Internal Geneva Mechanism

Aim of the Project :

To create the 3D model of the internal Geneva mechanism using Solid Works from the following 2D CAD drawings. -

Objectives:

1. To perform motion analysis by rotating the driver wheel at 10rpm and then 20rpm.
2. To obtain the following plots
   1. Contact force (between driving and the driven wheel) as a function of time.
   2. Angular Displacement of the driven wheel.
3. To compare the contact forces with and without "Precise contact" (Using 60 Frames per second for 10 rpm and then 120 Frames per second for 20 Rpm).
4. After completing the previous study, changing the rpm to 20 and to make a plot of the driver wheel's angular velocity and discuss the results.

THEORY:

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.

INTERNAL GENEVA DRIVE:

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.

Figure: Internal Geneva drive

Pictures of the 3D model:

Driven wheel

Driving wheel

Spindle

The whole assembly

Link for all the above files of parts and assembly:

https://drive.google.com/drive/folders/1RjOjeMtshNfffs6ITqyWHxvLXGpZL3Th?usp=sharing

RESULTS AND PLOTS:

Plots without "Precise Contact":

At Motor Speed = 10 rpm

• The plot of Contact forces between Driven and the driving wheel

• The Plot for the Angular Displacement of the Driven Wheel

• The plot of Driver Wheel's Angular Velocity:

At Motor Speed = 20 rpm

• The plot of Contact forces between Driven and the driving wheel

• The Plot for the Angular Displacement of the Driven Wheel

• The plot of Driver Wheel's Angular Velocity:

Plots with "Precise Contact": (fps = 60 for 10rpm and fps = 120 for 20 rpm)

At Motor Speed = 10 rpm, fps = 60

• The plot of Contact forces between Driven and the driving wheel

• The Plot for the Angular Displacement of the Driven Wheel

• The plot of Driver Wheel's Angular Velocity:

At Motor Speed = 20 rpm, fps = 120

• The plot of Contact forces between Driven and the driving wheel

• The Plot for the Angular Displacement of the Driven Wheel

• The plot of Driver Wheel's Angular Velocity:

Animation of the Internal Geneva Mechanism:

https://youtu.be/oF0j0XMv63s

Results and Discussions:

1. The 'Precise Contact' gives a more accurate result as we can see that the curves in the plots with precise contact becomes less fluctuating and smooth. Moreover, the values obtained from the precise contact plot are more flexible and true.
2. When the driver tries to enter in the slot of the driven wheel it faces a jerk as the entry is not smooth therefore, we can see also from the plot that at the entry to the slot gives rise to a sudden increase in the contact force which decreases as the driver enters into the slot.
3. We can also see from the contact force plots of the 2 RPM's that as we increase the rotation of the driver the jerk force/contact force at the entry also increases and then it drastically decreases.
4. The angular velocity plot reveals that at first, there is a jerk felt by the driven wheel and we see a peak in the angular velocity plot and then when it enters the slot there is an acceleration in the driven wheel but as the motion progresses there is a reduction in the velocity and hence retardation occurs. The maximum velocity is at the point where the pin of the driver reaches the bottom point of the driven wheel.
5. The angular velocity of the driven wheel also increases if we increase the rotation of the driver which is seen from the plot.
6. The angular displacement plot shows that the displacement occurs between -180 to 180 degrees and it repeats the pattern in each revolution starting from 0.

References:

https://en.wikipedia.org