Motion analysis of Internal Geneva Mechanism using SolidWorks
AIM
i) To create the internal geneva mechanism assembly in solidworks using the given 2D CAD models.
ii) To perform the motion analysis of the assembly by rotating the driving wheel and obtain various plots.
THEORY
Multibody dynamics
Multibody dynamics is a system consisting of solid bodies or links that are connected to each other so as to restrict their relative motion. The study of multibody dynamics in the analysis of movement of mechanism system under the influence of forces, in other words, dynamics.
Motion study
Motion studies are graphical simulations of motion for assembly models. It enables us to plot various forces between the bodies in contact, or several other properties like dislpacement, velocity, acceleration, momentum etc. in order to get a better idea about the motion of the system. In SolidWorks, motion study can be done after creating the assembly by selecting the motion study option under SolidWorks Add-ins menu. After that, various parameters like contact between the bodies, presence of spring, damper, motor etc. are defined and analysis is performed.
Geneva mechanism
The geneva mechanism, also known as Maltese cross, is a gear mechanism that converts a continuous rotary motion into an intermittent rotary motion. It is characterized by the alternate period of motion and rest of the driven wheel with no reversal in direction. The driving wheel is equipped with a pin that reaches into the slots made on the driven wheel. The driving wheel rotates only when there is a contact between the pin and the driving wheel.
There are basically 3 types of geneva mechanisms:
1) External Geneva mechanism
2) Spherical Geneva mechanism
3) Internal Geneva mechanism
In this study, we are concerned about the internal geneva mechanism. Based upon the 2D CAD drawings, 3D models are made. The dimensions of the parts are kept in millimetres.
Driving part
Driven part
The 2 components are then assembled by applying suitable mates. The centre distance was evaluated using the measure command and it came out to be 226.17mm. It is to be assured that the driving part does not come in contact with the driven part other than at the pin extruding from its surface. Also, the pin should be in contact with surfaces of the slots only.
Assembly
Once the assembly has been done, the motion analysis of the internal geneva mechanism is performed. A motor is provided to the driving part with clockwise rotation. Solid body contact is provided between the driving and the driven part and the material provided is dry steel.
For this study, the motor is set to rotate first at 10 RPM and then at 20 RPM and plots are obtained for angular displacement of driving and driven part, angular velocity of the driven part and the contact force between the components with and without the precise contact.
The plots thus obtained are given below:
MOTOR AT 10 RPM
1) Contact forces
Without precise contact
Enabling precise contact
2) Angular Displacement
3) Angular velocity
MOTOR AT 20 RPM
1) Contact forces
Without precise contact
Enabling precise contact
2) Angular displacement
3) Angular velocity
ANIMATION
Internal geneva mechanism (10 RPM, 60FPS)
https://drive.google.com/file/d/1Xq7bzoyQbq2HIIVk6ViGYZ3TELllisNk/view?usp=sharing
Internal geneva mechanism (20 RPM, 120 FPS)
https://drive.google.com/file/d/13PCQL_zJz_rtu_UCc-HYOQX1JPEPV1NN/view?usp=sharing
RESULTS
i) Since there are 4 slots in the driven part, for each revolution of the driven part there will be 4 revolutions of the driving part. The same can be seen in the graph showing the angular displacement of the components.
ii) The contact forces increase on increasing the angular velocity of the driving part
iii) Enabling the precise contact provides better results. From the plots, it can be seen that the contact forces are higher with the precise contact than without it.
CONCLUSION
The motion analysis of internal Geneva mechanism was completed using SolidWorks. There are several applications of geneva mechanism like film projectors and movie cameras where film is pulled through exposure gates through periodic starts and stops. Other applications include pen change mechanism in plotters, mechanical watches, banknote counting machines, automated sampling devices etc.
REFERENCES
https://en.wikipedia.org/wiki/Geneva_drive