Internal Geneva Mechanism

AIM: To do multibody dynamics (MBD) analysis of Internal Geneva Mechanism using SOLIDWORKS

OBJECTIVES:

1. Create 3D models for driver and driven wheels.
2. Perform motion analysis by rotating the driver wheel at 10 rpm first and then at 20 rpm.
3. Make plots on Excel by exporting the results obtained by these following plots:

• Contact force (between driving and the driven wheel) with and without “Precise contact" as a function of time.
• Angular displacement of the driven wheel.
• Driver wheel’s angular velocity.

THEORY:

The Geneva drive or a Maltese cross is a gear mechanism that translates a continuous rotation movement into intermittent rotary motion. The main difference between Geneva mechanism and gears is the unusual teeth of Geneva mechanism. In typical gears, the meshing between the driven and the driving part is continuous but in Geneva mechanism the meshing is not continuous and the motion is intermittent.

The internal Geneva mechanism has the driving wheel below the driven wheel which is often being much larger in size than the driving wheel. The angle by which the driving wheel has to rotate to effect one step rotation of the driven wheel is always smaller than 180 whereas in external Geneva mechanism it’s always greater than 180.

Motion studies are graphical simulations of motion for assembly models. Motion studies do not change an assembly model or its properties. They simulate and animate the motion we prescribe for a model. By using SolidWorks mates to restrict the motion of components in an assembly when we model motion. From motion studies, motion analysis can also be done for accurately simulating and analyzing the effects of motion elements. It can also be used to plot simulation results for further analysis.

PROCEDURE:

• For motion analysis we will first design a 3D model of driving and driven wheel separately using there dimensions. (all dimensions are in mm)

• After designing the parts separately, assembly is created using both the parts. After exporting both the parts in SolidWorks, the first part is made floating by right clicking on the first part and then selecting float option. After selecting temporary axis visible option, we will be using appropriate mate option to constrain a part.
• Now we will do motion analysis of the assembly for that we will choose SolidWorks motion option from SolidWorks Add-Ins. After this we will provide a motor at the center axis of the driving wheel, for that select the motor option and then select the axis of the driving wheel provide the rotation speed in rpm. Now we will assign solid body contact so that as the driving wheel rotates the driven wheel will also rotate accordingly, while assigning the contact we will assign a material to both the parts. For analyzing different parameters, select the results and plot option from the bottom menu bar and get plots of different parameters as follows:

1. Angular displacement of driven wheel.
2. Contact forces as a function of time (with an without precise contact)
3. Angular velocity of driving wheel.

• All the results are calculated first at 10 rpm and then at 20 rpm. Also contact forces with precise contact option should be done for 10 rpm at 60 FPS and for 20 rpm at 120 FPS.The option of “with precision contact” is selected from the settings button from the menu bar and from there the FPS can also be manipulated.

PLOTS:

• AT 10 RPM

• Here we can see that the angular displacement driven wheel is changing with respect to time. It is constant when the driven wheel is not rotating.

• The contact force between driving and driven wheel can be seen. It is zero when the stud present on the driving wheel is not in contact with the driven wheel that is when the stud leaves the groove till when it enters the next groove. As it is without precise contact option it is not that accurate.

• The contact force between driving and driven wheel can be seen. It is zero when the stud present on the driving wheel is not in contact with the driven wheel that is when the stud leaves the groove till when it enters the next groove. As it is with precise contact option it is very much accurate. Slight jerks can also be seen.

• The angular velocity is same throughout.

• AT 20 RPM

• Here we can see that the angular displacement driven wheel is changing with respect to time. It is constant when the driven wheel is not rotating. We can also see the gradual decrement and then sudden increment in the angular displacement.

• Here we can the contact forces at 20 rpm, as it is without precise contact the results are not that accurate. By comparing it with the data of contact forces at 10 rpm we can see that the time duration between contacts has reduced and because of it the jerk has also reduced.

• Here we can see that when the plot is made of contact forces with the use of precise contact the results are more accurate.

• Angular velocity is increasing linearly.

CONCLUSION:

• When we plot contact forces using the precise contact option we are able to get more accurate results. We can see the results from different plots and can analyze the effect of rpm on different parameters.

DRIVE LINK:

Flies on Internal Geneva mechanism

YOUTUBE LINK:

Animation of internal Geneva mechanism