Internal Geneva Mechanism
AIM : Design and motion analysis of an Internal Geneva Mechanism.
INTRODUCTION : The geneva mechanism is a type of gear mechanism that is used to translate a continuous rotary motion into an intermittent rotary motion characterized by alternate periods of motion and rest with no reversal in direction. It is also used for indexing (i.e. rotating a shaft through a prescribed angle).
external geneva mechanism
internal geneva mechanism
An internal Geneva drive is a variant on the design of mor common external geneva drive. 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.
ADVANTAGES :
Geneva may be the simplest and least expensive of all intermittent motin mechanisms. They come in a wide variety of sizes ranging form those used in instruments, to those used in machine tools to index spindle carriers weighing several tons. They have good motion-cruve characteristics compared to 'ratchets' but exhibit more jerk or instantaneous change in acceleration, than do better cam systems.
The geneva maintains good control of its load at all times since it is providded with locking ring surfaces. In addition if properly sized to the load, the geneva generally exhibits very long life.
DISADVANTAGES :
The geneva is not a versatile mechanism. It can be used to produce no less than 3 and usually not more than 18 dwells per revolution of the output shaft. Furthermore, once the no. of dwells has been selected, the designer is well locked into a given set of motion curves. The ratio of dwell period and motion period is also established once the number of dwells per revolution has been selected. Also, all geneva acceleration curves start and end with finite acceleration and deceleration. This means they produce jerk.
Following objectives are to be fulfilled through this project :
- Creating 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.
- Once we have completed the previous study, change the rpm to 20
- Using 60 Frames per second for 10 rpm and then 120 Frames per second for 20 Rpm.
Following 2D CAD drawings are used to create the 3D model of the internal Geneva mechanism.
PROCEDURE :
- Create 3D drawings of the driver and driven wheel using the 2d drawings provided.
- Make use of solidworks tools such as offset entities, circular sketch patterns etc to create the 3D drawings.
- Create a new assembly of these 2 parts and perform the following mating adjustments to properly align the driver and driven wheels:
- Coincident- between front plane of driver and front plane of assembly.
- Coincient- between axis of driver and top plane of assembly.
- Coincident- between axis of driver and right plane of assembly.
- Coincident- between axis of driven and right plane of assmebly.
- Distance mate- between front plane of driven and front plane of assembly and distance of 20mm.
- Distance mate- between axis of driver and axis of driven.
The final assembly obtained is as follows :
- Finally after obtaining the final assembly, choose Solidworks motion from the Solidworks Add-Ins and perform motion analysis.
- Apply torque using a motor on the driving wheel with an rpm of 10 and then 20 and then obtain the required plots.
- Run the simulation for the desired interval of time and save the motion simulation.
OUTPUT :
The final output after running the motion is obtained in the following form.
motion analysis by running the driver wheel at 10 rpm, 60fps without precise contact.
motion analysis by running the driver wheel at 10 rpm, 60fps with precise contact.
motion analysis by running the driver wheel at 20 rpm, 120fps with precise contact.
angular displacement of driver wheel at 20 rpm, 120fps without precise contact
angular displacement of driver wheel at 20 rpm, 120fps with precise contact.
The motion simulation of geneva mechanism can be viewed from the link below :