MOTION ANALYSIS OF INTERNAL GENEVA MECHANISM.

1. Objective:-

1.1 To Create 3D models for driver and driven wheels.

1.2 Perform motion analysis by rotating the driver wheel at 10rpm.

1.3 Obtain the following plots:
     - Contact force (between driving and the driven wheel) as a function of time.
     - Angular Displacement of the driven wheel.

1.4 Compare the contact forces with and without "Precise contact."
     - Showing image for precise contact selection.
     - Export the results that we get to Excel and make the plots over there.

2. Basic:-

The Geneva drive or Maltese cross is a gear mechanism that translates a continuous rotation movement into intermittent rotary motion. the difference between Geneva Drive mechanisms and other gears is that Geneva Drive mechanisms have unusual teeth. Unlike in typical gears, the interaction between the part being driven and the part doing the driving is not continuous and the resultant motion is intermittent. Geneva Drives work by having the driving part, or wheel, interlocking with the driven part intermittently. The number of spokes that the driven part has and the speed with which the driving part rotates determines the frequency and distance of travel. There is an elevated segment on the driving wheel which keeps the driven part in position till the next revolution is complete. The external Geneva Drive is by far the most common type, although there are other variations possible in the design. The external drive can be built to withstand higher levels of stress and are generally more robust and sturdy than other variants.

                     fig.1 External Geneva drive                                                      fig.2 Internal Geneva drive

 The Internal Geneva Drive has the driving part below the driven wheel, with the driving part often being much smaller than the driven wheel. 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.

3. Procedure:-

To do motion analysis of internal Geneva mechanism first we need to design 3D parts of the Geneva mechanism. Hence, to do that we are going to design individual part in Solidworks. i.e first part is the driving wheel and the second part is a driven wheel based on the following 2D drawing.

                                                            fig. 3 2D Drawing of Diver and Driven Wheel.

To do part design, open Solidworks software > choose a new option > choose a part design option. before designing part we need to verify our unit system which is as MMGS (i.e Millimeter, Gram, Second).

3.1 Driver part: 

We are going to design our first part which is Driving wheel based on 2D drawing. first select front plane and choose the option for sketching, draw a circle at Axis with diameter 420-millimetre diameter, similarly draw a small circle with diameter 40 millimetres and at a distance of 160 mm from the centre.

After completing 2D sketching we are going to use the Extrude option. to make the 2D drawing into 3D. Extrude Larger circle and smaller circle with 20 mm and 40 mm respectively, the final result of the first part is as shown below.

                                                                                      fig. 4 Driving Wheel 

After completing the driver part we are going to save it as a name of driving Wheel on our computer. 

3.2. Driven part:

our second part is driven part which is known as Driven wheel. again we are going to use front playing as a sketching plane. now Drawing a circle with diameter 1120. our driving wheel has slots for moving part of driver wheel. hence to draw a slot in the driven wheel we are going to draw x and y-axis as a reference. 

Using offset with  

after that, we are going to draw Internal circle with radius 120 millimetre 

which has Fillet of the 50-millimetre radius. 

Extruding with 40 millimetres. 

                                                                                       fig. 5 Driven Wheel 

After completing our driven part we are going to save it as a name of driving wheel in our computer.

3.3 Assembly Design:

To do motion analysis we are going to assemble the parts which we are designed previously. for more convenient we are using part 1 name for driving wheel and part 2 name for the driven wheel. first, we are going to open a new file and select assembly by taking the unit in consideration

Now, we are importing two parts in an assembly. 

Note:- the first part imported is always fixed to free it to right-click on Part-1 and click on float option, now we can move our part in our required direction. after that, we are going to use mate option to constrain or part.

we are going to select temporary Axis visible for more convenient in assembly. now we can see many axes in our parts. we are selecting a front plane of part-1 and assembly and choose to coincide option. another way to do that we can choose mate option in assembly and selecting a front plane of assembly and front plane of part-1

select Centre axis of part 1 and consider with write plane of assembly, similarly, we are going to coincide the centre axis of Part 1 with the top plain of assembly.

now we can see that our part 1 is fully constrained it can do only circular motion at its own axis.

further, we are going to a constraint our second part. to do that we are considering its Axis with the right plane of assembly and coinciding it. now we can see that our part is in line with part 1 but it still required proper constraint. hence we are going to find out the centre distance between part 1 and part 2 which is unknown to us, hence we are taking the approximate distance between the parts.

                                                                                       fig. 6 Assembly

3.4 Simulation and Analysis:

To do simulation and analysis (i.e motion analysis) we are going to choose the Solidworks motion option from Solidworks Add-Ins. now we can see that the motion analysis option which is shown below in the window of Solidworks.

To Rotate the Driving wheel we are going to assign motor for driving wheel. to do that select motor option in motion analysis and select part 1 axis. after that, we are going to assign solid body contact by selecting contact option and selecting part 1 and part 2. from Menu we can assign a material to the parts.

To analyse multiple parameters, we can use a plot option in motion analysis, the plot results as follows.

3.4.1 Analysis at 10 RPM:

1. Contact force:

The plot of contact force between driving and the driven wheel as a function of time is as follows.

The contact force is the same in magnitude in a constant interval of time.

2. Angular Displacement of the driven wheel:

From plot-2 we can see that the angular velocity of the driven wheel is initially decreased slightly after that we can see small sudden increment and gradual decrement overtime respectively.

3. Contact forces with Precise contact:

The contact force is continuously changing in a given time interval and we can calculate each and every point of a jerk as shown in the plot The Precise Contact gives a more accurate result.

3.4.2 Analysis at 20 RPM:

1. Contact force:

from the plot of contact force at 20 rpm and by comparing with contact force at 10 rpm, we can say that by increasing rpm of the motor we can reduce the jerk which will discuss in the "Error and error elimination section".

2. Angular Displacement of the driven wheel:

From plot-5 we can see that gradual decrement and sudden increment in the angular Displacement at 20 rpm. This trend is constant throughout the interval.

3. Contact forces with Precise contact at 120 FPS:

from the plot of contact force with the use of precise contact, we can say that the precise contact gives us more clear results. also, we can see small jerk also which is induced in operation.

3.4.3 Angular velocity of Driver wheel at 20 RPM :

As we saw in the plot-7 the angular velocity is lowest at the starting position of the rotation. As rotation of the Driver wheel increase the angular velocity also increases but, we can see that the constant line in the plot-7. The constant line indicates that when rotator/key is inserted in the slot of the driven wheel the angular velocity of the drive wheel is constant.

3.4.4 Comparing the contact forces with and without Precise contact:

By comparing the contact forces with and without 'Precise Contact' we can conclude that the Precise Contact gives a more accurate result as we can see that the plots. we can also see the small variation in the plot with the help of Precise Contact.

 3.4.5 Surface selection for Contact force:

Here, our surface selection for contact force is based on the rotation direction of drive and driven wheel. In our consideration, our drive and driven wheel both rotate in clockwise direction and gravity acts at the y-axis. Hence, we selected the surface of the rotator in drive wheel and surface of the driven wheel which is in contact with rotator in the clockwise direction as shown below.

                                                                          fig. 6 surface selection for contact force

4. Error & error elimination:- 

The common error we can get during the motion analysis is the jerk.

The first cause of jerk is due to improper alignment of the driven wheel and drive wheel. This may cause serious damage to the real-life situation. Hence to avoid this error we need to align the drive and driven wheel by changing the centre distance between them.

The second cause of jerk is due to low rpm of the motor. Because gravity is acting on the driven wheel the small distortion in the position of the drive wheel occurs. hence, the key is unable to insert in the slot smoothly. To avoid that we can increase the rpm of the motor or set up our assembly arrangement in such a way that the gravity effect is zero.

Note:- Always Remember the unit system while doing part design and Assembly Design.

5. Result and Conclusion:-

5.1 Final Output: 

5.2 Conclusion:

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 from the plot, a sudden increase in the contact force at the entry to the slot. As we increase the rpm the angular displacement is also increased. 

Reference:- 

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

The flow of Report:

1. Objective

2. Basic.

3. Procedure.

4. Error & error elimination.

5. Result and Conclusion.