Internal Geneva Mechanism
Aim: MODELLING AND SIMULATION OF A GENEVA MECHANISM IN SOLIDWORKS AND ALSO USE THE MOTION STUDY TO FIND THE CONTACT FORCE.
Flow of Content:
- Introduction
- Design
- Solidworks Part Design Procedure
- Motion Study
- Tasks
- Result and Explanation
- Errors or issues I faced
- Reference
Introduction:
What is Internal Geneva Mechanism?
: 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.
The external form is the more common, as it can be built smaller and can withstand higher mechanical stresses.
Application:
Geneva drive include the pen change mechanism in plotters, automated sampling devices, banknote counting machines, and many forms of indexable equipment used in manufacturing (such as the tool changers in CNC machines; the turrets of turret lathes, screw machines, and turret drills; some kinds of indexing heads and rotary tables; and so on). The Iron Ring Clock uses a Geneva mechanism to provide intermittent motion to one of its rings. Etc.
Design:
Fig: Internal Geneva Mechanism Assembly
Solidworks Part Design Procedure:
Reference Drawing for Parts Design;
Part 1- Driven Design:
Fig: Driven
Open the solidworks, select new file and select part, (Do select the unit system as mm), once you are in part; go to sketch and select the front plan.
Now, select the circle and draw three circles of radius 560mm, 520mm and 2120mm. Make 520mm radius circle for construction.
Now, from the constructional circle draw a straight line till 520mm Radius circle, and now select the offset Entity with bi-directional & cap end Arcs.
Now, trim the arc end at the center and after this, select circular sketch pattern of 4. Once get the pattern, make sure to define the sketch by making the pattern center coincidental with the circle center. Now remove the closing line for the pattern.
Now draw fillets of 50mm radius from the 120mm radius circle to the connecting pattern to it as shown below in the fig.
Now draw a bisecting construction line from the center point and keep it between the 2 patterns as shown below.
Now, select the center line of adjacent pattern to these construction line and make these 2 center line equal length. Now select the smart dimension and give the construction line an angle of 45deg. This is been sketch for the driver to set at it position.
Extrude the sketch except the center circle of 40mm and save it.
Part 2- Driver Design:
Fig: Driver
Draw 2 circle with radius of 210mm and 160mm (construction circle). Now the draw the small circle o 40mm dia on the construction circle at any point.
Extrude the sketch except 20mm and for the other 40mm circle, extrude it from the selected face 60mm upwards.
Assembly of Part 1 & Part 2:
Select new and select assembly, now inside assemble select the part to be assembled.
Driver and Driven as parts for assembly, the first part will automatically get fixed and thus free the part by clicking right click on the part and set free. Whereas the second part is already set free as default.
Now use the Mates as to constrain the parts correctly,
Mates with faces;
At last as you see in above picture, make a sketch for the driver with convert entities and for the driven as well to make them rotate on the same sketch
At the end, give the distance for the driver and the driven wheel centers i.e. depends on the design. (Please keep a small gap in the driver & driven face)
Motion Study:
Now go to SOLIDWORKS ADD ONs and select SOLIDWORKS MOTION
Now select the MOTION ANAYLSIS for the motion study.
Tasks:
Now select the driver wheel center axis and select the motor; the motor will be constant speed (Speed can be set as per your preferences) and then distance, it will be in degrees and here you can also assign the start and end time.
Then set the contact and select the 2 parts.
Now either play the study or calculate the study.
And on below link, the output is attached for Speed of 10RPM and 1000deg distance.
And for plot select on plot, select force-select contact force-select magnitude. Below is the plot for the study.
Plot:
Angular Displacement of the driven wheel.
As we can see in the graph the angular displacement o the driven wheel is has raised from zero ad increased gradually for some time until the second contact was made and it fluctuated rapidly because of wobbling and jerk.
Compare the contact forces with and without "Precise contact"; Showing image for precise contact selection:
60 Frames per second for 10 rpm with precision contact;
As plot shows, the precision contact has almost similar contact forces for few cycle.
60 Frames per second for 10 rpm without precision contact;
As plot shows, without precision contact, the forces were raised at cycle interval of contact and it is very varied as compare to precision contact.
Together 60 Frames per second for 10 rpm with & without precision contact;
120 Frames per second for 20 rpm with precision contact;
As plot shows, the precision contact has almost similar contact forces for few cycle.
120 Frames per second for 20 rpm without precision contact;
As plot shows, without precision contact, the forces were raised at cycle interval of contact and it is very varied as compare to precision contact.
Export to excel:
https://drive.google.com/file/d/1vvJzFv73FkGC3eO5wd2FrBa_2soqySb0/view?usp=sharing
120 Frames per second for 20 rpm with precision contact;
120 Frames per second for 20 rpm without precision contact;
60 Frames per second for 10 rpm with precision contact;
60 Frames per second for 10 rpm without precision contact;
At rpm to 20 made a plot of the driver wheel's angular velocity and discussing the results;
Angular Velocity is increasing gradually as we can see in the graph. Yes, the fluctuations there and it will be until the wobbling and the jerk is in the motion.
Result and Explanation:
Link, the output is attached for Speed of 10RPM and 1000deg distance.
I have tried examining and observing the differences when the distance between the driver and driven varies and also tried to examine the difference with the change in speed.
The two major problem that I find will affect the Geneva Mechanism with contact force is ‘Jerking’ and the “Wobbling’.
The reason the wobbling begins is due to release of the driver wheel from the driven as all of sudden it became motionless, whereas the jerking start with first contact of the pin with driven wheel and it can be observed in the entrance then at the neck push of the driven and at the exist.
The Jerking can be controlled by keeping the optimum distance between the Driver and Driven Wheel.
However, it can be observed in below plot, the jerking/or the effect of contact force varies at different points at different speed.
In above plots, we can see there are more than 1 cycles of the driver in touch with driven thus there are more protuberance or eruption the contact force and rest of the time it almost stands at low. However, there were few instance where due to wobbling (from the 2nd cycle wobbling is increasing) the contact force may have raised in the driver and driven arc faces (no gap condition) but we did not recorded those as we have chosen 2 contact faces only.
The major finding is, as we can with change in speed the maximum contact force occurrence has changed. (10RPM and 20 RPM).
But for almost all instances, the contact force is max in the beginning as compare to the end(opposite to the external Geneva Mechanism) and I can conclude, this is because of the induced jerk with each cycle and also due to induces wobbling with create more and more contact. While passing on the 2 faces.
Errors or issues I faced:
Not any issues or errors but I discovered how differently we can align the driver to it position, the way I did by using last week learning but there are more. Then I wasn’t sure from where I can export the excel file, I did try find the way on internet but later I found it was right under my sleeves.
Reference:
- https://en.wikipedia.org/wiki/Geneva_drive
- Solidwork MOOC’s from Linkedin, which I did in past month.(I used the matting technique for the driver and driven by using convert entities and then matting the wheel on the new sketch.