Internal Geneva Mechanism

Aim: To create 3D model for Internal Geneva mechanism and simulation of the mechanism for motion study

Introduction:

Geneva mechanism is an intermittent motion mechanism which produces the repeated intermittent motion from a driver under constant rotation. The mechanism consists of a driving unit with a pin and specific number of radial slots in the driven or output part. Both the parts rotate about their axis with a constant centre distance.

At the start of engagement, pin enters a particular slot tangentially and due to ever-changing direction of contact force the driven part rotates and halts until the pin leave that particular slot and enters the another one. There are two types of Geneva mechanism namely Internal and external Geneva Mechanism.

External Geneva mechanism is used when the sense of input and output link is different and internal one is used when sense is same.

Road Map for project completion

• Creation of 3D model using the given drawing and making an assembly by calculating centre distance using available equations.
• Setting and executing up motion study with output plots namely contact force between the bodies as a function of time, angular displacement of the driven wheel and comparing contact force with and without Precise Contact

Centre distance calculations

Expression used:

`d= R/(cos⁡(β_0/2))`

Where,

R = Crank Radius

d = centre distance

`β_0= 360/(n)`

Where,

n = number of slots

Given:

Number of slots = 4

R = 160 mm

Hence,

`β_0= 360/4=90°`

and

`d= 160/(cos⁡(90/2)) =226.27 mm`

Motion study setup:

First of all Solid works Motion must be enabled in Solid works Add-ins ribbon

Later you will see a ribbon in the downward section with entire motion analysis tools, right click on the motion study 1 to make new motion study make sure that motion analysis is checked.

First we will set the assembly at the point where pin is about to enter in the slot of the driven plate tangentially.

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Let us set up input, we are putting a motor on the driver link axis to rotate with a constant speed of 10 rpm

Now, we will select both bodies to establish contact and allotting material as dry steel to set motion study for 10 seconds and will start calculating the motion analysis. The link of video is given below

Link:

https://drive.google.com/file/d/1ZGrsYoZSTAy3LwZ9ccZY-nyWVXR1eSOy/view?usp=sharing

Now Let us create and discuss plots

Plot1: Angular Displacement of Driven wheel

In the plot above we can observe that the angular displacement decreases gradually while the two links are in contact and remains approximately constant while the pin leaves one slot and enters the another one.

Plots: Contact force comparison between the links of the mechanism at 10 RPM with and without precise contact

Plot2: Contact forces at 10 RPM without Precise Contact

In the Plot above we can see the contact forces increases suddenly while at the entry of driver’s pin in the driven link’s slot and remains constant while the bodies are in contact.

Contact forces increases suddenly due to jerk induced by inertia effect. We can nullify the effect while real implementation of bearings.

Plot3: Contact forces at 10 RPM with Precise Contact at 60 frames per second

Fig: Selecting precise contact at 60 fps

In the plot above, precise contact gives more accurate results

Plots: Contact force comparison between the links of the mechanism at 20 RPM with and without precise contact

Plot4: Contact forces at 20 RPM without Precise Contact

In the plot above, observation related to contact forces that it remains constant in fixed intervals which are less here as compared to 10 RPM plot due to increased Rotations. Jerk observations are clearer here to rectify which is the advantage over calculating forces at low speed.

Plot5: Contact forces at 20 RPM with Precise Contact at 120 frames per second

Fig: Selecting precise contact at 120 fps

In the above plot more accurate results are obtained

Plot6: Angular velocity of the driving wheel at 20RPM

In the plot above the angular velocity of the driving wheel increases gradually as the direction of net force the body is changing gradually because of the pin entering and leaving the slot.

CAD FILES

https://drive.google.com/drive/folders/18Jbscc7KEjR415FT4Tf0dO6vNckxvQR9?usp=sharing