MODELLING AND SIMULATION OF A GENEVA MECHANISM IN SOLIDWORKS AND ALSO USE THE MOTION STUDY TO FIND THE CONTACT FORCE

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
• Result and Explanation
• Errors or issues I faced
• Reference

Introduction:

What is Geneva Mechanism or Geneva Drive?

: 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.

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: Geneva Mechanism Assembly

Fig: Driver

Fig: Driven

Solidworks Part Design Procedure:

Reference Drawing for Parts Design;

Part 1- Driven Design:

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 10mm, 90mm and 200mm. Make 90mm circle for construction.

Now, from the constructional circle draw a straight line till last circle, and now select the offset Entity with bi-directional & cap end Arcs.

Now, trim the arc end and line on the last circle and after this, select circular sketch pattern of 5. 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 a construction line inside the pattern beside the reference pattern and again draw a construction line between these 2 pattern and placed this on 36deg from the both the construction lines as shown below in fig. this way we can define the 2 lines and sketch will be fully defined.

Now draw the 3 point circle for 2 point from the end of the 2 patterns and one should be the center of construction circles as shown below in the fig and make this circle as constructional. Now from the center of this circle, draw a circle of radius 52mm.

Now trim the extra lines and arc from the 53 mm circle except the arc on the 200mm circle as shown below in fig.

Now, again draw the circular sketch pattern for this arc of 5 and again trim the outer arc as shown below in the fig.

Extrude the sketch except the center circle of 10mm and save it.

Part 2- Driver Design:

Driver part design and the dimension are same as the construction circle we drew for the pattern arc i.e the 3 point circle and as we drew a circle of 106mm from the same center for the counter of the driver. Beside this circles, one circle of 10mm will be drawn at the meeting point of the construction between the face and the construction circle as shown below in the fig.

Extrude the sketch except the center circle of 10mm and for the other 10mm circle, extrude it from the selected face 20mm 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, give the distance for the front plane and driven wheel(select the face before giving the distance as the distance will depend on the alignment of the parts, as if the driver is above the driven the flip the direction of the distance mate.)

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.

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 20RPM and 1000deg distance.

https://youtu.be/fEw6Cosu448

And for plot select on plot, select force-select contact force-select magnitude. Below is the plot for the study

Plot:

Result and Explanation:

Video: Output for Speed 20RPM, Distance 1000deg and distance between the driven and driver is 110.22mm (no gap) https://youtu.be/fEw6Cosu448

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.

Plot: Output for Speed 20RPM, Distance 1000deg and distance between the driven and driver is 110.18mm (no gap)

Plot: Output for Speed 20RPM, Distance 1000deg and distance between the driven and driver is 110.22mm 0.04mm gap)

Plot: Output for Speed 30RPM, Distance 1000deg and distance between the driven and driver is 110.22mm (0.04mm gap)

Plot: Output for Speed 30RPM, Distance 1000deg and distance between the driven and driver is 110.20mm (0.02mm gap)

In above plots, we can see there are 4 cycles of the driver in touch with driven thus there are 4 protuberance or eruption the contact force and rest of the time it almost stands at zero. However, there were few instance where due to wobbling the contact force may have raised in the driver and driven arc faces(no gap condition)but we did not recorded those as the 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. (20RPM and 30 RPM)

The value of maximum contact force has changed to almost double in the speed change from 20RPM to 30RPM.

But for almost all instances, the contact force is least in the beginning and at the end 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:

I faced only one issue while building the assembly, it seems my matting were not similar to the explanation and thus I found my driven wheel below the driver wheel and thus while making the first distance mate, thus I chose flip direction of distance mate.

Reference: https://en.wikipedia.org/wiki/Geneva_drive