Internal Geneva Mechanism
AIM:
Design and motion analysis of 'INTERNAL GENEVA MECHANISM' using solidworks.
OBJECTIVE:
The objective of the project are:
- To create the 3-D models of the driver and the driven wheels.
- To do the motion analysis by rotating the driver wheel at different rpm.
- Obtain the following plots by using precise contact and without precise contact:
- Contact force (between driving and the driven wheel) as a function of time.
- Angular Displacement of the driven wheel v/s time.
- Angular velocity of the driver wheel v/s time.
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.
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.
PROCEDURE:
1)Initially the 2-D drawings of the driving and the driven wheels was made using the Solidworks 2017 with the dimension that was provided in the question.The reference dimension image is shown below:
Reference image of driving wheel:
Reference dimension image of driven wheel:
2)Then from this 2-D drawings 3-D CAD model was generated by using the extrude command.The 3-D CAD model for driving and driven wheels is shown below:
Isometric view of 3-D CAD MODEL of Driving Wheel:
Isometric view of 3-D CAD MODEL of Driven Wheel:
3)After finishing with both the parts we open the solidworks assembly to do the assembly of the two wheels.
4)In assembly first we insert both the components and we have to keep one thing in mind here is that the part we insert first has to made float so that we can give the desired mates to it.
5)After inserting both the parts we add some mate to both the parts as per the requirements.The isometric view of the assembly is shown below:
6)After finishing with the assembly we start the motion study analysis and analyse the Geneva Mechanism at different rpm of the motor.
MOTION STUDY ANALYSIS:
- For motion study the add-ins in Solidworks is clicked and Solidworks motion is selected. Then a motor/torque is provided to the driving wheel as required. Contact between solid bodies are provided by choosing required solid bodies and their material is selected as Steel(dry).
- Then calculate button is pressed, it will run and solve the analysis. After this the plots can be made from the results obtained by choosing desired parameters.
- After the plots has been made they are then exported to the spreadsheet and their plot is made on excel.
Analysis at 10 RPM:
The various plot without precise contact when the motor of 10 rpm is provided is shown below:
CONTACT FORCE BETWEEN DRIVING AND DRIVEN WHEEL V/S TIME:
From the above plot we can conclude that there is sharp rise in the force value when the driver wheel makes contact with the driven wheel.It is due to the jerk produced while making the contact.Also there is small bumps when the driver wheel looses its contact with the driven wheel.
ANGULAR DISPLACEMENT OF DRIVEN WHEEL V/S TIME:
From the above plot we can conclude that the initially the angular displacement decreases gradually and then there is a sudden rise in the displacement and then further it decreases gradually.
ANGULAR VELOCITY OF DRIVING WHEEL V/S TIME:
The above plot shows an almost constant angular velocity of the driving wheel except at some points.
Then the motion analysis with precise contact was done with 60 frames per second when the motor of 10 rpm was applied.
Image showing selection of precise contact option:
The various plot with precise contact when the motor of 10 rpm is provided is shown below:
CONTACT FORCE BETWEEN DRIVING AND DRIVEN WHEEL V/S TIME:
As we have applied the precise contact so the plot is smooth and less fluctuating results is obtained as shown above.
ANGULAR DISPLACEMENT OF DRIVEN WHEEL V/S TIME:
The above plot of the angular displacement of the driven wheel is more smooth as compared with the earlier plot of the angular displacement due to precise contact applied between the driving and the driven wheel.
ANGULAR VELOCITY OF DRIVING WHEEL V/S TIME:
The above plot has more precision as compared to the earlier plot of angular velocity of the driving wheel due to the precise contact.This helps us to interpret the results with better precision at certian time intervals.
After both the study for 10 rpm has been done and the plots are made we make the animation of the motion study analysis.The link for animation is shown below:
Analysis at 20 RPM:
The various plot without precise contact when the motor of 20 rpm is provided is shown below:
CONTACT FORCE BETWEEN DRIVING AND DRIVEN WHEEL V/S TIME:
From the above plot we can conclude that the jerk felt during contact of the wheels is more as we increase the rpm of the motor.There is more fluctuation in the force values as we increase the motor rpm.
ANGULAR DISPLACEMENT OF DRIVEN WHEEL V/S TIME:
The above plot follows the same pattern of intially decrease in the angular displacement of the driven wheel and then a sudden increase.
ANGULAR VELOCITY OF DRIVING WHEEL V/S TIME:
The angular velocity of the driving wheel is almost constant except at the some points.
Then the motion analysis with precise contact was done with 120 frames per second when the motor of 20 rpm was applied.
The various plot with precise contact when the motor of 20 rpm is provided is shown below:
CONTACT FORCE BETWEEN DRIVING AND DRIVEN WHEEL V/S TIME:
More smooth and sharper points is obtained due to precise contact of driving and the driven wheel.
ANGULAR DISPLACEMENT OF DRIVEN WHEEL V/S TIME:
The above plot depicts smoothness and accuracy when compared to its unprecised contact between the wheels.
ANGULAR VELOCITY OF DRIVING WHEEL V/S TIME:
The transition is smoother, although some variation exist. The higher RPM makes the mechanical motion of the mechanism much more reliable.
After both the study for 20 rpm has been done and the plots are made we make the animation of the motion study analysis.The link for animation is shown below:
CONCLUSION:
Thus we have made the 3-D CAD models of both the wheels and the assembly of them was done.Also the motion study of the Internal Geneva Mechanism was done and their plots were also obtained and exported to the excel spreadsheet.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.The jerks between the driver and the driven wheels is due to misalignment between driven and driver wheel or wrong placemnets of them. If these are avoided the plots obtained will be more accurate.
REFERENCES:
https://en.wikipedia.org/wiki/Geneva_drive
APPLICATION AND USES:
Stepper motors, Mechanical watches, Plotters, CNC machine, Iron ring clock etc.
LINK TO SOLIDWORKS PART & ASSEMBLY FILES IS GIVEN BELOW:
https://drive.google.com/drive/folders/1QvYZYhcQ2kCm15NR7L9-H2rW0QDo1hOh?usp=sharing