Internal Geneva Mechanism

Internal Geneva Mechanism

Aim

To create the 3D model of the internal Geneva mechanism, to perform motion analysis by rotating the driver wheel at 10rpm, 20rpm and analysing it by plotting different variables.

Objectives

1. To create 3D models for driver and driven wheels.
2. To perform motion analysis by rotating the driver wheel at 10 rpm and 20rpm.
3. To obtain a plot for contact force between driving and driven wheel.
4. To obtain a plot for Angular displacement of the driven wheel.
5. To compare the contact forces with and without “Precise contact”.
6. To obtain a plot for Angular velocity of the driving wheel.
7. To export the results to a spreadsheet.

Introduction

Geneva mechanism, also called Geneva Stop is one of the most commonly used devices for producing intermittent rotary motion, characterized by alternate periods of motion and rest with no reversal in direction. 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. An internal Geneva drive is a variant on the design. 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.

Motion studies are graphical simulations of motion for assembly models. You can incorporate visual properties such as lighting and camera perspective into a motion study. Motion studies do not change an assembly model or its properties. They simulate and animate the motion you prescribe for a model. You can use SOLIDWORKS mates to restrict the motion of components in an assembly when you model motion.

CAD Geometry

i) Part 1

ii) Part 2

iii) Assembly

Motion study setup

The motion features of the software can assist with a wide range of motion study depending on how complicated your requirements are, and they can be divided into three categories:

• Animation: If you simply wish to create some nice visuals for presentation or marketing without consideration of mass and gravity effects, then animation is for you.
• Basic Motion: For an extra layer of complexity that takes into consideration the effects of mass, springs, gravity and physical collision detection, then a Basic Motion study is more suited for your requirements.
• Motion Analysis: This is the top tier of motion study and takes into account a wider range of physical interactions such as impact effects, damping, force, momentum, etc.

Steps for motion analysis

1. Activate the motiton analysis plugin of solidwokrs in add-ins tab.
2. Select the motion analysis from the left drop down menu where there are other option including animation and basic motion. Animation will be selected as default so we need to select motiton analysis from the list.
3. The driver is rotating at 10rpm and 20rpm in different cases and this is achieved by selecting motor from the motion analysis options.
4. As the pin should move inside slot and they should not overlap during the motion analysis we should give information to solidwworks that these are solid contact bodies. For this we use 'Contact' option from motion analysis options.
5. Motiton study properties can be selected to input necessary accuracy details and also to enable precision contact.
6. After including all settings to analyse the motion click the calculate button.
7. Using 'Results and plots' option we can plot different graphs and get results.

Motor settings

Precision contact settings

Contact settings

Motion Study of the whole Internal geneva mechanism assembly (using precision and without precision).

NOTE: Angular displacement and angular velocity of driving wheel is plotted. Also contact force between pin and the slot is also plotted for each case.

a) Motion study of assembly with driver at 10rpm without precision contact (60fps)

b) Motion study of assembly with driver at 10rpm with precision contact (60fps)

c) Motion study of assembly with driver at 20rpm without precision contact (120fps)

d) Motion study of assembly with driver at 20rpm with precision contact(120fps)

Angular velocity of driving wheel

Comparison of contact force (With precision contact vs Without precision contact)

The force and represented slightly more accurate and the forces have incresed while using precision contact. The red data represents contact forcce with precision contact and blue represents contact forcce without precision contact.

Import of these data to excel

1. To import the data to spreadseet, first select the plot and open it. Now right click on the plot name in the list as shown in the figure below to find an option 'Export to spreadsheet'.
2. Now all the data and plot will be opened in an excel file.
3. We can further modify the graph to the sutability in excel after exporting this data.

Animation

Link : https://youtu.be/pZoh45GY8J4

Results and Explanation

1. The maximum angular displacement possible is 360 degree. As the driver wheel rotates, the driven wheel also rotates when the pin is locked in the slot and when the pin is not locked in the slot the driven stays stationary. So the angular displacement at those intervals remain almost flat. In the angular displacment vs time plot, the angle varies from 180 degree to -180 degree in effect 360degree in total. In comparison to 10rpm the graph of 20rpm 360degree of rotation is completed in less time (In 14 seconds the driven disc rotates upto -153degree (20rpm case), while the disc only rotates upto -39 degree in 10rpm case . This is because of more angular velocity which is evident. The use of precision contact doesnot much effect the angular displacement graph, some slight more accuracy is achieved which is negligible.
2. Angular velocity of driven wheel follows a same pattern of a curve. It remains zero at the time the pin is not locking the slot and angular velocity reaches its peak when pin reaches at the end of the slot. As rpm of the driving motor increases the angular velocity of driven slightly increases in the successive rounds as initially due to driction and contact force the anguar velocity is reduced. Precision contact gave more clear graph of angular velocity devoid of sudden peaks and troughs.
3. Contact force generally occurs at the entry of pin to the slot. Precision enabled study shows higher contact forces as compared to one without. As rpm increases the contact forces also increases.
4. For10 rpm, the angular velocity of the driver wheel is almost steady state with time.
5. The number of peaks in contact force increased linearly with RPM as expected.

Conclusions

3D model of the internal Geneva mechanism was created and motion analysis of the model was performed successfully.