Internal Geneva Mechanism

Project Title: Internal Geneva Mechanism

Aim:

1. To Create 3D models for driver and driven wheels using SOLIDWORKS.

2. To Perform motion analysis by rotating the driver wheel at 10rpm and 20 rpm respectively.

3. To Obtain the plots for:

• Contact force (between driving and the driven wheel) as a function of time.
• Angular Displacement and angular velocity of the driven wheel.

4. To Compare the contact with and without "Precise contact"

Introduction:

Geneva mechanism is commonly used indexing mechanism where an intermittent motion is required. The Inverse Geneva mechanism, which is a variation of the Geneva mechanism, is used where the wheel has to rotate in the same Direction as the crank. It requires less radial space and the locking device can be a circular segment attached to the crank that locks by wiping against a built up rim on the periphery of the wheel. The design and fabricating of a conventional Geneva mechanism is generally simple and inexpensive because there is no specially curved profile on any of the components except straight lines and circular arcs. However, due to the discontinuity of the acceleration at the beginning and ending positions, the shortcoming of using conventional Geneva mechanism is the large impact when the driving crank engages and disengages with the wheel slot.

Theory:

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.

Modelling and assembly:

The modelling of both the wheels as well the assembly is carried out using these two 2D sketches.

Fig-1

Fig-2

A CAD model of internal Geneva mechanism is created consisting of a driven wheel and a driving wheel using the sketches shown above. The two components are then assembled using assembly mates.

Fig-3

Motion analysis setup:

A motor is applied to the driving wheel and a contact body pair is created in between diving and driven wheel.

We have 4 cases in this.

1. Rpm of motor = 10 and it is without precise contact (60 Frames).

1. Rpm of motor = 10 and it is with precise contact (60 Frames).

1. Rpm of motor = 20 and it is without precise contact (120 Frames).

1. Rpm of motor = 20 and it is with precise contact (120 Frames).

Result and discussion:

For 10 RPM:

• Contact force between the wheels

• Contact force between the wheels with precision of 60 frames per sec

• Angular velocity of the driven wheel

• Angular velocity of the driven wheel with precision of 60 frames per second

• Angular displacement of the driven wheel

• Angular displacement of the driven wheel with precision of 60 frames per second

For 20 RPM:

• Contact force between the wheels

• Contact force between the wheels with precision of 120 frames per second

• Angular velocity of the driven wheel

• Angular velocity of the driven wheel with precision of 120 frames per second

• Angular displacement of the driven wheel

• Angular displacement of the driven wheel with precision of 120 Frames per second

CONCLUSION:

1. From these plots we see that contact force increases with increase in rpm.
2. Precise contact gives analytically better results.
3. Angular velocity of the driven wheel increases and then decreases and then remains constant for some time and this pattern repeats. (slightly different curve pattern for 10 RPM and 20 RPM)
4. Max angular velocity of the driven wheel increases as the RPM increases.
5. Angular displacement of the driven wheel remains constant as RPM increases.

The Animations for Internal Geneva Mechanism:

For 10 rpm:

https://youtu.be/Wro7es_yLCE

With Precision:

https://youtu.be/4M4KBw-wyro

For 20 rpm:

https://youtu.be/WtwOMRmhpmA

With Precision:

https://youtu.be/ND7ew8ytauo