Geneva Mechanism

INTRODUCTION

Geneva Mechanism or a Maltese cross or Geneva stop is the most commonly used mechanism to produce intermittent mechanism. Geneva mechanism is a unique form of Gear mechanism, but the motion is brought to stand still for certain fraction of period. This stoppage and start is periodic and there is no reverse motion.

Geneva mechanism has a driver wheel, driven wheel and the pin. The driver wheel has the pin and pin mates with the slot in Driven wheel. As long as the pin is in contact with the slot, motion is transmitted from Driver wheel to driven wheel, but when the slot loses contact with the pin, the driver wheel is brought to standstill.

Geneva mechanism is used predominantly in indexing operation. During stop condition, some operation is performed on the workpiece. The workpiece is moved when the operation is complete and the next workpiece is brought in. This mechanism results in efficient way of controlling the indexing and intermittent mechanism.

Mechanism can be achieved in two forms

a) Internal Geneva mechanism

b) External Geneva Mechanism

MOTION BODY SIMULATION

GEOMETERY

The Driver is modelled as follows

The driven wheel is modelled as follows

The Driver wheel and driven wheel is assembled, with string of Coincident and offset mating criterion.

Geneva Mechanism Assembly

ANALYSIS

4 cases are run. Comparison between 10RPM and 20RPM is done for Precision and Without precision settings. They are controlled using the settings.

ANGULAR DISPLACEMENT PLOT

Angular displacement for all 4 cases for the driven wheel is plotted as shown in the diagram.

For every one revolution of the Driving wheel, Driven wheel moves by 90 degrees,

For 10RPM Case, we would expect driving wheel to make 10 revolutions per minute, but driven wheel would make 1 revolution for 4 revolutions of the Driving wheel. Hence in 25s, it would make about ~1 revolution

Similarly for 20RPM case, driving wheel would make 20 revolutions per minute, driven wheel would make 5 revolutions, For 25s, it would make about 2 revolutions.

CONTACT FORCES

When pin contacts the slot, contact force is established between driving and driven wheel. However, contact force wouldnt be constant and reduces when the contact is broken. Precision contact when selected should ideally capture the exact contact nature between driving and driven wheel

But, graph shows 10RPM Without precision contact. This graph shows that simulation did not run well and the contacts were made abnormally,

Hence this result and focussing on other three,

From the setup, we would expect 8 periodic spikes for 20RPM, 4 periodic spikes for 10RPM

From the graph, 20RPM and 10RPM Precision seems to follow Theoretical predications, However, 20RPM without precision doesnt seem to follow

DISCUSSION ON THE RESULTS

1) The animated videos can be found in the link

External Geneva Mechanism: https://youtu.be/i0nzk0UB4Wk

Internal Geneva Mechanism: https://youtu.be/gybCcIS3EME

2) For x revolutions of the driver per minute, Driven wheel would make x/number of slots number of revolutions per minute. Example: For driven wheel of 4 slots, for driving wheel making 4 revolution per minute, driven would make 1 revolution

3) Contact force can be, similarly periodic. Abnormal periods of contact force indicates improper simulation

4) Precision contact when turned on produces better results and maintains possible scenario of contact. Without precision contact, Pin can be seen intruding into the driving wheel which is not possible in reality.