given below are the selections made :

Fig 6: Selection done in Motor,Contact & Motion Study Properties for 10 RPM

After the analysis of this we use precise contact for comparison of results.The selections are shown below:

Fig 7: Selection done in Motor,Contact & Motion Study Properties for 10 RPM

Study 2: Motion Analysis with 120 frames per second and motor at 20 rpm constant speed :-

• Setting up time frame for our motion study to 10 seconds.
  
• In motor,select axis of driver with 20 RPM in clockwise direction.
• In contact,select driver wheel and driven wheel.
• Select Steel (Dry) material for both driver and driven wheels.
• Select Friction.
• In Motion Study Properties, enter 120 fps without the use of precise contact.

Given below are the selections made :

Fig 8: Selection done in Motor,Contact & Motion Study Properties for 20 RPM

After the analysis of this we use precise contact for comparison of results.The selections are shown below:

Fig 9: Selection done in Motor,Contact & Motion Study Properties for 20 RPM

4. Motion Analysis:

(A) Study 1: Motion Analysis with 60 frames per second and motor at 10 rpm constant speed :-

Given below is a plot between Contact force and time without precise contact:

Fig 10: Plot for Contact Force Vs Time without Precise contact

Given below is a plot between Angular Displacement and time:

Fig 11: Plot for Angular Displacement Vs Time

Given below is a plot between Angular Velocity and time:

Fig 12: Plot for Angular Velocity Vs Time

Given below is a plot between Contact force and time with precise contact:

Fig 13: Plot for Contact Force Vs Time with Precise contact

(B) Study 2: Motion Analysis with 120 frames per second and motor at 20 rpm constant speed :-

Given below is a plot between Contact force and time without precise contact:

Fig 14: Plot for Contact Force Vs Time without precise contact

Given below is a plot between Angular Displacement and time:

Fig 15: Plot for Angular Displacement Vs Time

Given below is a plot between Angular Velocity and time:

Fig 16: Plot for Angular Velocity Vs Time

Given below is a plot between Contact force and time with precise contact:

Fig 17: Plot for Contact Force Vs Time with Precise contact

Animation:

The animation of Internal Geneva Mechanism for this project can be seen through the below link:
https://drive.google.com/file/d/13Tl_HrQxPc30uEvB7t-wWeiM-HN5bcDE/view?usp=sharing

Observation:

• Comparison between Contact Force vs Time in Case 1 & 2 :

Cases	Case 1-10 RPM at 60 FPS	Case 2-20 RPM at 120 FPS
Maximum Contact Force(N)	2639	3646

• Comparison between Angular Displacement of Driven wheel vs Time in Case 1 & 2 :

Cases	Case 1-10 RPM at 60 FPS	Case 2-20 RPM at 120 FPS
Maximum Angular Displacement(deg)	At 3.1 sec	At 1.5 sec

• Comparison between Angular Velocity of Driver wheel vs Time in Case 1 & 2 :

Cases	Case 1-10 RPM at 60 FPS	Case 2-20 RPM at 120 FPS
Maximum Angular Velocity(deg/sec)	Constant	Constant

Conclusion:

• The speed ratio in Internal Geneva Mechanism model is 4:1 as when the driver wheel rotates 4 times the driven wheel rotates once only.
• From analysis between the plot of Contact Force vs Time in case 1 & 2, we can observe that maximum contact force is present in Case 2.Also in case 2,there is major deflection 4 times in 10 seconds when compared to in case 1 with 2.It is due to oncrease in rpm of motor.
• Contact Force increases when entering the slot as it is the first time two surfaces come in contact.
• From analysis between the plot of Angular Displacement vs Time in case 1 & 2, we can observe that Angular Displacement reaches its maximum earlier in Case 2 due to increase in RPM.Even the graph seems a bit smoother due to increase in RPM of the motor.
• In Angular Displacement plots,the graph follow the same path/pattern after some time this is due to the fact that the same path is followed after sometime.
• From analysis between the plot of Angular Velocity vs Time in case 1 & 2, we can observe that Angular Velocity remain constant with bit of minor deviations as velocity is same even after increase in RPM the velocity at that RPM remains same with respect to time.
• From analysis between the plot of Contact Force with Precise Contact vs Time in both the cases it is seen that the graph seems a bit more smooth in Precise Contact graphs.This is due to the fact in precise contact all the analytical contacts are also considered. Eventhough time for calculation increases but precise solution is displaced more closer to real life scenario.