Internal Geneva Mechanism
Motion Study of Internal Geneva Mechanism:
Aim: To Create 3d models of the internal geneva mechanism components and assemble them to create a motion study and compare various results obtained from the study.
Procedure:
- Create the driver or driven components of the internal geneva mechanism
- Assemble them to form the Geneva mechanism
- Create a motion study for 10RPM at 60 frames per second.
- Create a motion study for 20RPM at 120 frames per second.
- Create plots for contact forces, Angular displacement, and angular velocity.
- Compare these results with and without precision contact.
Theory: Geneva mechanism is one of the most common mechanisms used to create intermittent motion using the driver and driven components of the mechanisms. The components are created and assembled in such a way that they enable the intermittent motion to occur. Here the continuous motion of the driver component is converted into intermittent motion of the driven component. The driver wheel usually has a pin that gets into the slots of the driven wheel and this helps the creation of the intermittent motion. These are usually seen in mechanical watches and movie projectors.
Types
- External Geneva Mechanism
- Internal Geneva Mechanism
- Spherical Geneva Mechanism
External Geneva Mechanism:
This is a type of geneva mechanism where the driven wheel is connected to the driver component externally. Given below are the pictorial representations of the external geneva mechanism.
Internal Geneva Mechanism:
This is a type of geneva mechanism where the driven wheel is connected to the driver component internally. Given below is the pictorial representation of the internal geneva mechanisms.
Spherical Geneva Mechanism:
This is a type of the geneva mechanism where the driven and driver components are spherical and are mostly designed in a way where the axes of the two components are perpendicular to each other. Below is the pictorial representation of the spherical geneva mechanism.
Motion Study Setup:
Given information:
According to the given dimensions we first create the Driver wheel as shown in the below picture:
Now we create the driven wheel according to the dimensions given:
Now we draw a circle tangent to the center lines of the two slots adjacent on the driven wheel and give it a dimension of 320mm diameter or 160mm radius as provided in the information picture of the question. This gives us the center of the driver wheel and we already have the center of the driven wheel. This circle is shown in the below sketch.
Now we measure the distance between the centers of the driver and driven wheels to find the distance between the axes of both the wheels. This value is found out to be 226.27mm. This is shown in the below sketch.
Now we assemble the two components by selecting the assemble option in Solidworks. Here the components are constrained by mating them with the planes of the assembly and fixing the distance between the central axes of the two components. Once all this is done, the final assembly looks something like the picture below.
Condition 1 (10RPM 60FPS):
Now we create a motion study and in the motion analysis, we give the following conditions of 10RPM at 60FPS with and without precision contact and also choose the material as steel (dry). Also, we fix a time period of this study as 10sec. These are shown in the below pictures.
After applying all these conditions we click on calculate the motion study button and then plot the graphs for contact force between driver and driven wheels as a function of time with and without precision contact, Angular displacement of the driven wheel and angular velocity of the driver component. The results obtained are showcased below.
contact force vs time without precision contact.
Contact force vs time with precision contact.
Angular displacement vs time of the driven wheel.
Angular velocity vs time for the driver wheel.
Condition 1 Analysis:
Observing the plots of the contact force vs time with and without precision contact, we can say that having the precision contact turned on helps us in giving more accurate results as the force components are distinct and variation of contact force while the components touch each other to create the movement is observed clearly at various points of time.
The angular displacement of the driven wheel is observed to be at its peak when the driver's pin is at the deepest point of the slot that it can reach and slowly keeps decreasing as the pin comes out of the slot. This variation can be clearly seen in the plot.
The angular velocity of the driver wheel is seen to be more or less stable all the time. This is because we gave a constant speed motor attached to the driver and only minor fluctuations are seen due to the contact with the driven wheel.
So these are some of the observations that we can make by looking at the plots of contact forces, angular displacement, and angular velocity of the components.
Condition 2 (20RPM 120FPS):
Now we create another motion study and in the motion analysis, we give the following conditions of 20RPM at 120FPS with and without precision contact and also choose the material as steel (dry). Also, we fix a time period of this study as 8sec. These are shown in the below pictures.
After applying all these conditions we click on calculate the motion study button and then plot the graphs for contact force between driver and driven wheels as a function of time with and without precision contact, Angular displacement of the driven wheel and angular velocity of the driver component. The results obtained are showcased below.
contact force vs time without precision contact.
contact force vs time with precision contact.
Angular displacement vs time of the driven wheel.
Angular velocity vs time of the driver wheel.
Condition 2 Analysis:
As discussed in the previous analysis of the first condition, the plot of the contact force with precision is more helpful in giving us a clear idea of how the force varies with time as the driven comes in contact with the driver. The highest force here is seen during the first contact of the driver wheel's pin with the slot of the driven wheel and it decreases as it goes deep into the slot and comes out of it. The precision of the calculation of the distance between the axes of the driver and driven would help in decreasing the impact of contact force while entering the slot and cause the movement.
When the Rpm of the motor is increased in this condition, we are able to get more cycles of movement in the same time period and it helps us analyze the motion over greater displacement compared to the first condition.
The angular displacement of the driven wheel and the angular velocity of the driver wheel has been seen to show similar trends in both the conditions.
Final Conclusion:
- Forces seen during the 10rpm analysis are lesser than the forces seen in the 20rpm condition.
- 20rpm give more cycles of movement compared to 10rpm in a given time period. So analyzing both the conditions helps in getting to know the variation of forces with increase in the speed of driver.
- Turning on the precision contact option helps in getting to know the forces during the contact more precisely and increase the accuracy of the mechanism.
So the Internal Geneva Mechanism is created, plots are made and the analysis is done.