Internal Geneva Mechanism

MOTION ANALYSIS OF “INTERNAL GENEVA MECHANISM” USING SOLIDWORKS

1. AIM :-To analyse the multi body dynamics for Internal Geneva Mechanism using SolidWorks.
2. OBJECTIVE :-

• To create 3D models for driver and driven wheels.
• To perform motion analysis by rotating the driver wheel at 10rpm.
• To obtain a plot for -

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

• To compare the contact forces with and without “Precise contact”.
• To obtain a plot for angular velocity of the driver (i.e. driving) wheel.
• To create motion study for following conditions -
1. 60 frames per second for 10 rpm motor speed.
2. 120 frames per second for 20 rpm motor speed.
• To export the obtained results to a spreadsheet.

3.INTRODUCTION :-

3.1 Multibody Dynamics:- A multibody dynamics (MBD) system is one that consists of solid bodies, or links, that are connected to each other by joints that restrict their relative motion.

3.2 Geneva Drive:- 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-shaped groove located in the outer wheel (driven wheel) that advances it by one step at a time. The main wheel also has an elevated circular blocking disc that “locks” the rotating driven wheel in position between steps.

The main difference between Geneva drive and other gears is that Geneva drive have unusual teeth. Unlike other gears, in Geneva drive the interaction between the driving and the driven part is not continuous and the resultant motion is intermittent.

3.3 Types of Geneva Drive :-

1. External Geneva Drive
2. Internal Geneva Drive
3. Spherical Geneva Drive

A.) External Geneva Drive :- In this External Geneva Drive, the driven wheel is connected with rotating drive wheel externally. This is the most popular Geneva Drive and used in many mechanical products like watch and film projector. It can withstand higher mechanical stresses.

fig.1: showing a six-position External Geneva Drive in operation

B.) Internal Geneva Drive :- In Internal Geneva Drive, the driven wheel is connected with the rotating drive wheel internally.

fig.2: Animation showing an Internal Geneva Drive in operation

Both the driver and the driven wheel rotate in the same direction. Its disadvantage is that it cannot be built smaller and cannot withstand higher mechanical stresses like external Geneva drive .

C.) Spherical Geneva Drive :- In Spherical Geneva Drive, the driven wheel is in spherical shape and is connected to rotating drive wheel externally. In this driving and the driven wheel are on perpendicular shafts i.e. input and output shafts are perpendicular to each other.

fig.3 Spherical Geneva Drive animation

3.4 Application of Geneva Drive :-

1. The most common application of Geneva Drive is Movie Projectors.
2. Bank Note Counting Machines.
3. Automated sampling machines.
4. Plotters- Plotters is a computer printer for printing vector graphics.
5. Wrist Watches.

4.PROCEDURE :-

For performing motion analysis of our Internal Geneva Mechanism, we will need 3D model of assembly of the Internal Geneva Mechanism.

So, first we need to design 3D model of driver and driven wheel. And for designing any 3D model, we have to design 2D sketch of the model first, after that we can proceed for designing 3D models.

fig.4: Dimensions for 2D drawing of Driverwheel

fig.5: Dimensions for 2D drawing of Driven Wheel

• All the dimensions are in the unit system of MMGS (i.e. millimeter, gram, second).
• All the sketches are in the front plane.

4.1 Driver wheel :

fig.6: Driver wheel

4.2 Driven wheel :

fig.7: Driven wheel

4.3 Assembly Design :

For performing assembly we need to import the 3D model of parts which have to be assembled; here those model parts are Driver wheel and Driven wheel.

First we are going to import the ‘Driver wheel’ only. After importing this we will see that our part is not able to move, it happens because when we import parts for assembly in SolidWorks then the part which is imported first will automatically get fixed. And if we want to make that part movable then we have to choose 'float' option instead of ‘fix’ for that particular part.

So, now we have imported our ‘driver part’ into assembly and made it to float. After that, we are going to use mate option to make our part constrain. Now we select the temporary axis to be visible, for our convenience in selection of location while performing assembly of parts. After that we are going to coincide the front plane of driver wheel and the front plane of assembly by using coincide option under mate. Now we coincide the center axis of driver wheel with the right plane of assembly. And we also coincide the center axis of driver wheel with the top plane of the assembly. Now we can see that our ‘driver wheel’ is fully constrained and can do only circular motion along it’s center axis, which we wanted. So, here our work is done with driver wheel.

Moving forward to ‘Driven wheel’, first going to import driven wheel 3D model into that assembly. We can see that it can move all around (i.e. float is already selected for driven wheel), so let's keep float selected for our driven wheel. Now, select temporary axis to be visible if not selected. And then we are going for mate selection by selecting front plane of driven wheel and front plane of assembly and providing a distance of 20 mm .

• Why 20 mm distance? -: Here, selecting 20 mm distance because our driven wheel have to be just above the driver wheel and the thickness of driver wheel is already given as 20 mm .

After that we are going to coincide the center axis of driven wheel with the right plane of assembly. Now we can see that our driven wheel came in the line with driver wheel but it is still not fully constraint; for this we have to provide a distance between both the center axes of driver and driven wheels.

For this, after taking and analyzing various distance measurements, I came on a conclusion that 224.50 mm distance between both the center axes of driver and driven wheel makes the assembly to work smoothly (i.e. without any unwanted collisions). Now, we can see that our driven wheel is also fully constraint.

fig.8: Internal Geneva Mechanism Assembly

4.4 Motion Study :

• Motion study can be done in SolidWorks by using ‘Motion Analysis’ under ‘SolidWorks Motion’ option from ‘SolidWorks Add-Ins’.

So, we are going to select ‘SolidWorks motion’ option from ‘SolidWorks Add-Ins’ and then going to select ‘Motion Analysis’ option which we can find below in the SolidWorks window.

5. BODY OF CONTENT :-

5.1 Study-1 (Motion Analysis with 60 frames per second and motor at 10 rpm constant speed) :

To rotate the driver wheel, we have to assign motor for the driver wheel first. To do that we are going to select motor and then rotary motor for motor type . Then selecting the center axis of driver wheel for motor position and giving clockwise direction for motor. Now, set up the motor speed to 10 rpm constant speed.

• Setting up time frame for our motion study to 10 seconds.

After that going to assign solid body contact by selecting contact option and selecting driver wheel and driven wheel. There we can also assign material to the models. So, here selecting ‘Steel (Dry)’ material for both driver and driven wheels.

Note: - Friction must be selected. [if we do not select friction then after few seconds of starting of motion, the driven wheel will not longer be in the contact with driver wheel; which will result failure of assembly].

Now, setting up the number of frames per second equal to 60 frames per second as -

Now, we will calculate the motion study and we get the different plots as by clicking on result and plots section as -

plot: Contact_Force Vs Time plot without ‘precise contact’ (60 frames per second, 10 rpm constant speed)

To Compare the contact force with “Precise Contact” :-

To do so we have to select the ‘Use Precise Contact’ option under ‘Motion Analysis’ and then calculate the motion study and we get the result as a plot for the same as -

plot: Contact Force Vs Time plot with ‘precise contact’ (60 frames per second, 10 rpm constant speed)

5.2 Study-2 (Motion Analysis with 120 frames per second and motor at 20 rpm constant speed) :

For this we will follow the same process as performed in our study-1 with a few changes in that, which are as follows -

First we will create new motion study and then set up the motor speed to 20 rpm constant speed.

• Set up time frame for our motion study to 10 seconds.

After that going to assign solid body contact between driver and driven wheels. And also assign material to the models. So, selecting again ‘Steel (Dry)’ material for both driver and driven wheels.

Now, setting up the number of frames per second equal to 60 frames per second as -

Now we will calculate the motion study and we get the different plots as by clicking on result and plots section as -

plot: Contact Force Vs Time plot without ‘precise contact’ (120 frames per second, 20 rpm constant speed)

To Compare the contact force with “Precise Contact” :-

To do so we have to select the ‘Use Precise Contact’ option under ‘Motion Analysis’ and then calculate the motion study and we get the result as a plot for the same as -

plot : Contact Force Vs Time plot with ‘precise contact’ (120 frames per second, 20 rpm constant speed)

7. ERRORS : -

The most common error is the collision between driver wheel and driven wheel at the corner of slot while driver wheel key (small cylindrical pin) enters in the slot of driven wheel at time of performing dynamic motion. It generates a vibration (jerk) in the assembly which can cause failure of assembly.

Causes due to which this jerk happens -

1. Improper alignment of driver and driven wheel.

• [ solution: - This can be solved by doing proper alignment of driver and driven wheel by changing the distance between their center axes ] .

1. It can also happen due to very less rpm of motor connected with driver wheel. Because gravitational force is already acting towards downwards on driver wheel, and if we provide very less rpm to motor then in this case the resultant force may varry large and may have some different direction, which can restrict smooth dynamic motion of driver wheel while entering into the slots of driven wheel.

• [ solution: - We can avoid it by increasing the force provided by motor to driver wheel, this can be done by increasing the rpm of motor ] .

8. CONCLUSIONS : -

1. The ‘Precise Contact’ gives us more accurate result as we can see from the plots of contact force Vs time with precise contact is less fluctuating and smooth as compare to the plots of contact force Vs time without precise contact.
2. Contact force at time of entry of driver wheel into slot of driven wheel increases with the increase in rpm of motor due to sudden contact takes place there at high speed of motor in a short interval of time.
3. We can also improve our model with help of ‘Motion Study’, because it gives us an idea about how our assembly is going to perform in real world scenario.
4. The Angular displacement plot of driven wheel shows that the displacement is repeating after a regular interval of time, because pin of driver wheel travels equal distance in every slot of driven wheel and also at the same time, the driver wheel is moving at a constant rpm (speed); and here driver wheel is the only one responsible for the rotation of driven wheel.
5. The Angular velocity plot of driver wheel tells us that as we increase the number of rotation of the driver wheel, the angular velocity of driver wheel also increases.

Link for parts and assembly :-

https://drive.google.com/drive/folders/1oUuYgtjsO8BDm85jfq6Ekbw9rPqDDKFG?usp=sharing

Animation vedio:-

https://youtu.be/doQAsj-oESc